BMSK · Public Zone
A reference book of the weather terms used in forecasts and warnings.
Choose a chapter to explore weather terms.
| Zone | Districts |
|---|---|
| North-West Zone | East Champaran, Gopalganj, Siwan, Saran, and West Champaran |
| North-Central Zone | Darbhanga, Madhubani, Muzaffarpur, Samastipur, Sheohar, Sitamarhi and Vaishali |
| North-East Zone | Araria, Kishanganj, Madhepura, Purnea, Saharsa and Supaul |
| South-Central Zone | Begusarai, Gaya, Lakhisarai, Nalanda, Nawada, Patna and Sheikhpura |
| South-West Zone | Arwal, Aurangabad, Buxar, Bhojpur, Jehanabad, Kaimur (Bhabua) and Rohtas |
| South-East Zone | Banka, Bhagalpur, Jamui, Katihar, Khagaria and Munger |
Geographical zones of Bihar
| Type | Description |
|---|---|
| One or two spells of rain | In a 24 hrs time, rainfall occurring with a frequency of 1–2 spells. |
| A few spells of rain | In a 24 hrs time, rainfall occurring with a frequency of more than 2 spells but with well-defined dry spells in between. |
| Intermittent rain | In a 24 hrs time, rainfall occurring with a frequency more than "A Few Spells" but is discontinuous and without the character of a shower. |
| Drizzle | Liquid precipitation in the form of water drops of very small size (radius between ~100 and 500 µm). |
| Rain | Liquid precipitation in the form of water drops of radius between ~500 and 2500 µm. |
| Shower | Solid or liquid precipitation from a vertically developed cloud; often characterised by short duration and rapid fluctuations of intensity (radius >2500 µm). |
| Hailstorm | Solid precipitation in the form of balls or pieces of ice (hailstones) with diameters ranging from 5 to 50 mm or more. |
| Thunderstorm | One or more sudden electrical discharges manifested by a flash of light (lightning) and a sharp rumbling sound (thunder). |
| Duststorm | An ensemble of particles of dust or sand energetically lifted to great heights by a strong and turbulent wind. Visibility below 1000 m. |
| Mist | A phenomenon of small droplets suspended in air. |
| Dew | Condensation of water vapour on a surface when temperature is reduced by radiational cooling to below the dew-point of the air in contact with it. |
| Fog | A phenomenon of small droplets suspended in air; visibility is one kilometre or less. |
| Frost | Occurs when the temperature of the air in contact with the ground or at thermometer-screen level is below the freezing-point of water. |
| Haze | An atmospheric phenomenon where dust, smoke and other dry particles obscure the clarity of the sky. |
| Smog | Smoke and fog together reduce the visibility. |
| Squally weather | Occasional or frequent squalls with rain or persistent strong gusty winds (mean wind speed ≥20 knots) with rain; associated with low pressure systems or monsoon onset. |
| Visibility | The greatest distance that a prominent object can be seen and identified by unaided, normal eyes. |
| Type | Description |
|---|---|
| Nowcasting | A short range forecast having a lead time/validity of less than 24 hrs. |
| Short range forecasts | Forecasts having a lead time/validity period of 1 to 3 days. |
| Medium range forecasts | Forecasts having a lead time/validity period of 4 to 10 days. |
| Long range / Extended Range forecasts | Forecasts having a lead time/validity period beyond 10 days. Usually issued for a season. |
| Local Forecast | Whenever any weather phenomenon is expected, its intensity, frequency and time of occurrence is indicated. Valid for a radius of 50 km around the station; updated 4 times a day. |
| Term | Description |
|---|---|
| Cyclonic Circulation (Cycir) | Atmospheric wind flow in upper levels associated with any low-pressure system. Counter-clockwise in the Northern Hemisphere, clockwise in the Southern Hemisphere. |
| Anticyclonic Circulation | Atmospheric wind flow in upper levels associated with any high-pressure system. Clockwise in the Northern Hemisphere, counter-clockwise in the Southern Hemisphere. |
| Low Pressure Area (LOPAR) | Area in the atmosphere where pressures are lower than surrounding regions at the same level; represented on a synoptic chart by one closed isobar (surface wind <17 Kts at sea). |
| Depression | Intense low-pressure system with 2–3 closed isobars at 2 hPa interval; wind speed 17–27 Kts at sea. |
| Deep Depression | Intense low-pressure system with 2–3 closed isobars at 2 hPa interval; wind speed 28–33 Kts at sea. |
| Cyclonic Storm | Intense low-pressure system with >4 closed isobars at 2 hPa interval; surface wind speed 34–47 Kts. |
| Severe Cyclonic Storm | Surface wind speed 48–63 Kts. |
| Very Severe Cyclonic Storm | Surface wind speed 64–119 Kts. |
| Super Cyclonic Storm | Surface wind speed ≥120 Kts. |
| Western Disturbance | Weather disturbance originating over the Mediterranean Sea, Caspian Sea and Black Sea, moving eastwards across north India; shows as cyclonic circulation/trough in mid and lower tropospheric levels. |
| Western Depression | Weather system originating over the Mediterranean/Caspian/Black Sea approaching northwest India; defined by two or more closed isobars on the surface. |
| Induced Low | Under the influence of a western disturbance, a low sometimes develops to the south of the system. |
| Induced Cyclonic Circulation | A cyclonic circulation sometimes developing to the south of a western disturbance. |
| Trough | A line or curve along which atmospheric pressure is minimum; pressure increases on both sides. |
| Trough in westerlies | A moving wave perturbation in mid-latitude regions moving west to east, affecting northern parts of India. |
| Trough in easterlies | A moving wave perturbation in the equatorial easterly wave, moving east to west. |
| Easterly Waves | A shallow trough disturbance in the easterly current of the tropics; its westward passage is followed by intensified cloudy, showery weather. Affects southern peninsular India. |
| Shear Line | A line across which there is an abrupt change in the horizontal wind component; a line of maximum horizontal wind shear. |
| Ridge | An elongated area of relatively high atmospheric pressure associated with maximum anticyclonic curvature of wind flow. |
| Wind-discontinuity | A line across which there is an abrupt change in wind direction. |
| Troposphere | The atmospheric layer in which all significant weather phenomena occur; characterised by decreasing temperature with height. |
| Lower troposphere level | Part of the troposphere up to 2.1 km above mean sea level (a.s.l.). |
| Mid troposphere level | Part of the troposphere from 2.1 km a.s.l. but below 7.6 km a.s.l. |
| Upper troposphere level | Part of the troposphere from 7.6 km a.s.l. to the height up to which temperature decreases with height. |
| Part of Day | IST | UTC |
|---|---|---|
| Early hours of (date) | 0000 – 0400 hrs | 1830 – 2230 hrs |
| Early morning | 0400 – 0600 hrs | 2230 – 0030 hrs |
| Morning | 0600 – 0800 hrs | 0030 – 0230 hrs |
| Forenoon | 0800 – 1200 hrs | 0230 – 0630 hrs |
| Around noon | 1100 – 1300 hrs | 0530 – 0730 hrs |
| Afternoon | 1200 – 1600 hrs | 0630 – 1030 hrs |
| Evening | 1600 – 2000 hrs | 1030 – 1430 hrs |
| Night | 2000 – 2400 hrs | 1430 – 1830 hrs |
| Condition | Octa Coverage |
|---|---|
| Clear sky | 0 Octa |
| Mainly clear | 1–2 Octa of sky covered |
| Partly cloudy | 3–4 Octa of sky covered |
| Generally cloudy | 5–7 Octa of sky covered |
| Cloudy | >7 Octa of sky covered |
Intensity of Rainfall
| Term | Amount (mm / 24 hrs) |
|---|---|
| No Rain | 0.0 |
| Very Light Rain | 0.1 – 2.4 |
| Light Rain | 2.5 – 15.4 |
| Moderate Rain | 15.5 – 64.4 |
| Heavy Rain | 64.5 – 115.5 |
| Very Heavy Rain | 115.6 – 204.4 |
| Extremely Heavy Rain | ≥ 204.5 |
| Exceptionally Heavy Rain | When amount is near the highest recorded rainfall at/near the station for the month or season (must exceed 12 cm). |
Weekly / Seasonal Rainfall Distribution — Regional Scale
| Term | % Departure from Normal |
|---|---|
| No Rain | −100% |
| Large Deficient | −60% or less |
| Deficient | Between −59% to −20% |
| Normal | Between −19% to +19% |
| Excess | Between +20% to +59% |
| Large Excess | +60% or more |
Rainfall Distribution — All India Scale
| Term | Condition |
|---|---|
| Normal | % departure of realised rainfall is within ±10% of the Long Period Average. |
| Below Normal | % departure is <−10% of the Long Period Average. |
| Above Normal | % departure is >+10% of the Long Period Average. |
| All India Drought Year | Rainfall deficiency >10% and 20–40% of the country is under drought conditions. |
| All India Severe Drought Year | Rainfall deficiency >10% and spatial coverage of drought is >40%. |
Spatial Distribution of Rainfall
| Distribution | No. of Places | Station Coverage |
|---|---|---|
| Isolated | One or two places | <25% of stations get rainfall. |
| Scattered | At a few places | 26–50% of stations get rainfall. |
| Fairly Widespread | At many places | 51–75% of stations get rainfall. |
| Widespread | At most places | 76–100% of stations get rainfall. |
| Dry | — | No station reported rainfall. |
| Term | Description |
|---|---|
| Monsoon | The seasonal reversal of winds and associated rainfall. Derived from Arabic "Mausim". The annual oscillation of the Sun between the Tropics of Cancer and Capricorn causes oscillation in temperature, pressure, wind, cloudiness, and rain. |
| Southwest Monsoon | The southwesterly wind flow occurring over most parts of India and Indian Seas from June to September. |
| Onset of Southwest Monsoon | Commencement of rainy season with establishment of monsoon flow. Normal dates: South Andaman Sea — 20 May; Kerala — 1 June; Mumbai — 10 June; New Delhi — 29 June; Entire country — 15 July. Bihar: 15 June. |
| Withdrawal of Southwest Monsoon | Cessation of southwest monsoon rainfall. Normal date from extreme west Rajasthan — 15 September. Bihar: 30 September. |
| Northeast Monsoon | With withdrawal of southwest monsoon from northern/central India by first half of October, winds change from southwesterly to northeasterly (October–December). Major rainfall period for south peninsula; contributes ~60% of annual rainfall in coastal Tamil Nadu. |
Terms used during Southwest Monsoon Season (June – September)
| Term | Condition |
|---|---|
| Weak / Subdued Monsoon | Rainfall less than half the normal (land); wind speed up to 12 knots (sea). |
| Normal Monsoon | Rainfall half to less than 1½ times the normal (land); wind speed 13–22 knots (sea). |
| Active Monsoon | Rainfall 1½–4 times the normal; at least two stations with ≥5 cm (west coast) or ≥3 cm (elsewhere); fairly widespread to widespread; wind speed 23–32 knots (sea). |
| Vigorous Monsoon | Rainfall >4 times the normal; at least two stations with ≥8 cm (west coast) or ≥5 cm (elsewhere); widespread; wind speed ≥33 knots (sea). |
| Northern Limit of Monsoon (NLM) | A line delineating regions where monsoon has advanced from regions where it is yet to set in. |
| Axis of Monsoon Trough | A NW–SE oriented trough across the northern plains. Normal position passes through Ganganagar–Allahabad–Kolkata to Head Bay. |
| Break Monsoon | Monsoon trough shifts northwards close to the Himalayan foothills, causing drastic reduction in rainfall outside foothills and southernmost Peninsula. |
| Off-shore Trough | The trough along the west coast generally running from Gujarat to Kerala coasts; generally seen from the surface to 0.9 km a.s.l. |
| Term | Description |
|---|---|
| Temperature | The condition of a body that determines its ability to communicate heat to or receive heat from other bodies. Measured by thermometer; India Meteorological Department uses the Celsius scale. |
| Air Temperature | Temperature measured in an enclosed space with free airflow, not directly exposed to sunlight, with the thermometer at 1.2 m above the surface. |
| Maximum Temperature | The highest air temperature recorded in a day. |
| Minimum Temperature | The lowest air temperature recorded in a day. |
| Dew Point Temperature | The temperature to which moist air must be cooled (at constant pressure and moisture content) for condensation to occur. Lower dew point indicates lesser moisture content. |
| Freezing Point | The constant temperature at which solid and liquid forms of pure water are in equilibrium at Standard Atmospheric Pressure. |
24-hr Temperature Change Nomenclature — Maximum Temperature
a) When normal maximum temperature is 40°C or below
| Nomenclature | Past 24-hr change |
|---|---|
| Little change | −1°C to +1°C |
| Rise | 2°C |
| Appreciable rise | 3°C to 4°C |
| Marked rise | 5°C to 6°C |
| Large rise | 7°C or more |
b) When normal maximum temperature is more than 40°C
| Nomenclature | Past 24-hr change |
|---|---|
| Little change | −1°C to +1°C |
| Rise | 2°C |
| Marked rise | 3°C to 4°C |
| Large rise | 5°C to 6°C |
24-hr Temperature Change Nomenclature — Minimum Temperature
a) When normal minimum temperature is 10°C or more
| Nomenclature | Past 24-hr change |
|---|---|
| Little change | +1°C to −1°C |
| Fall | −2°C |
| Appreciable Fall | −3°C to −4°C |
| Marked Fall | −5°C to −6°C |
| Large Fall | −7°C or less |
b) When normal minimum temperature is less than 10°C
| Nomenclature | Past 24-hr change |
|---|---|
| Little change | +1°C to −1°C |
| Fall | −2°C |
| Marked Fall | −3°C to −4°C |
| Large Fall | −5°C or less |
Heat Wave Criteria
Other Temperature-Related Terms
| Term | Description |
|---|---|
| Normal | Departure of min/max temperature from normal is ±1°C. |
| Above normal | Departure is +2°C. |
| Appreciably above normal | Departure is +3°C to +4°C (normal max ≤40°C). |
| Markedly above normal | Departure is +5°C to +6°C (normal max ≤40°C). |
| Hot Day | Max temperature ≥40°C and min temperature ≥5°C above normal, but not satisfying heat wave criteria. |
| Heat Wave | Departure of max temp from normal: +4°C to +5°C or more (where normal max >40°C); +5°C to +6°C (where normal max ≤40°C). Also declared when actual max ≥45°C regardless of normal. |
| Severe Heat Wave | Departure of max temp from normal: +6°C or more (normal max >40°C); +7°C or more (normal max ≤40°C). |
| Hot Wind (Loo) | Hot wind over the plains of northwest India that reduces moisture causing dehydration; prolonged exposure may prove fatal. |
| Cold Day | Declared when maximum temperature is ≤16°C in plains (foggy winter conditions with max temperature much below normal). |
| Cold Wave | Departure of wind chill effective minimum temperature (WCTn) from normal min: −5°C to −6°C (where normal min >10°C); −4°C to −5°C elsewhere. Also when WCTn <0°C regardless of normal min. |
| Severe Cold Wave | Departure of WCTn from normal min: −7°C or less (normal min >10°C); −6°C or less elsewhere. |
| Term | Description |
|---|---|
| Wind | Atmospheric motion characterised by direction and speed. The direction of the wind is the direction from which it approaches the station (e.g., Northerly wind — wind approaching from North). |
| Calm | The absence of apparent motion in the air. |
| Gales | A very strong wind (34 to 47 knots). |
| Squall | A sudden increase of wind speed by at least 3 stages on the Beaufort Scale, the speed rising to force 6 or more, and lasting at least one minute. |
| Gust | A rapid increase in the strength of the wind relative to the mean strength at the time. |
| Term | Description |
|---|---|
| Clouds (Low/Medium/High) | Aggregates of very small water droplets, ice crystals, or a mixture, with base above the earth's surface. High clouds (5–13 km): Ci, Cc, Cs. Medium clouds (2–7 km): Ac, As, Ns. Low clouds (0–2 km): Sc, St, Cu, Cb. |
| Condensation | The process of formation of a liquid from its vapour; in meteorology, the formation of liquid water from water vapour. |
| Convection | A mode of heat transfer within a fluid involving movement of substantial volumes of the substance; of fundamental importance in effecting vertical exchange of heat and other air-mass properties throughout the troposphere. |
| Freezing-Level | The lowest height above mean sea level at which, for a given place and time, the air temperature is 0°C. |
| Isobar | A line of constant (atmospheric) pressure. |
| Isohyet | A line of constant rainfall amount. |
| Isotach | A line of constant wind speed. |
| Isogon | A line of constant wind direction. |
| Isotherm | A line of constant temperature. |
| Streamline | A line parallel to the instantaneous direction of the wind vector at all points along it. |
| Jet Stream | A fast narrow current of air, generally near the tropopause, characterised by strong vertical and lateral wind shears; thousands of kilometres in length, hundreds in width, a few kilometres in depth. |
| Latent Heat | The quantity of heat absorbed or emitted, without change of temperature, during a change of state of unit mass of a material. Important in meteorology for ice–water–vapour state changes. |
| Radiation | The transmission of energy by electromagnetic waves. |
| Land and Sea Breezes | Local winds caused by unequal diurnal heating and cooling of adjacent land and water surfaces. Sea breeze blows onshore during day (land warmer); land breeze blows offshore at night (land cooler). |
| Tornado | A violent, rotating column of air in contact with both the surface of the earth and a cumulonimbus cloud. Often visible as a condensation funnel encircled by a cloud of debris. |
| Water Spout | An intense columnar vortex (usually funnel-shaped cloud) over a body of water connected to a cumuliform cloud. Generally a non-supercell tornado over water; weaker than most land counterparts. |
| Pressure | The weight of the air column above unit area at a given point. Measured by mercury barometer; expressed in hPa (1 hPa = 10⁶ Newton/m²). |
| Precipitation | The depth to which rain or snow would cover the surface (if no loss by evaporation/runoff/infiltration, with snow/ice melted). Expressed in mm or cm. |
Lightning
Lightning – Lightning is a giant spark of electricity in the atmosphere between clouds, the air, or the ground.
Types of Lightening
| Term | Description |
|---|---|
| Cloud-to-Ground Lightning (CG) | Discharge between a thunderstorm cloud and the Earth's surface. |
| Intra-Cloud Lightning (IC) | Lightning occurring within a single cloud. |
| Cloud-to-Cloud Lightning (CC) | Discharge occurring between separate clouds or charged regions associated with different clouds. |
| Cloud-to-Air Lightning (CA) | Discharge extending from a cloud into the surrounding air without reaching another cloud or the ground. |
| Positive Cloud-to-Ground Lightning (+CG) | Positive-polarity cloud-to-ground lightning. |
| Negative Cloud-to-Ground Lightning (−CG) | Negative-polarity cloud-to-ground lightning; the common CG polarity. |
Generated educational illustrations
Lightning Development Terminology
| Term | Description |
|---|---|
| Leader | Developing electrically conductive channel that establishes the pathway for a lightning discharge. |
| Stepped Leader | Leader that advances in discrete steps; especially characteristic of negative CG lightning. |
| Positive Leader | Positively charged conducting channel propagating through the atmosphere. |
| Negative Leader | Negatively charged conducting channel propagating through the atmosphere. |
| Upward Leader | Leader propagating upward from a ground object or structure. |
| Streamer | Small developing ionized electrical channel that can contribute to lightning-path formation. |
| Upward Streamer | Streamer developing upward from the ground or an object toward a descending leader. |
| Connecting Leader / Streamer | Developing connection between a descending leader and an upward streamer. |
| Return Stroke | Intense discharge traveling through the established channel after connection. |
| Dart Leader | Subsequent leader that travels along an already established lightning channel. |
| Continuing Current | Current that persists for a relatively extended period following a lightning stroke. |
| Lightning Channel | Ionized, electrically conductive pathway through which lightning current travels. |
Flash and Stroke Terminology
| Term | Description |
|---|---|
| Lightning Flash | A complete lightning event that may contain multiple individual strokes. |
| Lightning Stroke | An individual electrical discharge within a lightning flash. |
| Ground Strike Point | Location on the Earth's surface reached by a lightning discharge. |
| Multiple-Stroke Flash | A flash containing multiple successive strokes. |
Lightning Polarity Terminology
| Term | Description |
|---|---|
| Positive Lightning (+) | Lightning associated with positive charge transfer. |
| Negative Lightning (−) | Lightning associated with negative charge transfer. |
| Positive Cloud-to-Ground (+CG) | Positive-polarity cloud-to-ground discharge. |
| Negative Cloud-to-Ground (−CG) | Negative-polarity cloud-to-ground discharge. |
| Positive Stroke (+) | A lightning stroke with positive polarity. |
| Negative Stroke (−) | A lightning stroke with negative polarity. |
| Positive Flash (+) | Lightning flash characterized by positive polarity. |
| Negative Flash (−) | Lightning flash characterized by negative polarity. |
| Peak Current (kA) | Maximum electrical current associated with a lightning stroke/discharge. |
Special Lightning Terminology
| Term | Description |
|---|---|
| Bolt from the Blue | Cloud – ground (CG) flash that travels away from the main thunderstorm before reaching the ground. |
| Dry Lightning | Lightning occurring with little or no rain reaching the ground nearby. |
| Heat Lightning | Common term for lightning whose thunder cannot be heard because the storm is distant. |
| Sheet Lightning | Broad/diffuse illumination of a cloud by lightning, often associated with cloud flashes. |
| Staccato Lightning | Brief, sharply defined lightning flash, often associated with a short-duration CG discharge. |
| Ribbon Lightning | Optical appearance produced when successive strokes are displaced by wind. |
| Thundersnow | Thunder and lightning occurring during a snowstorm. |
| Ball Lightning | Rare reported luminous spherical phenomenon associated in some reports with thunderstorms. |
| Triggered Lightning | Lightning initiated or influenced by an artificial object or activity. |
High-Atmosphere Lightning Terminology
| Term | Description |
|---|---|
| Transient Luminous Events (TLEs) | Short-lived optical/electrical phenomena occurring above thunderstorms. |
| Red Sprite (Sprite) | Large, short-lived luminous electrical phenomenon high above a thunderstorm, commonly reddish/orange-red. |
| Blue Jet | Upward-propagating luminous discharge developing from the upper part of a thunderstorm. |
| ELVES | Rapidly expanding luminous rings high above thunderstorms associated with strong electromagnetic pulses from lightning. |
| Gigantic Jet (GJ) | Large electrical discharge extending from a thunderstorm toward the upper atmosphere. |
Lightning Protection & Safety Terminology
Protection terminology from the East Coast Lightning Equipment (ECLE) Lightning Terms Glossary. The entries below are presented as terminology + concise reference definitions rather than long paragraphs.
| Term | Concise Definition |
|---|---|
| Air Terminal | Metal component installed at the highest points of a structure to intercept lightning; commonly called a lightning rod. |
| Bonding Connections | Connections or fittings that interconnect a lightning protection system with conductive parts of a structure. |
| Conductors | Heavy-duty copper or aluminum conductors connecting air terminals, roof networks and the grounding system. |
| Electrical Resistance (Ω) | Measure of how strongly a material opposes electric current; lower resistance in conductors and grounding paths supports effective current dissipation. |
| Ground Electrode | Ground rod, ground plate or ground conductor providing electrical contact with the earth. |
| Grounding System | Network of conductors and electrodes designed to direct lightning/electrical currents safely into the earth. |
| Lightning Risk Assessment | Evaluation of a structure's vulnerability to lightning based on factors such as size, height, location, surroundings, construction, occupancy and fire susceptibility. |
| Lightning Strike | Sudden electrical discharge in which lightning directly contacts the ground or an object. |
| NFPA 780 | National Fire Protection Association standard providing requirements and guidance for lightning protection system design, installation and maintenance. |
| Sideflash | Lightning arc or jump from one conductive path to another because of a significant voltage difference between nearby objects. |
| Surge Protection Device (SPD) | Device designed to protect electrical/electronic systems from transient overvoltages by diverting excess energy away from protected circuits. |
| Bonding | Electrical connection between a conductive object and a grounded component of a lightning protection system to reduce potential differences. |
| Building Envelope | Elements of a structure that separate the exterior and interior environments; also called the building enclosure. |
| Down Conductors | Cables or suitably sized metal structural paths carrying lightning current from the roof-based protection system to the grounding system. |
| Equipotential Bonding | Electrical connection of conductive parts so they remain at a more uniform electrical potential, reducing dangerous voltage differences and sideflash risk. |
| Ground Ring | In-ground conductor system, also called a ground loop, that carries electrical current away from a building and dissipates it into the earth. |
| Lightning Protection System (LPS) | Engineered network of strike-termination components, conductors and grounding elements that provides a controlled path for lightning current. |
| Lightning Rods | Older/common name for air terminals. |
| Lightning Flash | Visible burst of light from a lightning discharge, typically consisting of multiple strokes in rapid succession. |
| Rolling Sphere Method (RSM) | Lightning protection design method using an imaginary sphere to identify portions of a structure vulnerable to direct lightning attachment. |
| Strike Termination | Collective term for metal objects positioned at the highest points of a lightning protection system to intercept lightning. |
| UL 96A | UL standard addressing installation requirements for lightning protection systems. |
| Zone of Protection | Area around a structure or object considered protected from direct lightning strikes by a properly designed and installed lightning protection system. |
Protection Terminology Categories
| Category | Related Terms |
|---|---|
| Strike Interception | Air Terminal • Lightning Rods • Strike Termination |
| Current Conduction | Conductors • Down Conductors |
| Grounding | Ground Electrode • Grounding System • Ground Ring |
| Bonding | Bonding • Bonding Connections • Equipotential Bonding |
| Surge Protection | Surge Protection Device (SPD) |
| Risk & Design | Lightning Risk Assessment • Rolling Sphere Method • Zone of Protection |
| Structural / Electrical | Building Envelope • Electrical Resistance • Sideflash |
| Standards | NFPA 780 • UL 96A |
Source: East Coast Lightning Equipment (ECLE), Lightning Terms Glossary
Understanding Lightning Protection
What is a Lightning Protection System (LPS)?
8.1 Lightning Protection Terminology – Detailed Reference
| Term | Detailed Meaning |
|---|---|
| Lightning Protection System (LPS) | Complete engineered system of strike-termination components, conductors, bonding and grounding intended to provide a controlled path for lightning current. |
| Air Terminal | Manufactured strike-termination device intended to provide an attachment point for lightning to the LPS. |
| Strike Termination | Collective term for conductive components positioned to provide intentional lightning attachment points. |
| Roof Conductor | Conductor forming part of the roof-level lightning protection network and connecting strike-termination components. |
| Down Conductor | Conductor that carries lightning current from the roof-level system toward the grounding system. |
| Grounding Electrode | Conductive element that establishes electrical contact with the earth, such as a rod, plate or other permitted electrode. |
| Grounding Electrode System (GES) | Interconnected grounding electrodes and conductors used to dissipate lightning current into the earth. |
| Ground Ring | Buried conductor arranged around or associated with a structure to provide an earth-current path. |
| Bonding | Intentional electrical connection between conductive systems to reduce differences in electrical potential. |
| Equipotential Bonding | Bonding arrangement intended to keep connected conductive parts at substantially similar potential during a lightning event. |
| Bonding Conductor | Conductor used to connect a metal body or system to the LPS or grounding/bonding network. |
| Sideflash | Arc or jump of lightning current from one conductive object/path to another because of a significant voltage difference. |
| Bonding Distance | Calculated separation/connection consideration used to determine when conductive objects should be bonded to reduce flashover or sideflash risk. |
| Surge Protective Device (SPD) | Device that limits transient overvoltage and diverts surge current to help protect connected electrical/electronic equipment. |
| Transient Overvoltage | Short-duration voltage rise caused by events such as lightning or switching. |
| Zone of Protection | Area considered protected against direct lightning attachment by the design of the LPS. |
| Rolling Sphere Method (RSM) | Lightning protection design method using an imaginary sphere to identify locations vulnerable to direct lightning attachment. |
| Protection Level | Design concept describing the required degree of lightning protection based on assessed risk and applicable criteria. |
| Lightning Risk Assessment (LRA) | Evaluation of the likelihood and consequences of lightning-related damage to a structure or system. |
| Building Envelope | Physical separation between indoor and outdoor environments, including roof, walls and other enclosure elements. |
| Equipotential Plane | Conductive plane or interconnected conductive arrangement intended to reduce potential differences across a protected area. |
| Surge Protection Coordination | Coordination of SPDs and system grounding/bonding so protective devices operate effectively. |
| Master Label / Inspection Certificate | Documentation or certification used by an inspection/certification program to indicate that an LPS has been evaluated against applicable requirements. |
Basic Drought Terminology
| Term | Definition |
|---|---|
| Drought | A prolonged period of abnormally low precipitation that results in water shortage and affects agriculture, water resources, ecosystems or society. |
| Drought Event | A period during which rainfall or water availability remains below the expected level and produces measurable impacts. |
| Drought Onset | The beginning or development of drought conditions in an area. |
| Drought Termination | The point at which drought conditions end following sufficient improvement in rainfall, soil moisture, water storage or other indicators. |
| Drought Duration | The length of time from the beginning to the end of a drought event. |
| Drought Severity | The degree to which drought conditions are below normal and the extent of their impacts. |
| Drought Intensity | The magnitude of dryness or water deficit during a drought period. |
| Drought Frequency | How often drought events occur within a specified period. |
| Drought Persistence | The continuation of drought conditions over an extended period. |
| Drought Recovery | The process through which rainfall, soil moisture, water bodies, groundwater, agriculture and ecosystems return toward normal conditions. |
Types of Drought
| Term | Definition |
|---|---|
| Meteorological Drought | A period of abnormally low precipitation compared with the normal or expected amount for a location and period. |
| Agricultural Drought | A condition in which available soil moisture and water supply are insufficient to meet the water requirements of crops, vegetation or livestock. |
| Hydrological Drought | A period of reduced water availability in rivers, reservoirs, lakes, streams and groundwater systems. |
| Socioeconomic Drought | A condition in which drought-related water shortages affect the supply and demand of goods and services. |
| Ecological Drought | A drought-related water deficit that causes stress or degradation of ecosystems and their functions. |
| Flash Drought | A drought that develops or intensifies rapidly, often due to reduced rainfall combined with high temperature, wind and increased evapotranspiration. |
| Short-Term Drought | Drought developing over weeks to several months, generally affecting soil moisture, vegetation and agriculture relatively quickly. |
| Long-Term Drought | Drought persisting for several months or longer, with increasing effects on water resources, groundwater, ecosystems and society. |
| Seasonal Drought | Drought occurring during a particular season when precipitation is substantially below the expected seasonal amount. |
Rainfall and Water Deficit
| Term | Definition |
|---|---|
| Rainfall Deficiency | The amount by which observed rainfall is below the normal or expected rainfall. |
| Rainfall Departure | The difference between observed rainfall and normal rainfall, expressed as an amount or percentage. |
| Rainfall Anomaly | The difference between observed precipitation and its long-term average for a specified period. |
| Normal Rainfall | The long-term average rainfall used as a reference for assessing whether rainfall is above or below normal. |
| Precipitation Deficit | A shortage of precipitation relative to the amount normally expected over a specified period. |
| Water Deficit | A shortage between available water and the amount required by people, agriculture, ecosystems or other uses. |
| Water Availability | The amount of water accessible for human, agricultural, ecological and other needs. |
| Water Shortage | A situation in which available water is insufficient to meet current demand. |
| Water Stress | A condition in which water demand places significant pressure on available water resources. |
| Water Scarcity | A situation in which available water resources are insufficient to meet the long-term needs of a population or ecosystem. |
Soil and Agriculture
| Term | Definition |
|---|---|
| Soil Moisture | The amount of water contained in the soil and available to plants. |
| Soil Moisture Deficit | A condition in which soil contains less water than required for healthy plant growth. |
| Plant Water Stress | Stress experienced by plants when available water is insufficient to meet their water requirements. |
| Crop Water Requirement | The amount of water required by a crop for normal growth and development. |
| Evaporation | The conversion of liquid water from soil, water bodies or other surfaces into water vapour. |
| Transpiration | The release of water vapour from plants, mainly through small openings in their leaves. |
| Evapotranspiration (ET) | The combined loss of water through evaporation from surfaces and transpiration from plants. |
| Potential Evapotranspiration (PET) | The amount of water that could be lost through evapotranspiration when sufficient water is available. |
| Actual Evapotranspiration (AET) | The amount of water actually lost through evaporation and plant transpiration under existing moisture conditions. |
| Evaporative Demand | The atmospheric demand for water through evaporation and plant transpiration. |
| Crop Stress | Reduction in crop growth or productivity caused by insufficient water or other environmental conditions. |
Drought Monitoring and Indices
| Term | Definition |
|---|---|
| Drought Index | A numerical indicator used to measure or classify the intensity, duration or severity of drought. |
| Standardized Precipitation Index (SPI) | An index that measures precipitation anomalies over a selected time scale and is widely used for drought monitoring. |
| Standardized Precipitation Evapotranspiration Index (SPEI) | A drought index that considers precipitation and atmospheric evaporative demand, including the influence of temperature. |
| Palmer Drought Severity Index (PDSI) | A drought index that uses precipitation, temperature and soil-water information to estimate moisture conditions. |
| Standardized Runoff Index (SRI) | An index used to identify and monitor drought conditions based on unusually low runoff. |
| Standardized Groundwater Index (SGI) | An indicator used to assess groundwater conditions relative to historical levels. |
| Vegetation Health Index (VHI) | A satellite-based indicator combining vegetation and temperature information to assess vegetation stress. |
| Normalized Difference Vegetation Index (NDVI) | A satellite-derived index commonly used to measure vegetation greenness and monitor vegetation stress. |
| Drought Monitoring | The continuous assessment of rainfall, soil moisture, water resources, vegetation and other indicators to identify and track drought conditions. |
| Drought Early Warning | The use of observations, monitoring, forecasts and indicators to provide information about developing or expected drought conditions. |
Climate and Water Resources
| Term | Definition |
|---|---|
| Aridity | A climatic characteristic describing a long-term lack of sufficient precipitation to support vegetation and other water needs. |
| Climate Variability | Natural variations in climate conditions occurring over different time periods and geographical scales. |
| Climate Change | Long-term changes in temperature, precipitation and other climate characteristics. |
| Aquifer | An underground layer of permeable rock, sediment or soil capable of storing and transmitting groundwater. |
| Groundwater | Water stored beneath the Earth's surface in soil and geological formations. |
| Surface Water | Water present on the Earth's surface, including rivers, lakes, reservoirs, ponds and wetlands. |
| Reservoir | A natural or artificial storage area used to collect and retain water. |
| River Flow | The volume of water moving through a river or stream over a given period. |
| Streamflow | The flow of water in a stream or river channel. |
| Groundwater Recharge | The process by which water enters and replenishes underground aquifers. |
| Water Storage | The amount of water retained in reservoirs, lakes, soil, snow or groundwater systems. |
Drought Risk, Mitigation and Management
| Term | Definition |
|---|---|
| Drought Risk | The potential for drought to cause harmful effects to people, agriculture, infrastructure, ecosystems and the economy. |
| Drought Impact | The observed consequence of drought on people, agriculture, water resources, ecosystems, health or the economy. |
| Drought Vulnerability | The degree to which a community, system, sector or ecosystem is likely to be affected by drought. |
| Drought Exposure | The people, assets, agricultural areas, ecosystems or infrastructure located in areas that may experience drought. |
| Drought Mitigation | Actions taken before or during drought to reduce its potential impacts. |
| Drought Adaptation | Adjustments in practices, systems or policies that help reduce vulnerability to current or expected drought conditions. |
| Drought Preparedness | Planning and actions undertaken before drought occurs to improve the ability to respond effectively. |
| Drought Management | Coordinated planning, monitoring, response, mitigation and recovery actions used to manage drought risk and impacts. |
| Water Conservation | The efficient and responsible use of water to reduce unnecessary consumption and preserve available supplies. |
| Water Harvesting | The collection and storage of rainwater or runoff for later use. |
| Rainwater Harvesting | Collection and storage of rainwater from roofs, land surfaces or other catchments for subsequent use or groundwater recharge. |
| Drought-Resilient Agriculture | Agricultural practices and systems designed to maintain productivity and reduce vulnerability during water shortages. |
| Adaptive Capacity | The ability of people, institutions, ecosystems or systems to adjust to drought and reduce its harmful consequences. |
Drought Impacts
| Term | Definition |
|---|---|
| Agricultural Impact | Effects of drought on crops, livestock, irrigation, agricultural productivity and farm income. |
| Water Supply Impact | Reduction in the availability or quality of water for domestic, agricultural, industrial or ecological uses. |
| Ecological Impact | Effects of drought on vegetation, wildlife, habitats, biodiversity and ecosystem functions. |
| Economic Impact | Financial losses resulting from reduced agricultural production, water shortages, business disruption and increased costs. |
| Social Impact | Effects of drought on communities, livelihoods, employment, migration and access to essential resources. |
| Public Health Impact | Effects of drought on human health through water shortages, poor water quality, heat, dust, food insecurity and other pathways. |
| Wildfire Risk | The increased potential for wildfire caused by dry vegetation, low soil moisture and hot or windy conditions during drought. |
| Water Quality | The physical, chemical and biological condition of water and its suitability for intended uses. |
Important Indian Drought Terms
| Term | Definition |
|---|---|
| Rainfall Departure (%) | The percentage difference between observed rainfall and normal rainfall. |
| Long-Term Average | The historical average rainfall or climate value used as a reference for comparison. |
| Normal | A reference climatological value against which current rainfall or other weather conditions are compared. |
| Moderate Drought | A drought category used in Indian meteorological classification when seasonal rainfall deficiency is within the moderate-deficiency range. |
| Severe Drought | A drought category used in Indian meteorological classification when seasonal rainfall deficiency exceeds the severe-deficiency threshold. |
Terminology on Cyclonic Disturbances over the North Indian Ocean
| Term | Definition |
|---|---|
| Afternoon | 1200 - 1600 HRS. IST |
| Around Noon | 1100 - 1300 HRS. IST |
| Average wind speed | Speed of the wind averaged over the previous 10 minutes (mean surface wind) as read from the anemogram or the 3 minutes mean determined with the non recording anemometer or estimated wind at sea by the mariners using the Beaufort scale. |
| Bulletin | Cyclone warning bulletin |
| Calm ( Glassy) sea condition | Wave height is zero corresponding to calm wind at the sea surface. |
| Calm (Rippled) sea condition | Wave height is 0- 0.1 metres corresponding to sea surface wind speed of 1 - 3 knots. |
| CDO | Central dense overcast. |
| Central pressure of a tropical cyclone | Surface pressure at the centre of the tropical cyclone as measured or estimated. |
| Centre fix of the tropical cyclone | The estimated location of the centre of a tropical cyclone. Fixation of the centre with the help of land based and other radars, satellite and conventional observations like surface and upper air observations, ships' reports, buoys report, commercial aircraft observations, etc. |
| Centre of the tropical cyclone | The centre of the cloud eye or, if not discernible, of the wind / pressure centre. |
| CMV | Cloud motion vector. |
| Confidence in the centre position | Degree of confidence in the centre position of a tropical cyclone expressed as the radius of the smallest circle within which the centre may be located by the analysis. “Position good” implies a radius of 30 nautical miles (55 kilometers) or less, “Position fair”, a radius of 30 to 60 nautical miles (55 to 110 km) and “Position poor”, a radius of greater than 60 nautical miles (110 km). |
| Convergence | Convergence is the process whereby air streams come together into a much smaller central area, either by opposing flow directions or by some faster elements catching up to those ahead. In the atmosphere, when winds converge over a relatively small area, the push of the air-molecule crowd forces those in the centre to form an upward air current or updraft. |
| Coriolis force | Once air has been set in motion by the pressure gradient force, it undergoes an apparent deflection from its path, as seen by an observer on the earth. This apparent deflection is called the "Coriolis force" and is a result of the earth's rotation. |
| CTT | Cloud top temperature. |
| Cyclogenesis | It is the development or strengthening of cyclonic circulation in the atmosphere (a low pressure system).Cyclogenesis is an umbrella term for several different processes, all of which result in the development of cyclonic disturbances (Depression and above). |
| Cyclone Alert* | A priority message for the Government officials containing tropical cyclone information and advisories issued generally 48 hours before the commencement of adverse weather. |
| Cyclone warning bulletin | A priority message for exchange of tropical cyclone information and advisories. |
| Cyclone warning | A priority message containing tropical cyclone warning and advisories issued generally 24 hours in advance of the commencement of adverse weather. |
| Cyclone | Tropical cyclone |
| Cyclonic disturbance | A non frontal synoptic scale low pressure system originating over tropical waters with organized convection and definite cyclonic wind circulation. It is called as a depression if the associated sustained maximum wind speed at surface level is 17-27 knots. It is called as a deep depression if the sustained maximum wind speed is 28-33 knots. |
| Cyclonic storm | A cyclonic disturbance in which the maximum average surface wind speed is in the range of 34 to 47 knots (62 to 88 kmph). |
| Depression | A cyclonic disturbance in which the maximum sustained surface wind speed is between 17 and 33 knots (31 and 61 kmph). If the maximum sustained wind speed lies in the range 28 knots (50 kmph) to 33 knots (61 kmph) the system may be called a "Deep Depression". * |
| Direction of movement of the tropical cyclone | The direction towards which the centre of the tropical cyclone is moving. |
| Divergence | Divergence is the opposite situation to convergence, the moving away of air from a common area. In the upper troposphere, an elevated zone of divergence can establish an updraft region below it. Upper-level divergence that can induce sustained updrafts from the surface is one sign that severe-weather forecasters look for as an indicator of cyclone formation or severe thunderstorm potential. |
| Early Hours | 0000 - 0400 HRS. IST |
| Early Morning | 0400 - 0600 HRS. IST |
| ECP | Estimated central pressure of the cyclone. |
| ESCAP | Economic Social Commission for Asia and the Pacific. |
| Evening | 1600 - 2000 HRS. IST |
| Exceptionally Heavy Rainfall | When the amount is a value near about the highest recorded rainfall at or near the station for the month or season. However, this term will be used only when the actual rainfall amount exceeds 12 cm. |
| Extremely Heavy Rainfall | 24 hours cumulative rainfall at a station is greater than 204.5 mm as recorded at 0830 hrs IST. |
| Extremely severe cyclonic storm | A cyclonic disturbance in which the maximum average surface wind speed is 90 knots (167 kmph) to 119 knot ( 221kmph). |
| Eye of the tropical cyclone | The relatively clear and calm area inside the circular wall of convective clouds, the geometric centre of which is the centre of the tropical cyclone. |
| Fairly Widespread rainfall | (51 – 75)% of area gets rainfall. |
| Forenoon | 0800 - 1200 HRS. IST |
| Gale force wind | Average surface wind speed of 34 knots or more. |
| GMDSS | Global Maritime Distress and Safety System. |
| GMT | Greenwich Mean Time. |
| Gust | Instantaneous peak value of surface wind speed recorded or expected. |
| Heavy Rainfall | 24 hours cumulative rainfall at a station is 64.5-115.5 mm as recorded at 0830 hrs IST. |
| High sea condition | Wave height is 6.0 - 9.0 metres corresponding to sea surface wind speed of 34 - 40 knots. |
| HPA | Hecta Pascal. It is the unit of atmospheric pressure. |
| INSAT | Indian National Satellite. |
| IR imagery | Infrared Channel imagery. |
| Isolated rainfall | Less than <25% of area gets rainfall. |
| IST | Indian Standard Time |
| ITCZ | The Intertropical Convergence Zone (ITCZ), is a belt of low pressure girdling Earth near the equator. It is formed by the vertical ascent of warm, moist air from the latitudes north and south of the equator. |
| Knots | Nautical miles per hour. 1 Knots = 1.86 KM per hour. |
| Light Rain rainfall | 24 hours cumulative rainfall at a station is 2.5–15.5 mm as recorded at 0830 hrs IST. |
| Low or low pressure area | An area enclosed by a closed isobar with minimum pressure inside when mean surface wind is less than 17 knots (31 kmph). |
| Maximum sustained wind | Maximum value of the average wind speed at the surface. |
| Maximum sustained surface wind speed | Highest 3 minutes surface wind occurring within the circulation of the system. These "surface" winds are those observed (or, more often, estimated) to occur at the standard meteorological height of 10 m (33 ft) in an unobstructed exposure (i.e., not blocked by buildings or trees). |
| Mean wind speed | Average wind speed. |
| Moderate Rain rainfall | 24 hours cumulative rainfall at a station is 15.6-64.4 mm as recorded at 0830 hrs IST |
| Moderate sea condition | Wave height is 1.25 - 2.5 metres corresponding to sea surface wind speed of 17 - 21 knots. |
| Morning | 0400 - 0800 HRS. IST |
| Name of the Tropical Cyclone | Once wind speed in a cyclonic disturbance attains a 34 kt threshold value it will be given an identification name by RSMC tropical cyclones New Delhi from the consolidated name list. |
| Night | 2000 - 2400 HRS. IST |
| No Rain | The amount of rainfall 0.0 mm. |
| Panel members or member countries or countries | Countries constituting the WMO/ESCAP Panel on Tropical Cyclones viz: Bangladesh, India, Maldives, Myanmar, Oman (Sultanate of), Pakistan, Sri Lanka and Thailand. |
| Phenomenal sea condition | Wave height is over 14.0 metres corresponding to sea surface wind speed of 64 or above knots. |
| Post Landfall Outlook | This bulletin is issued 12 hours before cyclone landfall and contains more specific forecasts about place and time of landfall. |
| Pre-Cyclone Watch | This bulletin contains early warning about likely development of a cyclonic storm and an indication of the coastal belt likely to experience adverse weather. It is issued 72 hrs in advance of the commencement of adverse weather. |
| Pressure gradient | The pressure gradient is a physical quantity that describes in which direction and at what rate the pressure changes the most rapidly around a particular location. The pressure gradient is a dimensional quantity expressed in units of pressure per unit length. |
| Rainfall at a one or two places | Less than <25% of total raingauge stations get rainfall. |
| Rainfall at a few places | (26 –50)% of total raingauge stations get rainfall. |
| Rainfall at many places | (51 – 75)% of total raingauge stations get rainfall. |
| Rainfall at most places | (76 – 100)% of total raingauge stations get rainfall. |
| Rough sea condition | Wave height is 2.5 - 4.0 metres corresponding to sea surface wind speed of 22 - 27 knots. |
| RSMC | Regional Specialized Meteorological Center. |
| Scattered | (26 –50)% of area gets rainfall. |
| Severe cyclonic storm | A cyclonic disturbance in which the maximum average surface wind speed is in the range of 48 to 63 knots (89 to 117 kmph). |
| Slight sea condition | Wave height is 0.5 - 1.25 metres corresponding to sea surface wind speed of 11 - 16 knots. |
| Smooth (waveless) sea condition | Wave height is 0.1-0.5 metres corresponding to sea surface wind speed of 04-10 knots. |
| Speed of movement of the tropical cyclone | Speed of movement of the centre of the tropical cyclone. |
| Squally wind | When sudden increases of wind speed occur in squalls with the increased speed reaching a minimum of 22 knots (40 kmph) and persist for at least one minute. |
| SST | Sea surface temperature. |
| State of sea or sea condition | The state of sea is described qualitatively as calm, smooth, slight, moderate, rough, very rough, high, very high and phenomenal sea. However these qualitative terms correspond to a range of height of waves based on the surface wind speed. |
| Storm season | The periods April to May and October to December during which most of the cyclonic storms occur in the Bay of Bengal and Arabian Sea. |
| Storm surge | The difference between the actual water level under the influence of a meteorological disturbance (storm tide) and the level, which would have been reached in the absence of the meteorological disturbance (i.e. astronomical tide). (Storm surge results mainly from the shoreward movement of water under the action of wind stress. A minor contribution is also made by the hydrostatic rise of water resulting from the lowered barometric pressure.) |
| Storm tide | The actual water level as influenced by a weather disturbance. The storm tide consists of the normal astronomical tide and the storm surge. |
| Super cyclonic storm | A cyclonic disturbance in which maximum wind speed is 120 knots and above (222 kmph and above). |
| T Number | T stands for tropical cyclones. It indicates the intensity of cyclonic disturbance bases on Dvorak’s technique. It varies from 1.0 to 8.0 at an interval of 0.5. |
| TCAC | Tropical Cyclone Advisory Centre. |
| Tropical cyclone advisory | A priority message for exchanging information, internationally, on tropical cyclones in the Bay of Bengal and the Arabian Sea. |
| Tropical cyclone | Generic term for a non-frontal synoptic scale cyclone originating over tropical or subtropical waters with organized convection and definite cyclonic surface wind circulation. |
| Tropical depression | Depression. |
| Tropical storm | Tropical cyclone. |
| Tropical Weather Outlook | A priority message for exchange between the Panel countries of synoptic and satellite inferences for the Bay of Bengal and the Arabian Sea region. |
| UTC | Universal Time Co-ordinate. |
| Vertical Wind Shear | Difference of horizontal component of wind between 200 hPa and 850 hPa level. |
| Vertical Wind Shear Tendency | 24 hours change in vertical wind shear. |
| Very Heavy Rainfall | 24 hours cumulative rainfall at a station is 115.6-204.4 mm as recorded at 0830 hrs IST. |
| Very High sea condition | Wave height is 9.0 - 14.0 metres corresponding to sea surface wind speed of 41 - 63 knots. |
| Very light Rainfall | 24 hours cumulative rainfall at a station is Trace –2.4 mm as recorded at 0830 hrs IST. |
| Very Rough sea conditon | Wave height is 4.0 - 6.0 metres corresponding to sea surface wind speed of 28 - 33 knots. |
| Very severe cyclonic storm | A cyclonic disturbance in which the maximum average surface wind speed is 64 knots (118 kmph) to 89 knot (166kmph). |
| VIS imagery | Visible Channel imagery. |
| Visual storm signals | Visual signals displayed at coastal ports to warn ships of squally winds, gales and tropical cyclones. |
| Vorticity | Vorticity is a mathematical concept used in fluid dynamics. It can be related to the amount of "circulation" or "rotation" (or more strictly, the local angular rate of rotation) in a fluid. The average vorticity ζav in a small region of fluid flow is equal to the circulation Γ around the boundary of the small region, divided by the area of the small region. |
| Weather warning | Meteorological message issued to provide appropriate warnings of hazardous weather conditions. |
| Wide spread rainfall | (76 – 100)% of area gets rainfall. |
| WMO | World Meteorological Organization |
| WV imagery | Water vapour Channel imagery. |
| Zone of disturbed weather | A zone in which the pressure is low relative to the surrounding region and there is convective cloud masses which are not organized. |
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