Glossary Of Terms in Meteorology (Meteorology Notes)


Meteorology Notes

 

 

GLOSSARY

Term Definition
Types Of Forcast  
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 Forecast Forecasts having a lead time /validity period beyond 10 days. Usually this is being issued for a season.  IMD issues Long Range Forecast for southwest Monsoon rainfall and onset date  for  Kerala,  Northeast  Monsoon  Rainfall  and  Winter precipitation over Northwest India.
 

Seasons

Meteorological seasons over India are:

Winter Season: January – February

Pre Monsoon Season: March – May

Southwest Monsoon Season: June – September

Post Monsoon Season: October – December

Monsoon  The seasonal reversal of winds and the associated rainfall.

This word is derived from the Arabic word “Mausim”.

The annual oscillation in the apparent position of the Sun between the Tropics of Cancer and Capricorn causes the annual oscillation in the position of the thermal equator (region of  maximum  heating)  on  the  Earth’s  surface.This  is associated   with  the  annual  oscillation  of  temperature,

pressure, wind, cloudiness, rain etc.  This is the cause of the monsoons.

On the Earth’s surface, there are asymmetries of land and Ocean. The differential heating of land and Ocean cause variations  in  the  intensity  of  the  annual  oscillation  of  the thermal equator and hence regional variations in the intensity of monsoon.

Prominent monsoon region includes Africa, South Asia and north Australia.

Southwest Monsoon The southwesterly wind flow occurring over most parts of India and Indian Seas gives rise to southwest monsoon over India from June to September.
Onset of southwest monsoon Commencement of rainy season with the establishment of monsoon flow pattern

Normal date for Onset of southwest monsoon

South Andaman Sea : 20 May

Kerala: 1 June

Mumbai: 10 June

New Delhi: 29 June

Entire country: 15 July

Withdrawal of southwest monsoon Cessation of southwest monsoon rainfall

Normal date of withdrawal from extreme west Rajasthan is 15 September.

Northeast Monsoon With the withdrawal of the southwest monsoon from the northern and central India and the northern parts of the Peninsula by the first half of the October, the wind pattern rapidly changes from southwesterly to northeasterly and hence the term ”Northeast Monsoon” is used to describe the period October to December. This is the major period of rainfall in south peninsula. In Tamil Nadu, this is the main rainy season, accounting for nearly 60 % of annual rainfall in the coastal districts.
Weekly/Seasonal Rainfall Distribution  Meteorology Notes
Excess Percentage departure of realised rainfall from normal rainfall is + 20% or more.
Normal Percentage departure of realised rainfall from normal rainfall is between    – 19 % to + 19 %.
Deficient Percentage departure of realised rainfall from normal rainfall is between – 20 % to – 59 %.
Scanty Percentage departure of realised rainfall from normal rainfall is between – 60 % to – 99 %.
No Rain Percentage departure of realised rainfall from normal rainfall is –100 %
Temperature
Temperature The temperature of a body is the condition which determines its ability to communicate heat to other bodies or to receive heat from them. Meteorologists are interested in the temperature of the air, of the soil and of water bodies. Temperature is measured by means of a thermometer. Thermometers are graduated in different scales. The scale accepted for use in the India Meteorological Department is the Celsius scale giving temperature in degrees Celsius.
Air Temperature(reported by surface meteorological observatories The temperature measured in an enclosed space allowing free flow of air and not directly exposed to sunlight where the thermometer is kept at a height of 1.2 m above the surface.
Dew Point temperature It is defined as the temperature to which moist air must be cooled, during a process in which pressure and moisture content of the atmosphere remain constant. Lower value of the dew point temperature indicates lesser moisture content of the atmosphere.
Freezing point The constant temperature in which the solid and liquid forms of pure water are in equilibrium at Standard Atmospheric Pressure.
Hot Day Whenever the maximum temperature remains 40o C or more and minimum remains 5o C or more above normal, provided, it is not satisfying the heat wave criteria.
Heat Wave Departure of maximum temperature from normal is + 4o C to + 5o C or more for the regions where the normal maximum temperature is more than 40o C and departure of maximum temperature from normal is + 5o C to + 6o C for regions where the normal maximum temperature is 40o C or less (Heat Wave is declared only when the maximum temperature of a station reaches at least 40O C for plains and at least 30O C for Hilly regions). When actual maximum temperature remains 45°C or more irrespective of normal maximum temperature, heat wave is declared.
Cold Day In the plains of north India, foggy conditions prevail during winter for several days or weeks. The minimum temperature on these days remains above normal, while maximum temperature remain much below normal. This creates cold conditions for prolonged period. When maximum temperature is less than or equal to 16°C in Plains, it will be declared “Cold Day”.
Cold Wave Wind chill factor is taken into account while declaring the cold wave situation.

The wind chill effective minimum temperature (WCTn) is defined as the effective minimum temperature due to wind flow. For ex. When the minimum temperature is 15 OC and the wind speed is 10 mph, WCTn will be 10.5 OC.

Departure of WCTn from normal minimum temperature is from –5oC to–6oC where normal minimum temperature > 10oC and from –4oC to –5oC elsewhere, Cold Wave is declared. For declaring cold wave etc. WCTn only is used and when it is < 10OC only, cold wave is considered (this criteria does not hold for coastal stations).

Also cold wave is declared when WCTn is < 0oC irrespective of the normal minimum temperature for those stations.

Severe Cold Wave Departure of WCTn from normal minimum temperature is –7oC or less for the regions where normal minimum temperature is > 10oC and –6oC or less elsewhere. (departure of WCTn from normal minimum temperature is from –5oC to –6oC where normal minimum temperature > 10oC and from – 4oC to     –5oC elsewhere)
Pressure The pressure of the atmosphere at any point is the weight of the air column which stands vertically above unit area with the point as its centre. For  meteorological purposes, atmospheric pressure is usually measured by means of a mercury barometer where the height of the mercury column represents the atmospheric pressure. The pressure is expressed in hPa (Hecta Pascal) which is defined as equal to 106 Newton/sq. m.
Precipitation Precipitation whether it is rain or snow is expressed as the depth to which it would cover a horizontal projection of the earth’s surface, if there is no loss by evaporation, run–off or infiltration and if any part of the precipitation falling as snow or ice were melted. It is expressed in the units of mm or cm.
Rainfall Liquid rainfall is expressed as the depth to which it would cover a horizontal projection of the earth’s surface, if there is no loss by evaporation, run–off or infiltration. It is expressed in terms of mm or cm.

It is assumed that the amount of precipitation collected in the gauge is representative of a certain area around the point where the measurement is made. The choice of the instrument and the site itself, the form and exposure of the measuring gauge, the prevention of loss of precipitation by evaporation and the effects of wind and splashing are some of the important points to be considered in the correct measurement of precipitation.

Snowfall Snowfall is measured either as the depth of snow which has fallen in a stated period, or melted and measured as water. The depth of snow is usually measured in millimetres and its water equivalent in millimetres and tenths obtained by dividing the snow depth by 10, assuming the density of snow as 0.1.
Relative Humidity Relative Humidity is the ratio of the actual quantity of moisture at a certain temperature and pressure to the maximum it can hold at the same temperature and pressure. It is usually multiplied by 100 and expressed in percent.
Spatial distribution of Rainfall  Meteorology Notes
Widespread (Most places) 75 % or more number of stations of a region (usually a meteorological sub-division) reporting at least 2.5 mm rainfall.
Fairly widespread (Many places) 51% to 74 % number of stations of a region (usually a meteorological sub-division); reporting at least 2.5 mm rainfall.
Scattered (at a few places) 26 % to 50% number of stations of a region (usually a meteorological sub-division) reporting at least 2.5 mm rainfall.
Isolated (At isolated places) 25% or less number of stations of a region (usually a meteorological sub-division) reporting at least 2.5 mm rainfall.
Mainly dry No station of a region reported rainfall
Synoptic Systems  Meteorology Notes
Cyclonic Circulation (Cycir) Atmospheric wind flow in upper levels associated with any low pressure system. The wind flow is counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere.
Anti Cyclonic Circulation Atmospheric wind flow in upper levels associated with any high pressure system. The wind flow is clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere
Low Pressure area (Lopar)/ Well marked Lopar Area in the atmosphere in which the pressures are lower than those of the surrounding region at the same level and is represented on a synoptic chart by a system of one closed isobar (wind speed on the surface < 17 Kts when the system is at sea or one closed isobar in the radius of 3 Deg. from the centre over land)
Depression Intense low pressure system represented on a synoptic chart by two or three closed isobars at 2 hPa interval and wind speed from 17 to 27 Kts at sea and two closed isobars in the radius of 3 Deg. from the centre over land.
Deep Depression Intense low pressure system represented on a synoptic chart by two or three closed isobars at 2 hPa interval and wind speed from 28 to 33 Kts at sea and three to four closed isobars in the radius of 3 Deg. from the centre over land
Cyclonic Storm Intense low pressure system represented on a synoptic chart by more than four closed isobars at 2 hPa interval and in which the wind speed on surface level is in between 34 – 47 Kts.
Severe Cyclonic Storm Intense low pressure system represented on a synoptic chart by more than four closed isobars at 2 hPa interval and in which the wind speed on surface level is in between 34 – 47 Kts.
Very Severe Cyclonic Storm Intense low pressure system represented on a synoptic chart by more than four closed isobars at 2 hPa interval and in which the wind speed on surface level is in between 64 – 119 Kts.
Super Cyclonic Storm Intense low pressure system represented on a synoptic chart by more than four closed isobars at 2 hPa interval and in which the wind speed on surface level is 120 Kts. and above
Western Disturbance Weather disturbances noticed as cyclonic circulation/trough in the mid and lower tropospheric levels or as a low pressure area on the surface, which occur in middle latitude westerlies and originate over the Mediterranean Sea, Caspian Sea and Black Sea and move eastwards across north India.
Western Depression Weather system which originate over the Mediterranean Sea, Caspian Sea and Black Sea and approach northwest India and is defined by two or more closed isobars on the surface.
Induced Low Under the influence of the western disturbance, sometimes a low is developed to the south of the system called as induced low
Induced Cyclonic Circulation Under the influence of the western disturbance, sometimes a cyclonic circulation is developed to the south of the system called as induced cyclonic circulation
Trough A line or curve along which the atmospheric pressure is minimum. Pressure increases on both sides of the line or curve.
Trough in Westerlies A moving wave perturbation in mid latitude regions which are present throughout the year which move from west to east and entire globe. These systems generally affect the northern parts of India.
Trough in Easterlies A moving wave perturbation in the equatorial easterly wave, moving from east to west .
Easterly Waves A shallow trough disturbance in the easterly current of the tropics, more in evidence in the upper level winds than in surface pressure, whose passage westwards is followed by a marked intensification of cloudy, showery weather. The southern peninsular region is affected by easterly waves.
Shear Line A line or narrow zone 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, almost always associated with and most clearly identified as an area of maximum anticyclonic curvature of wind flow.
Terms used in Local Forecast  Meteorology Notes
Local Forecast In local forecast, whenever any weather phenomenon is expected, its intensity, frequency and time of occurrence is indicated. In the absence of a weather phenomenon, the local forecast describes anticipated sky conditions. The other parameters for which the local forecast issued include maximum temperature and/or minimum temperature, rainfall, wind and special phenomenon. It is valid for a radius of 50 km around the station and is updated 4 times in a day.
Sky Conditions Reported in terms of Octa wherein the sky is divided into 8 equal parts
0 octa Clear Sky
1-2 octa of sky covered Mainly Covered
3-4 octa of sky covered Partly Cloudy
5-7 octa of sky covered Cloudy
> 7 octa of sky covered Overcast
Weather phenomena  Meteorology Notes
Intermittent Rain In a 24 hrs time, rainfall occurring with a frequency more than that defined in “A Few Spells” but is discontinuous and without presenting the character of a shower.
Drizzle Liquid precipitation in the form of water drops of very small size (by convention, with radius of water drops between about 100 and 500 μm).
Rain Liquid precipitation in the form of water drops of radius between about 500 and 2500 μm.
Shower Solid or liquid precipitation from a vertically developed cloud is designated a shower and is distinguished from the precipitation, intermittent or continuous, from layer clouds. Showers are often characterized by short duration and rapid fluctuations of intensity.(by convention, with radius of water drops more than 2500 μm).
Hail Solid precipitation in the form of balls or pieces of ice (hailstones) with diameters ranging from 5 to 50 mm or even 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. Surface visibility is reduced to low limits; the qualification for a synoptic report is visibility below 1000 m.
Mist Mist is a phenomenon of small droplets suspended in air
Dew Condensation of water vapour on a surface whose temperature is reduced by radiational cooling to below the DEW -POINT of the air in contact with it
Fog Fog is a phenomenon of small droplets suspended in air and the visibility is one kilometer or less
Frost 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 (‘ground frost’ or ‘air frost’, respectively). The term is also used of the icy deposits which may form on the ground and on objects in such temperature conditions (glaze, hoar-frost).
Haze Haze is traditionally 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 Squally weather is meant to cover occasional or frequent squalls with rain or persistent type of strong gusty winds (mean wind speed not less than 20 knot) accompanied by rain. Such conditions are associated with low pressure systems or onset and strengthening of monsoon.
Intensity of Rainfall

 

Descriptive Term used Rainfall amount in mms

 

No Rain 0.0

 

Very light Rain 0.1- 2.4

 

Light Rain 2.5 – 7.5

 

Moderate Rain 7.6 – 35.5

 

Rather Heavy 35.6 – 64.4

 

Heavy Rain 64.5 – 124.4

 

Very Heavy Rain 124.5 – 244.4

 

Extremely Heavy Rain >244.5

 

Exceptionally Heavy Rain 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

 

 

Meteorology Notes

Arunaksha Nandy

 

 

 

 

 

 

 

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