Glossary

• Types of Seasons

  • Winter Season	January – February
    Pre Monsoon Season	March – May
    Southwest Monsoon Season	June - September
    Northeast Monsoon Season	October - December

• Sky Conditions

  • 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

• Rainfall
  • The latest practices followed are summarised below:
    Spatial Distribution of Rainfall

    Distribution	No. of Places	Description
    Isolated	One or two Places	<25% of area or less gets rainfall
    Scattered	At a few Places	(26–50)% of area gets rainfall
    Fairly Widespread	At many Places	(51–75)% of area gets rainfall
    Wide spread	At Most place	(76–100)% of area gets rainfall
    Dry	-	No station reported rainfall

• 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.

• Weekly/Seasonal Rainfall Distribution on regional scale

  • 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 %

• Rainfall distribution on All India scale

  • Normal	percentage departure of realised rainfall is within ± 10 % of the Long Period Average
    Below Normal	percentage departure of realised rainfall is < -10% of the Long Period Average
    Above Normal	percentage departure of realised rainfall is > 10% of the Long Period Average
    All India Drought Year	When the rainfall deficiency is more than 10% and when 20 to 40% of the country is under drought conditions, then the year is termed as All India Drought Year
    All India Severe Drought Year	When the rainfall deficiency is more than 10% and when the spatial coverage of drought is more than 40% it is called as All India Severe Drought Year

• Monsoon

  • "MONSOON" has originated from the Arabic word “MAUSIM” which means season. It is most often applied to the seasonal reversals of the wind direction along the shores of the Indian Ocean, especially in the Arabian Sea, that blow from the southwest during one half of the year and from the northeast during the other half.

    Onset & Advance of Monsoon

    The guidelines followed for declaring the onset of monsoon over Kerala and its further advance over the country are enlisted below:

    Onset over Kerala

    a)Rainfall If after 10th May, 60% of the available 14 stations enlisted*, viz. Minicoy, Amini,Thiruvananthapuram, Punalur, Kollam, Allapuzha, Kottayam, Kochi,Thrissur, Kozhikode, Thalassery,Kannur, Kasargode and Mangalore report rainfall of 2.5 mm or more for two consecutive days, the onset over Kerala be declared on the 2nd day, provided the following criteria are also in concurrence.

    b) Wind field Depth of westerlies should be maintained upto 600 hPa, in the box equator to Lat. 10ºN and Long. 55ºE to 80ºE. The zonal wind speed over the area bounded by Lat. 5-10ºN, Long. 70-80ºE should be of the order of 15 – 20 Kts. at 925 hPa. The source of data can be RSMC wind analysis/satellite derived winds.

    3. Outgoing Longwave Radiation (OLR) INSAT derived OLR value should be below 200 wm-2 in the box confined by Lat. 5-10ºN and Long. 70-75ºE.

    Further Advance of Monsoon over the Country

    a) Further advance is declared based on the occurrence of rainfall over parts/sectors of the sub-divisions and maintaining the spatial continuity of the northern limit of monsoon, further advance is declared.

    The following auxiliary features are also looked into:

    b) Along the west coast, position of maximum cloud zone, as inferred from the satellite imageries is taken into account.

    c) The satellite water vapour imageries are monitored to assess the extent of moisture incursion.

    Northern Limit of Monsoon (NLM)

    Southwest monsoon normally sets in over Kerala around 1st June. It advances northwards, usually in surges, and covers the entire country around 15th July. The NLM is the northern most limit of monsoon upto which it has advanced on any given day.

    Withdrawal of SW Monsoon

    a)Withdrawal from extreme north-western parts of the country is not attempted before 1st September.

    b) After 1st September:
    The following major synoptic features are considered for the first withdrawal from the western parts of NW India.

    i) Cessation of rainfall activity over the area for continuous 5 days.
    ii) Establishment of anticyclone in the lower troposphere (850 hPa and below)
    iii) Considerable reduction in moisture content as inferred from satellite water vapour imageries and tephigrams.

    Further Withdrawal from the Country

    i) Further withdrawal from the country is declared, keeping the spatial continuity, reduction in moisture as seen in the water vapour imageries and prevalence of dry weather for 5 days.
    ii) SW monsoon is from the southern peninsula and hence from the entire country only after 1st October, when the circulation pattern indicates a change over from the southwesternly wind regime.

    Criteria for Declaring Onset of Northeast Monsoon

    For declaring onset of Northeast Monsoon following criteria are considered:
    (1) Withdrawal of south west Monsoon upto 15º N.
    (2) Onset of persistent surface easterlies over Tamil Nadu coast.
    (3) Depth of easterlies upto 850 hpa over Tamil Nadu coast.
    (4) Fairly widespread rainfall over the coastal Tamil Nadu and adjoining areas.
    (5) Onset is not to be declared before 10th October even, if the conditions described above exist.

Activity/ strength of Northeast Monsoon
• The monsoon activity over a subdivision is described as below: Weak Monsoon:Rainfall less than half of the normal. Normal Monsoon:Rainfall half to less than one and a half (1½) times the normal.
  Active Monsoon:
  i) Rainfall 1½ to 4 times the normal. ii) Rainfall in atleast two stations should be 3 cm in Coastal Tamil Nadu and south coastal Andhra Pradesh and 2 cm elsewhere. iii) Rainfall in that sub-division should be fairly widespread or widespread.
  Vigorous Monsoon :
  i) Rainfall exceeding 4 times the normal. ii) Rainfall in atleast two stations should be 5 cm in Coastal Tamil Nadu and south coastal Andhra Pradesh and 3 cm elsewhere. iii) Rainfall in that sub-division should be fairly widespread or widespread.
• Strength of Southwest Monsoon
  • Descriptive term over the Sea
  • Description of monsoon condition over the sea is based on wind speed (in knots) reported or inferred to be existing.
    Weak monsoon Upto 12 knots
    Moderate monsoon 13 to 22 knots
    Strong monsoon 23 to 32 knots
    Vigorous monsoon 33 knots and above.
    Descriptive term over land area
  • Weak monsoon:Rainfall less than half of the normal.
  • Normal monsoon:Rainfall half to less than 1½ times the normal (mention of ‘normal’ monsoon may not be necessary in general).
  • Active/ strong monsoon
    i) Rainfall 1 ½ to 4 times the normal.
    ii) The rainfall in at least two stations should be 5 cm, if that sub-division is along the west coast and 3 cm, if it is elsewhere.
    iii) Rainfall in that sub-division should be fairly widespread to widespread.
  • Vigorous monsoon
    i) Rainfall more than 4 times the normal.
    ii) The rainfall in at least two stations should be 8 cm if the sub-division is along the west coast and 5 cm if it is elsewhere.
    iii) Rainfall in that sub-division should be fairly widespread or widespread
    
• Temperature
  • Description of 24 hrs temperature changes.
  • Maximum temperature:(a) When the normal maximum temperature of a station is 40º C or below.
    Nomenclature Past 24 hours change
    Little change -1º C to 1º C
    Rise/ fall 2º C
    Appreciable rise/ fall 3º C to 4º C
    Marked rise/ fall 5º C to 6º C
    Large rise/ fall 7º C or more
  • (b) When the normal maximum temperature of a station is more than 40º C.
    Nomenclature Past 24 hours change
    Little change -1o C to 1 o C
    Rise/ fall 2º C
    Marked rise/ fall 3º C to 4º C
    Large rise/ fall 5º C or more
    Minimum temperature
  • (a) When the normal minimum temperature of a station is 10º C or more.
    Nomenclature Past 24 hours change
    Little change 1º C to -1º C
    Fall/ rise 2º C
    Appreciable fall/ rise 3º C to 4º C
    Marked fall/ rise 5º C to 6º C
    Large fall/ rise 7º C or less
  • (b) When the normal minimum temperature of a station is less than 10º C.
    Nomenclature Past 24 hours change
    Little change 1º C to -1º C
    Fall/ rise 2º C
    Marked fall/ rise 3º C to 4º C
    Large fall/ rise 5º C or less
• Criteria for Heat Wave
  • Heat wave need not be considered till maximum temperature of a station reaches at least 40º C for Plains and at least 30º C for Hilly regions.
  • a)When normal maximum temperature of a station is less than or equal to 40º C
  • Heat Wave Departure from normal is 5º C to 6º C Severe Heat Wave Departure from normal is 7º C or more
  • b)When normal maximum temperature of a station is more than 40º C
  • Heat Wave Departure from normal is 4º C to 5º C Severe Heat Wave Departure from normal is 6º C or more
    When actual maximum temperature remains 45ºC or more irrespective of normal maximum temperature, heat wave should be declared.
• Winds
  • Wind Air in motion is called wind which is haracterized by direction and speed. The direction of the wind is the direction from which the wind approaches the station (Example Northerly wind – Wind approaching the station from North)
  • Gales A gale is a very strong wind (34 knots or more)
  • Squall A sudden increase of wind speed by atleast 3 stages on the Beaufort Scale, the speed rising to force 6 or more, and lasting for atleast one minute is called a squall
  • Gust A rapid increase in the strength of the wind relative to the mean strength at the time. The wind speed should be at least 17 knots or more
• Types of forecast

  • Nowcasting	A short range forecast having a lead time/validity of less than24 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 Rangeforecasts	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

  • 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

  • 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.
    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.

• Seasons

  • 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 %

• Rainfall distribution on All India scale

  • Normal	percentage departure of realised rainfall is within ± 10 % of the Long Period Average
    Below Normal	percentage departure of realised rainfall is < 10% of the Long Period Average
    Above Normal	percentage departure of realised rainfall is > 10% of the Long Period Average
    All India Drought Year	When the rainfall deficiency is more than 10% and when 20 to 40% of the country is under drought conditions, then the year is termed as All India Drought Year
    All India Severe Drought Year	When the rainfall deficiency is more than 10% and when the spatial coverage of drought is more than 40% it is called as All India Severe Drought Year

• 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.
    Maximum Temperature	The highest air temperature recorded in a day.
    Minimum Temperature	The lowest air temperature recorded in a day.
    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.
    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.