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Tuesday, January 26, 2010

Rainfall

The latest practices followed are summarised below:

Spatial Distribution of Rainfal l

Distribution No. of Places Description

Isolated One or two Places <25% of stations gets rainfall

Scattered At a few Places (26–50)% of stations gets rainfall

Fairly Widespread At many Places (51–75)% of stations gets rainfall

Wide spread At Most place (76–100)% of stations 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


 

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 2º C

Appreciable rise 3º C to 4º C

Marked rise 5º C to 6º C

Large rise 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 -1 C to 1 C

Rise 2º C

Marked rise 3º C to 4º C

Large rise 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 -2º C

Appreciable fall -3º C to -4º C

Marked fall -5º C to -6º C

Large fall -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 -2º C

Marked fall -3º C to -4º C

Large fall -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

c) When actual maximum temperature remains 45ºC or more

Irrespective of normal maximum temperature, heat wave should be

Declared.


 


 

Hot day

In the northern plains of the country, dust in suspension occurs in many

Years for several days, bringing minimum temperature much higher than

Normal and keeping the maximum temperature around or slightly above

Normal. Sometimes increase in humidity also adds to this discomfort. Nights

Do not get cooled and become uncomfortable. To cover this situation, hot day

Concept has been introduced as given below:

Whenever, the maximum temperature remains 40ºC or more and

Minimum temperature is 5º C or more above normal, it may be defined as Hot

Day provided it is not satisfying the heat wave criteria given above.

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