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Tuesday, January 26, 2010

Criteria to decide

Criteria for describing Hot Day for coastal stations

When maximum temperature departure is 5º C or more from normal, Hot Day

may be described irrespective of the threshold value of 40º C. If the threshold

value of 40º C is reached Heat Wave may be declared.

When a station satisfies both the Heat Wave and Hot Day criteria, then Heat

Wave should be given higher priority and be declared.

Hot Wind

Hot wind reduces moisture causing dehydration and a prolonged exposure

may prove to be fatal.

The phenomena of Loo over the plains of northwest India is very well known. It

is also described in the Weather bulletins and appropriate warnings are being

issued at present. The present practice should continue.

Criteria for Cold Wave

Wind chill factor plays an important role and brings down the actual minimum

temperature depending upon the wind speed. The actual minimum

temperature of a station should be reduced to "wind chill effective minimum

temperature (WCTn)" based on wind chill factor using the enclosed table

Enclosure I, WMO No. 331/ Tech note No. 123 on "The assessment of human

bio-climate. A limited review of physical parameters, 1972).

For declaring "Cold Wave" and "Cold Day" WCTn should only be used.

If WCTn is 10°C or less, then only the conditions for cold wave should be

considered.

a) When normal minimum temperature is equal to 10°C or more.

Cold Wave Departure from normal is -5°C to -6°C.

Severe Cold Wave Departure from normal is -7°C or less

b) When normal minimum temperature is less than 10°C.

Cold Wave Departure from normal is -4°C to -5°C.

Severe Cold Wave Departure from normal is -6°C or less.for describing Hot Day for coastal stations

When maximum temperature departure is 5º C or more from normal, Hot Day

may be described irrespective of the threshold value of 40º C. If the threshold

value of 40º C is reached Heat Wave may be declared.

When a station satisfies both the Heat Wave and Hot Day criteria, then Heat

Wave should be given higher priority and be declared.

Hot Wind

Hot wind reduces moisture causing dehydration and a prolonged exposure

may prove to be fatal.

The phenomena of Loo over the plains of northwest India is very well known. It

is also described in the Weather bulletins and appropriate warnings are being

issued at present. The present practice should continue.

Criteria for Cold Wave

Wind chill factor plays an important role and brings down the actual minimum

temperature depending upon the wind speed. The actual minimum

temperature of a station should be reduced to "wind chill effective minimum

temperature (WCTn)" based on wind chill factor using the enclosed table

Enclosure I, WMO No. 331/ Tech note No. 123 on "The assessment of human

bio-climate. A limited review of physical parameters, 1972).

For declaring "Cold Wave" and "Cold Day" WCTn should only be used.

If WCTn is 10°C or less, then only the conditions for cold wave should be

considered.

a) When normal minimum temperature is equal to 10°C or more.

Cold Wave Departure from normal is -5°C to -6°C.

Severe Cold Wave Departure from normal is -7°C or less

b) When normal minimum temperature is less than 10°C.

Cold Wave Departure from normal is -4°C to -5°C.

Severe Cold Wave Departure from normal is -6°C or less.


 

When WCTn is 0°C or less, Cold Wave should be declared irrespective of

normal minimum temperature of the station. However, this criteria is not

applicable for those stations whose normal minimum temperature is below

0°C.

Cold Wave conditions for coastal stations

For coastal stations the threshold value of minimum temperature of 10ºC is

rarely reached. However, the local people feel discomfort due to wind chill

factor which reduces the minimum temperature by a few degrees depending

upon the wind speed. The "Cold Day" concept may be used following the

criteria given below:

Criteria for describing "Cold Day" for coastal stations

i) Actual minimum temperature of a station be reduced to WCTn.

ii) This WCTn should be used to declare "Cold Wave" or "Cold Day".

iii) When minimum temperature departure is -5ºC or less over a station,

"Cold Day" may be described irrespective of threshold value of 10ºC

iv) However, when a threshold of 10ºC is reached "Cold Wave" be declared.

v) When a station satisfies both the Cold Wave and Cold Day criteria, then

Cold Wave has a higher priority and has to be declared.

Heat wave/ Cold Wave and hot Day/ Cold Day are area specific phenomena.

Therefore they may be described for a Met. Sub-division or a part thereof,

when at least two stations satisfy the criteria

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