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Tuesday, January 26, 2010

Forecast

Types of forecast 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 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.

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.

Seasons

Winter Season

Pre Monsoon Season

Southwest Monsoon Season

Northeast Monsoon Season

January – February

March – May

June - September

October - December

Sky Conditions

Reported in terms of Octa wherein the sky is divided into 8 equal parts

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

Weather phenomena

Types 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 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.

Visibility The greatest distance that prominent object can be seen

and identified by unaided, normal eyes.

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