• MONSOON CIRCULATION DOMINATION.• SIX MONTHS WINDS BLOW FROM

COOLER HUMID OCEANS TO DRY LAND.• OTHER SIX MONTHS REVERSAL WINDS

BLOW FROM DRY ASIAN LAND MASS TO WARM INDIAN OCEAN

CLIMATOLOGY OF INDIA

SEASONS OF INDIA

• TWO MAIN SEASONS.– SUMMER SOUTHWEST MONSOON– WINTER NORTHEAST MONSOON

• THE TRANSITION BETWEEN THE TWO ARE INTERMIDIATE SEASONS.– PRE MONSOON SEASON– POST MONSOON SEASON

SEASONS BASED ON TEMPERATURE AND RAINFALL

• COLD SEASON. NE MONSOON SEASON FROM DEC TO MAR.

• HOT SEASON. PREMONSOON SEASON FROM APR TO MAY.

• WET SEASON. SOUTHWEST MONSOON FROM JUN TO SEP.

• COOL SEASON. POST MONSOON SEASON FROM OCT TO NOV.

WINTER SEASON•COLD SEASON.

•DEC TO MAR.

•COLDEST PORTION OF THE YEAR.

•SUBSIDING ANTICYCLONIC AIR PREVAILS MOSTLY OVER THE ENTIRE COUNTRY.

•WEATEHR CLEAR.

•CORE OF WESTERLY STJ SHIFTS WELL. SOUTH OF HIMALAYAS. FURTHEST DOWN SOUTH POSITION TO 22° N IN FEB.

PRESSURE PATTERN: WINTER SEASONPR DISTRIBUTION

JANUARY

UPPER WINDS WINTER SEASON

900 mb 700 mb

500 mb300 mb

UPPER WINDS : WINTER SEASON

WEATHER

• MOSTLY COLD DRY NORTHERLIES OR NORTHEASTERLIES, DUE TO COOLING OF ASIAN PLAIN, OVER THE COUNTRY EXCEPT SOUTH EAST INDIA.

• CLEAR SKIES FINE WEATHER LOW HUMIDITY & LOW TEMPERATURES.

• FINE SPELL BROKEN BY PASSAGE OF WESTERN DISTURBANCES FREQUENCY 5-6 PER MONTH.

• MAX FREQUENCY AND SOUTHMOST POSITION OF WDs IN JAN & FEB. MOSTLY IN OCCLUDED STAGE.

• FALL IN PR AND RISE IN TEMP INDICATES APP OF WARM FRONT.

• CLOUDS LOWER TO AS & ST WITH RAIN AND DRIZZLE WITH DEPRESSION OVER STATION.

• CHANGE OF WIND DIRN, CUMULIFORM CLOUDS, THUNDERSHOWERS, RISE IN PR AND FALL OF TEMPERATURE SIGNIFY PASSAGE OF COLD FRONT.

• AFTER PASSAGE OF WDs, STRONG, DRY & COLD NORTHEASTERLIES OR NORTH WESTERLIES SET IN.

• DUE HIGH HUMIDITY IN THE WAKE OF WDs RADIATION FOG FORMS AND WHILE LIFTING OF IT STRATUS CLOUDS FORM.

• COLD WAVE LASTS ABOUT 2 TO 3 DAYS.• IN SOUTH, TROUGH EXTENDS FROM KERALA TO

GUJARAT AT TIMES TO RAJASTHAN WHEN ACCENTUATED.

• A CUT OFF LOW FORMS AND MOVES IN EASTERLY DIRECTION CAUSING RAIN IN CENTRAL INDIA & SOUTH UP.

• IN SOUTH EAST PENINSULA, NORTH EASTERLIES ACQUIRE MOISTURE OVER BAY OF BENGAL & GIVE A FEW THUNDER STORMS.

• SOMETIMES A CYCLONIC STORM MOVING WESTWARDS CAUSES HEAVY RAIN IN EXTREME SOUTHEAST TAMILNADU.

AVIATION WEATHER HAZARDS• VISIBILITY.

– RADIATION FOG. CONDITIONS FAVOURABLE AFTER PASSAGE OF WDs.

– MOST SUSCEPTIBLE AREAS ARE NORTHWEST INDIA, UP, BIHAR, NORTH BENGAL & ASSAM.

– ADVECTION FOG IN SUNDERBANS, COASTAL BELTS AND BRAHMAPUTRA VALLEY.

– MIST OR HAZE IS COMON IN NORTH AND CENTRAL INDIA.

• ICING– FREEZING LEVELS LOWEST. IN NORTH BETWEEN 2 KM TO 4 KM

& IN SOUTH ABOUT 5 KM AMSL. ICING AT LOWER LEVELS.

• STJ.– CORE AT AROUND 25° NORTH BETWEEN 200 TO 300 hPa. – AV SPEEDS 80 Kts. WHEN PRONOUNCED SPEEDS 150 TO 200 Kts

PREMONSOON SEASON• HOT SEASON, TRANSITIONAL SEASON.• APR TO MAY.• EXTREME HOT TEMPERATURES OVER NORTH INDIA AND

WIDESPREAD DUST HAZE.• DUST STORMS OVER WEST INDIA AND THUNDER

SQUALLS ( KALBAISAKHIS) OVER EASTERN INDIA.• TRACKS OF WDs MOVE NORTHWARDS WITH PROGRESS

OF THE SEASON.• LAND AND SEA BREZE IS PROMINENT OVER COASTAL

AREAS.• THUNDERSHOWERS IN SOUTH INCREASE WITH

PROGRESS OF THE SEASON• A FEW TROPICAL CYCLONES FORM IN NDIAN SEAS &

MOVE NORTH OR NORTHEAST TO BENGAL BANGLA & BURMA.

• WESTERLY STJ WEAKENS AND MOVE NORTH OF HIMALAYAS BY END OF THE SEASON.

PRESSURE DISTRIBUTION

• WINTER PATTERN REPLACED BY SUMMER PATTERN.• SUN CROSSES EQUATOR NORTHWARDS INDIAN

LANDMASS GETS PROGRESSIVELY HEATED.• PERIOD OF CONTINUOUS RISE OF TEMP AND DECREASE

OF PRESSURE IN NORTH AND INTENSIFICATION OF SOUTHERN ANTICYCLONE.

• PRESSURE PATTERN OVER THE COUNTRY DIFFUSED PR DIFFERENCE BETWEEN EXTREME NORTH AND SOUTH IS 2 TO 3 hPa ONLY.

• WINDS LIGHT AND VARIABLE.• A FEW TROPICAL CYCLONES FORM IN INDIAN SEAS.• A TROUGH OF LOW COVERS ALMOST ENTIRE CENTRAL

REGION OF THE SUBCONTINENT.• RELATIVELY HIGH PRESSURE OVER ARABIAN SEAS AND

BAY OF BENGAL.

WEATHER

• RAINFALL IN EAST INDIA. 5 TO 25 CM OF RAIN.• WEATHER MOSTLY CONVECTIONAL TYPE.• LARGE SCALE THUNDERSTORM ACTIVITY.• DUST STORMS IN PUNJAB J&K HARYANA RAJASTHAN

UP & NORTH MP IN CASE OF NO HUMIDITY.• VIOLENT LOCAL THUNDER SQUALLS OVER BENGAL

ASSAM ORISSA & BIHAR (NORWESTERS).• TROPICAL CYCLONES IN ARABIAN SEA MOVE WEST

WARDS NOT AFFECTING INDIA.• MAY RECURVE AND HIT MAHARASHTRA GUJARATH.• TROPICAL STORMS IN BAY OF BENGAL BETWEEN LAT

10° TO 15° NORTH AND MOVE NORTH OF NORTHEAST AND STRIKE BENGAL BANGLADESH AND BURMA

• SOME MOVE NORTHWESTWARDS AND CROSS TAMILNADU COAST WEAKEN OR MAY CROSS OVER TO ARABIAN SEA AND REVIVE.

AVIATION WEATHER HAZARDS• THUNDERSTORMS.

– VIOLENT CONVECTIONAL TYPE DUE TO INTENSE INSOLATION & INFLUX OF MOISTURE.

– LOCAL AND SHORT LIVED. OCCUR IN LATER PART OF THE DAY OR EARLY EVENINGS.

– NORWESTERS MOST DESTRUCTIVE. FROM MID MARCH TO JUNE IN AFTERNOONS OR EVENINGS.

– AREAS OF FORMATION ARE CHHOTA NAGPUR HILLS AND MOUNTAIN DISTRICTS OF NORTH BENGAL.

– IN PENINSULA THUNDERSTORM FREQUENCY INCREASES IN KERALA AS SEASON PROGRESSES.

• DUST HAZE.– PUNJAB RAJASTHAN JAMMU HARYANA NORTH MP & UP– VIS 2 Km OR LESS FOR DAYS ALTOGETHER. VISIBILITY LESS

THAN A FEW HUNDRED METRES IN DUST FOG.– VERTICAL EXTENT OF DUST 3 TO 5 Km AT TIMES 7 TO 8 Km.

• DUST RAISING WINDS.– STRONG SURFACE WINDS 30 Kts GUSTING TO 40 TO 50 Kts.– CONFINED MOSTLY TO DAY TIME BETWEEN 0900 TO 1700 hrs.– VISIBILITY REDUCED TO A FEW HUNDRED METRES IN BLOWING

DUST.– IN CASE OF STRONG PRESSURE GRADIENT DUST RAISING

WINDS CONTINUE EVEN AT NIGHT.

• DUST STORMS.– ANDHIS OF NORTH INDIA.– DUST SCOOPED UP BY THE SQUALL FROM THE THUNDER

STORMS.– THE SQUALL SPEEDS 50 TO 60 Kts OCCASSIONALLY 70 TO 80

Kts.– NO MOISTURE SO NO RAIN OR VERY LITTLE RAIN. ONLY

DOWNDRAUGHTS WITHOUT MUCH PRECIPITATION. VISIBILITY ONLY A FEW METRES.

– CU AND CB PREREQUISITES FOR SUCH DUSTSTORMS.– OVER PUNJAB, RAJASTHAN, JAMMU, HARYANA, NORTH MP, UP

AND AT TIMES BIHAR & BENGAL.

SOUTHWEST MONSOON

•WET SEASON.

•JUNE TO SEPTEMBER.

•COUNTRY (OUTSIDE WESTERN & EASTERN HIMALAYAS AND SOUTHWEST PENINSULA TIP) RECIEVES 75% OF RAINFALL DURING THIS SEASON.

•SETS IN AT HE EXTREME SOUTHWEST TIP OF THE PENINSULA BY END OF MAY.

•WESTERLY STJ SHIFTS NORTH OF HIMALAYAS AND TROPICAL EASTERLY JET SETS IN OVER THE PENINSULA.

•MAIN CHARACTERISTICS ARE MODERATE TO HEAVY RAINS VERY LOW CLOUDS HIGH HUMIDITY AND SULTRY WEATHER.

PRESSURE DISTRIBUTION

•PRESSURE GRADIENT AND WIND FLOW SHOWS A COMPLETE REVERSAL AS COMPARED TO THE WINTER.

•INTENSE LOW PRESSURE OVER THE NORTHWESTENS PART OF THE COUNTRY AND ADJOINING AFGHANISTAN SPREADING EASTWARD AS AN EXTENSIVE TROUGH UPTO THE NORTHWEST BAY OF BENGAL COVERING THE ENTIRE INDOGANGETIC PLAIN.

•THE PRESSURE GRADUALLY INCREASES TO INDIAN OCEAN.

•HIGH PRESSURE AREA TO THE SOUTH.

•TROUGH OF LOW PRESSURE AREA IS KNOWN AS MONSOON TROUGH.

•THIS TROUGH GETS MODIFIES WHEN DEPRESSIONS FORM IN NORTHWEST BAY. THESE DEPRESSIONS MOVE WEST OR NORTHWEST WARD ALONG THE AXIS OF TROUGH UPTO RAJASTHAN WEAKEN AND MERGE INTO SEASONAL LOW.

WEATHER• MONSOON CURRENT SETS IN BY MIDDLE OF MAY IN

ANDAMANS.• BEGINNING OF JUN EXTENDS NORTHWARDS AS TWO

DISTINCT CURRENTS: – ARABIAN SEA BRANCH.– BAY OF BENGAL BRANCH.

• IN NEXT TWO WEEKS THE MONSOON CURRENTS COVERS THE ENTIRE COUNTRY EXCEPT THE NORTHWEST INDIA.

• BY MIDDLE OF JULY THE ENTIRE SUBCONTINENT IS UNDER THE EFFECT OF MONSOON.

• MORE OR LESS GENERAL RAIN THOUGH OUT THE COUNTRY.

• ARABIAN SEA BRANCH GIVES SOME RAIN IN GUJARAT AND WEST RAJASTHAN.

• BAY OF BENGAL BRANCH CAUSES HEAVY RAINS IN BURMA & EASTERN HIMALAYAS.

• FROM BENGAL THIS BRANCH IS DEFLECTED WESTWARDS AS EASTERLIES PARALLEL TO HIMALAYAS.

• THE WESTERLY ARABIAN SEA BRANCH AND EASTERLY BAY OF BENGAL BRANCH MEET ALONG THE MONSOO TROUGH GIVING MODERATE TO HEAVY RAINS.

• THE AXIS AT WHICH THE ARABIAN SEA CURRENT AND MODIFIED BAY MONSOON AIR MEETS IS KNOWN AS THE MONSOONFRONT. THE ORIENTATION OF THIS DECIDES RAINFALL IN THE INDO GANGETIC PLAIN.

• DEPRESSIONS IN BAY MOVE WESTWARDS ALONG TROUGH OF LOW GIVING WIDESPREAD AND HEAVY RAINS. RAINFALL HEAVIEST IN THE SOUTHWESTERN SECTOR OF THE DEPRESSION.

• MONSOONFRONT AT THE FOOTHILLS INCREASES RAIN OVER HILLS AND REDUCTION IN PLAINS. THIS IS BREAK MONSOON.

AVIATION WEATHER HAZARDS

• LOW CLOUDS. 150 TO 200 M AGL IN MODERATE CONDITIONS. IN DEPRESSION 30 TO 60 M AGL

• STRONG SURFACE WINDS. NORMAL MONSOON CONDITIONS 25 TO 30 Kts DEPRESSIONS 40 TO 50 Kts.

• VISIBILITY. VISIBILITY IS GENERALLY GOOD. DROPS TO A FEW HUNDRED METRES IN HEAVY RAINS.

• EASTERLY TROPICAL JET STREAM. LIES BETWEEN 10° TO 15° NORTH LATITUDE. CORE BETWEEN 150 TO 100 hPa. AVERAGE CORE SPEEDS 60 TO 70 Kts OCCASSIONALLY 100 TO 120 Kts.

POSTMONSOON SEASON• COOL SEASON. TRANSITIONAL SEASON.• OCT TO NOV.• TEMPERATURES START DECREASING, BY

OCT COOL IN MOST PALCES.NIGHTS ARE COOL AND PLEASANT.

• ZONAL WESTERLIES SHIFT SOUTH OF HIMALAYAS. WDs’ TRACKS SHIFT SOUTH.

• STJ APPEARS OVER EXTREME NORTH.• TROPICAL EASTERLY JET DISAPPEARS

FROM OVER THE PENINSULA.• TAMILNADU GETS MAX RAIN IN THIS

SEASON.

PRESSURE DISTRIBUTION• PATTERN IS DIFFUSED AND ILL DEFINED.• SUMMER PATTERN OF PRESSURE AND WINDS

UNDERGO MODIFICATIONS PRIOR TO CHANGE OVER TO WINTER PATTERN.

• TRACKS OF MIDLATITUDE DEPRESSIONS SHIFT SOUTHWARDS WITH PROGRESS OF THE SEASON. START AFFECTING INDIA.

• IN SOUTH OCTOBER IS THE MONTH OF MAXIMUM FREQUENCY OF CYCLONIC STORMS IN BAY OF BENGAL.

• IN NOV SEVER CYCLONIC STORMS FORM AND HIT TAMILNADU COAST. MOVE NORTH OR NORTHEAST WARDS STRIKING BENGAL BANGLA OR BURMA. A FEW MAY CROSS OVER AND REVIVE IN ARABIAN SEA AND AFTER RECURVING HIT MAHARASHTRA & GUJARATH.

WEATHER• WITH RETREAT OF MONSOON COLD DRY CONTINENTAL

AIR SETS IN OVER NORTH- WEST INDIA.• CLEAR WEATHER & GOOD VISIBILITY.• WITH ADVANCE OF SEASON, DAWN AND DUSK MIST OR

HAZE IS EXPERIENCED.• WDs START AFFECTING EXTREME NORTH (1 TO 2 PER

MONTH) CAUSING CLOUDING & PPTN IN KASHMIR AND PUNJAB. SNOWFALL IN GREAT HIMALAYAS IN OCT.

• IN NORTHEAST INDIA WEATHER GENERALLY IMPROVES. BUT IN CASE OF CYCLONIC STORM IN BENGAL OR BANGLADESH INCURSION OF MOISTURE CAUSES CLOUDING AND WEATHER.

• BAY STORMS CROSS EITHER TN OR MOVE NORTHEAST SKIRTING ORISSA WEATHER OF ENTIRE COASTAL BELT OF PENINSULA GETS AFFECTED. GIVING EXTENSIVE CLOUDING WITH VERY LOW CLOUDS HEAVY RAIN AND THUNDERSQUALLS.

AVIATION WEATHER HAZARDS

• LOW VISIBILITY. DUE TO STRATIFIED SMOKE NEAR TOWNS AND INHABITED PALCES AT DAWN AND DUSK.

• TROPICAL STORMS. SEVERE CYCLONIC STORMS DEVELOP IN THE BAY OF BENGAL AND IN ARABIAN SEA IN THE LATITUDE BELT OF 10 TO 15° N AND AFFECT THE ENTIRE REGION.

• SUBTROPICAL JET STREAM. MEAN POSITION OF WESTERLY STJ STARTS SHIFTING SOUTH AND MAY BE LOCATED BETWEEN 30 TO 35° N NEAR ABOUT 200hPa LEVEL. AVERAGE SPEEDS 60 TO 70 Kts MAX SPEEDS 100 TO 150 Kts.