1. PRE-MONSOON SEASON
FG OFFR BHAVNA VERMA

2. PRE MONSOON INTRODUCTION ONSET PRESSURE PATTERN UPPER WINDS
TEMPERATURE
RAINFALL
WEATHER
AVIATION WEATHER HAZARDS
CASE STUDIES

3. PRE MONSOON
THE TWO MAIN SEASON OF INDIA ARE SOUTH WEST MONSOON AND NORTHEAST MONSOON. BUT BETWEEN THESE TWO SEASONS THERE IS A TRANSITION PERIOD WHERE THE REVERSAL OF PRESSURE AND WIND PATTERN CHANGES. SO BASED ON THESE PATTERNS THE SEASON IN INDIA ARE AS FOLLOWS:-
WINTER/NORTH EAST MONSOON DEC-MAR
SUMMER / PRE-MONSOON AUG-MAY
SOUTH-WEST MONSOON JUN-SEP
POST-MONSOON OCT-NOV

4. PRE-MONSOON
THE PRE-MONSOON OR HOT SEASON IS A TRANSITIONAL SEASON WHEN THE SEA LEVEL PRESSURE DISTRIBUTION OVER SOUTH ASIA SHOWS A GRADUAL TRANSITION, DURING MARCH-MAY FROM THE WINTER CONDITIONS TO THE CHARACTERISTIC PATTERN OF SUMMER MONSOON CIRCULATION. THIS PERIOD IS MARKED BY HIGH TEMPERATURES AND EXTENSIVE HEAT LOWS OVER LAND AREAS.

5. CAUSAL FACTORS
THE SUN CROSSES EQUATOR ON MARCH AND START MOVING NORTHWARDS GIVING RISE TO HIGH TEMPERATURES
WITH THIS MOVE THE EQUATORIAL TROUGH MIGRATES TO ITS NORTH MOST POSITION 30° N OVER SOUTH ASIA DURING SUMMER
THIS MIGRATION RESULTS IN THERMAL DIFFERENCES BETWEEN THE LAND AND THE SEA

6. FEATURES
WITH THIS SHIFT THE HEATING ZONE STARTS SHIFTING AND HEAT LOWS DEVELOPED OVER THE SUB CONTINENT
LOWS EXTENDS FROM SAHARA TO CENTRAL ASIA ACROSS ARABIA, IRAN, AFGANISTAN, PAKISTAN, NORTH WEST INDIA AND EVEN UP TO MYANMAR
BY MAY THE CONDITIONAL LOW PRESSURE COMPLETELY DOMINATE ASIA
THE LOWEST PRESSURE FOR THE WHOLE BELT IS OVER CENTRAL PART OF PAKISTAN

7. FEATURES
ITS MAIN CENTRE OVER INDIA IS NEAR 30°N,75°E WITH AN EXTENSION AS A TROUGH UPTO ORISSA.
BY END OF MAY THE TROUGH OVER THE PENINSULA HAS SHIFTED TO 79° E ALONG THE MADRAS COAST.

8. FEATURES
OVER INDIA THE MAXIMUM SHIFTING OF HEAT WAS OVER NW INDIA KNOWN AS HEAT LOWS DUE TO
TAPERING SHAPE OF PENINSULAR INDIA.
THE HIMALAYAN BARRIER TO THE NORTH.
THE ASSAM HILLS OF NORTH EAST.
BLOCKING OF COLD AIR MASS INCURSION FROM NORTH OF HIMALAYA IN THE LOWER TROPOSPHERE ALSO MAKES THE HEAT LOW MORE INTENSE IN THIS REGION.

9. FEATURES OVER INDIA
THE HEAT LOW IS SHALLOW, EXTENDING UP TO ABOUT 1.5 KM
IT IS OVERLAIN BY A WELL MARKED RIDGE EXTENDING TO THE UPPER TROPOSPHERE
THE SUBSIDENCE IN THE UPPER RIDGE MAINTAIN THE CLEAR SKIES REQUIRED FOR THE HEAT LOW
THE HEATING CONTINUES UNINTERRUPTEDLY IN MAY IN THE ABSENCE OF ANY RAIN OR CLOUDING OVER THE AREA OF HEAT LOW

10. FEATURES OVER INDIA
LAND AND SEA BREEZE EFFECT IS REGULAR AND PROMINENT.
FORMATION OF FEW TROPICAL CYCLONES TAKES PLACE IN THE INDIAN SEAS

12. PRESSURE PATTERN PRESSURE DECREASES DUE TO RISE IN TEMPERATURE
HIGH PRESSURE OVER NW INDIA IS DIFFUSED
2-3 hPa DIFFERENCE BETWEEN NORTH & SOUTH
WINDS ARE LIGHT/VRB
LAND AND SEA BREEZE BECOME PROMINENT
A FEEBLE LOW OVER CENTRAL PARTS OF THE SUBCONTINENT
HIGH PRESSURE OVER ARABIAN SEA AND BAY OF BENGAL

13. UPPER WINDS FEATURES

14. UPPER WINDS FEATURES

15. UPPER WINDS FEATURES
IN APRIL, A TROUGH LINE RUNS ALONG 77 ºE AT 900 HPA OVER THE PENINSULA AS AT SEA LEVEL
WEAK HIGH CELLS PROBABLY EXIST OVER THE CENTRAL BAY AND THE ARABIAN SEA
THE SUB TROPICAL RIDGE APPEARS OVER LAND AROUND ºN AT THE 850 HPA LEVEL AND EXTENDS UP TO 700HPA
IT GRADUALLY SHIFTS SOUTHWARDS TO 8º N AT THE 200 HPA LEVEL
A TROUGH IS ALSO PRESENT OVER NORTHEAST INDIA BETWEEN 900 HPA TO 800 HPA

16. UPPER WINDS FEATURES
WESTERLIES STRENGTH WITH HEIGHT OVER NORTHERN INDIA, REACHING MAXIMUM SPEEDS AROUND 40KT NEAR 25º N AT 300HPA AND 50-60KT AT 200 HPA
THE SUBTROPICAL WESTERLY JET STREAM IN THE UPPER TROPOSPHERE IS FOUND LITTLE NORTH OF DELHI
SOME UPPER LEVEL EASTERLIES ARE FOUND OVER SOUTHERN PENINSULAR INDIA, BUT THEY GENERALLY WEAKEN DURING PRE MONSOON MONTHS

17. SURFACE TEMPERATURES
THE HIGHEST TEMPERATURES OVER INDIA OCCUR DURING THE PRE-MONSOON SEASON
THE HOTTEST REGIONS EXTEND FROM WEST PAKISTAN TO EAST MP
THE MEAN MAXIMUM TEMPERATURE IS OF ORDER ºC
TEMPERATURE UP TO 50ºC ARE RECORDED OVER DESERT AREA

18. A P R I L

19. A P R I L

20. UPPER AIR TEMPERATURE
THERMAL HIGH IS DEVELOPS AT 850 HPA OVER CENTRAL INDIA NEAR ABOUT 22ºN,80ºE.
TEMPERATURES DECREASES IN ALL DIRECTIONS.
AT 700HPA,THE THERMAL HIGH SHIFTS SOUTHWARDS AND TEMPERATURE GRADIENT TOWARDS NORTH.
A RIDGE APPEARS FROM NORTH ARABIAN SEA TO THE CASPIAN SEA.

21. UPPER AIR TEMPERATURE
AT 500 HPA THE TEMPERATURE DECREASES TO THE NORTH AND UNIFORM TO THE SOUTH.
AT 300 HPA WEAK THERMAL HIGH IS OVER THE SOUTH PENINSULA.
IT IS SEEN AS A WEAK THERMAL HIGH OVER SOUTHERN PENINSULAR INDIA AT 300 HPA.
AT 200 HPA THE THERMAL FIELD BECOMES FLAT WITH VARYING TEMPERATURES IN THE RANGE -52ºC TO -48ºC.
ABOVE 200 HPA, THE TEMPERATURE GRADIENT IS REVERSED, WITH A MARKED INCREASE IN TEMPERATURE FROM CENTRAL PARTS OF THE PENINSULA TOWARDS THE NORTH.

22. APRIL

23. RAINFALL
IN THE PRE-MONSOON THE AREAS THAT RECEIVE SIGNIFICANT RAINFALL, CHIEFLY FROM FREQUENT THUNDERSTORMS
DURING APRIL THE EASTERN HALF OF THE COUNTRY RECEIVES 10CM
THE NORTH-EASTERN PARTS RECEIVE OVER 30CM OF RAINFALL
IN MAY THE WHOLE COUNTRY RECEIVES WELL OVER 17 CM RAINFALLS, WITH MAXIMUM OCCURRING OVER NORTH EASTERN PARTS REACHING MORE THAN 50CM

24. RAINFALL

25. WEATHER SYSTEM WESTERN DISTURBANCES FREQUENCY REDUCES
DUST STORM OVER NW INDIA TRIGGERED BY WESTERN DISTURBANCES
TROPICAL STORMS
FREQUENCY INCREASES
FORMS BETWEEN 8ºN AND 15ºN
MOVES N/NW SOMETIMES RECURVE AND MOVE NE AFFECTING BANGLADESH AND BURMA
ARABIAN SEA CYCLONE ALSO MOVE N/NW AND RECURVE NE WARDS

26. WEATHER THE SEASON IS MAINLY CHARACTERISED AS:- DUST HAZE OR ANDHIS
NORWESTERS OR KALBAISAKHIS
THUNDERSHOWERS

27. AVIATION WEATHER HAZARDS
THUNDERSTORMS
DUST HAZE
DUST RAISING WINDS
DUST STORMS

28. THUNDERSTORMS
CAUSED BY INTENSE INSOLATION,WHEN THERE IS INFLUX OF MOISTURE.
LOCAL IN NATURE AND COMPARATIVELY SHORT LIVED.
OCCUR IN THE LATTER PART OF THE DAY OR EARLY EVENING.

29. DUST-HAZE
DUST IN SUSPENSION REDUCES VISIBILITY TO 2KM OR LESS FOR DAYS TOGETHER.
AT TIMES VISIBILITY REDUCES TO FEW METERS
THE VERTICAL EXTENT OF THE DUST LAYER VARIES BETWEEN 3-5 KM THOUGH

30. DUST RAISING WINDS THEY ARE STRONG SURFACE WINDS OF ORDER 30KT.
THE VISIBILITY REDUCES TO FEW METERS.
DUST RAISING WIND CONTINUES DURING NIGHT WHEN THERE IS STRONG PRESSURE GRADIENT.

31. WEATHER OVER NORTHWEST INDIA
IN WESTERN INDIA THE MOISTURE CONTENT IS LESS WHICH RESULTS IN DUST STORM AND DUST HAZE
THEIR FREQUENCY IS 3 TO 5 PER MONTH
THESE STORMS ARE LOCALLY KNOWN AS ANDHIS
THE AFFECTED AREAS ARE PUNJAB, JAMMU HARYANA, RAJASTHAN, UTTAR PRADESH AND NORTH MADHYA PRADESH

32. WD L

33. DUST STORMS
THEY ARE RESULT OF DUST SCOOPED UP FROM EARTH SURFACE DUE TO THUNDERSTORMS.
THE SPEED OFTEN REACHES 40-50KT.
VISIBILITY DETERIORATES TOO FEW METERS.
THEY ARE OF SHORT DURATION AND ARE OFTEN FOLLOWED BY A FEW DROPS OF RAIN.

34. DUST STORM
IN INDIA, DUSTSTORMS ARE ALSO CALLED ANDHI BECAUSE OF THE DARKNESS CREATED BY THE DUST WALL AS IT PASSES OVER A STATION.
DUST STORMS OCCUR OVER PUNJAB, HARYANA, DELHI AND WESTERN UTTAR PRADESH DURING THE MONTHS OF APRIL, MAY & JUNE. & AT TIMES IN OCT & NOV ALSO.
DUST-STORMS ALSO OCCUR IN THE REGION EXTENDING WESTWARDS ACROSS PAKISTAN AND ARABIA TO THE ARID REGIONS OF AFRICA LIKE SUDAN, CHAD ETC.
IN AFRICA AND ARABIA THE PHENOMENON IS CALLED HABOOB.
IN DUSTSTORMS, DUST IS RAISED BY THE DOWN DRAFT OF CB OR A SQUALL LINE.

35. DUST STORM 06 APR 15

36. DUST STORM 06 APR 15

37. MSL 06 APR 15

38. 925 HPA 06 APR 15 C

39. 850 HPA 06 APR 15

40. 700 HPA 06 APR 15

41. WEATHER OVER NORTHEAST INDIA
IN EASTERN PARTS OF INDIA VIOLENT LOCAL THUNDER SQUALL OFTEN ACCOMPANIED BY HAIL AND RAIN OCCURS
THEY ARE LOCALLY KNOWN AS NORWESTERS OR KALBAISAKHIS
THEY OCCURS WHERE DEEP HUMID AIR OF OCEANIC REGION MEETS HOT DRY LAND WINDS
THE AREAS WHERE THIS TYPE OF THUNDERSTORMS OCCURS ARE BENGAL, ASSAM, ORRISA AND EAST BIHAR

42. WD C

43. WHAT IS NOR’WESTER?
IN THE MONTH OF MARCH, APRIL AND MAY (PRE-MONSOON PERIOD) THUNDERSTORMS THAT USUALLY ORIGINATES IN WB AND CHOTONAGPUR AREA, SOME OF WHICH REACH TO SEVERE THUNDERSTORM OR TORNADO VIOLENCE, ARE TERMED AS NOR’WESTER BECAUSE OF THEIR TENDENCY TO MOVE FROM NORTH-WEST.
IN BENGALI NOR’WESTER ARE POPULARLY KNOWN AS ‘KAL BAISHAKHI’ DUE THEIR BLACK APPEARANCE AND PERIOD OF FORMATION IN THE BENGALI MONTH BAISHAKH.

44. CAUSES OF NOR’WESTER
A LOW-LEVEL HUMID AND WARM LAYER OF AIR EXTENDING FROM SURFACE TO 5,000 FT OR 7,000 FT.
AN UPPER LEVEL DRY AND COLD LAYER OF AIR.
ONE OR MORE OF THE FOL TRIGGERING ACTIONS:
* INSOLATION
* INFLOW OF MOIST AIR AT LOWER LEVELS
* EASTWARD PASSAGE OF WD
* CONVERGENCE OF AIR OR KATABATIC FLOW TO ENABLE THE LOW LEVEL MOIST AIR TO ASCEND.

45. L ** A LOW PRESSURE OVER WB & BIHAR DURING MONTHS OF MARCH TO MAY.
30,000 ft
FORMATION MECHANISM
Cold & dry air
20,000 ft
** A LOW PRESSURE OVER WB & BIHAR DURING MONTHS OF MARCH TO MAY.
65 to 75 % RH
Warm & moist air
7000 ft L
** WARM & MOIST AIR FROM SURFACE TO 7,000 FT.
Surface
** COLD & DRY AIR WITHIN 20,000 TO 30,000 FT .
** INVERSION LAYER BETWEEN 10,000 TO 15,000 FT.
Cold & dry air
Inversion Layer
Low level warm and
moist air, 65 to 75 %

46. LIFE SPAN IN 30% CASES, DURATION RANGES FROM 1 TO 1.5 HOURS.
IN REST 10% CASES, DURATION RANGES MORE THAN 4 HOURS.

47. WX ASSOCIATED WITH NOR’WESTER
MODERATE TO HEAVY RAINFALL. THE AVERAGE RAINFALL 12 MM BUT IN SOME CASES IT MAY CROSS 50MM OR MORE.
MODERATE TO STRONG GUSTY/SQUALLY WINDS. THE MOST COMMON WIND SPEED IS 25 TO 50 KNOTS. IN FEW CASES, WIND SPEED MAY REACH 100 KNOTS OR MORE.
MODERATE TO SEVERE HAIL STORMS. SOME OF THE NOR’WESTER ARE ACCOMPANIED BY HAIL WITH SUFFICIENTLY LARGE SIZE TO DAMAGE THE AIRCRAFT AND OTHER INSTALLATIONS. SMALL HAILSTONES CAUSE SUPERFICIAL DAMAGE BUT LARGE ONE MAY MAKE HOLES ON THE BODY OF AIRCRAFT.

48. WX ASSOCIATED WITH NOR’WESTER
MODERATE TO SEVERE ICING. MODERATE TO SEVERE ICING IS
POSSIBLE IN A HIGHLY DEVELOPED NOR’WESTER.
VERTICAL AND HORIZONTAL WIND SHEAR. THE SPEED OF
UPDRAFT (VERTICAL CURRENT) IN A NOR’WESTER MAY BE AS HIGH AS 30 TO 40 FEET PER SECONDS.
MODERATE TO SEVERE LIGHTNING.
DAMAGE OF AERIALS, HINGE BEARINGS AND DENTING.
BACKGROUND CRACKLE ON THE RADIO RECEIVER.
DAMAGE OF ELECTRICAL AND COMPASS SYSTEMS.
TEMPORARY BLINDNESS OF PILOT.

49. WX ASSOCIATED WITH NOR’WESTER
LOW CLOUD. THE HEIGHT OF THE BASE OF THE LOWEST CLOUD MAY COME DOWN AS LOW AS 300FT OR BELOW .
THICKNESS OF CB CLOUD. TOP OF THE CB CLOUD MAY REACH AS HIGH AT 40,000FT. IN SOME CASES TOP MAY REACH UPTO 60,000FT.

50. 21 APR 2014 (KKD NORWESTER)

51. MSL 21 APR 15

52. 925 HPA C

53. 850 HPA

54. RADAR IMAGERY

55. WEATHER OVER SOUTHERN INDIA
IN THE PENINSULA, BEFORE THE ONSET OF SOUTHWEST MONSOON FREQUENT INCURSION OF MOISTURE GIVES RISE TO PER MONSOON THUNDERSHOWERS
THESE ARE LOCAL IN NATURE AND SHORT-LIVED
THEY OCCURS FROM MID-MARCH TO JUNE, MAINLY DURING AFTERNOONS AND EVENINGS

56. Peninsular discontinuity
During Pre-monsoon months, especially in Mar-Apr, a wind discontinuity (up to 1.0 km) extends from S-Kerala to Vidarbha and SE MP. This discontinuity shows E-W oscillations. In extreme cases it may be close to E-Coast of peninsula or along W-coast.
This discontinuity is also a line of moisture discontinuity. The air to the south and east is generally moist while the air to the west is much drier particularly in Northern-Peninsula.

57. Peninsular Discontinuity
Normal Position of Peninsular Discontinuity
Thunderstorm activity over peninsula is generally confined to the east of wind discontinuity. When the discontinuity is well marked, Thunderstorm activity may increase and even widespread Thunderstorm may occur over few portion of south peninsula.
Induced system moving across central parts of the country when comes in phase with the bubble of circulation formed along the peninsular discontinuity leads to the intensification and increased weather activities in respect of both the systems.

58. PENNISULAR DISCONTINUITY 06 APR 2014 ASSOCIATED CLOUDING

59. 925 HPA AND 850 HPA

60. Thank you

61. MICROBURST
A MICROBURST IS A SMALL, VERY INTENSE DOWNDRAFT THAT DESCENDS TO THE GROUND RESULTING IN A STRONG WIND DIVERGENCE
THE SIZE OF THE EVENT IS TYPICALLY LESS THAN 4 KILOMETERS ACROSS
MICROBURSTS ARE CAPABLE OF PRODUCING WINDS OF MORE THAN 100 MPH CAUSING SIGNIFICANT DAMAGE
THE LIFE SPAN OF A MICROBURST IS AROUND 5-15 MINUTES

62. DISSIPATING CB
NO PROBLEM ?

63. LIGHTNING
LIGHTNING IS PRODUCED IN THUNDERSTORMS WHEN LIQUID AND ICE PARTICLES ABOVE THE FREEZING LEVEL COLLIDE, AND BUILD UP LARGE ELECTRICAL FIELDS IN THE CLOUDS.
ONCE THESE ELECTRIC FIELDS BECOME LARGE ENOUGH, A GIANT "SPARK" OCCURS BETWEEN THEM (OR BETWEEN THEM AND THE GROUND) LIKE STATIC ELECTRICITY, REDUCING THE CHARGE SEPARATION. THE LIGHTNING SPARK CAN OCCUR BETWEEN CLOUDS, BETWEEN THE CLOUD AND AIR, OR BETWEEN THE CLOUD AND GROUND.
AS LONG AS A THUNDERSTORM CONTINUES TO PRODUCE LIGHTNING, THE STORM STILL HAS ACTIVE UPDRAFTS AND IS STILL PRODUCING PRECIPITATION.
THE TEMPERATURE INSIDE A LIGHTNING BOLT CAN REACH 50,000 DEGREES F.

64. CLOUD TO GROUND LIGHTNING USUALLY OCCURS NEAR THE BOUNDARY BETWEEN THE UPDRAFT REGION (WHERE THE DARKEST) CLOUDS ARE AND THE DOWNDRAFT / RAINING REGION (WITH THE LIGHTER, FUZZY APPEARANCE).

65. INGREDIENTS OF THUNDERSTORM
ALL THUNDERSTORMS REQUIRE THREE INGREDIENTS FOR THEIR FORMATION:-
INSTABILITY
MOISTURE
LIFTING MECHANISM.

66. DUST PRODUCING REGIONS

67. CONVECTIVE DUSTSTORM
Conditions for the occurrence of convective type of Dust storm are the same as for Thunderstorm:-
Steep lapse rate through very deep layers of the atmosphere.
Large vertical velocities.
Presence of conditional instability. Instability may increase due to advection of warm air in the lower levels and colder air in the higher levels.
Low level convergence and upper level divergence with a favourable synoptic situation.

68. PRESSURE GRADIENT D’STORMS
Dust is injected into the atmosphere by the high winds caused by strong pressure gradients in low pressure systems.
Pressure gradient D’storms last longer than convective D’storms (Andhi and Haboob), sometimes even two to three days.
On such occasions, the winds in the very low levels (upto 1.5 km) are also very strong-usually of the order of 30 kts. And sometimes reaching as much as 50 kts.