1. BLOCKING HIGH

2. SEQUENCE INTRODUCTION FORMATION CASE STUDIES OF BLOCKING HIGH
TYPES OF BLOCKING HIGH
OMEGA
REX
RING OF FIRE
SPLIT FLOW
CONCLUSIONS

3. SEQUENCE INTRODUCTION FORMATION CASE STUDIES OF BLOCKING HIGH
TYPES OF BLOCKING HIGH
OMEGA
REX
RING OF FIRE
SPLIT FLOW
CONCLUSIONS

4. INTRODUCTION BLOCKING HIGH IS:
FORMATION
CASE STUDIES OF BLOCKING HIGH
TYPES OF BLOCKING HIGH
CONCLUSIONS
INTRODUCTION
BLOCKING HIGH IS:
SURFACE ANTICYCLONES IN MIDDLE LATITUDES
PREVENTING– OR BLOCKING- THE EASTWARD PROGRESS OF INCOMING SYSTEMS

5. DEFINITIONS
2. NAMIAS (1947).
  (A) IT IS A RETARDATION OF THE ZONAL CIRCULATION IN A LIMITED SECTOR OF THE HEMISPHERE AND AT ALL LEVELS IN THE TROPOSPHERE, SPREAD WESTWARD.
(B) SLOW PROGRESS LASTING FOR WEEKS AND AFFECTING LARGE AREAS.
  (C) WAVES OF BLOCKING IN FAIRLY CLOSE SUCCESSION.
  (D) THE BLOCKING SECTOR USUALLY COMPRISES OF
A WARM ANTICYCLONE IN HIGH LATITUDE AND
A COLD DEPRESSION IN LOWER LATITUDES.

6. DEFINITIONS
ELLIOT & SMITH (1947) : STATE OF CIRCULATION IN WHICH THE NORMAL ZONAL FLOW IS INTERRUPTED IN A SECTOR BY PERSISTENT STRONG MERIDIONAL TYPE OF FLOW. A PERSISTENT HIGH AT HIGH LATITUDES OBSTRUCTING THE NORMAL EASTWARD PROGRESS OF MIGRATING CYCLONES AND ANTICYCLONES IS TERMED BLOCKING.

7. DEFINITIONS 4. BERGGREN, BOLIN & ROSSBY (1949).
INTRODUCTION
FORMATION
CASE STUDIES OF BLOCKING HIGH
TYPES OF BLOCKING HIGH
CONCLUSIONS
DEFINITIONS
4. BERGGREN, BOLIN & ROSSBY (1949).
  (A) DESCRIBED BLOCKING IN TERMS OF INSTABILITIES IN THE NORMAL ZONAL CIRCULATION LEADING TO THE ABNORMAL GROWTH OF UPPER WAVES, WITH THE RESULT PARTIAL ‘CUTTING-OFF’ OF WARM ANTICYCLONIC AND COLD CYCLONIC VORTICES.

8. DEFINITIONS
  (B) THE ‘WESTWARD ADVANCE’ DEPENDS ON THE RAPID DEVELOPMENT AND GRADUAL STAGNATION EACH TIME FURTHER WEST OF A SERIES OF WAVES FROM THE WEST.
  (C) THE UPPER FLOW PATTERN IS CHARACTERISED BY SPLITTING OF THE MAIN ZONAL CURRENT (THE JET STREAM) INTO TWO DISTINCT BRANCHES, ONE BEING DEFLECTED NORTH ROUND THE ANTICYCLONIC VORTEX AND ONE TO THE SOUTH ROUND THE COLD LOW.

9. SEQUENCE INTRODUCTION FORMATION CASE STUDIES OF BLOCKING HIGH
TYPES OF BLOCKING HIGH
OMEGA
REX
RING OF FIRE
SPLIT FLOW
CONCLUSIONS

10. INTRODUCTION
FORMATION
CASE STUDIES OF BLOCKING HIGH
TYPES OF BLOCKING HIGH
CONCLUSIONS
FORMATION
5. NORMALLY, THE FLOW PATTERN IN THE MIDDLE & UPPER TROPOSPHERE IN MID LATITUDES CONSISTS IN A SUCCESSION OF TROUGHS & WEDGES. ON OCCASION, A WEDGE MAY INTENSIFY & DEVELOP A CLOSED ANTICYCLONIC CIRCULATION. 6. QUITE FREQUENTLY THE DEVELOPMENT OF A BLOCKING HIGH IS ACCOMPANIED BY THE FORMATION OF ONE OR MORE CUT-OFF LOWS IN NEIGHBOURING REGIONS.

11. INTRODUCTION
FORMATION
CASE STUDIES OF BLOCKING HIGH
TYPES OF BLOCKING HIGH
CONCLUSIONS
FORMATION
7. IT IS TYPICAL OF THESE DEVELOPMENTS THAT THE INITIAL STATE MAY BE IDENTIFIED WITH UPPER AIR TROUGHS AND WEDGES, WHICH ARE OUT OF PHASE WITH THE THERMAL PATTERN. 8. UNDER FAVOURABLE CIRCUMSTANCES, THE CUT-OFF HIGH ACQUIRES APPRECIABLE DIMENSIONS AND INTENSITY. 9. ZONAL CURRENT DEVELOPS TWO DISTINCT BRANCHES AND SHARP TROUGHS OR CUT-OFF LOWS DEVELOP TO THE SOUTHWEST AND SOUTHEAST.

12. FORMATION
(A) STAGE I.
WHEN THE JET STREAM IS WELL TO THE NORTH OF ITS MEAN POSITION, THE RIDGES AND TROUGHS ARE NORMALLY TILTED OR EVEN FRACTURED

13. FORMATION
(B) STAGE II. WHEN THE TWO WAVES GET INTO PHASE, THE CORE OF THE JET STREAM BECOMES DISPLACED SOUTHWARD.
(C) STAGE III. THE CIRCULATION PATTERN STARTS ACQUIRING CELLULAR PATTERN.
(D) STAGE IV. THE JET CORE SHIFTS SOUTHWARDS TO ITS LOWEST LATITUDE WHEN THE CIRCULATION PATTERN HAS ACQUIRED THE CELLULAR PATTERN AS SHOWN IN FIGURE 3 STAGE IV. BLOCKING HIGH AND CUT OFF LOW FORMATION IS SEEN IN THIS STAGE.

14. AREAS OF BLOCKING.
1. OFF THE WEST COAST OF EUROPE AND NORTH AMERICA. 2. ATLANTIC BLOCKS ARE MORE FREQUENT THAN PACIFIC BLOCKS. 3. WEST ASIA IS ANOTHER ZONE OF WEAK BLOCKING SITUATIONS 4. THE AVERAGE DURATION – 16 DAYS OVER ATLANTIC. 5. FREQUENCY MAXIMUM IN APRIL AND MINIMUM ABOUT AUGUST OR SEPTEMBER.

15. SEQUENCE INTRODUCTION FORMATION CASE STUDIES OF BLOCKING HIGH
TYPES OF BLOCKING HIGH
OMEGA
REX
RING OF FIRE
SPLIT FLOW
CONCLUSIONS

16. COMPILATION OF CASE STUDIES OF BLOCKING HIGH

17. INTRODUCTION
FORMATION
CASE STUDIES OF BLOCKING HIGH
TYPES OF BLOCKING HIGH
CONCLUSIONS
BLOCKING ACTION IN THE MIDDLE TROPOSPHERE AND ITS EFFECT UPON REGIONAL CLIMATE BY
REX D F (1950)

18. CONCLUSION
THE UPPER JET STREAM SPLITS INTO TWO DEFINITE BRANCHES IN EACH CASE.
DOWNWIND OF THIS BIFURCATION, THERE IS USUALLY A MACRO TURBULENT REGION, A MAIN FEATURE OF WHICH IS QUASI STATIONARY WARM RIDGE OR ANTICYCLONE.
THIS ANTICYCLONE MAY MOVE E’WARD OR (MORE SLOWLY) W’WARD OR REMAIN ESSENTIALLY STATIONARY, IN COMPANY WITH SURFACE HIGH (KNOWN AS ‘BLOCKING HIGH’), WITH WHICH IT IS ASSOCIATED.

19. CONCLUSION
4. WITHIN THE BLOCKED SECTOR, THE ZONAL INDEX MAY EITHER INCREASE OR DECREASE WITH BLOCKING, DEPENDING ON THE LATITUDE BAND WITH RESPECT TO THE AXIS OF THE BLOCK FOR WHICH IT IS COMPUTED. 5. A SHARP TRANSITION FROM A ZONAL TO A MERIDIONAL FLOW EXISTS IN EACH CASE. 6. THE FOREGOING CHARACTERISTICS MUST EXIST WITH RECOGNISABLE CONTINUITY FOR AT LEAST 10 DAYS.

20. GUPTA AND RAJAGOPALAN (1970)
INTRODUCTION
FORMATION
CASE STUDIES OF BLOCKING HIGH
TYPES OF BLOCKING HIGH
CONCLUSIONS
FORMATION OF UPPER AIR ANTICYCLONES AND THEIR BLOCKING ACTIVITY DURING WINTER OVER ASIA BY
GUPTA AND RAJAGOPALAN (1970)

21. CONCLUSION
1. WITH THE HELP OF NHEC CHARTS STUDIED FOR A PERIOD OF SIX YEARS THEIR CONCLUSIONS ARE AS FOLLOWS:- (A) PREFERENTIAL AREA BETWEEN 65°E - 70°E FOR THE DEVELOPMENT OF ANTICYCLONE IN THE UPPER W’LIES. (B) WARM AIR ADVECTION FROM WEST SOUTHWEST. (C) OUTBREAK OF POLAR AIR TOOK PLACE OVER CENTRAL ASIA THROUGH SIBERIA. (D) ASIAN BLOCKS ARE WEAK COMPARED TO ATLANTIC AND PACIFIC BLOCKS.

22. CONCLUSION
(E) ONLY 40% OF THE ANTICYCLONE WHICH FORMED IN MID LATITUDES, CIRCULATION OVER ASIA AND ADJOINING EUROPE DEVELOPS AS BLOCKING SYSTEM. (F) THE BLOCKING SYSTEMS WHICH DEVELOP SOUTH OF 48°N AND EAST OF 60°E ARE FOUND TO AFFECT THE CIRCULATION OVER INDIAN SUB CONTINENT. AS EAST OF THESE BLOCKS COLD LOWS FORM OVER NORTH INDIA AND NEIGHBOURHOOD. (G) STJ SPLITS N’WARD ALONG THE TROUGH ON THE WEST OF THE ANTICYCLONE, MAIN JET SPLITS SOUTH.

23. INTRODUCTION
FORMATION
CASE STUDIES OF BLOCKING HIGH
TYPES OF BLOCKING HIGH
CONCLUSIONS
White & Clark (1974).
(A) A SHARP TRANSITION IN THE WESTERLY FROM A ZONAL TYPE UPSTREAM TO MERIDIONAL TYPE DOWNSTREAM MUST BE OBSERVED.
  (B) THE BASIC WESTERLY CURRENT MUST SPLIT INTO TWO BRANCHES EACH TRANSPORTING APPRECIABLE MASS.
  (C) THE DOUBLE JET MUST FORM MORE THAN 45º OF LONGITUDE AND LAST FOR GREATER THAN10 DAYS.
  (D) A BLOCKING RIDGE EXISTS WHEN A SHARP TRANSITION IN THE MID-LATITUDE WESTERLY FROM ZONAL TYPE UPSTREAM TO MERIDIONAL TYPE DOWNSTREAM OCCURS WITH AMPLITUDE EXCEEDING 5º LATITUDE.

24. Conclusion
(A) LARGE-SCALE PROGRESS, USUALLY EMBRACING SEVERAL SYNOPTIC SYSTEMS.
(B) AND ALL INCLUDE AN ELEMENT OF PERSISTENCE IN THEIR DEFINITION. ELLIOT & SMITH STIPULATE THREE DAYS AND REX TEN DAYS AS A MINIMUM PERSISTENCE; WHILE OTHERS DO NOT GIVE A PRECISE FIGURE.

25. SEQUENCE INTRODUCTION FORMATION CASE STUDIES OF BLOCKING HIGH
TYPES OF BLOCKING HIGH
OMEGA
REX
RING OF FIRE
SPLIT FLOW
CONCLUSIONS

26. TYPES OF BLOCKING HIGH
1. IN ORDER TO KNOW MORE ABOUT BLOCKING HIGH AND ITS ASSOCIATED WEATHER PHENOMENA METEOROLOGIST JEFF HABY CLASSIFIED BLOCKING INTO FOUR MAIN TYPES AND EXPLAINED ABOUT WEATHER PHENOMENA AND SECTOR ASSOCIATED WITH THAT TYPE. THE FOUR TYPES OF BLOCKING HIGH ARE :- (A) OMEGA BLOCK (B) REX BLOCK (C) RING OF FIRE (D) SPLIT FLOW

27. SEQUENCE INTRODUCTION FORMATION CASE STUDIES OF BLOCKING HIGH
TYPES OF BLOCKING HIGH
OMEGA
REX
RING OF FIRE
SPLIT FLOW
CONCLUSIONS

28. OMEGA BLOCK

29. OMEGA BLOCK
1. IT RESEMBLES THE GREEK LETTER OMEGA . 2. THE REGION UNDER THE OMEGA BLOCK EXPERIENCES DRY WEATHER AND LIGHT WIND FOR AN EXTENDED PERIOD OF TIME WHILE RAIN AND CLOUDS ARE COMMON IN ASSOCIATION, WITH THE TWO TROUGHS ON EITHER SIDE OF THE OMEGA BLOCK.

30. OMEGA BLOCK
3. OMEGA BLOCKS MAKE FORECASTING EASIER SINCE ONE CAN PINPOINT AREAS THAT WILL BE DOMINATED BY DRY OR RAINY WEATHER FOR SEVERAL DAYS.
4. BECAUSE OF THE CLOSED NATURE OF THE ANTICYCLONIC CIRCULATION

31. SEQUENCE INTRODUCTION FORMATION CASE STUDIES OF BLOCKING HIGH
TYPES OF BLOCKING HIGH
OMEGA
REX
RING OF FIRE
SPLIT FLOW
CONCLUSIONS

32. REX BLOCK H L

33. REX BLOCK
1. A REX BLOCK RESEMBLES A “HALF FIGURE 8” OR “BACKWARD S”. 2. A REX BLOCK SETS UP WITH A STRONG HIGH-PRESSURE RIDGE ADJACENT TO A STRONG LOW-PRESSURE TROUGH. A LARGE RIDGE IS NORTH OF A LARGE TROUGH. 3. THIS CAUSES THE AIR TO SWING IN LOOPS NEAR THE SAME LONGITUDE BUT FORCES THE AIR TO A MUCH LOWER LATITUDE.

34. REX BLOCK
4. THE WIND FLOW MAKES LITTLE PROGRESS TO THE EAST DUE TO THE “HALF FIGURE 8” .
5. THEY ARE MOST OFTEN FOUND OVER WESTERN EUROPE AND THE WEST COAST OF THE UNTIED STATES.

35. Associated weather Conditions
1. UNSETTLED STORMY WEATHER FOUND NEAR LOW PRESSURES WHILE DRY CONDITIONS ARE TYPICAL WITH HIGH PRESSURE. 2. STRONG, PARTICULARLY PERSISTENT REX BLOCKS CAN CAUSE FLOODING IN THE SOUTHERN PART OF THE BLOCK AND SHORT TERM DROUGHT IN NORTH.

36. SEQUENCE INTRODUCTION FORMATION CASE STUDIES OF BLOCKING HIGH
TYPES OF BLOCKING HIGH
OMEGA
REX
RING OF FIRE
SPLIT FLOW
CONCLUSIONS

37. RING OF FIRE

38. RING OF FIRE
1. THE AIR IS MOST STABLE AT THE CENTER OF THE HIGH PRESSURE. AT THE EDGES OF THE HIGH PRESSURE, THE CAP WILL BE WEAK ENOUGH TO ALLOW AFTERNOON CONVECTION TO OCCUR. 2. THIS AFTERNOON CONVECTION MAKES A RING AROUND THE HIGH PRESSURE CELL. 3. THE AFTERNOON THUNDERSTORMS WILL ALSO ROTATE IN A CLOCKWISE MANNER AROUND THE HIGH PRESSURE CELL.

39. SEQUENCE INTRODUCTION FORMATION CASE STUDIES OF BLOCKING HIGH
TYPES OF BLOCKING HIGH
OMEGA
REX
RING OF FIRE
SPLIT FLOW
CONCLUSIONS

40. SPLIT FLOW
1. A SPLIT FLOW OCCURS WHEN THE JET STREAM BRANCHES INTO TWO SEPARATE BRANCHES.
2. WEATHER SYSTEMS FLOW QUICKLY THROUGH EACH BRANCH OF THE JET BUT THE WEATHER PATTERN BECOMES STAGNANT IN THE REGION BETWEEN THE TWO JETS.

41. SPLIT FLOW
3. THE SOUTHERN BRANCH IS AS STRONG OR STRONGER THAN ITS HIGHER LATITUDE COUNTERPART. 4. PREVALENT DURING EL NINO YEARS, A SPLIT SIGNATURE IS A TRUE BLOCK. 5. RIDGE CENTERS, OFTEN APPEAR ABOVE 45 N LATITUDE AND ACT TO KEEP STORMS IN THE SUBTROPICAL/ LOWER POLAR FLOW FROM PHASING WITH THE TRADITIONAL POLAR WESTERLIES.

42. IN-SITU
1. IN CASES WHERE A STTONG CLOSED LOW SEPERATES FROM A HIGH VELOCITY ARCTIC/ POLAR JEST STREAM, 2 IMPRESSIVE BAROCLINIC ZONE MAY SET UP IN THE CORRIDOR BETWEEN THE STORM AND THE FAST UPPER FLOW.
2. THE PRESENCE OF THE RIDGING SLOWS THE FORWARD MOTION OF THE DISTURBANCES WHILE AT THE SAME TIME ACTING TO HOLD COOLER AIR IN PLACE.
** BAROCLINIC ZONE: THE ZONE IN WHICH A TEMPERATURE GRADIENT EXISTS ON A CONSTANT PRESSURE SURFACE

43. BLOCKING IN SOUTHERN HEMISPHERE
1. THERE IS NO PREFERRED LONG TIME SCALE.
2. DURATION OF BLOCKING IS SHORTER THAN NORTHERN HEMISPHERE.
3. PERSISTENCE TIME IS 5 DAYS.
4. PREFERRED AREAS NEW ZEALAND, SOUTH AMERICA AND INDIAN OCEAN.
5. BLOCKING TENDS TO OCCUR IN LOWER LATITUDES IN SOUTHERN HEMISPHERE AND FOR SHORTER DURATION.

44. DISSIPATION OF BLOCKING HIGH
1. BLOCKING SPELL ENDS WHEN THE BLOCKING ANTICYCLONE SINKS INTO LOWER LATITUDES, WHERE IT USUALLY PERSISTS WITH A SIMULTANEOUS DECREASE IN AMPLITUDE OF THE UPPER RIDGE. 2. SOME TIMES THE BLOCKING PATTERN SIMPLY WEAKENS AND FADES AWAY. 3. IN SOME CASES, THE BREAKDOWN PATTERN IS MORE COMPLEX MAIN POINTS BEING NARROWING OR SECLUSION OF THE UPPER RIDGE ALLOWING THE W’LIES TO ADVANCE INTO MID LATITUDES.

45. IMPORTANT POINTS FOR FORECASTING OF BLOCKING
1. THE INITIATION OF A TYPICAL BLOCKING SITUATION IS USUALLY PRECEDED BY INTENSE CYCLOGENESIS TO THE WEST AND DEVELOPMENT OF A COLD CONTINENTAL ANTICYCLONE TO THE EAST OF THE BLOCKING AREA. 2. THE ESTABLISHMENT OF A BLOCK USUALLY REQUIRES SEVERAL DAYS TO ACCOMPLISH.

46. IMPORTANT POINTS FOR FORECASTING OF BLOCKING
3. ONCE A BLOCK HAS BEEN ESTABLISHED, IT WILL DOMINATE THE CIRCULATION PATTERN FOR MANY DAYS.
4. THE GREAT STABILITY AND THE SLOW MOVEMENT OF THE BLOCKS ARE FAVOURABLE FOR THE FORECASTING OF LENGTHY PERIODS WITHOUT MANY CHANGES.

47. RELATIONSHIP BETWEEN ZONAL INDEX AND BLOCKING HIGH
1. THE STRENGTH OF THE ZONAL CIRCULATION IS CONVENIENTLY MEASURED BY THE MEAN PRESSURE DIFFERENCE BETWEEN TWO LATITUDE CIRCLES. 2. IT IS CUSTOMARY TO USE THE PRESSURE DIFFERENCE BETWEEN 35 AND 55 N TO CHARACTERISE THE MID LATITUDE BELT AND THIS DIFFERENCE HAS BEEN CALLED THE ZONAL INDEX.

48. RELATIONSHIP BETWEEN ZONAL INDEX AND BLOCKING HIGH
3. THE TRANSITION FROM HIGH TO A LOW ZONAL INDEX AND VICE-VERSA ARE ASSOCIATED WITH MERIDIONAL VARIATIONS IN THE LOCATION OF THE JET STREAM IN THE UPPER TROPOSPHERE.

49. Zonal Index and Meridional variations in Jet Stream Location
1. THE TRANSITION FROM A HIGH TO A LOW INDEX AND VICE VERSA, ARE ASSOCIATED WITH MERIDIONAL VARIATIONS IN THE LOCATION OF THE JET STREAM IN THE UPPER TROPOSPHERE AND ON OCCASIONS MARKED TRENDS MAY EXIST FOR PERIOD OF SEVER 2. THE TYPICAL INDEX CYCLE IS BROUGHT ABOUT BY THE DIFFERENTIAL MOVEMENT OF TWO WAVE TRAINS AT DIFFERENT LATITUDES IN THE UPPER TROPOSPHERE AL WEEKS

50. SEQUENCE INTRODUCTION FORMATION CASE STUDIES OF BLOCKING HIGH
TYPES OF BLOCKING HIGH
OMEGA
REX
RING OF FIRE
SPLIT FLOW
CONCLUSIONS

51. CONCLUSIONS
1. BLOCKING IS A LARGE SCALE PROCESS 2. IT USUALLY EMBRACING SEVERAL SYNOPTIC SYSTEMS. 3. THE NORMAL ZONAL FLOW IS INTERRUPTED BY STRONG MERIDIONAL TYPE OF FLOW AND THE UPPER JET STREAM SPLITS INTO TWO DEFINITE BRANCHES. 4. THE BLOCKING ANTICYCLONE MAY MOVE E’WARDS OR SLOWLY W’WARDS OR REMAIN STATIONARY IN COMPANY WITH THE SURFACE HIGH KNOWN AS BLOCKING HIGH.

52. CONCLUSIONS
5. ALL BLOCKING HIGH SITUATIONS HAVE AN ELEMENT OF PERSISTENCE. 6. THE MAIN PROCESSES IN THE FORMATION OF BLOCKING CAN BE EITHER A PROGRESSIVE GROWTH OF AN UPPER RIDGE ASSOCIATED WITH A DEVELOPING SURFACE RIDGE OR ANTICYCLONE OR A SIMPLE DISRUPTION OF A PRE EXISTING UPPER RIDGE .

53. CONCLUSIONS
7. THE INITIATION OF A BLOCKING IS USUALLY PRECEDED BY INTENSE CYCLOGENSIS TO THE WEST AND DEVELOPMENT OF A COLD CONTINENTAK ANTICYCLONE TO THE EAST, OF THE BLOCKING AREA.
8. THE AREAS OF BLOCKING ARE ATLANTIC AND PACIFIC WITH A MARKED ANNUAL VARIATION IN THE FREQUENCY, THE MAXIMUM IS ABOUT APRIL MONTH AND MINIMUMS IN AUGUST AND SEPTEMBER.

54. THANK YOU