Atmospheric depressions sometimes initiate tropical cyclones in the pre- and post-monsoon season in the Bay of Bengal, which move to land and create havoc. Their intensity and pattern vary individually.

After hourly monitoring of the weather parameters of cyclone Sidr of 15 November 2007, which created severe disaster and weather variability in the Bangladesh plains and hills of Meghalaya Plateau in India a noticeable depression of cool air masses in the starting phase of the cyclone was found, which created the atmospheric disturbance for a shorter time. As a result, the psychrometric index fell slightly with significantly higher thermal efficiency values in the starting phase of cyclone, thus inviting speedy wind with heavy rains in its last phase.

The pattern of cyclonic rainfall was highly influenced by southerly (northward) speedy winds, especially on the windward areas of the cyclone. Two-peaked pattern of diurnal rainfall was observed at Cherrapunji and its surroundings, which was the active path of Sidr on the windward slopes of the Plateau. On the other hand, leeward areas of the Plateau experienced one-peaked pattern of diurnal rainfall with lower intensity.

Some important conclusions have been drawn from the present study of weather variability that occurred during the cyclone Sidr. The following particular inferences have been drawn from the analysis -

• Upper air results showed that at Dhaka and Guwahati, very wet conditions prevailed in the middle part of the upper layers at 450 hPa during the starting phase of the cyclone.
• Rainfall pattern of all stations was highly influenced by the southerly and southeasterly speedy winds during the cyclone in its active path across the Meghalaya Plateau.
• Rainfall intensity had a positive relationship with storm duration during the cyclone. It shows the concentration of rainfall for longer duration in the areas of intense cyclonic activity on the windward side of the Meghalaya plateau.
• There was a change in rainfall pattern over space as the cyclone weakened. Spatial variability in rainfall intensity was found to be much higher than expected. It may also be noted that under this condition of high spatial variability, the complex nature of temperature–pressure relationship arising out of orographic conditions is important for the spatial distribution of rainfall over the Meghalaya Plateau. Hence, further research needs to be done on the developments that favour these conditions.

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