The onset of sea breeze is usually marked by an increase in wind speed, a fall in surface temperature and a rise in humidity as also the sudden change in wind direction. A study of sea breeze over Kalpakkam is carried out during the period from March, 1999 to January, 2000. Measurement of meteorological variables viz; wind speed (at 5 levels),wind direction (at 2 levels), temperature (at 2 levels) and relative humidity (at 1 level) are obtained from a 30 m tall micrometeorological tower located at 12.34.28 N latitude and 80.10.27 E longitude about 700 m inland from the eastern coast of India. The aim of the present work is to study the daily, monthly and seasonal variations of sea-breeze onset parameters and trends,and,compare these with earlier studies base on available data over Chennai(also located on eastern coast of India at some distance from Kalpakkam). The data are hourly averaged to study the various aspects. The parameters studied are the time of onset of sea-breeze,percentage rise in relative humidity, fall in surface temperature, wind speed range (at 30 m level) and wind direction variations at the time of onset. In addition, duration of sea-breeze each day, number of days in a month with sea-breeze and monthly frequency of time of onset etc. are also examined. The results are compared with the earlier studies carried out in 1938-40 and 1945-46 over Chennai for most of these parameters.

The criteria adopted to identify onset of sea-breeze is fall in relative humidity of 5% or more, fallin temperature of 0.7 degree celsius or more, increased wind speed and sudden significant variation in the wind direction. It has been observed that during most days maximum frequency of onset of sea-breeze falls between 15 to 18 hour. Maximum frequency of the wind speed at 30 m level at the time of onset has been observed between the range of 4.5-6.3 m/s. Maximum monthly frequency of relative humidity rise and temperature fall at 3 m level has been fond in the range of 15-19% and 0.7-1 degree celcius respectively. However, data indicates that relative humidity fall of 25% or more and temperature fall as 3.5 degree celcius or more also exists with low frequencies. Wind direction variation of 100 degree is often observed which may be higher in several cases. Marked differences in the trends between the current study and those obtained from the previous studies is also noticed and attempts are made to understand and explain these trends.