If you walk on the beach shore on a summer evening, you would feel the winds or breeze making the weather pleasant. The movement of air around us is referred to as ‘wind’. Earth is one big mass of land and water which gets some heat energy from the sun. Due to its rotation on its axis, some parts face the sun for a period of time while others don’t. This uneven heating of earth’s surface is the reason for the flow of wind.
On a hot sunny day solid objects like buildings, vegetation, roads, sand and rocks get heated up faster than water. The sun’s rays cannot penetrate deep into the soil but only heat up the top layer.
The water in the ocean reflects much of sun’s rays falling over it. Also the rays that get absorbed reach some depth where it gets distributed over a large area. Hence the increase in temperature of sea water is very much less compared to the increase in the objects on the land. This means the water in the sea would remain much cooler during the day.
Before going further, let’s see what air pressure means. Air pressure is the force (per unit of area) exerted on the surface by weight of air above the surface. More the air molecules, more is the pressure.
Though land objects absorb heat quickly, they also radiate them very quickly. Which means the air above gets heated up and becomes light. The warm air starts to rise and low pressure is created above the surface of land. The air above the surface of water being cold is much dense and heavy causing high pressure over the surface. This pressure difference causes winds to blow from sea to land during the day.
What happens after sun set? The thermal coefficient of water is high meaning water cannot radiate heat quickly. Though water absorbs less heat during the day, it retains the heat. Now the sea is warmer than the land. So, the winds blow from high pressure area (land) to low pressure area (sea).
Winds blowing from the land to the sea is called land breeze and that blowing from the sea to the land is called sea breeze.