Air pressure at the Earth's surface is different in different places and this is partially due to the different amounts of heat they receive from the sun. When the Sun heats up the Earth, the air above warms, becomes less dense, expands and rises. The air above is pushed upwards and then spreads out horizontally. Because of this horizontal air movement there is less air above the ground where the heating took place and this leads to an area of low pressure. As the air rises, it cools. As it cools, the air becomes more dense and sinks. This sinking means that there is more air above the ground in this area and a region of high pressure is formed. Air moves from high pressure areas to low pressure regions to even out the pressure differences, generating winds and atmospheric circulation as a result.
The same behaviour happens on a smaller scale during the formation of sea breezes during the day and land breezes during the night and these are shown in the animation below. During the day, the land heats up faster than the sea, warming the air above. During the night, the sea cools slower than the land so the air is warmer over the sea compared to the land. In both cases, the warm air rises and leaves an area of low pressure (L) below. Air from higher pressure (H) regions moves towards the low pressure regions to even out the pressure differences. At higher altitudes the air is transported back in the opposite direction.
