Convection is the flow of heat in a liquid or gas from a region of high temperature to a region of lower temperature where the whole medium moves and carries heat energy with it. It is caused by a change in density.
When a liquid or gas is heated, the fluid in close contact with the heat source receives the energy first. The molecules move faster with more kinetic energy and push each other further apart, this causes the fluid to expand in this region. The expanded fluid is now less dense than the surrounding fluid and rises above the cooler and more dense fluid. The cooler fluid sinks to the bottom and a convection current is set up.
- In sunshine, the land warms up more quickly than the sea
- The air over the land becomes warmer than the air over the sea
- The warmer air over the land rises and the cooler air above the sea moves inland, forming a convection current or breeze blowing in from the sea – this is called a sea breeze
- At night, the land cools more quickly than the sea
- The warmer air is now over the sea
- This air rises and the cooler air moves from the land towards the sea – this is called a land breeze