Stand on a beach on a sunny July afternoon. The sand burns your feet, but the lake is still cool. By evening the sand has gone cold while the water feels almost warm. The same sunshine fell on both, yet they behaved very differently. Explaining that difference, and how heat moves from the Sun to the ground to the air and around the planet, is the key to understanding wind, weather and climate.
Heat, temperature and thermal energy
Thermal energy is the total kinetic energy of the particles in a substance. Temperature measures the average kinetic energy of those particles: how fast they are moving on average. Heat is thermal energy that is transferred from a warmer object to a cooler one. Heat always flows from hot to cold on its own, and it can move in three ways: radiation, conduction and convection.
| Method | How it works | Climate examples |
|---|---|---|
| Radiation | Energy travels as electromagnetic waves; needs no particles, so it can cross empty space | Sunlight reaching Earth; the ground giving off infrared radiation at night |
| Conduction | Faster particles collide with slower neighbouring particles and pass on energy; needs direct contact | Hot pavement warming the thin layer of air touching it; cold lake water chilling the air just above it |
| Convection | Warmer fluid (gas or liquid) expands, becomes less dense and rises; cooler, denser fluid sinks to take its place, forming a loop called a convection current | Rising warm air forming clouds; sea breezes; global wind belts; overturning of lake and ocean water |
In the climate system the order is usually radiation, then conduction, then convection. The Sun's radiation passes through the mostly transparent air and is absorbed by the ground and water. The warm surface heats the air touching it by conduction. Air is a poor conductor, so conduction only warms a thin layer; convection then carries that warm air upward and mixes heat through the lower atmosphere. This is why the lower atmosphere is mostly heated from below, not directly by sunlight.
Why water warms and cools slowly
Specific heat capacity is the energy needed to raise the temperature of 1 g of a substance by 1 °C. Water's is about 4.18 J/(g·°C), roughly five times that of dry sand or rock (about 0.8 J/(g·°C)). So the same amount of sunlight raises the temperature of water far less than land. Water also mixes, spreading heat down through many metres, and some of the energy goes into evaporation instead of raising temperature. Land heats only a thin surface layer. All three effects make bodies of water slow to warm and slow to cool.
From uneven heating to wind
Because land and water, and the equator and the poles, are heated unevenly, some air becomes warmer than other air. Warm air rises, leaving lower air pressure at the surface; cool air sinks, creating higher pressure. Wind is air moving from areas of high pressure to areas of low pressure. A sea breeze (or lake breeze) is a small convection cell: on a sunny day the land warms faster than the water, air rises over the land, and cooler air flows in from the water. At night the pattern reverses into a land breeze, because the land cools faster.
- Global convection cells: air rises at the hot equator, flows toward the poles high up, sinks around 30° latitude and flows back along the surface. Similar cells exist at middle and polar latitudes.
- The Coriolis effect: because Earth rotates, moving air and water are deflected (to the right in the Northern Hemisphere), which turns north–south flows into the trade winds, westerlies and polar easterlies.
- Prevailing westerlies: over most of Canada, including Ontario, the prevailing winds blow from west to east, so weather systems usually arrive from the west.
- The jet stream: a fast, winding river of air high in the atmosphere along the boundary between cold polar air and warmer air to the south; its position steers storms across Canada.
Water moves heat in the same ways. Surface ocean currents are pushed mainly by the prevailing winds. Deep currents are driven by differences in density: cold, salty water is denser and sinks, while warmer or fresher water stays near the surface. Together, winds and ocean currents carry heat from the tropics toward the poles; without them the tropics would be much hotter and the poles much colder. You will look at ocean currents in more detail later in this unit.