How Bodies of Water Change Weather and Climate

Remember from Grade 7

In Grade 7 science you learned that convection causes lake breezes, and in Grade 7 geography you learned that nearness to water is one of the LOWERN factors that shape climate. This lesson explains the science behind those effects and looks at more ways bodies of water change the atmosphere.

A beach puzzle

On a sunny July afternoon, the sand at a Lake Huron beach is too hot to walk on, but the water feels cool. At night, the sand is cool, but the water still feels warm. Same sun, same day. Why do the land and water behave so differently?

Water is slow to heat and slow to cool

• High specific heat capacity. It takes about 4.2 joules to warm 1 g of water by 1 °C, several times more than for the same mass of sand or rock. So water warms slowly and, once warm, stores a lot of energy and cools slowly.
• Mixing. Waves and currents spread heat down through the water, while on land the heat stays in a thin top layer.
• Transparency. Sunlight shines several metres into clear water, spreading the energy through a deep layer.
• Evaporation uses energy, which cools the water surface.

Result: big lakes and oceans act like a giant thermal battery. They soak up heat in summer and release it in fall and winter.

Effects on atmospheric conditions

1. Moderate temperatures. Places near large water have cooler summers, milder winters and a smaller yearly temperature range. Vancouver and Winnipeg are at almost the same latitude, but Vancouver (next to the Pacific) averages about +4 °C in January, while Winnipeg (far inland) averages about −16 °C. In July, they are similar (about 18 °C and 20 °C).

2. Longer growing seasons. Lake Ontario stays cool in spring, which keeps nearby fruit trees from blossoming too early, before the last frost. In fall, the warm lake delays the first frost. This helps make the Niagara Peninsula one of Canada's main regions for tender fruit like peaches and grapes.

3. Lake breezes. On a sunny day, land heats faster than water. Warm air over land rises, and cooler, denser air from over the lake moves in to replace it: a lake breeze. At night it can reverse into a land breeze.

4. More moisture. Evaporation from lakes and oceans adds water vapour to the air, raising humidity and providing the water for clouds and precipitation downwind.

5. Lake-effect snow. In late fall and early winter, the Great Lakes are still warmer than the Arctic air blowing over them. The lake warms the bottom of the air and adds moisture. The air rises, cools, and forms bands of clouds that dump heavy snow on the downwind shore. In Ontario, snowbelts include areas east of Lake Huron and Georgian Bay (such as around Owen Sound and London) and east of Lake Superior (such as around Sault Ste. Marie). When a lake freezes over, lake-effect snow mostly stops.

6. Fog. When warm, moist air moves over cold water, it cools and its vapour condenses into fog. The Grand Banks off Newfoundland are among the foggiest places in the world, where air warmed over the Gulf Stream meets the cold Labrador Current.

7. Ocean currents carry heat over long distances. The warm Gulf Stream and North Atlantic Drift help keep western Europe mild; the cold Labrador Current cools Atlantic Canada's coast.