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Regional Weather Thermals • Thermals are a rising current of warm air caused by convection – it is also known as an updraft. • The sun heats up the land and the air just above the land heats up and rises. This rising, warm air is our thermal. • Balloonists and gliders depend on thermals to help give them lift. Sea Breezes • Wind can be caused by the land near a large body of water heating up and creating thermals. • The air over the land rises and when it rises, the air over the water moves in to take its place. • The air in the thermals cools and drops down to take the place of the air that moved in from over the water. Sea Breeze Formation Land Breezes • Land breezes are the reverse of sea breezes. They occur later in the day as the temperature is dropping. • The land cools much quicker than the water (which stays warmer longer). • The air above the warmer water rises as thermals and is replaced by cool land air. • The warm air then cools and replaces the air that was once over the land. Land Breeze Formation Lake-Effect Snow • Lake-effect snow occurs during the winter when a wind sweeps across a large body of water that has retained some heat from the warmer weather earlier in the year. • The wind picks up some moisture from the lake and carries it up over the land. • The land is much colder than the lake so the moisture cools and condenses and falls as precipitation (snow). • Much of the snowfall experienced by the Durham Area is the result of Lake-effect snow. Lake-Effect Snow Formation Chinook Winds • Chinook winds are seen in Western Canada and are caused by the mountains and “orographic lifting”. • The orographic clouds form by moisture condensing as it rises up the mountain. This condensation releases heat that stays in the air and travels down the other side of the mountain and out over the prairies as a dry, warm air mass. Chinook Wind Formation Precipitation & Humidity Precipitation • Precipitation is any water that reaches the ground in either a liquid or solid form. • The type of precipitation that falls depends on the temperature on the ground and in the atmosphere. • Types of precipitation may include… • • • • • • Drizzle Rain Freezing Rain Snow Ice Pellets (Sleet) Hail Humidity • Humidity is the measure of the amount of water in the atmosphere. • Low humidity means that the evaporation of sweat from our body can take place easily because there is a lot of room in the air for it. • A high humidity means that sweat sticks to us because the air already has a lot of moisture in it and there is no room for extra moisture from us. • Relative humidity is a measure of the moisture in the air against the maximum amount of moisture the air could actually hold at a given temperature. It measured as a percent. FIN