Regional Weather and Precipitation 14.4-14.5 pgs. 553-557 Thermals A local convection current set up during the day is called a thermal or thermal updraft (usually weak in the morning, strong in the afternoon) Thermals Thermals Sea Breezes are caused when a thermal forms near a sea, an ocean, or lake. Solar energy warms the land faster than the water-and the water provides cool air Thermals Land Breezes form when the land cools more quickly than the water The air above the water rises and is replaced by cooler air from above the land As a result, a convection thermal flows from the land to the water Lake-Effect Snow An air mass moves across a large body of water and picks up moisture When the moisture laden air mass warms and rises as it travels across the water where the temperature is lower the moisture becomes snow Chinook Winds Chinook Winds are warm, dry winds in the Rocky Mountains. Westside of the mountains is the windward side Chinook Winds Eastside is warmer and dryer Orographic lifting causes water to condense-this releases heat warming the air Precipitation Drizzle: Fine water droplets between 40 μm and 0.5 mm in diameter-like rain, may be light, moderate or heavy Precipitation Rain: Made up of falling water droplets between 0.5 mm -5 mm in diameter. If a larger drop forms- it breaks into pieces as it falls Precipitation Freezing Rain: Raindrops are close to freezing and strike a cold object on/or near the ground- the drops freeze almost instantly Precipitation Snow: Forms when water vapour crystallizes on tiny particles of dust and other solid substances such as smoke. Forms when air temperature and water vapour has cooled to 0o C Precipitation Hail: is a solid form of water created in cumulonimbus clouds high in the troposphere. Frozen rain drops move up and down in highly active thunderclouds. Hailstones grow larger each time an updraft raises them through cold water droplets at high altitude Precipitation Dew: Air cools and the water vapour condenses on a cool surface near the ground. If the surface is very cold the water vapour deposits frost.
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