12. Precipitation, Air Masses, and Fronts

Photo 23: A Thunderstorm in its developmental stage, Lacombe, Alberta. 2011-06-29

Cloud formation

The average cloud droplet has a diameter of 10 µm and a rain droplet 1000 µm (1mm). Cloud droplets are light and remain suspended in the air, or fall a short distance and are evaporated. A falling droplet is pulled by gravity and resisted by friction, when both forces equal each other a droplet is either suspended or falls at a constant speed. Larger droplets have greater terminal velocities.

Water condensates around small particles in the Atmosphere, creating droplets up to 20 µm. If droplets of various sizes exist in a warm cloud (above 0°C), collision-coalescence occurs due to colliding droplets and so forming larger precipitation droplets. Water in a cold cloud crystalizes, unless the water has no solid nucleus to form a crystal around, and then it will remain liquid even if the temperature of the droplet is below 0°C. Cold clouds (below 0°C) form in higher altitude areas. Precipitation from these clouds will often start as snow, and depending on the temperature conditions on the surface, will fall as rain or snow.

Thunderstorms

Thunderstorms are often created on hot days and have a short lifespan of only a few hours. Due to high surface temperatures, hot air will rise quickly, at speeds from 10 m/s up to 30 m/s. As the hot air rises, it also cools; cold air can hold less moisture than hot air, and therefore droplets will form. Due to the strong updraft of hot air, these droplets will continue rising. This is called the developmental stage. The Maturing stage occurs when enough raindrops from and fall to the surface to cause a cold downward draft. During this stage, the cloud still grows at the top and empties at the bottom. The final stage is the dissipating state. The cloud empties at the bottom and is sometimes dispersed at the top by upper air winds, creating a broad anvil like top. Photo 23 shows a developing thunderstorm on a hot day.

Photo 24: Orographic lifting, Rocky Mountains near Jasper, Alberta. 2011-05-19

Orographic lifting

Alberta has relatively low precipitation compared to British Columbia due to orographic precipitation that falls in the Rocky Mountains. Based on the concept that cold air can hold less moisture, Oceanic clouds that drift through the Rocky Mountains have to rise in order to go over the mountains. This rising process causes cooling. The windward slope of a mountain receives the precipitation and the leeward sides relatively little. Places like Kamloops in the interior of BC are relatively dry too, because they lay beyond the Coastal-Cascade mountain range. Oceanic clouds, moved westward by the Westerlies wind currents, empty partially in this mountain range and lower again on the leeward side, thus warming up and having more holding capacity for moisture. Once these clouds reach the second mountain range, the Rocky Mountains, they will largely empty. Therefore Alberta has relatively little precipitation compared to BC. Photo 24 shows clouds against the windward side of the Rocky Mountains.