Topic 4_Ch_10"Precipitation"

Precipitation occurs when water droplets or ice crystals become large enough to overcome the updrafts in rising air that form clouds. For precipitation to qualify, it must fall to the ground without evaporating. There are several types of precipitation including rain, drizzle, sleet, freezing rain, snow, and hail, all influenced by the atmospheric temperature profile.

A key distinction between cloud droplets and raindrops is their size; typical cloud droplets have a radius of around 10 µm, whereas typical raindrops are approximately 1000 µm (1 mm). Larger droplets are necessary for precipitation to occur. Terminal velocity is attained when the force of gravity is balanced by the air resistance in still air, with smaller drops experiencing a larger surface area to mass ratio, resulting in slower terminal velocities. The velocity of updrafts varies, with stratiform clouds exhibiting updrafts of around 10 m/s.

A drop must be sufficiently large to overcome updrafts and must not evaporate before reaching the ground. In cold temperatures (below 0°C), phenomena such as the Bergeron–Findeisen Process take place, where both ice crystals and water droplets coexist, and the saturation vapor pressure is higher over liquid water than ice.

During precipitation formation, larger drops fall faster than their smaller counterparts, not all collisions result in coalescence, and larger collector drops (greater than 20 µm) promote efficient collision and coalescence. Sequences of coalescence are more probable when drops differ in size or are similar. Growth of cloud droplets occurs predominantly in deeper clouds with strong updrafts, leading to larger droplets, while thin clouds yield smaller droplets, like drizzle.

In the temperature range between 0°C and –40°C, clouds can provide ideal conditions for both ice crystals and water droplets. The processes of ice crystal growth include accretion, where ice crystals grow by colliding with supercooled water droplets, and aggregation, which occurs by colliding with other ice crystals. The type of precipitation that reaches the surface is primarily determined by the kind of cold clouds present, which are mostly composed of ice.

Very dry air below precipitation can lead to effects like virga, where precipitation evaporates before reaching the ground, resulting in small drizzle-sized drops. Sleet forms when precipitation melts in a warm layer and then refreezes into ice pellets upon descending through a sufficiently deep cold layer (greater than 250 m). On the other hand, freezing rain occurs when the cold layer is shallow, leading to supercooled water that freezes upon contact with surfaces.

Hail formation begins with small ice particles and is significantly influenced by strong updrafts in cumulonimbus clouds, which lead to the recycling of particles and contribute to hail growth primarily through accretion. Hail is generally larger than 5 mm and is classified as severe when exceeding 2 cm in size. Understanding the distribution and character of precipitation is essential for comprehending its various impacts on weather and climate.