Last week we looked at clouds, including what they’re made of, how they form, and the different types. We said that clouds form when water vapor condenses/deposits on tiny particles (e.g., dust, smoke, salt) called cloud condensation nuclei / ice nuclei.
When those water droplets or ice crystals grow large enough, they will begin to fall out of the cloud.
Types of Precipitation
- Falling water droplet + other water droplets = coalescense, leading to rain
- Falling ice crystal + supercooled water droplets (liquid water existing below 0 °C or 32 °F) = accretion, leading to graupel (< 3 to 4 mm)…if the graupel gets lofted up in the cloud (more on this later), it may continue to collect water droplets and we will get hail
- Falling ice crystal + other ice crystals = aggregation, leading to snowflakes
- If the lowest layer of the atmosphere is warm enough (above 0 °C) and thick enough, the snowflakes can melt and will just fall as rain
- If there is a thick warm layer (above 0 °C) above a thin cold layer above the ground, the snow will melt but the raindrops will become supercooled, and you will get freezing rain as the liquid water freezes on contact with the ground
- If there is a thin warm layer (above 0 °C) above a cold layer above the ground, the snow will partially melt but then refreeze before hitting the ground as sleet
More info and illustrations on the temperature profiles needed for winter precipitation will come later in the series.
The final element of weather is visibility, but we will not treat this separately as it is dependent on other elements such as precipitation.
Next Monday we will move on to time zones!