How Do Clouds Form and Produce Precipitation?
Clouds form when rising air cools to its dew point and vapor condenses on tiny particles. Precipitation needs those droplets to grow enormously, either by colliding and merging in warm clouds or by ice crystals growing at the expense of supercooled droplets in cold clouds. The air temperature below decides whether it lands as rain, snow, sleet or freezing rain.
Clouds are the weather you can see without any equipment at all. This unit teaches you to name them, explains how a cloud full of tiny droplets finally lets go of rain or snow, and sorts out the winter mess of sleet, freezing rain and graupel. We finish with fog, the cloud that sits on the ground and causes more trouble than it looks like it should. Bring your eyes and maybe a window.
What You’ll Learn
- Name the ten basic cloud types and what their Latin roots mean.
- Recognize which clouds produce lightning, thunder and large hail.
- Explain how collision-coalescence and the Bergeron process turn droplets into rain and snow.
- Understand supercooled water and why most summer rain starts as ice.
- Tell sleet, freezing rain, snow and graupel apart by the temperature profile that makes them.
- Describe how radiation, advection and steam fog form, and how to drive safely in fog.
Lessons In This Unit
- 5.1The Ten Cloud TypesThe ten basic cloud types are sorted by height. High clouds are cirrus, cirrocumulus and cirrostratus. Middle clouds are altocumulus and altostratus. Low clouds are stratus,...Beginner · 9 min
- 5.2How Rain And Snow FormCloud droplets are far too small to fall, so they must grow about a million times in volume. In warm clouds, bigger drops collide and merge with smaller ones. In clouds colder...Intermediate · 9 min
- 5.3Snow, Sleet, Freezing Rain And GraupelBoth start as snow that melts in a warm layer aloft. Sleet forms when the cold layer near the ground is deep enough to refreeze the drops into ice pellets that bounce. Freezing...Intermediate · 9 min
- 5.4Fog And Low CloudsFog is a cloud touching the ground. It forms when air near the surface becomes saturated, either by cooling to its dew point, as in radiation fog on clear, calm nights or...Beginner · 8 min
Questions People Ask
Which cloud produces thunderstorms?
Cumulonimbus is the thunderstorm cloud and the only one that produces lightning, thunder and large hail.
What is the difference between sleet and freezing rain?
Both melt in a warm layer aloft. Sleet refreezes into pellets in a deep cold layer near the ground, while freezing rain lands as liquid through a shallow cold layer and freezes on contact.
Is fog a cloud?
Yes. Fog is a cloud whose base touches the ground, with visibility below 1 kilometer.
By the end of this unit, you will never look at a gray sky the same way. Sorry in advance to everyone you go on walks with. — Jen
