Kenn Brody
July, 2022
Those fleecy white mounds of insubstantial dream stuff floating serenely on a blue sky? Yes, clouds. Everybody knows they are made of moisture. But there is so much more to them.
What do you do when you want to know the weather? Look it up on your cell phone? Turn on the Weather Channel on you TV? Clouds tell you a lot about the near-term weather right over your head, if you understand them. As a cruising sailor, making passages to places far from the weather forecasters, I learned to read clouds. You can, too. Here’s how:
Clouds form when a bubble of air rises to the height where water vapor, a gas, becomes a liquid, fog. This defines the base of the cloud.
The bubble of air starts at ground level or at the surface of the sea. It generally has the temperature of the stuff underneath it, like a parking lot pavement or the sun-warmed surface water. As it rises, due to the thinning of the atmosphere, it expands. As it expands it gets cooler. As long as it stays hotter than the air around it, it continues to rise.
Dry air naturally gets cooler as the altitude increases. Every 1000 feet it gets 3.5 F cooler. Meteorologists call this the adiabatic lapse rate. If our bubble starts at, say, 10 F warmer than the air around it, and it stays warmer than the dry adiabatic lapse rate, it rises just as a hot air balloon rises. Eventually that bubble, depending on the amount of water vapor it contains, reaches the dew point, where it forms droplets of water. Initially, these are very small, so small that the slight updraft keeps them aloft. This is the cloud base. By looking at the height of the cloud base you can tell how much water vapor was in that bubble. Warm air in the tropics can hold many times as much water as cold air in the arctic, so the cloud base is denser. The denser the cloud, the more fog it has, the thicker it is likely to be.
Weathermen describe clouds according to layers in the atmosphere at which they form. The lowest layer is the cumulus, which tend to be those puffy, piled up cloud forms. Next higher is stratus, which tend to form a blanket of cloud in the sky. On top, at the ordinary limits of cloud formation, are cirrus clouds, which are no longer water but ice. This picture shows the various cloud layers and their altitude in the atmosphere.
How high can clouds go?
Suppose that bubble of moist air is large enough to survive winds and turbulence, say a mile wide. It rises to cloud base and the vapor forms water. That transformation releases an enormous amount of heat, the heat of vaporization, roughly 1070 BTUs per pound of water (2.27 MJ/kg). This heat energy increases the temperature of the bubble compared to its surrounding air. Our bubble goes up like a rocket. You can see the vertical “chimneys” of fast rising air as thunderheads form in clouds. If you are in an airplane you experience these as powerful turbulence. They can exceed 400 feet per minute. It’s like being in a runaway elevator.
Chimneys can punch through the various layers of the atmosphere. If it punches through to the point where water freezes, more heat is released, about 180 BTUs per pound (333.55 KJ/kg). This creates a vortex of rising air which forms growing balls of ice. If that ice gets to the size where it falls out of the top of the cloud, you get hail. The size of the hail is a good indicator of the strength of the updraft. When you read about baseball-size hail in Texas, you know those updrafts are at hurricane strength, but without the hurricane.
At a certain altitude, the temperature of the atmosphere starts getting warmer instead of cooler. That is the end of the troposphere and the beginning of the stratosphere. The temperature inversion is the limit of cloud formation. There are exceptional formations that breach that limit, but that is a topic for another article.
Clouds that appear in multiple layers, cloud that move with the prevailing wind, clouds that grow or dissipate, tell you about approaching cold fronts or warm fronts. Funnel clouds tell you about potential tornadoes. A pattern of squall lines can tell you about an approaching hurricane, and whether you are on the navigable or the dangerous side of its path.
Why should you care? Weather reports are created first by weather models. Because those models have to simulate the entire hemisphere or the entire globe, they are limited in the grid size they use. Typically, US weather models use a grid size of 7 to 10 kilometers. A prediction of a 50% chance of rain does not mean you will have rain half the time. It probably means that somewhere in that 10 kilometer grid it will pour and you may stay bone dry.
You can do better just by looking up and reading the clouds over your head.
Thanks for reading this article. Look for more from this author.
Ref: https://weather.mailasail.com/Franks-Weather/Grid-Length-Resolution
