CloudSpeak: What Clouds Are Saying
Every day, there's a wealth of information floating lazily above the horizon. Clouds are almost diagrams of weather patterns, depicting how heat and humidity affect the minuscule droplets of water they are made of. On this Earth Day 2021, we take a look at what the clouds are telling us as they drift above these Hawaiian islands.
"Clouds are my favorite thing to talk about," says Alison Nugent. "Really, they're so beautiful. And so informative."
Nugent teaches atmospheric sciences (weather) at UH Manoa. She says clouds come in two categories, stratiform and convective. In the Hawaiian Islands, we usually have convective, which means in buoyant circulation, or in motion.
"When I think about convective clouds I move my fingers in these little circles like cauliflower or like boiling water in a pot. Convective clouds have a lot of moving circulation of air. Stratiform means layered. Stratiform clouds are pretty flat and usually pretty ugly."
You've noticed that often billowy convective clouds have a flat bottom--that's called the lifting condensation level.
"When air rises up from the ground surface, it reaches 100% relative humidity at that level. That depends on two things, the temperature, and the amount of moisture in the air."
The bottom of convective clouds table at about 800 meters to one kilometer above the earth. A lot of our mountains are higher than that, so they get cloud cover. Then, Nugent says, Hawaiian islands have something called a trade wind inversion. Normally, air cools as it rises, but the trade inversion causes a slight warming, usually around four kilometers up. That prevents clouds from forming above that height.
"And so cloud base is usually around 1 kilometer, cloud top around 4 kilometers on a typical average day."
Nugent says, the clouds might be taller when the usual trade wind inversion is not in play.
"When we see things like cumulonimbus clouds like clouds that grow from the flat level around one kilometer all the way up to the top of the troposphere, around eighteen kilometers, when we see those really tall clouds, that tells me the atmosphere is really unstable. It tells me that we don't have a trade wind inversion, it tells me that there's some sort of atmospheric system nearby that is enabling the clouds to get up that high. When you have cumulonimbus clouds, you always have rain. Cumulo nimbus clouds are rainmakers."
Think about miles of open ocean, and suddenly, bits of land, the Hawaiian islands.
"The sun shines on the land, it shines on the ocean, it heats from the bottom and that heat wants to rise up. In the process of water vapor condensing to form liquid water, that energy change releases heat. So when you see a cloud, you know there has been heat released into the atmosphere. The cloud has released its instability. It has gotten rid of that extra energy from that surface heating."
That cloud puff has done its job. Nugent says, 5 to fifteen minutes after sunset can offer the most thrilling effects, as the sun slides around the curve of the earth, lighting higher and higher levels of clouds.
"And then when the sun sets, the clouds dissipate too. That's one of my favorite things to observe because it's so clear. Like there's no more fuel in your car, right, there's no more heating in the ground, so the clouds will just kind of dissipate."
Making way, perhaps, to gaze at the stars tonight.
Professor Nugent uses a free online text with a chapter about clouds, if you want to learn more. You really should because there's the whole issue of classificarion by altitude. A former student, Shintaro Russell, put together a reference listing of local cloud forms and names. it's charming and fun and you'll feel smart next time you see a cloud.