Robert Lawry, Australian Bureau of Meteorology
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Where do clouds come from and why do they all have different shapes? – Ryan Potts, age 7, Canberra.
Hi Ryan, great question!
When it comes to understanding clouds, it helps first to understand water.
You know the water you drink out of a glass? That’s liquid water. There is also solid water, such as ice from the freezer or a slushy.
But water also exists as a gas. It’s called water vapour. You can’t can’t see, taste or feel this water but it’s everywhere. It’s all around you right now. Water vapour is in the air we breathe. When it’s warm and there is a lot of water vapour in the air, it can feel very sticky and sweaty.
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Now let’s go back to clouds.
For clouds to form, air needs to be cooled to a temperature at which the water vapour turns into liquid water. The best way to do this is to make the air rise, because the higher we go in the atmosphere the colder the temperature. There are many reasons air might lift, but one reason is because during the day the sun heats up the surfaces around us.
Imagine the oval near your school on a warm day. Starting in the morning the sun warms the surface of the oval and before too long the entire oval is warming up.
Warm air weighs less than cooler air. So a big bubble of warm air, filled with water vapour, slowly lifts off your school oval.
As the bubble of air (filled with water vapour) rises upwards, it starts to cool down. The higher it goes the cooler it gets.
Eventually, well off the ground above your school, the bubble of air has cooled so much that the water vapour turns into liquid water. We call this point the condensation level. When the water vapour turns into tiny specks of liquid water, a cloud forms.
Clouds are simply liquid water: very, very small drops of liquid water. So small in fact, that they can be held up in the air by rising air currents.
Back on the school oval: the day keeps getting warmer, more and more bubbles of rising air race upwards, cooling as they rise. When these bubbles of air reach the condensation level, more cloud forms.
Clouds formed by rising warm air currents are called “convection clouds”. Because of all the rising air coming up, these clouds can be bumpy on the top, sometimes producing very high thick clouds looking like cotton wool or cauliflower heads.
When air rises very slowly and gently over an area and reaches the condensation level you get a cloud that is very smooth looking, like this:
Sometimes air goes up past the condensation level then falls back below the condensation level, then up, then below, again and again. This creates clouds that are stripy, often with lines between the clouds.
The way the air moves creates all the different clouds we see.
All the grey clouds that you see contain liquid water. However, as we discussed earlier, water can also exist as a solid (ice). Clouds that are very high are extremely cold and may appear pure white. These clouds contain ice.
I used to wonder what it would feel like to touch a cloud. Would it be fluffy? Hard, soft, warm or cold?
Well, we don’t need to wonder. Because every time we see fog, we are looking at cloud.
Fog is simply air that has cooled to the point where the water vapour has turned into liquid water. That forms fog – which is really just a cloud on the ground.
So next time you see fog, go outside and touch a cloud!
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Robert Lawry, Hydrologist, Australian Bureau of Meteorology
This article is republished from The Conversation under a Creative Commons license. Read the original article.