Cloud Answers

by Deb on February 17, 2012


After you’ve checked out this post, I strongly recommend a visit to the Cloud Appreciation Society, they have fantastic guides for cloud viewing and a beautiful gallery you can spend hours browsing through 🙂

1.     Noctilucent clouds are the highest type of cloud and one of the most beautiful. It literally means ‘night shining’ and usually they are invisible. They are made of tiny ice crystals all the way up in the mesosphere, around 80km high. Most clouds are in the lowest 15-20km. There is some question about whether they are a new phenomenon, they weren’t noticed until 1885 after Krakatoa exploded and they seem to be moving further south now. Is this because we are just noticing or is it another signal of our changing climate?

noctilucent cloud

Courtesy of Martin Koitmäe

They can be seen in the evening because the sun shines up through them. The observer is in night because the sun is below the horizon, however because the clouds are so high they still catch the sunlight.

noctilucent observation

2.     Contrails is short for condensation trails, they are artificial clouds created by aircraft. There are two different ways they form. Firstly the engine exhaust contains lots of water vapour from burning fuel. It may locally push the humidity of the air high enough to form visible clouds as the droplets cool in the frigid air. The other dust particles coming from the engine are convenient nuclei for the droplets to condense on and act as seeds for the long thin clouds.


Courtesy of Sergey Kustov

The second method does not increase local humidity but decreases local temperature and pressure, causing the water vapour already there to condense. Aircraft wings create lift by decreasing air pressure, this sets up vortices or whirlwinds at the tip of each wing. In the core of each vortex is an area of very low pressure which causes the temperature to drop.

When many airlines were grounded after September 11 there was a lot of research on changes in weather caused by contrails. There appears to be a measurable effect caused by the extra cloud cover they create.

3.     When we were kids we thought that the blacker a cloud was the more rain it had. Unfortunately, you can’t estimate the amount of rain from a cloud by its size or colour. Obviously size does play a part, but it also depends on the temperature and winds where all that water will be dropped. And colour is related to density and therefore the amount of water, but the position of the sun and if there are any higher level clouds above it will also affect it. It’s quite sad, we had lots of fun trying to work out when and how much it would rain.

4.     A cloud forest is a rainforest at a higher altitude. They are usually tropical and occur in the same areas as rainforests. One of the causes of rainforests can be a mountain range that forces the air to go up and over it, cooling the air and dumping moisture as rain. Higher on the slopes of those mountains the humidity remains as cloud or fog, forming a cloud forest. They tend to create ‘islands’ of biodiversity because they are separated by lowlands with a different climate and they are very vulnerable to climate change.

5.     The photograph is a fallstreak hole or punch hole cloud.  What has happened is that the cloud is made of super-cooled water, water that is below freezing point but hasn’t frozen because there are no nuclei to encourage crystals to form. Something has seeded one part of the cloud and formed crystals, which then seed other crystals. They spread symmetrically, forming the lovely circle. Once the crystals form they become a small local snowfall or melt on the way down for rain, although often it evaporates before hitting the ground.

They are known to be caused by planes, especially turbo-props that disturb the clouds on the way through, but they can also form naturally.

This appears to be video of a fallstreak hole forming.


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{ 2 comments… read them below or add one }

Penni Roden via Facebook February 17, 2012 at 7:26 am

I took a photo of a cloud yesterday that looked like a perfect mushroom – never seen anything quite like it before. It was thin, high cloud, and had a huge storm behind and above it – it bucketed down and hailed a few minutes after I saw the cloud. Oddest thing I’ve seen in the sky for a while.
And my kids love contrails – we live in perfect contrail country – the planes that leave Sydney heading S and W fly directly over us, and the altitude and clear skies make for perfect “jet stream” days.


Science@home via Facebook February 17, 2012 at 2:08 pm

That sounds amazing. We often see rain that doesn’t reach the ground and lightning above the clouds but the clouds themselves are fairly normal.


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