Science Questions

Why do storm clouds have such a clean, flat top?

Sun, 10th Aug 2008

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Shabnam, Canada asked:

Why do storm clouds have such a clean, flat top?


Sarah -  We actually had some great responses to this on our forum from but basically the story is that thunderstorms are formed of what are known as cumulo-nimbus clouds.  These grow from the billowy cumulus clouds like the ones you see in cartoons and these grow upwards.  The reason they do this is because if the sun heats up the ground you get thermals which is like warm air and it rises, pushing up the clouds.

Ben -  Are these the same thermals that birds use to lift themselves really high up in the sky, and also in hang-gliders?

Sarah -  Yes. Big heavy birds like vultures, ones that live in quite hot areas, will wait for the thermals to rise before they can get up in the air because theyíre quite heavy so they find it hard to take off.

Ben -  Once these thermals have lifted the clouds how do they get the flat top?

Sarah -  They keep growing upwards and then reach what is known as the tropopause which is the part of the atmosphere between the troposphere which is the bit nearest to the ground and the stratosphere which is the next layer up.

Ben -  What happens there?

Sarah -  What you get here is known as an inversion which is where you get cold air sitting over warm air.  What everyoneís used to is the warm air rising and therefore being above the cold air.  This doesnít happen in the stratosphere because it gets colder and colder as you go up.  This means that the clouds canít go any farther as it hits the wall of warm air.  It spreads out along the top and itís very flat.  Obviously because the climate is very complicated there are other factors involved but thatís basically the story.


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Shabnam asked the Naked Scientists: Love your program and listen at work all the time again and again!! :) Now my question is Why thunder storm cluds have such a clean flat top at their anvil section. If it is because of temperature change between the stratosphere and traposphere, I expect to still see some uneven surface on the top like other couds that form in lower layers. Why is it like a straight line? Thanks Shabnam What do you think? Shabnam, Fri, 1st Aug 2008

Shabnam, you are not too far off.
The clouds in question are Cumulonimbus incus (many different clouds, and accessory clouds make up what you think of as a thunder cloud). These occur when the cumulonimbus cloud encounters an inversion, most likely this inversion is actually the tropopause. The tropopause acts as a ceiling prevents further ascent by the cloud, most of this cloud expands upwind, and the rest downwind, thus you have your classic "anvil" (Cumulonimbus incus), Fri, 1st Aug 2008

Paul, thanks, but what is a "tropopause"?

Chris chris, Sun, 3rd Aug 2008

The Tropopause is the transition boundary between the troposphere and the stratosphere. There are "pauses" between each layer of atmosphere (stratopause, mesopause...) and they are the area where the maximum changes in thermal characteristics, chemical composition, movement, and density occur., Sun, 3rd Aug 2008

Thanks for the description. I kind of have a basic knowledge of meteorology and I have tried to find more detailed answers but none of the websites get into that much detail. With regards to my question, why is it such a flat top. Let's say it is because of temperature, it is strange that it can affect and stop all the water molecules at the same level. As far as I know the thunderstorms have a lot of upward energy, what happens to all that energy when it reaches tropopause? shouldn't the transition be more gradual? Shabnam, Wed, 13th Aug 2008

Hi Shabnam, lets give it another try.

The short, easy answer is that it is deflectd sideways, thus the cloud spreads out forming the anvil. The longer answer is to go back to before a cumulonimbus cloud forms:

Cumulonimbus start life as lowly innocent looking cumulus clouds, born in an unstable air. this is when a parcel of air is more buoyant than the surrounding air - including the air above. The lighter air will tend to rise until a height is reacehd where the air has the same degree of buoyancy, this buoyancy is controlled by temperature, hence the importance of the variations throughout the atmosphere. If you want to know more about this then you can read up on LAPSE RATES on Wiki, or ask further questions in this forum.

So we have the unstabe air, but we also have convection. The ground has different coverings, the warmest of these are called thermal sources, these become covered by air that is warmer than its surroundings, and because they are also less dense they begin to rise in the form of bubbles. A series of these bubbles make up a convection current.

After the bubbles have risen once or twice their own diameter they are spent, although new and larger bubbles are created by a combination of wakes of smaller bubbles. so the higher they go, the larger the bubble but the difference in temperature between the bubble and its surrounding environment is smaller. Eventually, some of these bubbles will cool ( I have left out any mention of ELR AND DALR Lapse rates, as this will only get more technical and write and read) enough to reach saturation, any further rise will produce clouds. Initially these will be rather small and ragged, they will be cumulus fractus (cu fra), they will continue to develop as more bubbles join them. you can see this process yourself simply by watching a developing cu cloud from your garden.

thats how a cumulus cloud forms, but what about CUMULONIMBUS, i hear you impatiently asking...

As a rule these develop from large cumulus clouds, cumulus congestus (cu con), they also need temperatures of -10c and below. once the process starts it is very quick, maybe in 10 to 20 minutes most of the cloud that is above 0c will have started the process of GLACIATION.

So what about that flat top? Well, all that text was a drawn out way to get back to the simple answer, the cloud simply spreads out beneath the inversion / tropopause, its energy is diverted / deflected by the inversion. I know this may seem too easy, and i think you may need to see some sort of experiment or drawings to show how this happens.

The easiest way i think to grasp this is to simply think of how the air reacts when a clod front meets a warm front, THIS topic may help.

Having said that, there are instances where the inversion is breached by the extreme instability.

Does that go anyway to answering your question?, Thu, 14th Aug 2008

Understanding Inversions

The term "temperature inversion" describes an increase in air temperature with height above the earth's surface. Several atmospheric processes form inversions at a variety of altitudes.

Some inversions are weak and dissipate quickly while others can hang around for several days depending on the weather pattern.

An inversion is an extremely stable layer of the atmosphere, which inhibits upward air motion. That is, inversions often act like a cap on the atmosphere.

Inversions often prevent the formation of clouds and rain showers, but they don't always guarantee sunny weather. Inversions caused by warm fronts are often accompanied by rain and inversions over the Plains can contribute to severe thunderstorms and tornadoes. Below are the most common scenarios in which inversions form.

Ozone causes stratospheric inversions

A natural layer of ozone is found in the stratosphere, which is approximately 9 to 30 miles (12- 49 kilometers) above the earth's surface. The stratosphere contains about 97 percent of the ozone found in the atmosphere. Ozone consists of three oxygen atoms.

It is very caustic and considered a pollutant if it is generated near the earth's surface as a result of air pollution, but it is extremely valuable in blocking out harmful ultraviolet radiation in the stratosphere.

Ozone absorbs ultraviolet radiation, which breaks each ozone molecule into a single oxygen atom and a molecule of oxygen (with two atoms). This reaction gives off heat, which warms the stratosphere. The warming caused by ozone reacting with ultraviolet radiation is why temperatures stop decreasing and even begin increasing with altitude in the stratosphere. (Related: Understanding ozone).

Nocturnal inversions often bring fog

Clear, calm fall nights are perfect for creating inversions near the ground.

The ground cools much quicker than the air, which cools the air near to the ground. But the air higher up is slower to cool. On calm nights, when the wind isn't stirring up the air, the air close to the ground can grow quite cool compared with the air above, creating a "nocturnal" because this happens at night, or "radiation" inversion because heat is radiating away from the ground much quicker than from the air higher up.

If the air near the ground often cools to its dew point temperature, water vapor in the air condenses to form tiny water droplets, which drift in the air to create a layer of radiation fog. (Related: Understanding clouds and fog).

Fronts can cause inversions
Temperature inversions usually form near the boundaries of warm and cold air that define fronts.

While most fronts do create inversions, they are most pronounced in warm fronts. These inversions are known as frontal inversions. They form near the warm-cold air boundaries as warm air overruns colder air at the surface. The warm air riding up and over the cold air creates a relatively thin layer of air near the cold air in which temperatures increase with height.

This is one case where an inversion does not always bring quiet, pleasant weather. Fronts are often accompanied by precipitation as the force causing the warm air to rise is usually sufficient to break through the thin inversion. This is especially true with cold fronts.

Downward motion can create strong inversions
When you see an "H" on a surface weather map, it shows the air pressure is higher than in surrounding areas. The high pressure is created by descending air.

As air descends it is compressed and warmed, which can form some of the strongest inversions, known as "subsidence inversions" because they occur where they air is subsiding.

Such inversions most commonly form during the summer when large domes of high pressure, such as the Bermuda High, tend to dominate the weather over the USA. Since inversions keep air from rising and mixing with cleaner air above, subsidence inversions can cause long-lasting air-pollution episodes., Thu, 14th Aug 2008

While we are on the topic of cloud formation, what does this look like ? ... RD, Thu, 14th Aug 2008

Not a UFO. Are you obsessed with lennies, RD?, Thu, 14th Aug 2008

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