Monday, February 23, 2015

Weather Monday 2/23/15: Precipitation Part 1


Lightning in the desert postcard for an exchange.

Imagine that it is mid July and 92 degrees F (33 degrees C) and humid. You hear thunder. Only 3 1/2 miles away it is snowing. Yes I said snowing. How can that be? Is this some kind of Hollywood stunt? No it is not. It is snowing in the cloud!



image A thunderstorm is a particular type of cloud called a cumulonimbus cloud. The bottoms of these clouds could start at about 1000 feet above the surface and go up to 40,000 feet (up where jet airplanes are flying or the height of Mt Everest). It is so cold inside these clouds since parts of them are so high up that it could be snowing inside these clouds. There is basically a weather factory inside the cloud…rain, snow, hail.

This is a photo I took of a cumulonimbus cloud while storm chasing in the Midwest back in 2004.


So with that introduction I am going to begin to talk about precipitation today.

Why does precipitation fall to the ground?

In one word….gravity. Gravity makes snow or rain fall just like it makes all things fall. So why do cloud particles stay suspended up in the sky? Why don’t they just fall? Likewise we could ask why the dish sitting on your kitchen table does not fall to the floor. We have some upward force that opposes the downward pull of gravity. In the case of your dish sitting on the table the table itself pushes upward on the dish to prevent it from falling. Right now your chair is pushing upward on you to oppose gravity that is pulling you downward. You are happily balanced and reading my blog :)

So what opposes gravity in the cloud? It is called the updraft. The motion of the air inside of a cloud is upward and why it is called an updraft. In fact that is how we get a cloud to begin with. Air must rise to create a cloud. Air that rises cools, gets saturated with moisture, and creates a cloud. Air must always cool to reach saturation. That’s why we see fog, frost,dew (which show that the air has achieved saturation) form at night. It gets colder overnight.

Cloud droplets are typically very small…way less than a millimeter. A millimeter is a tiny dot or about the thickness of a dime.  A typical raindrop is much larger. That means that as long as the particles are small the updraft can keep them in the cloud. They have to grow much larger in order for them to get heavy enough for gravity to make them fall to the ground. In other words, gravity has to become stronger than the updraft. This is like you trying to sit on a chair that will not support your weight. Gravity is stronger than the upward chair force and the chair collapses and you fall to the floor. Something similar has been seen in the areas of the USA that have received so much snow. Gravity (the weight of all that snow) gets larger than the upward support from the roof and the roof collapses.

So the droplets inside the cloud due to the upward motion do collide with each other and stick together and grow larger. They can grow large enough to become heavy enough to fall as rain.

image This is how you get big dust bunnies in your house. The tiny dust particles stick together as air currents move them around the room. I swear this photo is not taken in my house!





Types of precipitation

We have liquid and frozen types of precipitation as you can see in this list.

Liquid precipitation

  • drizzle, rain

Frozen precipitation

  • snow, sleet, freezing rain and hail

Today I am only going to discuss the liquid ones. These kinds of precipitation come from relatively warm clouds where there is only liquid water droplets inside the cloud. Snow forms inside a cloud that is colder and has both liquid water and ice inside the cloud. I will discuss that next week.

Drizzle consists of tiny droplets. Raindrops are larger. You can probably guess which one requires a larger updraft to achieve. Did you guess raindrops? You are right! So if there is a weaker updraft then the droplets do not have to grow as big for gravity to overwhelm the updraft. So they will fall even though they are fairly small.

Does precipitation always reach the ground?

The answer is no. If it is very dry below the cloud the droplets can evaporate before reaching the ground. This is called virga. You can see a photo of virga here. See the streaks below the cloud and how they don’t extend to the ground?

Well that is enough for today..Stay warm, my friends in the bitter cold parts of the world. If you are not having cold weather you are lucky. We are getting tired of it.

Thanks for reading.


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