Friday, Oct. 26, 2012

I'll be working on the uiz for next week this weekend and needed some inspiration.  "Stairway to Heaven" by Led Zeppelin was just what was needed.

The Humidity Problems Optional Assignment turned in on Wednesday has been graded and was returned in class today.  You'll find answers to the questions and some explanation here.

After learning about clouds on Wednesday this seemed like a logical place to briefly discuss the 2 most common types of satellite photographs.

You'll find the following figures discussed on pps 99-100 in the photocopied ClassNotes.

IR satellite photographs

When you see satellite photographs of clouds on the TV weather you are probably seeing an infrared satellite photograph.

1. An infrared satellite photograph detects the 10 μm IR radiation actually emitted by the ground, the ocean and by clouds.  You don't depend on seeing reflected sunlight, so  clouds can be photographed during the day and at night, 24 hours per day.  You may recall that 10 μm radiation is in the middle of the atmospheric window, so this radiation is able to pass through air without being absorbed.  If clouds don't get in the way, you can see the ground and the ocean on an IR photograph.

2.   Clouds do absorb 10 μm radiation and then reemit 10 μm IR radiation upwards toward the satellite and down toward the ground.  It is the radiation emitted by the top surface of the cloud that will travel through the atmosphere and up to the satellite.  The top surface of a low altitude cloud will be relatively warm.  Warmer objects emit stronger IR radiation than a cool object (the Stefan Boltzmann law).  This is shown as grey on an IR satellite photograph.  A grey unimpressive looking cloud on an IR satellite photograph may actually be a thick nimbostratus cloud that is producing rain or snow.

3.   Cloud tops found at high altitude are cold and emit IR radiation at a lower rate or lower intensity.  This shows up white on an IR photograph.

4.   Two very different clouds (a thunderstorm and a cirrostratus cloud) would both appear white on the satellite photograph and would be difficult to distinquish.  Meteorologists are interested in locating thunderstorms because they can produce hazardous severe weather.  This can't be done using just IR photographs.

5.   The ground changes temperature during the course of the day.  On an infrared satellite animation you can watch the ground change from dark grey or black (afternoon when the ground is warmest) to lighter grey (early morning when the ground is cold) during the course of a day.  Because of water's high specific heat, the ocean right alongside doesn't change temperature much during the day and remains the same shade of grey throughout the day.

Here's a link to an IR satellite photograph loop on the UA Atmospheric Sciences Dept. webpage.

Visible satellite photographs

1. A visible satellite photograph photographs sunlight that is reflected by clouds.  It shows what you would see if you were out in space looking down at the earth.  You won't see clouds on a visible satellite photograph at night.

2. Thick clouds are good reflectors and appear white.
The low altitude layer cloud and the thunderstorm would both appear white on this photograph and would be difficult to distinquish.

3. Thinner clouds don't reflect as much light and appear grey.

Here's a summary (that wasn't shown in class)

The figure below shows how if you combine both visible and IR photographs you can begin to distinquish between different types of clouds.

You can use this figure to answer the satellite photograph question that is on the Quiz #3 Study Guide.

There is a 3rd type of satellite photograph, a water vapor image.  This is just for your information purposes and wasn't discussed in class.

This is also an IR satellite photograph, but the satellite detects and photographs 6.7 μm radiation.   This type of image can show air motions in regions where there aren't any clouds because the 6.7 um radiation (Point 1) is absorbed by water vapor.  The water vapor then emits IR radiation upward toward the satellite where it can be photographed.  Water vapor from lower in the atmosphere emits more strongly and appears grey (Point 2), water vapor from high in the atmosphere emits weak radiation and appears white (Point 3).

The last big topic we will cover before next week's quiz is precipitation formation and types of precipitation.  Only two of the 10 main cloud types (nimbostratus and cumulonimbus) are able to produce significant amounts of precipitation.  Why is that?  Why is it so hard for clouds to make precipitation?

This figure shows typical sizes of cloud condensation nuclei (CCN), cloud droplets, and raindrops (a human hair is about 50
μm thick for comparison).  As we saw in the cloud in a bottle demonstration it is relatively easy to make cloud droplets.  You cool moist air to the dew point and raise the RH to 100%.  Water vapor condenses pretty much instantaneously onto a cloud condensation nucleus to form a cloud droplet.  It would take much longer (a day or more) for condensation to turn a cloud droplet into a raindrop.  You know from personal experience that once a cloud forms you don't have to wait that long for precipitation to begin to fall.

Part of the problem is that it takes quite a few 20 μm diameter cloud droplets to make one 2000 μm diameter raindrop.  How many exactly?  Before answering that question we will look at a cube (rather than a sphere).

How many sugar cubes would you need to make a box that is 4 sugar cubes on a side?

It would take 16 sugar cubes to make each layer and there are 4 layers.  So you'd need 64 sugar cubes.  Volume is length x width x height.

The raindrop is 100 times wider, 100 times deeper, and 100 times taller than the cloud droplet.  The raindrop has a volume that is 100 x 100 x 100 = 1,000,000 (one million) times larger than the volume of the cloud droplets.  It takes about a million cloud droplets to make one average size raindrop.

Fortunately there are two processes capable of quickly turning small cloud droplets into much larger precipitation particles in a cloud.

The collision coalescence process works in clouds that are composed of water droplets only.  Clouds like this are only found in the tropics.  We'll see that this is a pretty easy process to understand.

This process will only produce rain, drizzle, and something called virga (rain that evaporates before reaching the ground).

The ice crystal process produces precipitation everywhere else.  This is the process that makes rain in Tucson, even on the hottest day in the summer (summer thunderstorm clouds are tall and reach into cold parts of the atmosphere, well below freezing.  Hail and graupel often fall from these storms; proof that the precipitation started out as an ice particle).  There is one part of this process that is a little harder to understand.

This process can produce a variety of different kinds of precipitation particles (rain, snow, hail, sleet, graupel, etc).

Here's how the collision coalescence process works.  The picture below shows what you might see if you looked inside a warm cloud with just water droplets:

The collision coalescence process works in a cloud filled with cloud droplets of different sizes.  The larger droplets fall faster than the small droplets.  A larger-than-average cloud droplet will overtake and collide with smaller slower moving ones.

This is an accelerating growth process.  The falling droplet gets wider, falls faster, and sweeps out an increasingly larger volume inside the cloud.  The bigger the droplet gets the faster it starts to grow (think of a growing ball of snow as it rolls down a snow-covered hill and picks up snow, grows, and starts to roll faster and faster;  or think of an avalanche that gets bigger and moves faster as it travels downslope)

A larger than average cloud droplet can very quickly grow to raindrop size.

The figure shows the two precipitation producing clouds: nimbostratus (Ns) and cumulonimbus (Cb).  Ns clouds are thinner and have weaker updrafts than Cb clouds.  The largest raindrops fall from Cb clouds because the droplets spend more time in the cloud growing. In a Cb cloud raindrops can grow while being carried upward by the updraft and also when falling in the downdraft.

Raindrops grow up to about 1/4 inch in diameter.  When drops get larger than that, wind resistance flattens out the drop as it falls toward the ground.  The drop begins to "flop" or "wobble" around and breaks apart into several smaller droplets.  Solid precipitation particles such as hail can get much larger (an inch or two or three in diameter).