Since we've been learning how to identify clouds this seemed like a logical time to learn a little bit about the 2 most common types of satellite photographs.  You'll find this 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.  You may recall that 10 μm radiation is in the middle of the atmospheric window, so emitted 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 emit 10 μm IR radiation upwards toward the satellite and down toward the ground.  The top surface of a low altitude cloud will be relatively warm.  Warmer objects emit IR radiation more intensely than cooler objects (the Stefan Boltzmann law).  Relatively strong IR emissions appear 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 weaker radiation (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 grey throughout the day. 

Land and ocean temperatures during the warmest and coolest part of the day are shown at the top of this figure.  Sketches of how the land and ocean might appear on an IR photograph are shown below.  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 "sees" 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.  The figure below wasn't shown in class.

The figure below shows how, if you examine 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 and wasn't mentioned in class (they are very occasionally shown on the TV weather forecast)

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?

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).

It would take 64 individual sugar cubes to make a 4 cube x 4 cube x 4 cube cube.  That is because the bigger cube is 4 times wider, 4 times deeper, and 4 times taller.  Volume is the product of all three dimensions.  (27 sugar cubes would be needed to make a 3 cube x 3 cube x 3 cube box etc)

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.

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