Monday Oct. 30, 2006

The Experiment #3 reports, the Scientific Paper reports, and the revised Expt. #2 reports were all collected today.  The Expt. #3 and the Scientific Paper reports will be graded in about 1 week.  The revised reports may take a little longer. 

Mid term grade summaries and more of the 1S1P reports were returned today also.

A printed copy of the Quiz #3 Study Guide was handed out in class.  Quiz #3 is on Wednesday this week.  Reviews will be held Monday and Tuesday afternoon (see the study guide for times and locations)
We have a few odds and ends to finish up today.  We'll start with weather radar.  Satellite photographs are a good way of observing clouds (especially out over the ocean).  By using both visible and infrared light satellite photographs you can get a good idea of cloud type (as we will see later in today's class).  However satellite photographs don't really tell you whether a cloud is producing precipitation or not.


An ordinary radar periodically transmits a short burst of microwave radiation.  This radiation penetrates a cloud but is reflected by precipitation particles.  The radar keeps track of what direction the antenna is pointing and determines how long it takes for a signal to go out and return.  The radar also measures the strength of the return signal.  Conventional radar can thus locate the precipitation and provide an estimate of its intensity. 

The radar antenna slowly spins as it is transmitting so it scans a full 360 degrees in a minute or two. 

Information from a single radar or a combination of data from many radars are drawn on weather maps (the PPI display above shows the data from a single radar, the radar would be at the center of the picture).  This would show where precipitation is occurring.  The radar data is often combined with satellite photographs.  Colors are used to indicate the intensity of the precipitation.  Yellows, oranges and reds generally indicate the heaviest precipitation (often coming from thunderstorms).

In research the radar can be used to scan vertically through a storm, this produces an RHI display.

By detecting changes in the frequency of the reflected signal, a doppler radar can measure the speed at which precipitation particles are moving toward or away from a radar antenna.  By combining data from 2 or more radars (and some complicated computer processing), three-dimensional wind motions inside a cloud can be mapped out. 

Doppler radars can detect a rotating thunderstorm updraft (a mesocyclone) that could indicate a thunderstorm capable of producing tornadoes.  Small mobile doppler radars are being used to try to measure wind speeds in tornadoes.  Police use doppler radar to measure the speeds of automobiles on the highway.

Radar can also be used to learn something about the kinds of precipitation particles inside a cloud.
Next we looked briefly at the three types of satellite photographs and what they tell you about clouds or wind motions in the atmosphere.  You'll find these discussed on pps 99-100 in the photocopied class notes.  You'll also find this topic discussed on pps 236-240 (pps 233-237 in the 3ed) in the text (Chap. 9).

An infrared satellite photograph detects the IR radiation actually emitted by clouds.  You don't depend on seeing reflected sunlight, so  clouds can be photographed during the day and at night.  Because the satellite detects 10 micrometer radiation, IR radiation emitted by the ground is also visible in regions where there aren't any clouds.

White on an IR photograph means the top radiating surface of the cloud is cold (found at high altitude).  It is sometimes hard to distinquish the tops of strong thunderstorms from high altitude cirrus or cirrostratus clouds even though the clouds are very different.  Warm, low level clouds appear grey.  A grey unimpressive looking cloud on an IR satellite photograph may actually be a thick nimbostratus cloud that is producing a lot of rain or snow.

Thick clouds produce a white image on a visible satellite photograph.  Thin clouds appear grey.  Note a thunderstorm appears white on both IR and VIS satellite photographs.  By comparing images of clouds on both visible and IR images you can begin to distinquish between different kinds of clouds. 

The origin of the patchy cloud pattern seen behing cold fronts that are out over the ocean is shown below.  This figure wasn't shown in class.

Cold air warms and is moistened as it passes over warmer ocean water.  The air can eventually become bouyant and rise enough that a cloud forms.  Clouds develop best when there is a big temperature difference between the air and the water.  This is essentially the same as the Lake Effect you may have read about if you did a 1S1P report on air masses.

A water vapor satellite photograph is similar to an IR satellite photo (both types photograph IR radiation).  In this case both water vapor and clouds emit IR radiation (it is a slightly different wavelength than in a normal IR photograph) that is detected and displayed by the satellite.  Water vapor found at low altitude is warm and appears grey on the photograph (often hard to see on the satellite photograph).  High altitude water vapor is cold and appears white.  But remember the high altitude air is cold and there isn't much water vapor up there.  These photographs show air motions in regions where there aren't any clouds, motions that would otherwise be invisible.

Now that you have read through this information on satellite photographs, here is a sample satellite photograph question.

As mentioned above, grade summaries were distributed in class.  You should first check to see that quiz grades have been recorded correctly and that you have been given credit for the optional assignments, 1S1P reports (not all of the Assignment #2 reports have been graded), and experiment/book reports that you have turned in. 

The computer has also tried to predict what grade you will end up with at the end of the semester if you keep performing as you have been so far this semester (it uses an average report grade if you haven't yet turned in an experiment report or your report hasn't been graded yet).  There is a little "guesswork" involved here, these really are grade estimates.  Your grade can improve or could get worse between now and the end of the semester.
We finished up with some of the material on Newton's Laws of Motion in Chapter 6 that we will be covering after the next quiz. This material will not be covered on Quiz #3 and will be moved to the Friday Nov. 3 notes.