IL-7: Fronts and Cyclones
Part I Types of Fronts
A front is defined as the boundary between two airmasses. The major types being:
mT – maritime Tropical (moist and warm)
mP – maritime Polar (moist and cold)
cT – continental Tropical (dry and warm)
cP – continental Polar (dry and cold)
When any of these airmasses meet, they form a front. Depending on which airmasses meet and which airmass is moving forward (and which is moving out of the way) you will have different kinds of fronts.
The two main types of fronts are cold fronts, where cold air is advancing forward toward warm air, and warm fronts, where warm air is advancing forward toward cold air. This video gives a brief overview of these two types of fronts:
What are weather fronts? (Links to an external site.)
Question 1: Describe three (3) differences between cold and warm fronts.
There are other types of fronts, however. When a cold or warm front stalls, or stops moving, we call this a stationary front. Also, in some cases, a cold front can catch up to a warm front and overtake it. When this happens, we call it an occluded front. This page reviews these four types of fronts, along with the types of weather they cause:
http://okfirst.mesonet.org/train/meteorology/Fronts.html (Links to an external site.)
Question 2: Which type of front tends to produce the most violent weather among all frontal types?
Question 3: Why do you think that type of front has the most violent weather?
One final type of front is called a dry line. A dry line is a boundary between warm dry (cT) and warm moist (mT) airmasses. Dry lines are a common location for the formation of thunderstorms in the spring in the central United States.
Part II Midlatitude Cyclones
A midlatitude cyclone is a strong low pressure area located somewhere between 20-70 degrees north or south latitude. These cyclones usually have one or more fronts associated with them. This page describes an idealized model of a midlatitude cyclone in the northern hemisphere:
http://ww2010.atmos.uiuc.edu/%28Gh%29/guides/mtr/cyc/def.rxml (Links to an external site.)
Question 4: Why does there tend to be warm air ahead of the midlatitude cyclone?
As we learned previously, winds rotate counterclockwise around a low pressure system in the northern hemisphere. Therefore, we can use wind barbs on a weather map to help identify the location of the low. This page describes this process:
http://ww2010.atmos.uiuc.edu/%28Gh%29/guides/mtr/cyc/wnd.rxml (Links to an external site.)
This rotation of the winds, transports airmasses from the south and from the north and wraps them around the midlatitude cyclone. This acts to intensify the cyclone and strengthen the fronts, or the boundaries between these airmasses. This page describes this process:
http://ww2010.atmos.uiuc.edu/%28Gh%29/guides/mtr/cyc/arms.rxml (Links to an external site.)
Question 5: a. What is the general wind direction in the warm sector of a midlatitude cyclone?
b. What is the general wind direction in the cold sector of a midlatitude cyclone?
We can also use satellite imagery to identify the location of and weather associated with midlatitude cyclones. This satellite image shows a very well developed cyclone over the Great Lakes region:
http://earthobservatory.nasa.gov/NaturalHazards/view.php?id=52297 (Links to an external site.)
In this image, we see a typical “comma-shaped” cloud. This page shows some other examples of these cloud patterns:
http://ww2010.atmos.uiuc.edu/%28Gh%29/guides/mtr/cyc/sat.rxml (Links to an external site.)
Here is a video showing a satellite loop of the development of one of these comma clouds:
Strong Extratropical Cyclone Over the US Midwest, October 25-27, 2010 (Links to an external site.)
Notice that the cloud pattern changes over the life of the storm and is never constant.
Part III Searching for Cyclones
Using what we have learned, we will now try to find some midlatitude cyclones over North America.
Go to this page, showing the current location of fronts over the contiguous U.S.
http://www.wpc.ncep.noaa.gov/noaa/noaa.gif (Links to an external site.)
Right-click (or ctrl-click) and select “Save image.” Upload the map when you submit the assignment.
You can you this page to help you interpret the map if needed: http://www.wpc.ncep.noaa.gov/html/fntcodes2.shtml (Links to an external site.)
Question 6: Describe the location of any low pressure centers on the map, within the contiguous U.S.
Question 7: For EACH of the low pressure centers described in question 6, describe the location and type of any fronts that seem to be associated with each low. (i.e. is touching the low or extending out from the low).
Now we will look at the current satellite imagery.
Go to this page for satellite imagery:
http://weather.rap.ucar.edu/satellite/ (Links to an external site.)
Select “Infrared (color)” from the radio buttons on the left.
Select “Large Size” from the radio buttons above the map.
Click on “Contiguous U.S.” on the map.
Right-click (or ctrl-click) and select “Save image.” Upload the map when you submit the assignment.
This map shows cloud top temperatures. Blues are higher colder cloud tops, while greens and yellows are lower warmer cloud tops. Oranges and reds are areas of no or very low clouds.
Now we will compare the weather map to the satellite image. Look the two images side by side.
Question 8: For EACH of the low pressure areas described in question 6, describe the clouds at the location of the low pressure center.
Question 9: For EACH of the fronts described in question 7, describe the clouds along the fronts.
Question 10: Describe how you could use your knowledge of the general circulation of the atmosphere, airmasses and fronts, and the weather map and satellite image you downloaded above to forecast the weather for Salt Lake City, UT over the next 24 hours.
Question 11: Using the method you describe in question 10, what would be your best guess for what the weather will be like in Salt Lake City, UT over the next 24 hours? Be as detailed as possible, AND describe WHY you came up with this forecast.
