Clim 301 HW09

Homework #9: Fronts

Due November 19

This lab is an introduction to locating fronts on the surface chart. You should have read Chapter 5 in Vasquez.

The assignment is to locate and draw the major fronts on the provided map of surface observations.

The map is linked HERE.

The surface chart is for 01Z on April 7, 2010. You should review front locations for the times prior using the maps archive HERE.

Draw the fronts using the standard notation and color, as discussed in the lab, and as shown below:

Note that the "bumps" on the front are supposed to point in the direction the front is moving. Thus, the triangles on a cold front are pointing from cold towards warm air (since, by definition, a cold front is the boundary where cold air is pusing into warm air). The semi-circles on a warm front are pointing from warm air towards cold air. For a stationary front, it is customary to point the triangles towards the warm side of the front, and the semi-circles towards the cold side of the front.

On your map, draw your initial estimate of the front locations lightly in pencil. See the discussion below for guidlines on locating fronts.

After estimating the locatation of the fronts, draw isobars at 2mb intervals (the same interval as the computer analysis below). When drawing the isobars, keep in mind that the front usually is a location of low pressure -- a pressure trough. See page 221 in Vasquez.

Then, finalize the front locations and types by drawing the fronts using the proper colors and sybols.


Analyzing fronts requires looking at every meteorological variable. See Chapter 5 in Vasquez for additional guidance.

Pressure: Pressure usually drops ahead of fronts and rises after the front passes. With a strong cold front, this can be dramatic. In some cases, the pressure will rise slowly ahead of a front, but more rapidly after the front passes. If the isobars are drawn on the map, the front will usually be found in a "trough" of lower pressure. It may be appropriate to show the isobar contour lines "kinked" at the front. See page 221 in Vasquez.

Temperature: Fronts often seperate air masses of different temperature. Within an air mass, the isotherms will be widely spaced, indicating gradual changes in temperature over distance. Near a front, the isotherms will be closely spaced, indicating a rapid change in temperature. The front is usually located on the warm edge of the closely spaced (also referred to as packed) isotherms.

Moisture: Fronts often seperate air masses having different amounts of moisture. When the moisture is the only significant change across a front, it is sometimes called a "dryline". The front will be located on the moist edge of the "packed" dewpoint lines.

Winds: The winds typically shift speed and/or direction when a front passes. On the surface map, winds will usually be different in direction on one side of a front compared to the other. For a strong front, the wind shift line is usually the best way to fine-tune the location of the front. Almost always locate a strong front along a wind shift line, unless there are good reasons not to. Mesoscale features, such as squall lines, can result in front placement where where the winds are not shifting direction as clearly.

Clouds and Precip: Lines of clouds seen on satellite images, or lines of showers seen on radar, will often be associated with a front.

The most useful aids for fine tuning the location of a front is the trough of low pressure and the wind shift line. So look especially closely at the Isobars and the Streamlines. Weak fronts can be difficult to locate precisely, as they are often more diffuse.

Another important tool for locating front positions is to follow the fronts over time, from map to map. Fronts will move at a regular pace, and not "jump" long distances.

The type of front is determined by examining the winds, and by following the front over time from map to map. When winds across the front (the component of the wind that is perpendicular to the front) are stronger one one side of the front than the other, the front is being "pushed" predominantly by those winds. When the winds on either side of the front appear fairly balanced, and the front over time is not moving, or moving very little, then the front can safely be labeled as stationary.


To aid in locating the fronts for your homework, the following computer analyses are provided, including the isobars. Take the time to appreciate the difference between the computer analysis of the data and your own analysis. With practice and experience, "by hand" analysis of the surface chart will be more informative that what the computer can provide.

Sea Level Pressure

Temperature

Dewpoint

Winds