AOS 101 Weather and Climate Lisha M. Roubert University of Wisconsin-Madison Department of Atmospheric & Oceanic Sciences

Fronts: The Boundaries between Air Masses  A front is defined as the transition zone between two air masses of different density.  Why do we have this transition zone between air masses? Initially when air masses come in contact with each other they cannot mix together as quickly because they posses different properties (such as temperature, moisture, etc.). Upon coming in contact the air masses retain their properties for a period of time and thus we observe fronts separating the air masses. Fronts can exist as long as the air masses they separate remain distinct.

Fronts: The Boundaries between Air Masses  Different air masses which affect North America, as well as other continents, tend to be separated by frontal boundaries.  In this image, the Arctic front separates the Arctic from Polar air masses, while the Polar front separates Polar air from warm air masses. (cA is continental arctic; cP is continental polar; mP is maritime polar; cT is continental tropic; and mT is maritime tropic.)

Frontal Zone  A frontal zone is a sloping surface that separates two air masses. The area where the front meets the ground is called the frontal zone, which is the region where the contrasts between the air masses are usually most prominent.

Front classifications  Fronts are classified by the temperature changes that result after an air mass passes over a given location. Weather map symbols: 1. cold front; 2. warm front; 3. stationary front; 4. occluded front; 5. surface trough; 6. squall/shear line; 7. dry line; 8. tropical wave

Cold Front: front in which cold air is replacing warm air at the surface  Some of the characteristics of cold fronts include the following:  Cold fronts tend to move faster than all other types of fronts.  Cold fronts tend to be associated with the most violent weather among all types of fronts.  Cold fronts tend to move the farthest while maintaining their intensity.  Cold fronts tend to be associated with cirrus well ahead of the front, strong thunderstorms along and ahead of the front, and a broad area of clouds immediately behind the front (although fast moving fronts may be mostly clear behind the front).  Cold fronts can be associated with squall lines (a line of strong thunderstorms parallel to and ahead of the front). Cold fronts usually bring cooler weather, clearing skies, and a sharp change in wind direction.

Warm Front: front in which warm air replaces cooler air at the surface  Some of the characteristics of warm fronts include the following:  Warm fronts tend to move slowly.  Warm fronts are typically less violent than cold fronts.  Although they can trigger thunderstorms, warm fronts are more likely to be associated with large regions of gentle ascent (stratiform clouds and light to moderate continuous rain).  Warm fronts are usually preceded by cirrus first (1000 km ahead), then altostratus or altocumulus (500 km ahead), then stratus and possibly fog.  Behind the warm front, skies are relatively clear (but change gradually).  Warm fronts are associated with a frontal inversion(warm air overrunning cooler air).  If a warm front exists on a weather map, it will be northeast of the cold front and often, to the east of a surface low pressure area. Clouds and precipitation are quite prevalent to the north of the warm front. /terc/content/visualizations/es2002/es2002pa ge01.cfm?chapter_no=visualization

Stationary Fronts  Stationary front- a front that does not move or barely moves.  Stationary fronts behave like warm fronts, but are more quiescent.  Many times the winds on both sides of a stationary front are parallel to the front.  Typically stationary fronts form when polar air masses are modified significantly so as to lose their character (e.g., cold fronts which stall).

Occluded Front  Because cold fronts move faster than warm fronts, they can catch up to and overtake their related warm front. When they do, an occluded front is formed.  Occluded fronts are indicative of mature storm systems (i.e., those about to dissipate).  The most common type of occlusion in North America is called a cold-front occlusion and it occurs when the cold front forces itself under the warm front. The weather ahead of the cold occlusion is similar to that of a warm front while that along and behind the cold occlusion is similar to that of a cold front. Development of an Occluded Front

Fronts on Weather Maps Weather map symbols: 1. cold front; 2. warm front; 3. stationary front; 4. occluded front; 5. surface trough; 6. squall/shear line; 7. dry line; 8. tropical wave

Lab Setup  Today we will be simulating a front in the rotating tank.  Dyed salty water is in the middle of the cylinder so we can create a density difference where more dense water is in the cylinder and less dense water is in the rest of the tank.  The table is set into rapid rotation at a speed of about 14 rpm.  We will observe what happens under 2 different scenarios: the first time with rotation, the second time without rotation.

For next time:  Lab report due with the same format as the previous lab report you wrote.