ATSC 110 Meteorology Chapter 11
Properties of a Warm Front
A warm front represents the location where advancing warm, moist or dry air is replacing retreating cold air. A warm front is represented on a surface weather map by a solid red line with semicircles pointing in the direction of movement. Surface winds typically have an easterly component ahead of the front and a southerly component after frontal passage.
What is a weather front?
A weather front is a transition zone between two distinct air masses of different properties. These properties are associated with the air mass temperature and moisture content. The different temperatures and moisture contents of the various air masses will cause the air masses to have different densities. A weather front is located in the area that separates the two different air masses.
What causes an air mass to move and in what direction it will move?
An air mass will usually move in response to the winds aloft. This is why the position of the jet stream in relation to an area on the surface plays a key role in forecasting the weather for that area. If a NW to SE jet stream aloft is aligned from northwestern Canada to North Dakota during the winter months, cP air masses from that source region will break lose and move towards the northern plains.
Cold Front
Blue Triangles
Stationary Front
Blue Triangles/ Red half Circles
How do continental and maritime air masses differ in respect to moisture content?
Continental air masses form over land/ice and maritime air masses form over open water. Therefore the continental air masses will be fairly dry air masses and the maritime air masses will be fairly moist air masses.
Best region for "Lake Effect" snow
Eastern and Southeastern shore lines along the Great Lakes are favorable areas for lake effect snow since the prevailing winds over the lakes is from the northwest.
What type of weather is associated with a Stationary Front
It depends, in some cases two dry air masses, one cold (cP) and one warm (cT) are separated by the front therefore no precipitation occurs along or near the front. In other cases a warm moist air mass (mT) will be advected over the top of a cold moist air mass (mP) and an extensive area of light precipitation will develop on the cold side of the stationary front. The precipitation type will depend on the vertical temperature between the surface and the cloud producing the precipitation.
What time of year for the development of "Lake Effect" snow
Late fall to early winter months
Weather associated with a Warm Front
Like the cold front, the type of weather associated with a warm front will depend on the season, moisture content of the air mass be lifted, and the temperature profile of each air mass. Because warm fronts have a gentler slope than cold fronts, the precipitation pattern is typically stratiform with large areal coverage. The precipitation type will depend on the temperature profile of the interacting air masses. Warm fronts with a strong temperature gradient can on occasion produce thunderstorms on the cold side of the front.
Properties of a Stationary Front
No movement. It is identified on a weather map by alternating red semicircles and blue triangle with the red semicircles pointing towards the cold air and the blue triangles pointing towards the warm air. Surface winds tend to flow parallel to the front, but in opposite directions on either side of the front. Upper-level winds, for the most part, move parallel to the surface stationary front.
Are the mid-latitudes a good source region for air mass development?
No, Surface temperatures and moisture characteristics vary considerably in the mid-latitudes making this region a poor source region for air mass development. The mid-latitudes are better known for the region of the world where air masses collide which in turn creates some of the world's most violent weather.
Precipitation associated with a Cold Front
Precipitation amounts and types can vary depending on the time of season, moisture content of the air mass ahead of the cold front, and temperature gradient across the frontal boundary. With significant moisture and a respectable temperature gradient, cold fronts in the summer will trigger thunderstorm activity ahead of, or along the frontal boundary. In the winter, snow squalls are possible along and just behind the cold front when ample moisture is available and the temperature gradient is significant. If moisture is limited, the cold front can pass without the development of any precipitation event.
Occluded Front
Purple Triangles and Half Circles
Warm Front
Red Half Circles
What five criteria are used to determine the location of a weather front on a surface weather map?
Significant temperature changes over a relatively short distance Change in the air's moisture content Noticeable shifts in wind direction Pressure patterns in the isobar contours Clouds and precipitation patterns
What type of weather would you expect with a Occluded Front?
The weather would resemble that of a warm front on most occasions. The one exception would be with the passage of the "Cold Occlusion" where heavy precipitation and thunderstorm activity is possible when there is a a greater contrast in temperatures between the colliding air masses.
What is an air mass "source region"?
This is a region where an air mass will originate. In order for a huge mass of air to develop uniform characteristics, its source region should be generally flat and of uniform composition with light surface winds. Therefore, ideal source regions are usually those areas dominated by surface high pressure systems. The following map identifies the regions of air mass development and the general movement of these air masses once they leave the source region.
Where is the best location for a dry line to form?
This unique situation sets up most often across the southern and central high plains where cT air masses from the desert southwest collide with mT air masses from the Gulf of Mexico.
What is "Lake Effect" snow?
When a cold dry air mass (cP or cA) moves over a relatively warmer body of water, such as the Great Lakes, heavy snow showers called "Lake Effect Snow" will form on the downwind shore line.
How does an Occluded Front form? How is it represented on a Surface Weather map?
When a cold front catches up to and overtakes a warm front, the frontal boundary created between the two air masses is called an "Occluded Front". On the surface weather map, it is represented as a solid purple line with alternating triangles and semicircles, both pointing in the direction that the front is moving.
Continental Polar
cold, dry, stable
Cold Occlusion
coldest air is located behind the cold front
Warm Occlusion
coldest air is located beneath the warm front
Maritime Polar
cool, moist, unstable
What is an air mass?
extremely large body of air whose properties of temperature and moisture are fairly similar in any horizontal direction at any given altitude.
Frontogenesis
formation, strengthening, or regeneration of a weather front. Usually caused by an increase in the temperature contrast across the frontal boundary.
Continental Tropical
hot, dry, stable aloft, unstable near surface
What is a dry line? How does a dry line form?
not a warm front or cold front, but represents a narrow boundary where there is a steep horizontal change in moisture. Will separate a hot dry air mass (cT) to the west from a hot moist air mass (mT) to the east.
Continental Arctic
very cold, dry, stable
Maritime Tropical
warm, moist, usually unstable
Frontolysis
weakening or dissipation of a weather front. Occurs when the temperature contrast across a frontal boundary weakens.
Properties of a Cold Front
zone where cold dry air will replace warm moist or warm dry air. The cold front is drawn on a weather map as a solid blue line with blue triangles pointing in the direction of movement. Surface winds tend to be southwest to south direction ahead of the front to a west-northwest or northerly direction behind the front. Atmospheric pressure will be falling before the cold front arrives and start to rise after the cold front passes.
