Meteorology Chapter 11

Name the five air masses and know the characteristics and two letter abbreviation for each.

1. Continental Arctic (cA) very cold, dry, unstable 2. Continental Polar (cP) cold, dry, stable 3. Continental Tropical (cT) hot, dry, stable aloft, unstable near surface 4. Maritime Polar (mP) cool, moist, unstable 5. Maritime Tropical (mT) warm, moist, usually unstable

What is a Dry Line? How do they form? Where is the best location for the formation of a Dry Line?

A Dry Line is not a warm front or cold front, but represents a narrow boundary where there is a steep horizontal change in moisture. Dry Lines will separate a hot dry air mass (cT) to the west from a hot moist air mass (mT) to the east. 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. With good upper level support, thunderstorms will develop along the Dry Line during the late afternoon hours. DIAGRAM A simple diagram identifying air mass characteristics on either side of a dry line. DIAGRAM Identifying a dry line on a surface weather map.

Identify the properties of a Cold Front

A cold front represents a 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 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. Note the isobar pattern in this diagram. The cold front is aligned in the area where you have kinks in the isobars and associated cyclonic (counter-clockwise) curvature. DIAGRAM DIAGRAM

Identify the properties of a Stationary Front

A stationary front has essentially 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. Little or no movement When the winds aloft are blowing at an angle across the surface frontal boundary, clouds and precipitation can form on the northern (cold) side of the stationary front. This is true if enough moisture is present in the warm air mass that is being lifted over the colder air mass. DIAGRAM

Identify the 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. This diagram represents conditions associated with a warm front during the winter season. Notice that the front is aligned in the area where the isobars are curving cyclonic or counter-clockwise. DIAGRAM DIAGRAM Note that the lighter warm air is forced to ride up an over the more dense cold air near the surface. AIr being lifted by this process is called overrunning.

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 is an air mass?

An air mas is an extremely large body of air whose properties of temperature and moisture are fairly similar in any horizontal direction at any given altitude

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.

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.

What type of weather is associated with the 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 type of weather is 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.

Are the mid-latitudes a good source region for air mass development?

No Notice the void across much of the United States on the previous diagram. 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.

What type of precipitation is 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.

Name the four weather fronts and identify the symbols used to represent the fronts on a surface weather map.

Stationary Front: opposite triangles/circles Warm Front: circles Cold Front: triangles Occluded Front: triangles/circles DIAGRAM

What is the difference between a Cold and Warm occlusion?

The difference between the two different types of occlusions will be related to the temperature (density) of the air masses. This will determine which air mass will be forced to rise when they meet. If the coldest air is located behind the cold front, then it will be a Cold Occlusion. If the coldest air is located beneath the warm front, then it will be a Warm Occlusion. DIAGRAM DIAGRAM

Define Frontogenesis

The formation, strengthening, or regeneration of a weather front. Usually caused by an increase in the temperature contrast across the frontal boundary.

What five criteria are used to determine the location of a weather front on a surface weather map?

The main area of concentration should be in regions where we have discontinuity in the data field. The first two were covered in an early question. 1. Significant temperature changes over a relatively short distance 2. Change in the air's moisture content 3. Noticeable shifts in wind direction 4. Pressure patterns in the isobar contours 5. Clouds and precipitation patterns

Define Frontolysis

The weakening or dissipation of a weather front. Occurs when the temperature contrast across a frontal boundary weakens.

What type of weather would you expect with an 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 greater contrast in temperatures between the colliding air masses.

What is an air mass "Source Region"?

This is a region where an air mas will originate. In order for a huge mass of air to develop uniform characteristics, its source region should be generally flat an of uniform composition with light surface winds. Therefore, ideal source region 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. DIAGRAM

What is "Lake Effect Snow"? How does it form? Where is the best region of development in relation to the Great Lakes? What time of year is most favorable for development?

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. 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. The best time of year for the development of Lake Effect Snow would be in the late Fall to early Winter months. DIAGRAM

How does an Occluded Front form? How is an Occluded Front 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. DIAGRAM