February 11, 2008
Put up web page (listed in assigned reading) titled “500 mb Height Pattern and Surface Weather”. Much of the first part is a review of what we have already covered. You should be able to understand it now. Look at material beginning in 6th paragraph.
o Review/Continue side two of handout, weather terminology
o Use diagrams to show what to look for in the 500 mb pattern to indicate regions with strong upper level divergence and potentially strong winter storms
n Shortwaves
o We will not spend too much time on shortwaves because they can be difficult to identify on 500 mb maps. But they are important in determining weather conditions.
o Shortwaves are much smaller than longwaves … they are smaller scale ripples in the 500 mb pattern, which are superimposed on the larger longwave pattern.
§ Draw a simple diagram
o There are shortwave ridges and shortwave troughs, however, we are most interested in shortwave troughs because they are often associated with precipitation and storms.
o Shortwaves are smaller waves that move along longwaves in the same direction as the winds at 500 mb. In fact shortwaves are “steered” around by the largescale 500 mb air flow. As a rule of thumb, shortwaves move at about half the speed of the 500 mb winds.
o Shortwaves tend to strengthen as they move through a longwave trough and weaken as they move through a longwave ridge.
§ Show shortwave figure from textbook (linked on lecture notes page)
n 500 mb closed lows and cut-off lows
o We have already looked at closed lows on 500 mb maps. Review quickly.
o Show example, linked on supplemental reading page.
o Draw diagram to show how cut-off lows can develop
§ Explain cut-off lows can bring a prolonged period of cool, rainy weather if they become stationary for several days. Relatively common for those to form just off southern California coast and influence our weather.
Air Masses
n An air mass is an extremely large body of air, often covering thousands of square kilometers, which is characterized by nearly uniform conditions of temperature and humidity. In general there are 4 main types of air masses: cold and dry, cold and humid, warm and dry, warm and humid.
n Air masses form when air remains stagnant over a uniform surface (e.g., an ocean or flat land areas). The longer air remains stagnant over a region, the more likely it is to take on the properties of the underlying surface.
n Air masses that affect the continental United States usually form either north or south of the continental US where conditions are favorable for air mass formation. Over the continental US, air usually does not remain stagnant long enough for air masses to form.
o Look at table 8.1 and figure 8.2 in textbook to learn the names given to the various air masses (and their source regions) that affect US weather.
§ Table 8.1 classifies air masses as stable or unstable. Don’t worry we have not discussed stability yet. If an air mass is unstable, it means that conditions are favorable for the formation of showers and possibly thunderstorms, while in a stable air mass, the formation of showers and thunderstorms is unlikely.
§ The Rocky Mountains and other tall mountains in the western United States can protect regions west of the continental divide from extremely cold air masses (e.g., coastal CA, OR, and WA and even here)
o Try to extract the basic information about air masses from the textbook. I think the book goes into some specifics about US weather that are difficult to follow unless you have prior knowledge or experience in analyzing US weather. We will not have time to explain all the details in this class.
n After air masses form, they move in response to the air flow in the middle and upper troposphere. We have already considered how to “visualize” the movement of warm and cold air masses by examining the height pattern on a 500 mb map.
o A trough indicates where a cold air mass has moved south and a ridge indicates where a warm air mass has moved north.
Weather Fronts
n A front is the boundary or transition zone between two air masses. Much of the interesting weather (rain, snow, strong winds, storminess, etc.) occurs along and near fronts where air masses clash, rather than in the center of a large air mass. In fact, the original use of the term front was made by analogy with a WWI battle front between troops … weather front is a battle or clashing of two air masses.
n Fronts are only marked on surface weather maps. However, it is important to realize that fronts are 3-dimensional and extend vertically as well as horizontally.
o Above the ground, frontal surfaces slope so that the warm air mass overlies the colder air mass. This is always the case, since the warmer air mass will be less dense (and hence lighter) than the colder, more dense air mass.
o Draw side view diagram of frontal surface
o Because the warm air mass is forced to rise vertically upward along frontal boundaries, we often observe precipitation along and near weather fronts. In the center of a large air mass, this type of forced lifting of air does not take place.
n Basic types of fronts
o Fronts are classified based on how the front is moving along the surface of the Earth.
o A stationary front shows little or no movement along the ground
§ Draw weather map symbol for a stationary front
o A cold front indicates that at the surface the cold air mass is advancing into a region formerly covered by a warmer air mass
§ Draw weather map symbol for a stationary front
o A warm front indicates that at the surface the warm air mass is advancing into a region formerly covered by a colder air mass.
§ Draw weather map symbol for a warm front
o Go over a handout which describes the typical characteristics of warm and cold fronts.
§ Also look at figures 8.15 (cold front) and 8.18 (warm front) in textbook.