Simulations

The study of air masses, storms, and fronts falls in the discipline known as synoptic meteorology. Synoptic meteorology includes the study of weather systems that are on the order of 1500 kilometers (or about 1000 miles) across. Thus, they are much smaller than the global circulations that govern world climate but larger than thunderstorms, snow flurries, and individual clouds.

When air lingers over a region for a number of days, it acquires characteristics associated with the underlying surface, and is called an air mass. Boundaries between air masses are called fronts, and low pressure areas tend to form along them.

If the surface is warm and moist, the air in the region becomes warm and moist also. If the air then moves to a different region, it takes these properties along, gradually changing in response to the new area of residence or passage. Here we study the nature of the large air masses found around the world, air masses on the order of 1500 kilometers in diameter.

Cold air masses originate over the polar and near-polar regions, whereas warm air masses form in the tropics and subtropics. Air masses acquire moisture through evaporation, which depends on the temperature of the evaporating surface, as well as on available moisture. Thus, air masses originating over tropical oceans are the moistest, whereas air masses that form over cold, dry land are the driest.

Cold air masses are called polar to represent air masses that originate in the northern parts of the northern hemisphere. Those that originate over the very coldest part of the polar regions are called Arctic. Warm air masses originate in low latitudes and are called tropical air masses.

An air mass that originates over a landmass is called continental; if it originates over water, it is called maritime. The names of the major air masses are derived from various combinations of the designations polar, tropical, maritime and continental: thus, a warm moist air mass is maritime tropical (mT), and so on, as shown in this table.

Table 8.1 Air Mass Designations

Fronts are boundaries between air masses possessing different temperature characteristics. Other contrasts exist in the vicinity of fronts, including wind shifts, pressure troughs, moisture (dew point) differences, cloudiness, and areas of precipitation with varying characteristics. On weather charts, cold fronts and warm fronts are placed at the warm edge of the temperature transition zone. Thus, the change in temperature occurs on the colder side of the front.

The movement of the air on the cold side of any front determines the front's movement. If cold air advances, the front is called a cold front. If cold air retreats and allows warm air to advance, it is a warm front. If the cold air is neither advancing nor retreating, the front is stationary. When a cold front overtakes a warm front, an occluded front results. Occlusions are the most complex fronts to study because the weather at the ground may be caused by hidden frontal locations aloft.

Cold fronts have relatively steep slopes and commonly cause convective precipitation as cold air pushes retreating warm air aloft. In contrast, warm fronts typically have gentler slopes, which fosters development of layered clouds that often produce steady rain or snow. Stationary fronts are often associated with clouds and precipitation due to overrunning, even though there is little or no frontal motion. While fronts may remain stationary for an extended period, they eventually move or dissipate.

Frontal cyclones are large low pressure systems that form on the polar front and are major producers of clouds and precipitation in middle latitudes. The Norwegian cyclone model describes the life cycle of these storms from formation and development through occlusion and dissipation. The conveyor belt model depicts the three-dimensional motion of air streams through frontal cyclones.

The development stage is one of special interest because of its relevance to weather forecasting. Divergence and vorticity changes related to upper-level long wave and short wave flow patterns are important indicators of storm development, intensification, and dissipation.

Introduction:
We start with a look at major air mass source regions, and take a snapshot view of the properties of air masses as they existed on a sample day in late spring. We'll look at this weather situation from several directions: through plots of temperature and dewpoint, by looking at moisture as seen from a geostationary satellite, then by examining fronts, low and high pressure areas and the weather associated with these features. After this we'll focus on a summary review of the major players on the weather maps we see on television. Finally, we'll see how those features are influenced by various systems higher in the atmosphere. We will begin to assess how future weather will unfold by looking at how the three dimensional atmosphere is depicted and predicted by some of the numerical meteorological models.

Simulation08_01 (15800.0K)