Michael Pidwirny
LABORATORY 4: MID-LATITUDE CYCLONES, WEATHER MAPS, AND FORECASTING
LEARNING GOALS
In this laboratory, we will examine the process of mid-latitude cyclone development and will learn some basic techniques of weather forecasting in the middle latitudes.
Upon completion of this laboratory you will be able to:
- Understand the role of air masses, fronts, and mid-latitude cyclones in producing weather on our planet.
- Interpret present climatic conditions from synoptic weather maps.
- Use computer model forecasts to predict weather in the immediate future.
AIR MASSES AND FRONTS
An air mass is a large body of air of relatively similar temperature and humidity characteristics covering thousands of square kilometers. Typically, air masses are classified according to the characteristics of their source region or area of formation. A source region can have one of four temperature attributes: Equatorial, tropical, polar, and arctic (this terminology is at first somewhat confusing because “polar” does not in fact refer to air masses originating at the poles). Air masses are also classified as being either continental or maritime in terms of moisture characteristics. Combining these two categories, several possibilities are commonly found associated with weather in North America (Figure 4.1): maritime polar (mP), continental polar (cP), maritime tropical (mT), continental tropical (cT), and continental arctic (cA).
Frequently two air masses, especially tropical and polar, develop a sharp boundary or interface. Such an area of intensification is called a frontal zone, or more commonly, a front. At a front, warmer air will override the denser, colder air. This means that where air masses converge, the warmer air is uplifted. This uplifting causes condensation and cloud formation to occur and the possibility of precipitation along the frontal boundary. Notice in Figure 4.1 where air masses with different characteristics usually meet, and where frontal lifting might occur.
Different types of front occur; see Table 4.1 for weather conditions associated with the common ones.
Table 4.1. Generalized weather conditions associated with surface fronts.
| AS A COLD FRONT PASSES: | |
| Temperature: | decreases |
| Moisture: | absolute humidity and dewpoint decreases as front passes |
| Wind Direction: | SW winds change to NW winds |
| Air pressure: | may decrease during frontal passage, then rise afterwards |
| Clouds and Precipitation: | cumuliform clouds common; heavy showers; possible thunderstorms; then rapid clearing |
| AS A WARM FRONT PASSES: | |
| Temperature: | increases |
| Moisture: | absolute humidity and dewpoint increases as front passes |
| Wind Direction: | SE winds change to SW winds |
| Air pressure: | may decrease during frontal passage, then stabilize |
| Clouds and Precipitation: | gradual increase and lowering of stratiform cloud cover followed by extended period of steady, widespread precipitation; partial or full clearing after fron