SCIN 137 AMU week 4 lesson ATMOSPHERIC CIRCULATIONS & AIR MASSES, FRONTS, AND MIDDLE-LATITUDE CYCLONES Introduction to Meteorology American Military university
- Lesson Overview
This week we investigate atmospheric circulation. This is one of my favorite topics in Meteorology because it is one that most people have never seen before, and it’s quite interesting. We all are familiar with “weather” and many people are familiar with “climate”, but to most people a “Hadley Cell” is unknown, as is the ITCZ, and reasons why monsoons, sea breezes or Santa Ana’s flow. I hope you enjoy it! You’ll learn why the trade winds flow as they do and why in the U.S. the predominant flow is the Westerlies……
Students will be able to:LO-26. Describe the global general circulation of the atmosphere. LO-27. Explain why the general circulation produces large scale precipitation patterns. LO-28. Discuss deviations from the general circulation and the effects of these deviations. LO-29. Identify various kinds of air masses and their sources. LO-30. Describe the various kinds of fronts and their characteristics. LO-31. Explain the Polar Front Theory. LO-32. Discuss middle latitude cyclones and explain how they are different from tropical cyclones.The following activities and assessments need to be completed this week:
- Read Barry and Chorley: Chapters 6 and 7
- Week 4 Lesson
- Week 4 Forum
- Week 4 Lab
- Week 4 Quiz
- COMET Modules (Optional):
- Thermally-forced Circulation I: Sea Breezes
- Jet Streams
In this lesson we will focus on atmospheric circulations as well as air masses and fronts. We will examine winds on different scales, from local to global. You will learn how the ocean influences the weather. Large masses of air can move as a unit and form fronts when different types of air masses meet. Middle-latitude cyclones can be one of the consequences of two fronts colliding. Topics to be covered include:
- Local wind systems
- Global winds
- Atmospheric-ocean interactions
- Air masses
- Middle-latitude cyclones
When summer beachgoers drive to the Jersey Shore they often pass through a rainstorm around 20 miles from the beach. Soon they pass out of the storm and enjoy clear weather at the beach. On the way home later that afternoon, they pass through a rainstorm again near the same location. Why is this common? The answer is the sea breeze.
In an earlier lesson you learned how the sun warms the land in the daytime, which in turn warms the air close to the surface. Warmer air is less dense than colder air. The air over the oceans is cooler and denser as water has a higher heat capacity making it slower to warm. This cooler, moister air from over the ocean moves into the less dense air over the warmer land, forcing the warm air up. As the warm air rises, it may reach cooler surroundings that result in condensation and precipitation.
At night this process reverses as the land cools more quickly than the water. The land breeze now moves along the surface from the land to the ocean. There are many other types of local winds that you can read about in your textbook.
Local winds can vary greatly from one season to another or even from daytime to nighttime. These winds are swirls and eddies within the larger air currents. The rotating high and low pressure zones you see on weather maps are swirls and eddies within the global river of air called the general circulation of the atmosphere. Remember that everything we say about weather in general refers to trends, so here we are talking about the average wind flow over the long term.
Besides the pressure gradient force and the Coriolis force, winds are also influenced by temperature gradients, which results in the complex patterns shown here. Remember from an earlier lesson how the total incoming solar energy is balanced by the total outgoing solar energy, but that energy is not equally spread out on the planet. The poles are colder than the tropics. As you might expect, the cold air of the poles tends to flow towards the warmer regions and the warm air at the equator moves towards the cooler poles. The global wind belts are created due to global long term pressure systems: Polar High (90 degrees) Subpolar Low (60 degrees) Subtropical High (30 degrees) and Equatorial Lows (Equator). In particular the Prevailing Westerlies (both north and south of the equator) are created from air flowing from the subtropical highs at 30 degrees toward the subpolar lows at 60 degrees north or south.
Air Masses and Source Regions
An air mass is a very large body of air that can cover up to thousands of square kilometers. Its temperature and humidity are fairly uniform across any given height above the ground. In the map you can see a large high-pressure air mass covering much of the central and eastern areas. The temperatures and humidity would vary somewhat in that very large area, they would be similar. Predicting where and when such masses form and move is a major part of weather forecasting.
The characteristics of an air mass are acquired in the source region, which is the surface area over which the air mass originates. The ideal source region has a uniform surface (all land or all water), a uniform temperature, and is an area in which air stagnates to form high-pressure systems. The properties (temperature and moisture content) that an air mass acquires in its source region are dependent upon a number of factors. These include the time of year (winter or summer), the nature of the underlying surface (whether land, water, or ice covered), and the length of time it remains over its source region.
Air Mass Designations
Air masses are categorized based on their humidity (humid or dry), temperature (cold or warm), source region (P for polar, T for tropical regions). If they form over land, it will be dry and given a letter “c” for continental. If they form over water, it will be moist and given the letter “m” for maritime. Where do you think an air mass designated cP originated? Over land in a polar region. In the winter, we designate the very cold air mass as cA for continental arctic.
Select a hot spot to learn more.
Maritime Tropical Air Masses
Marine Tropical Air in Winter
Maritime tropical air from the Gulf of Mexico often moves into the land east of the Rockies. In the winter, the cold arctic and polar air dominates, but occasionally maritime tropical air can move into the plains in winter. Because the incoming air is very moist and warmer than the surface, low clouds and fog often form. This usually only lasts a few days in winter before another cold air mass comes in from the north. When the two air masses meet, the warm air gets pushed up commonly causing heavy precipitation and storms.
Marine Tropical Air in Summer
In the summer the warm, moist air from the Bermuda High in the Gulf and eastern Atlantic Ocean can make the Great Plains and Midwest very hot and humid place. Thunderstorms are nearly daily occurrences along the Gulf Coast in the summer. A persistent weak flow of upper-level anticyclone air can even push this moist air all the way to the Rockies to cause afternoon thunderstorms, but the rain sometimes evaporates before it reaches the ground. You can watch this short video on the Bermuda High.