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6.2: Air Masses and Weather

In chapter 3, we discussed how warm air had more energy, and therefore formed areas of low pressure in the atmosphere, while cold air was more dense and formed areas of high pressure in the atmosphere. In chapter 4, we discussed atmospheric and oceanic circulation and global temperatures. In the previous lesson, we talked about the water cycle and water resources on earth. In this unit, we are going to bring together these discussions about the atmosphere/hydrosphere interactions in our exploration of weather and global climate.

First, let's get some very important definitions out of the way. Weather and climate are not the same thing. Weather is the short-term, day to day condition of the atmosphere, while climate is the long term (over decades) average of weather conditions in a region. Climate change can only be documented by noting a rise the average annual global temperature over many years. Another way to think about is that weather is a series of short-term events, while climate is a long-term trend. A person who studies and predicts the weather is called a meteorologist. A person who studies long-term trends in weather and how climate changes over many years is called a climatologist.

Air masses are classified based on their moisture and temperature. Air masses pick up moisture when they are over bodies of water, and they loose that moisture when they are over land. Therefore, wet air masses are designated m for maritime, and dry air masses are designated c for continental. The temperature of an air mass is designated as follows: A for arctic, P for polar (high latitude, but not as high as arctic), T for tropical, E for equatorial and AA for Antarctic.

The following air masses affect the United States. Write down their abbreviations in the parenthesis below:

  • Maritime Polar (___) affects the north west and north east coasts all year.

  • Continental Polar (___) affects the north central USA in the summer and more severely in the winter.

  • Maritime Tropical (___) affects the south west and south east coasts all year.

  • Continental Tropical (___) affects the south central US during the summer.

Clouds and violent weather occur when air masses collide. When a cold mass pushes under a warm air mass, the warm air is cooled adibatically (recall the average lapse rate of 6.4 degrees C per kilometer from lecture 3). The moist warm air quickly reaches its dew point and clouds form. If you watch the weather report on the evening news, you will note that the cold fronts are denoted by lines with triangles pointing in the direction that the front is advancing. Warm fronts are denoted by lines with half-circles pointing in the direction that the front is advancing.

Another important atmospheric lifting mechanism is orographic lifting. Orographic lifting occurs when air is forced up to high altitudes as it is pushed up a mountain front. The air cools adibatically, and clouds form, and precipitation often results. The windward side of the mountain, therefore, receives much more precipitation than the leeward side. This is known as the ‘rainshadow effect’. One example of orographic lifting is the Sierra Nevada mountains. Moist air blows off of the Pacific ocean and is forcibly lifted upslope. This results in significant rain and snow on the western side of the Sierras (Tahoe, for example), and very little of the precipitation makes it over to the east side of the Sierras. This results in a dry desert on the eastern slope. If you have ever driven over highway 80 to Reno, or highway 120 past Yosemite, you will have seen this transformation first hand. The western side of the Sierras boast lush forests, while the eastern slope is a dry, sage brush desert.

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