Fronts: Moving Air Masses

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Author: Janice VanCleave

Masses of air that stay in place for a length of time take on the characteristics of temperature and humidity of the region of the Earth beneath them. The temperature and humidity differences between adjoining regions result in movement of air masses and changes in local weather.

In this project, you will learn about air masses and demonstrate how differences in their density cause warm and cold fronts. You will learn how source regions give rise to the names of air masses and how these names can be combined to describe the humidity and temperature of the source region. You will discover how symbols on weather maps indicate movement of fronts and how clouds can be used to identify fronts. You will study dew point and how this temperature is affected by fronts, and its effect on cloud formation.

Getting Started

Purpose: To model a warm front.


  • 1-cup (250-ml) measuring cup
  • tap water
  • blue food coloring
  • spoon
  • one 20-ounce (600-ml) clear plastic bottle
  • 1 cup (250 ml) of liquid cooking oil


  1. Fill the measuring cup with water.
  2. Add three drops of food coloring to the water and stir.
  3. Pour the water into the bottle.
  4. Fill the measuring cup with oil.
  5. Tilt the bottle and slowly pour the oil into the bottle (see Figure 27.1).
  6. Observe the movement of the oil into the bottle.
  7. Fronts: Moving Air Masses


The oil moves across the top of the blue water.


An air mass is a large body of air with about the same temperature and humidity throughout. Air masses form when air stays over a region long enough to take on the temperature and humidity characteristics of that region. It takes a week or more for an air mass to form.

The density of air masses varies with the temperature and humidity of the air. Warm air masses are less dense than cold air masses, and humid air masses are less dense than dry air masses. When air masses with different densities meet, the two masses do not mix. As with oil and water, a distinct boundary forms between the air masses. In the experiment, the oil represented a warm air mass and the colored water a cold air mass. As with the oil and water, warm, less dense air moves over cold, denser air.

Vilhelm Bjerknes (1862-1951), a Norwegian physicist and meteorologist, coined the term front to describe the boundary between warm and cold air masses. The leading edge of a warm air mass advancing into a region occupied by a cold air mass is called a warm front. A cold front occurs when a cold air mass advances into a region occupied by a warm air mass. If the boundary between the cold and warm air masses doesn't move, it is called a stationary front. The boundary where a cold air mass meets a cool air mass under a warm air mass is called an occluded front. At a front, the weather is usually unsettled and stormy, and precipitation is common.

Try New Approaches

  1. Model a cold front produced by the movement of a cold air mass into a region occupied by a warm air mass. Do this by repeating the experiment, but place the oil in the bottle first, then slowly pour in the colored water.
  2. Does the volume of the air masses affect the results? Repeat the original experiment twice, first using 11/2 cups (375 ml) of water and 1/2 cup (125 ml) of oil, then using 1/2 cup (125 ml) of water and 11/2 cups (375 ml) of oil.
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