Lesson Objectives

• Describe the characteristics of air masses and how they get those characteristics.
• Discuss what happens when air masses meet.
• List the differences between stationary, cold, warm, and occluded fronts.

Vocabulary

• air mass
• cold front
• front
• occluded front
• squall line
• stationary front
• warm front

Introduction

The weather in a location often depends on what type of air mass is over it. Another key factor is whether the spot is beneath a front, the meeting place of two air masses. The characteristics of the air masses and their interactions determine whether the weather over an area is constant, or whether there are rapid changes.

Air Masses

An air mass is a batch of air that has nearly the same temperature and humidity (Figure below). An air mass acquires these characteristics above an area of land or water known as its source region. When the air mass sits over a region for several days, or longer, it picks up the distinct temperature and humidity characteristics of that region.

Air Mass Formation

Air masses form over a large area; they can be 1,600 km (1,000 miles) across and several kilometers thick. Air masses form primarily in high pressure zones, most commonly in polar and tropical regions. Temperate zones are ordinarily too unstable for air masses to form. Instead, air masses move across temperate zones so the middle latitudes are prone to having interesting weather.

What does an air mass with the symbol cPk mean? The symbol cPk is an air mass with a continental polar source region that is colder than the region it is now moving over.

Air Mass Movement

Air masses are slowly pushed along by high-level winds. When an air mass moves over a new region, it shares its temperature and humidity with that region. So the temperature and humidity of a particular location depends partly on the characteristics of the air mass that sits over it.

Storms arise if the air mass and the region it moves over have different characteristics. For example, when a colder air mass moves over warmer ground, the bottom layer of air is heated. That air rises, forming clouds, rain, and sometimes thunderstorms. How would a moving air mass form an inversion? When a warmer air mass travels over colder ground, the bottom layer of air cools and, because of its high density, is trapped near the ground.

In general, cold air masses tend to flow toward the equator and warm air masses tend to flow toward the poles. This brings heat to cold areas and cools down areas that are warm. It is one of the many processes that act towards balancing out the planet’s temperatures.

An online guide from the University of Illinois about air masses and fronts is found here: http://ww2010.atmos.uiuc.edu/%28Gh%29/guides/mtr/af/home.rxml.

Fronts

Two air masses meet at a front. At a front, the two air masses have different densities and do not easily mix. One air mass is lifted above the other, creating a low pressure zone. If the lifted air is moist, there will be condensation and precipitation. Winds are common at a front. The greater the temperature difference between the two air masses, the stronger the winds will be. Fronts are the main cause of stormy weather.

The rest of this section will be devoted to four types of fronts. Three of these fronts move and one is stationary. With cold fronts and warm fronts, the air mass at the leading edge of the front gives the front its name. In other words, a cold front is right at the leading edge of moving cold air and a warm front marks the leading edge of moving warm air.

Stationary Front

At a stationary front the air masses do not move (Figure below). A front may become stationary if an air mass is stopped by a barrier, such as a mountain range.

A stationary front may bring days of rain, drizzle, and fog. Winds usually blow parallel to the front, but in opposite directions. After several days, the front will likely break apart.

Cold Fronts

When a cold air mass takes the place of a warm air mass, there is a cold front (Figure below).

Imagine that you are standing in one spot as a cold front approaches. Along the cold front, the denser, cold air pushes up the warm air, causing the air pressure to decrease (Figure above). If the humidity is high enough, some types of cumulus clouds will grow. High in the atmosphere, winds blow ice crystals from the tops of these clouds to create cirrostratus and cirrus clouds. At the front, there will be a line of rain showers, snow showers, or thunderstorms with blustery winds (Figure below). A squall line is a line of severe thunderstorms that forms along a cold front. Behind the front is the cold air mass. This mass is drier so precipitation stops. The weather may be cold and clear or only partly cloudy. Winds may continue to blow into the low pressure zone at the front.

The weather at a cold front varies with the season.

• spring and summer: The air is unstable so thunderstorms or tornadoes may form.
• spring: If the temperature gradient is high, strong winds blow.
• autumn: Strong rains fall over a large area.
• winter: The cold air mass is likely to have formed in the frigid arctic so there are frigid temperatures and heavy snows.

Warm Fronts

At a warm front, a warm air mass slides over a cold air mass (Figure below). When warm, less dense air moves over the colder, denser air, the atmosphere is relatively stable.

Imagine that you are on the ground in the wintertime under a cold winter air mass with a warm front approaching. The transition from cold air to warm air takes place over a long distance so the first signs of changing weather appear long before the front is actually over you. Initially, the air is cold: the cold air mass is above you and the warm air mass is above it. High cirrus clouds mark the transition from one air mass to the other.

Over time, cirrus clouds become thicker and cirrostratus clouds form. As the front approaches, altocumulus and altostratus clouds appear and the sky turns gray. Since it is winter, snowflakes fall. The clouds thicken and nimbostratus clouds form. Snowfall increases. Winds grow stronger as the low pressure approaches. As the front gets closer, the cold air mass is just above you but the warm air mass is not too far above that. The weather worsens. As the warm air mass approaches, temperatures rise and snow turns to sleet and freezing rain. Warm and cold air mix at the front, leading to the formation of stratus clouds and fog (Figure below).

Occluded Front

An occluded front usually forms around a low pressure system (Figure below). The occlusion starts when a cold front catches up to a warm front. The air masses, in order from front to back, are cold, warm, and then cold again.

Coriolis Effect curves the boundary where the two fronts meet towards the pole. If the air mass that arrives third is colder than either of the first two air masses, that air mass slip beneath them both. This is called a cold occlusion. If the air mass that arrives third is warm, that air mass rides over the other air mass. This is called a warm occlusion (Figure below).

The weather at an occluded front is especially fierce right at the occlusion. Precipitation and shifting winds are typical. The Pacific Coast has frequent occluded fronts.

Weather is explored in this video at National Geographic Video, Natural disaster, Landslides, and more: Weather 101

Lesson Summary

• An air mass takes on the temperature and humidity characteristics of the location where it originates. Air masses meet at a front.
• Stationary fronts become trapped in place; the weather they bring lasts for many days.
• At a cold front, a cold air mass forces a warm air mass upwards.
• At a warm front, the warm air mass slips above the cold air mass.
• In an occluded front, a warm front overtakes a cold front, which creates variable weather.

Review Questions

1. What type of air mass is created if a batch of air sits over the equatorial Pacific Ocean for a few days? What is the symbol for this type of air mass?

2. What conditions must be present for air to sit over a location long enough to acquire the characteristics of the land or water beneath it?

3. How does latitude affect the creation of air masses in tropical, temperate, and polar zones?

4. Why are the directions fronts move in the Southern Hemisphere a mirror image of the directions they move in the Northern Hemisphere?

5. How is a stationary front different from a cold or warm front?

6. What sorts of weather will you experience as a cold front passes over you?

7. What sorts of weather will you experience as a warm front passes over you?

8. How does an occlusion form?

9. What situation creates a cold occlusion and what creates a warm occlusion?

Further Reading / Supplemental Links

• Cold Front animation, Goddard Space Flight Center: http://svs.gsfc.nasa.gov/vis/a000000/a002200/a002203/index.html
• The University of Illinois online guide to weather basics: http://ww2010.atmos.uiuc.edu/%28Gl%29/guides/mtr/af/frnts/wfrnt/prcp.rxml

Points to Consider

• How do the various types of fronts lead to different types of weather?
• Why are some regions prone to certain types of weather fronts and other regions prone to other types of weather fronts?
• Why does the weather sometimes change so rapidly and sometimes remain very similar for many days?