Air Masses

Air masses are distinctive bodies of air that are a homogenous mix of temperature, humidity, and stability. If you have ever watched a weather report and heard the meteorologist talk about a ‘cold front’ or ‘warm front’ moving through – they are talking about an air mass!

Air masses are created when different regions on Earth’s surface impart temperature and moisture characteristics to the overlying air. The region transferring these characteristics is known as the source region.

As air masses interact with other air masses and with new topographies and regions, they produce weather patterns – for instance, by bringing the temperature and moisture characteristics of their source region to a new location.  

An air mass will initially reflect its source region characteristics. However, the longer an air mass remains over a region, the more definite its physical attributes will be come. As a result, air masses can change to reflect the temperature and moisture characteristics of new regions they pass over. For example, moist, tropical air masses moving northward carry humidity and warmth, but over time will lose those characteristics as they move into cooler, drier regions.

Air masses take on the characteristics of their source regions: maritime air masses tend to be humid and less stable as they move over oceans, while continental air masses are drier and more stable. Polar air masses are cooler while tropical air masses are warmer.

Atmospheric Lifting

Atmospheric lifting happens when air masses are lifted up into the atmosphere, resulting in cooling. (Remember how temperatures are cooler in higher altitudes?)

As the temperatures cool at higher altitudes, water vapor in the air mass can condense to form clouds and sometimes precipitation.

There are four types of atmospheric lifting: Convergent, Convectional, Orographic, and Frontal.

There are four types of atmospheric lifting: Convergent, Convectional, Orographic, and Frontal. When atmospheric lifting occurs, the air mass is lifted up into the atmosphere, resulting in cooling, condensation, cloud formation, and sometimes precipitation.

Convergent lifting

Convergent lifting happens when air is flowing from different directions to the same low-pressure area – in other words, converging in the same area. This causes air to be displaced upward.

It is important to note here that wind always moves from high pressure to low pressure areas!

Convergent lifting is very common in equatorial regions because southeast and northeast trade winds converge, forming what is known as the intertropical convergence zone (ITCZ). This makes the tropics an area of extensive convergent uplift, resulting in a lot of cloud development (particularly vertical cumulonimbus clouds, a.k.a. storm clouds) and high average annual precipitation.

The intertropical convergence zone, or ITCZ, exists at the convergence of southeast and northeast trade winds in Earth's tropical region near the equator.

Global satellite imaging shows a band of clouds in the ITCZ - this cloud formation is the result of convergent lifting where the southeast and northeast trade winds converge.

Convectional lifting

Convectional lifting happens when an air mass moves from a cooler to a warmer region. The heat from the warmer surface causes warming and lifting in the air mass – something you may remember from high school science is that heat rises! As the air mass rises, it cools, resulting in condensation and cloud development.

A classic example of convectional lifting would be clouds forming over plowed fields. The dark, bare soil of a plowed field absorbs more heat than surrounding fields with grass or other crops growing (remember albedo?). When air moves from the cooler regions with heavy vegetation to a hotter plowed field surface, convection causes the air to warm and to lift, generating cloud formation.

Orographic lifting

Orographic lifting happens when an air mass is forcibly lifted upwards as it is pushed against a mountain. (In fact, oro means ‘mountain’!) The sudden lifting of the air mass causes cooling, resulting in condensation and cloud development.

Due to the presence of the mountain, different weather conditions occur on either side. On the windward slope of the mountain – the side where the air mass meets the mountain – the air is lifted and cools, causing moisture to condense and forming precipitation. Mountains can also form a barrier, potentially keeping this weather system on its windward side.

As moist, warm air lifts due to orographic uplift, moisture in the air condenses to create clouds.

On the leeward slope of the mountain – the side opposite where the air mass meets the mountain – the air mass descends and heats as it lowers. The increased heat causes remaining moisture in the air to evaporate. This causes a rain shadow – the area on the leeward side of the mountain gets a significantly reduced amount of precipitation.

An example of a rain shadow can be seen in central Washington state. West of the Cascade Mountain Range, Washington receives a significant amount of cloud cover and precipitation. However, east of the Cascades, Washington’s climate is much hotter and drier.

Frontal lifting

The leading edge of an advancing air mass is called its front. When two air masses meet, each has different temperature, pressure, humidity, wind direction and speed, and cloud development. The leading edge of a cold air mass is a cold front, while the leading edge of a warm air mass is a warm front.

Recall that warm air rises, and cold air tends to stay closer to the ground. When two fronts meet, the air mass with warmer temperatures is forced to rise while the denser, cooler air mass stays close to the ground. This is known as frontal lifting. The abrupt lifting of the warmer, moister air of a warm air mass can cause condensation, cloud formation, and precipitation along the front edge of the front. Precipitation can be heavy accompanied by hail, lightning, and thunder. In particularly violent frontal lifting caused by a fast-moving cold front, a squall line can develop with turbulent winds, intense precipitation, and the possibility of tornado development.

After the Hydrosphere RAQ, we will be using the information from this module to learn more about midlatitude cyclonic systems – otherwise known as tropical storms, or hurricanes!