Air masses are large bodies of air that take on the characteristics of the surface below them. They play a vital role in determining the weather and climate of different parts of the world. But how exactly are these air masses formed? What factors contribute to their properties? Let’s take a closer look at the air mass formation process.

What is an Air Mass?

An air mass is a large body of air, often hundreds or even thousands of miles across, that takes on the temperature and moisture characteristics of the surface below it. Air masses are classified based on two factors:

Source Region

The source region is where the air mass originates. There are three main source regions:

  • Polar (P) – These air masses form over polar or arctic regions and are cold.
  • Tropical (T) – These air masses form over tropical oceans near the equator and are warm and moist.
  • Continental (c) – These air masses form over continental landmasses and are dry.


Air masses are further classified based on their temperature:

  • Arctic (A) – Very cold, polar air masses.
  • Polar (P) – Cool, polar air masses.
  • Tropical (T) – Warm, tropical air masses.

So an air mass like cP represents a cool, dry air mass that originated over a continental landmass.

How Do Air Masses Form?

Air masses form when air remains stationary over a surface for an extended period, usually a minimum of 5-7 days. During this time, the air mass takes on the temperature and moisture properties of the surface through the processes of heat exchange and moisture exchange.

Heat Exchange

As air sits over a surface, it exchanges heat with that surface through conduction, convection, and radiation:

  • Conduction – Direct transfer of heat between the air and the surface.
  • Convection – Transfer of heat through vertical mixing and rising of warm air.
  • Radiation – Heat transfer via electromagnetic waves like sunlight.

For example, an air mass sitting over a hot, tropical ocean will gain heat through conduction and convection from the warm water. An arctic air mass will lose heat through radiation as sunlight is reflected off snow and ice.

Moisture Exchange

Similarly, air masses exchange moisture with the surface through the processes of evaporation and transpiration:

  • Evaporation – Transfer of liquid water from the surface into water vapor in the air.
  • Transpiration – Release of moisture from plants into the air.

A tropical air mass picks up lots of moisture through evaporation from the warm ocean waters. A continental polar air mass, however, remains very dry since there is minimal moisture to evaporate over ice sheets.

Key Factors Influencing Air Masses

Several key factors influence the properties and formation process of air masses:

Source Region Geography

  • Water vs. land – Air masses over water tend to be more moist and moderate in temperature than those over land. Water has a higher heat capacity and evaporates more readily.
  • Ocean vs. continent – Continental interior locations are subject to more extreme temperatures than ocean locations at the same latitude.
  • Topography – Areas of high elevation generally are cooler than low elevation regions, influencing the air temperature.
  • Ice/snow cover – The high reflectivity of ice and snow keeps air temperatures low by reflecting incoming solar radiation.

Prevailing Winds

  • Air masses are transported around the globe by major wind belts like the trade winds and westerlies. The source region location relative to these prevailing winds impacts the properties of the air mass.
  • Air masses can also be modified during transport by passing over surfaces that alter their properties.


  • During winter, polar and arctic air masses are colder due to less solar heating over the poles.
  • The equator-to-pole temperature gradient drives global circulation patterns. Air masses exchange more vigorously during winter due to increased temperature contrasts.

Weather Fronts

  • The collision of different air masses along weather fronts tends to sharpen the contrast in properties between the air masses.
  • Fronts act as barriers to air mass movement, allowing air masses to remain stationary longer.

Urban Heat Islands

  • Cities can generate their own localized air masses as concrete and buildings absorb and re-radiate heat. This urban heat island effect causes urban areas to be warmer than rural surroundings.

Key Properties of Air Masses

The source region where an air mass forms gives it distinct properties that affect the resulting weather when transported elsewhere:


  • Air mass temperature is determined by the temperature of the underlying surface via conduction and radiation heating.
  • Arctic and polar air masses are very cold while tropical air masses are hot. Continental air masses have more extreme temperatures than marine air masses.

Moisture Content

  • Moisture content is a function of evaporation and transpiration at the source region.
  • Tropical air masses are moist while continental polar air masses are dry.


  • Stable air resists vertical motion while unstable air readily rises, influencing cloud formation and turbulence.
  • Tropical air masses are generally unstable due to high moisture content. Polar and arctic air masses tend to be very stable.

Wind Speed

  • Air mass source regions with large temperature contrasts drive stronger thermal winds.
  • Arctic and polar outbreaks bring stronger winds due to increased temperature gradients.

Tracking and Forecasting Air Masses

Meteorologists track air masses using surface observations of temperature, moisture, and wind direction/speed. They also utilize satellite imagery by noting cloud patterns and features like regions of high/low pressure.

Forecasting models simulate the movement and evolution of air masses by incorporating data on:

  • Current air mass locations and properties
  • Evolution of the equator-to-pole temperature gradient based on time of year
  • Interaction with weather fronts
  • Projected pathways based on prevailing wind patterns

By analyzing these factors, the likely progression and modification of air masses can be predicted days or weeks in advance. This allows for accurate forecasts of associated weather conditions as air masses move into new regions.

Air Masses and Weather

The combination of temperature, moisture content, stability, and wind speed gives an air mass its distinct weather characteristics:

Maritime Polar Air Masses

  • Cool, moist air from oceans
  • Generates clouds and precipitation but not severe weather

Continental Polar Air Masses

  • Cold, very dry air from polar regions
  • Can produce cold snaps and frost when it invades warmer areas

Maritime Tropical Air Masses

  • Warm, very humid air from tropical oceans
  • Generates steady rain and thunderstorms

Continental Tropical Air Masses

  • Hot, dry air from desert regions
  • Leads to major heat waves and drought conditions

Artic Air Masses

  • Extremely cold, dry air from the arctic
  • Causes bitterly cold outbreaks with dry conditions when it reaches mid-latitudes

So in essence, air masses transport the weather and climate conditions from their source regions to wherever they travel downstream around the globe. By understanding air mass behavior, meteorologists can better predict important weather events.


Air masses are large bodies of air defined by their temperature, moisture, stability, and wind speeds – characteristics acquired from the underlying surface at their source region. Key factors like prevailing winds, geographic location, time of year, weather fronts, and urban landscapes all influence the formation and properties of air masses. Careful tracking and analysis of air masses allows meteorologists to generate accurate weather forecasts for areas expecting an influx of arctic, polar, tropical, or other air mass types. So the next time you experience an unusual cold snap or heat wave, it’s likely courtesy of a big mass of air that originated far upstream!

5 Frequently Asked Questions about Air Masses

What are the 3 main types of air masses?

The 3 main air mass types are polar, tropical, and continental:

  • Polar air masses form over polar and arctic regions and are cold.
  • Tropical air masses form over warm tropical oceans near the equator and are warm and moist.
  • Continental air masses form over large land areas and are dry with extreme temperatures.

How long does it take for an air mass to form?

Air masses require an extended period of time, usually 5-7 days or more, stationed over a surface to acquire the temperature and moisture properties of that surface. This time is needed for thorough heat and moisture exchange via conduction, radiation, evaporation, etc.

What causes an air mass to move from its source region?

Air masses are mobilized by major wind belts like the prevailing westerlies and trade winds. The pole to equator temperature gradient also drives circulation that pushes cold and warm air masses toward the equator and poles respectively.

What happens when two different air masses meet?

When two different air masses collide, it creates a weather front. This boundary between hot and cold air masses brings significant weather changes like precipitation and thunderstorms. The temperature contrast can also strengthen winds.

How do air masses impact the weather?

Air masses bring the weather conditions from their source regions to wherever they move downstream. For example, arctic air outbreaks transport bitterly cold dry air southward, while tropical air masses bring heat, humidity, and rainfall. Tracking air masses allows for better weather prediction.