Air mass: Definition and description

Introduction

An air mass is an air layer spread over a wide area whose temperature, humidity, and hydrostatic stability are almost equal horizontally everywhere. The variation in the distribution of solar energy results in different climates in different parts of the world. Climate change is seen with the change of seasons in almost all regions of the world. This weather change depends largely on the properties of the air mass.

• Depths of air masses can range from a few hundred meters to several hundred kilometers.
• Earth’s major air masses arise in polar and subtropical regions.
• Winds and air currents are the basic cause.
• The area of each air mass is millions of square kilometers and is movable.
• An invisible boundary is created between two different properties of air mass when they are located close to each other. This invisible wall is known as the front. 

When the air mass reaches a new region it transforms the existing weather condition by clashing with another mass that has a different temperature and humidity. Metrologic identifies them according to their regional source (1) & (2).

There are four categories

1. Arctic

2. Tropical

3. Polar

4. Equatorial

What is an air mass?

A massive air layer whose physical characteristics, especially temperature and humidity, and lapse rate are almost the same horizontally over hundreds of kilometers, is called an air mass. The theoretical idea is that it is a barotropic atmospheric state where the constant surface of air pressure is parallel to the constant surface of the air temperature. At any vertical cross-section, the isotherm and isobar are parallel to each other (3).

According to A. N. Strahler and A. H. Strahler, a layer of air between which the temperature and humidity of the rising air are equal almost everywhere over a wide area is called air mass. 

Interesting facts

1. An air mass is organized by its temperature and water vapor. 

2. It covers hundreds or thousands of miles.

3. They are classified according to latitude and their continental or marine source region.

4. Colder air masses are called polar or arctic and warmer air masses are considered tropical.

5. The north, south, and polar air masses are cold.

6. They are humid in marine and monsoon areas.

7. Equatorial air masses are usually heated.

8. The monsoon air mass is humid and unstable.

9. Continental tropical air masses are extremely hot.

10. When it moves away from its source region, its character under plants and ponds can change rapidly.

11. Continental air mass usually blows over North America over Canada. 

12. They can be changed in different ways. This kind of change can be noticed in the forest, in front of different trees (2) & (4). 

Factors

The temperature and humidity of each air mass depend on four factors. These are as follows 

1. Air mass source region

When a particular region of the earth’s surface acquires the same temperature and the same humidity that region is called the source region. The main features of the source region of the air mass are

• The source region of the air mass will be homogeneous i.e. the forest area, snow-covered land, and the topography of the land will be of the same type.
• This region will be warmer and cooler.
• The topography of this region will be the same i.e. the height, slope, relative humidity, etc. of the region will be the same. 
• Wind velocity in this region needs to be low so that the air mass can receive temperature and humidity from this region for a long time.
• Here the air below the source region must be light.
• The source region of the air mass will cover a large area.
• When air masses move from one source region to another, not only do these air masses change themselves, but they also change the climate of the region over which they flow. 

2. Air mass modification

As an air mass exits its source region, it exchanges different levels of heat and humidity with the earth’s surface during flow and in the dynamic processes of the atmosphere. So initially it was a barotropic air mass, which gradually evolved into a baroclinic. The slope of the horizontal heat means that air, like a solid, cannot circulate without any internal change.

The actual path and orbit in the upper and middle troposphere will be separated from the airflow adjacent to the surface below it. This is because the upper air is influenced by the movement of westerly winds with an increase in altitude. There are two main processes that change the air mass. These are

• Thermal dynamic modification
• mechanical modification

3. Front

Front refers to the boundary between two air masses. If two air masses with different temperatures and humidity move towards each other, then they do not merge together but separate along a sloping boundary line, this is called a front. The font gradually slopes and joins the surface. This is called a surface front. 

4. Age of air mass

After leaving the source region, heat and moisture exchange with the air mass and surface. A mixture of moving air masses occurs. As a result of the change in the original properties, it once became completely extinct.

The time that can be observed for this condition to occur is called the age of the air mass. How long it retains its original properties depends on the size, the quality of the orbit, heat, and the aqueous properties of the source region, etc. (1) & (3).

Classification

Peterson divides them into five main categories based on the geographical location of the source region of the air mass.

• Tropical
• Polar
• Equatorial
• Arctic
• Antarctic

Trewartha divides the air mass into two main classes according to the geographical location of the air mass.

• Tropical
• Polar

Air masses are also divided into six classes depending on the summer and winter seasons.

1. Continental arctic and Antarctic 

It is very cold and dry. This type of air mass is known by the symbol of cA, Caa. These air masses form in winter in the perpetually snow-covered regions of the northern and southern hemispheres. During this time the temperature drops far below freezing.

2. Continental polar 

These are cool and dry. Cp is the symbol. Here the sun sets diagonally all year round and cool breezes can be observed for the cold icy terrain. 

3. Maritime polar

They are cool, but humid with marine effects. The symbol is mP.

4. Continental tropical

It is warm and dry. cT is the symbol for this. When they move north, it causes dry summers in the Mediterranean region. 

5. Maritime tropical 

The source region of this warm and humid air mass is the oceanic region of the tropics. mT is the symbol of this air mass. 

6. Maritime equatorial

Maritime equatorial air masses are formed on water bodies in the equatorial region. They are warm and very humid. It is the meeting point of the inter-tropical convergence zone (3). 

Cause

The main cause for the formation is that the air must remain stable and calm for a long time on a wide and homogeneous landform in order to control the local characteristics of the region. 

It needs to be roughly homogenous or composed of the same substance on the surface of the earth. As a result, the surface temperature and humidity of the topography are roughly the same. The wind speed needs to be low to create air mass. As a result, the heat and humidity of the surface composed of the same substance are allowed to be carried into the air above it. 

Wind velocity is low in high-pressure regions of the earth. So the source regions of the air masses are located in these high-pressure regions. Winds and air currents are responsible for the movement of air masses (4).

Effects of air mass

1. Air masses influence the global climate zone. For example, tropical winds emanating from the warm waters of the Atlantic ocean, the Caribbean sea, and the Gulf of Mexico, initially between 10 and 3 degrees north of latitude, are major contributors to rainfall in most parts of North America, east of the Rocky Mountains.

2. When it moved vertically, the weather condition changes. This phenomenon is called convectional lifting. 

3. Marine originates from the ocean, so it creates humid weather conditions.

4. When the air moves the air mass, they move their weather conditions such as hot, cold, dry, moist, etc. from the source area to a new area. 

5. As the air mass reaches a new region, it can collide with another air mass with different temperatures and humidity. This can lead to strong storms. 

6. There are some mountains that act as barriers such as the Andes of South America, the rocky of North America, etc. When they are obstructed by these mountains, the air temperature drops. As the water vapor cools, it forms fog, and rain or snow may fall on the mountain’s windward slope (2) & (4). 

Q&A

1. What causes air masses to move?

It is created when it stays stabilized over a region for a few days or a few weeks. To move this weather pattern needs to change, and one major element of air mass movement is the upper-level winds. Cold air masses move toward the equator which is apparently southward and warm air masses move toward the poles which are also apparently northward. Coriolis force is the only cause to move them on a diagonal.

2. When two, unlike air masses, meet, what usually keeps them separate?

A front separates and acts as a boundary between these two different air masses.

3. Which is not a mechanism that causes air masses to rise?

Sublimation

4. What are air masses?

If the air of a particular place on the surface of the earth is covered by millions of square kilometers and its horizontal temperature, humidity and altitude are roughly the same, then that part of the atmosphere is called air mass. 

5. Which of the following is considered to be a boundary between two different air masses?

A front is a boundary between two different air masses. 

Written By: Manisha Bharati