20 Air Masses and Fronts

M K Nanda

1. Learning outcomes
2. Introduction
2.1. Source of air mass
2.2. Conservation of characteristics of an air mass
2.3. Classification and nomenclature of air mass
2.4. Examples of some typical types of air mass
3. Impact of air mass
4. Modification of air mass
5. Front
5.1. What is front
5.2. Frontogenesis and frontolysis
5.3. Classification of front
5.3.1. Cold front
5.3.2. Warm front
5.3.3. Stationary front
5.3.4. Occluded front
6. Weather associated with different types of fronts
7. Summary

1. Learning outcomes

• After studying this module, you shall be able to:
• know about the air mass and its origin
• know about the conventional nomenclature system of air mass know about the front and how it forms
• differentiate among different types of front get an idea about the air mass, front and their impact on weather in general

2. Introduction

An air mass is a body of air with fairly homogenous characteristics extending over a large area, spreading over more than 1000 kilometers across the surface of the Earth. The depth may extend to few kilometers and can reach from ground level to the height of the troposphere.

When a large body of air remains over a specific area for a long period, the temperature and humidity characteristics of the air is influenced by the underlying surface and an air mass forms. The air mass is characterized by the properties of temperature, moisture (humidity), and lapse rate which remain fairly homogeneous throughout the air mass. Horizontal changes of these properties are usually very gradual.

Air masses form an integral part of the global planetary wind system. Therefore, they are associated with one or other wind belt. When two air masses of different density and temperature meet each other, the transition region is characterized by discernible gradient of different atmospheric properties. These transition zones are called as fronts which are the harbinger of major weather changes on the surface of the earth.

2.1. Source of air mass

Air masses form over large surfaces with uniform temperatures and humidity, called source regions. In these regions the surface terrain varies little. Usually, the high pressure condition prevails over the region that stagnates for several days. During this time, the air mass takes on characteristics of the underlying surface. Low wind speeds let air remain stationary long enough to take on the features of the source region, such as heat or cold. The typical features of the source region are –

• The surface is fairly homogeneous. The homogenous regions can be the vast ocean surface or vast plains and plateaus.
• The source region should be extensive with gentle, divergent air circulation (slightly at high pressure). Areas with high pressure (anticyclone regions), but little pressure difference or pressure gradient are ideal source regions.
• The main source regions are the high pressure belts in the sub tropics (for tropical air masses) and around the poles (for polar air masses). There are no major source regions in the  mid-latitudes as these regions are dominated by cyclonic and other disturbances. The regions of convergence are not ideal source regions. Because the air is not stagnant and there is a tendency of upward movement of air.
• There are five major source regions. They are – a) Warm tropical and subtropical oceans, b) subtropical hot deserts, c) Relatively cold high latitude oceans, d) Very cold snow covered continents in high latitudes, e) Permanently ice covered continents in the Arctic and Antarctica.

2.2. Conservation of characteristics of an air mass

The source region establishes heat and moisture equilibrium with the overlying air mass. When an air mass moves away from a source region, the upper level maintains the physical characteristics for a longer period. This is possible because air masses are stable with stagnant air which do not facilitate convection. Conduction and radiation in such stagnant air is not effective.

Air masses are generally identified according to their source region. Basically there are four categories for air masses: arctic, tropical, polar and equatorial. Arctic (or Antarctic) air masses form in the Arctic (Antarctic) region and are very cold. Tropical air masses form in low-latitude areas and are moderately warm. Polar air masses take shape in high-latitude regions and are cold. Equatorial air masses develop near the Equator, and are warm. Air masses are also identified based on whether they form over land or over water. Maritime air masses form over water and are humid. Continental air masses form over land and are dry.

2.3. Classification and nomenclature of air mass

Air masses are classified on weather maps using two or three letters. First letter: A lowercase letter describes the amount of moisture in the air mass which is controlled by their source region: m — maritime air mass (moist) that forms over oceanic region c — continental (dry) that forms over the land mass Second letter: An uppercase letter describes the geographic origin (source region) of the air mass:

E —    Equatorial,

T —      Tropical,

P —      Polar,

A —     Arctic or Antarctic

• S — Superior—a unique situation with dry air formed by a powerful downward motion in the atmosphere, where adiabatic drying and warming of air mass takes place.

Third letter: A lowercase letter describes the Thermodynamic process or the relative warmth or coldness of the air mass.

k —    the air mass is colder than the ground below it

w–    the air mass is warmer than the ground below it

Sometimes, and with some air masses, the thermodynamic classification may change from night to day. A particular air mass may show k characteristics during the day and w characteristics at night and vice versa. Very often air masses are identified by the type of clouds within them. Cold air masses usually show cumuliform clouds, whereas warm air masses contain stratiform clouds.

2.4. Examples of some typical types of air mass

Some of the typical types of air mass as per the classification are given below

• cAk : Continental arctic air that is colder than the surface over which it lies. mAk Maritime arctic air that is colder than the surface over which it lies.
• cPk : Continental polar air that is colder than the surface over which it is moving.
• mPk : Maritime polar air that is colder than the surface over which it is moving.
• mTw : Maritime tropical air that is warmer than the surface over which it is moving.
• cTw : Continental tropical air that is warmer than the surface over which it is moving.
• Ek : Maritime equatorial air that is colder than the surface over which it is moving. Ew Maritime equatorial air that is warmer than the surface over which it is moving.

However, in most cases air mass is represented by its first two letter and sometimes only with the middle letter only. Some of the popular types of air masses with their source regions are given in figure- 1.

3. Impact of air mass

• Air masses have very vital role in regulating the atmospheric energy balance and also in influencing world weather.
• The air masses carry atmospheric moisture from oceans to continents and cause precipitation over land masses.
• They transport latent heat, thus modifying the latitudinal heat balance that is developed by differential insolation.
• Most of the migratory atmospheric disturbances such as cyclones and storms originate at the Frontal zone between different air masses and the weather associated with these disturbances is determined by characteristics of the air masses involved.

Each type of air mass has a typical weather condition associated with it.A continental polar air mass (cP) originated from the Arctic basin, Eurasia or Antarctica are characterized by dry, cold and stable conditions. The weather during winter is frigid, clear and stable. On the other hand, maritime polar air masses (mP) from oceanic source region are cool, moist and unstable. The weather during winters with such air mass is characterized by high humidity, overcast skies and occasional fog and precipitation. However, during summer cP air mass has less stable weather with lesser prevalence of anticyclonic winds and lesser snow whereas, with mP air mass the weather is clear, fair and stable.

Similarly, continental tropical air masses (cT) originated from tropical and sub-tropical deserts of Sahara in Africa, and of West Asia and Australia are dry, hot and stable throughout the year. On the other hand, the maritime tropical air masses (mT) from oceanic origin (Mexican Gulf, the Pacific and the Atlantic oceans) are warm, humid and unstable. The weather during winter has mild temperatures, overcast skies with fog. During summer, the weather is characterized by high temperatures, high humidity, cumulous clouds and convectional rainfall.

4. Air mass modification

When an air mass moves out of its source region, its properties may change remarkably as influenced by the characteristics of the underlying surface, the trajectory of the motion and the time factor, all of them acting in combination. The important modifying factors are discussed below:

Temperature of the underlying surface (over which the air mass moves) modifies the temperature and stability of the air mass. For example, the warm tropical air mass when moves over a colder surface, the cold surface cools the lower layers of the air mass and increases the stability. This stability extends to the upper layers in time, and condensation in the form of fog or low stratus cloud normally occurs. Similarly, when the continental polar air mass moves over a warmer surface the instability increases and that instability consequently spreads to upper layers.

Moisture of the underlying surface modifies the air mass moisture content as a result of evaporation (addition of moisture) or condensation and precipitation (removal of moisture). For example, the passage of cold air over a warm water surface decreases the stability of the air with resultant vertical currents. The passage of warm, moist air over a cold continental surface increases the stability and could result in fog as the air is cooled and moisture is added by evaporation.

Topography of surface also modifies the air mass. On the windward side of the mountain there is removal of moisture through precipitation with a decrease in stability. On the leeward side as the air descends the stability increases and the air becomes warmer and drier.

Trajectory (whether cyclonic or anticyclonic) of an air mass after it has left its source region affects its stability. In case of cyclonic trajectory, the air mass stability in the upper levels is decreased as a reflection of cyclonic relative vorticity. The stability of the lower layers is not greatly affected by this process. On the other hand, if the trajectory is anticyclonic, its stability in the upper levels is increased as a result of subsidence associated with anticyclonic relative vorticity.

When air masses move along the winds, they carry their weather conditions (heat or cold, dry or moist) from the source region to a new region. When the air mass reaches a new region, it might clash with another air mass that has a different temperature and humidity. This can create a severe storm.

5. Front

5.1. What is front?

Front is a zone of transition between two air masses of different density and temperature and is associated with major weather changes. In reality, the point at which two air masses touch is not a well defined, abrupt separation. It rather extends in both horizontal and vertical direction. As a transition region between two adjacent air masses of different densities, a frontal zone is characterized by significant density gradient. A frontal zone is bounded by a frontal surface. The frontal surface is the surface that separates two air masses. It is the surface next to the warmer air (less dense air). Since the temperature distribution is the most important regulator of atmospheric density, a front almost invariably separates air masses of different temperatures.

5.2. Frontogenesis and frontolysis

When new fronts are created or old fronts are regenerated, it is called frontogenesis. It is important to note that fronts don’t appear all of a sudden. The process of frontogenesis is rather a slow and gradual process. When winds converge towards a point it would lead to frontogenesis. Frontogenesis takes place only when two conditions are met. First, two air masses of different densities must exist adjacent to one another; and second, a prevailing wind field must exist to bring them together. Frontogenesis is likely to occur when the wind blows in such a way that the isotherms become packed along the leading edge of the intruding air mass. The wind flow is cross isothermal and flowing from cold air to warmer air.

On the other hand, the dying of a front is called Frontolysis. Like frontogenesis, this process also does not happen all of a sudden. Frontolysis must happen for quite some time to destroy the existing front. The divergence of the wind from a point or dilation from a line is helpful to the process of frontolysis. Therefore, frontolysis is likely to occur when fronts move into regions of divergent air flow. That is why on crossing the subtropical high-pressure regions, the fronts generally disappear.

5.3. Classification of front

A front is classified by determining the instantaneous movement. The direction of movement of the front for the past 3 to 6 hours is often used. Classification is based on movement relative to the warm and cold air masses involved. The different type of fronts is as follows.

5.3.1.   Cold front

A cold front is defined as the transition zone where a cold air mass is replacing a warmer air mass. The leading edge of the cold airmass typically has a shape like a dome. Cold fronts generally move from northwest to southeast (in north hemisphere). The air behind a cold front is noticeably colder and drier than the air ahead of it. When a cold front passes through, temperatures can drop more than 15 degrees within the first hour. There is typically a noticeable temperature change from one side of a cold front to the other. An abrupt temperature change over a short distance is a good indicator that a front is located somewhere in between. The typical features of cold front are summarized as – Cold air is advancing with time, replacing warm air in the frontal zone.

• Cold fronts form a steep slope, typically 1:100.
• Frontal surface is located at the leading edge of the strong temperature gradient.
• Frontal zone is characterized with a strong dewpoint gradient and a pressure trough. The pressure drops as the front approaches and rises after it passes.
• A sharp wind shift, generally occurs from winds with southerly component to winds with westerly or northwesterly component, will occur at front. Northwesterly winds are generally observed behind a cold front, and southwesterly in ahead of the front.
• Cold fronts are indicative of heavy precipitation events, rainfall or snow, combined with rapid temperature drops. Precipitation may be behind, along, or ahead of front (or may not be present) but when present, it will be organized along a line.
• They move faster than the warm front, up to 30 mph i.e. about 50 km hr-1.

Symbolically, a cold front is represented by a solid line with triangles along the front pointing towards the warmer air and in the direction of movement. On colored weather maps, a cold front is drawn with a solid blue line.

5.3.2.   Warm front

A warm front is defined as the transition zone where a warm air mass is replacing a cold air mass. Warm fronts generally move from southwest to northeast and the air behind a warm front is warmer and more moist than the air ahead of it. When a warm front passes through, the air becomes noticeably warmer and more humid than it was before. The warm front is usually much slower than the cold front. The typical feature of warm fronts are summarized as –

• Warm air displaces colder air.
• Warm front forms a light slope, typically (1:200).
• Warm front moves slower (than the cold front), average velocity is about 20 km hr-1.
• Normally, shallow horizontal stratus clouds and light precipitation occurs. Frontal fogs may occur as falling raindrops evaporate in the colder air near the surface. Sleet and freezing rain may also be formed.
• The intensity of precipitation depends on the stability of the warm air. If the warm air is conditionally unstable, thunderstorms may develop over the warm front. If the warm air is stable, the clouds will be layered.
• If the temperature in the cold air is freezing or below freezing, ice pellets, snow or freezing rain may occur.
• Symbolically, a warm front is represented by a solid line with semicircles pointing towards the colder air and in the direction of movement. On colored weather maps, a warm front is drawn with a solid red line.

5.3.3.   Stationary front

Fronts are sometimes stationary. Stationary front occurs between a pair of air masses (warm and cold) when neither of air mass is strong enough to replace the other. They tend to remain essentially in the same area for extended periods of time, and waves sometimes propagate along the frontal boundary. Although the boundary is stationary, air on both sides of the boundary can be moving. With a stationary front, air on the cold side of the front will always be flowing parallel to the front.

A wide variety of weather can be found along a stationary front, but usually clouds, prolonged precipitation, and storm trains are found there. “Storm train” refers to storm moving over a region in a relatively short period of time, and often causing severe rain and flash flood. When there is a lot of water vapor in the warmer air mass, significant amounts of rain or freezing rain can occur.

The typical feature of stationary fronts are summarized as –

• In stationary front two non similar air masses remain side by side, with neither encroaching upon the other.
• The cold air on the north side of the front is moving parallel to the front, while the warm air to the south moving toward the front and get lifted along the frontal boundary.
• If the warm air is conditionally unstable, a line of showers and thunderstorms may develop in the warm side of the front.
• If the warm air is stable, widespread layered clouds may form over the front, with rain falling on the cold side of the front.

On a weather map, this is shown by an inter-playing series of blue spikes pointing one direction and red domes pointing the other.

5.3.4.   Occluded front

If a cold front catches up to and overtakes a warm front, the frontal boundary created between the two air masses is called an occluded front, or, simply, an occlusion (meaning “closed off”).

Occluded fronts usually form around mature low pressure areas during the process of cyclogenesis when the cold front overtakes a warm front. When this occurs, the warm air is separated (occluded) from the cyclone center at the Earth’s surface.

A developing cyclone typically has a preceding warm front (the leading edge of a warm moist air mass) and a faster moving cold front (the leading edge of a colder drier air mass wrapping around the storm). North of the warm front is a mass of cooler air that was in place before the storm even entered the region. As the storm intensifies, the cold front rotates around the storm and catches the warm front. Thus forms an occluded front, which is the boundary that separates the new cold air mass (to the west) from the older cool air mass already in place north of the warm front. Symbolically, an occluded front is represented by a solid line with alternating triangles and circles pointing to the direction in which the front is moving. On colored weather maps, an occluded front is drawn with a solid purple line.

Changes in temperature, dew point temperature, and wind direction can occur with the passage of an occluded front. In the map below, temperatures ahead (east of) the front were reported in the low 40’s while temperatures behind (west of) the front were in the 20’s and 30’s. The lower dew point temperatures behind the front indicate the presence of drier air. A noticeable wind shift also occurs across the occluded front. East of the front, winds were reported from the east-southeast while behind the front, winds were from the west-southwest.

Occluded front is of two types: cold occluded and warm occluded.

Cold occluded: When the air behind the warm front is colder than the air ahead of it this is known as a cold-type occluded front, or cold occlusion. Here the cold air mass underrides and lifts off the ground both the warm front and the warm air mass. The frontal passage brings weather similar to that of a cold front: heavy, often showery precipitation with winds shifting to west or northwest.

Warm occluded: When the air ahead of the warm front is colder than the air behind the cold front, the milder, lighter air behind the cold front is unable to lift the colder, heavier air off the ground. As a result, the cold front rides “piggyback” along the sloping warm front. This produces a warm-type occluded front, or a warm occlusion. The surface weather associated with a warm occlusion is similar to that of a warm front.

Fig. 5: Structure of warm and cold occluded fronts

6. Weather associated with different types of fronts

Fronts are associated with development of typical weather conditions. The weather feature, characteristic of the particular front type is briefly described in table-1.

Table 1: Weather associated with different types of front

Weather Parameters	Cold Front	Warm Front	Occluded Front
Winds	Gusty wind	Variable wind	Variable wind
	Direction shifting from	Direction generally shifts	Direction generally shifts
	south-southwest to west-	from south-southeast to	from southeast-south to
	northwest	south-southwest	west to northwest
Temperature	Sudden drop while front	Steady rise	Dropping in cold type
	is passing and steadily		occluded
	dropping afterwards		Rising in warm type

7. Summary

• An air mass is a body of air with fairly homogenous characteristics extending over a large area, spreading over more than 1000 kilometers across the surface of the Earth, the depth may vary from few kilometers to the height of the troposphere.
• The source region is ideally an extended plain surface where air stagnates for several days under prevailing high pressure condition. The source region establishes heat and moisture equilibrium with the overlying air mass.
• Air masses are generally classified according to their moisture content, source region and thermal characteristics as denoted by three letters, <lower case letter for moisture characteristics > <upper case letter for source region> <thermal characteristics>.
• The air mass regulates the atmospheric energy balance and also influences the weather.
• The properties of air mass may change remarkably as it moves under the influence of the characteristics of the underlying surface, the trajectory of the motion and the time factor, all of them acting in combination.
• Front is a zone of transition between two air masses of different density and temperature and is associated with major weather changes.
• Creation of new front is called frontogenesis and degeneration front is called frontolysis. Fronts are classified as cold Front, warm Front, stationary front and occluded front.
• A cold front is the transition zone where a cold air mass is replacing a warmer air mass. Cold fronts have steep slope with characteristic gusty wind and sudden drop in temperature. Cold fronts are indicative of heavy precipitation events, rainfall or snow, combined with rapid temperature drops.
• A warm front is the transition zone where a warm air mass is replacing a cold air mass. The warm front usually moves much slower than the cold front and usually forms shallow horizontal stratus clouds and light precipitation. Frontal fogs may occur as falling raindrops evaporate in the colder air near the surface.
• Stationary front occurs between a pair of air masses (warm and cold) when neither of air mass is strong enough to replace the other for extended periods of time. Stationary fronts are usually characterized by clouds, prolonged precipitation, and storm trains.
• If a cold front catches up to and overtakes a warm front, the frontal boundary created between the two air masses is called an occluded front. Occluded fronts usually form around mature low pressure areas during the process of cyclogenesis when the cold front overtakes a warm front. Occluded fronts are characterized by development of Nimbostratus cloud or sometimes towering Cumulus to Cumulonimbus cloud formation with light, moderate or heavy continuous precipitation.