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Maritime Arctic (mA) and Maritime Polar (mP): These are cold air masses which form over the Arctic and acquire moisture as they move south over the cold waters of the North Atlantic and North Pacific oceans. The Maritime Polar air mass is air, which has moved farther out and over the ocean and, through contact with the ocean surface, has become more warm and moist than Maritime Arctic air.

Maritime Tropical (mT): Because the continent of North America narrows down towards its southern extremity, most of the tropical air in the southern latitudes is in contact with the warm ocean surface and becomes hot and moist. Source regions for Maritime Tropical air are: The Gulf of Mexico, the Caribbean Sea, and the Tropic of Cancer regions of the North Atlantic and North Pacific oceans.

In winter, the most common air masses that form over North America are Maritime Polar, Maritime Arctic and Continental Arctic Continental Polar seldom appears over this continent. In most cases, Maritime Arctic and Maritime Polar air masses found over the continent have entered from the west. Maritime air masses found over the Atlantic Ocean affect only the East Coast. The air mass in the southern portion of the continent is Maritime Tropical.

In summer, snow and ice melt, leaving numerous small lakes in the northern reaches of Canada and Alaska. These lakes provide moisture that affects the forming air masses. In summer, therefore. Maritime Arctic is the principal air mass and Continental Arctic rarely appears. Maritime Polar which enters from the Pacific Ocean and Maritime Tropical are the other common summer air masses.


There are three main factors that determine the weather in an air mass moisture content, the cooling process and the stability of the air. Some air masses are very dry and little cloud develops. Maritime air has a high moisture content and cloud, precipitation and fog can be expected.

Even if the air is moist, condensation and cloud formation occur only if the temperature is lowered. The cooling processes that contribute to condensation and the formation of clouds are (1) contact with a surface cooling by radiation, (2) advection over a colder surface and (3) expansion brought about by rifling. It follows, therefore, that cloud formation within an air mass is not uniform. Clouds may form, for example, in an area where the air is undergoing orographic lift even though the rest of the air mass is clear.

Stability of the air is of prime importance. In stable air, layer cloud and poor visibility are common. Good visibility and cumulus cloud are common in unstable air.


Although the characteristics of an air mass are determined by the area over which it forms, as an air mass moves away from its source area, it is modified by conditions over which it passes. If the modification is extensive both horizontally and vertically' the air mass is given a new name. Continental Arctic air, for example, by moving out over the oceans, becomes moist and may be renamed Maritime Polar or Maritime Arctic.

Temperatures may vary considerably within an air mass as the air is warmed while passing over relatively warm surfaces and cooled while passing over relatively cool surfaces.

Moisture content may also vary. The air mass will pick up moisture as it passes over lakes, wet ground or melting snow. Moist air passing over a mountain range may lose a great deal of moisture as precipitation as it ascends the western slope of the range. The air will be much drier when it reaches the prairies.

Stability characteristics also may vary considerably. Air warming from below by radiation becomes unstable as convective currents develop. Air that is cooled from below becomes more stable as vertical motion is blocked. Changes in stability alter weather conditions quite considerably.

These variations in the properties of a particular air mass during its life are not unusual. As a result, it is impossible to say that a certain weather pattern will always occur in an air mass that is classified, for example, as Continental Arctic. Nevertheless, it is possible to define in broad terms some typical characteristics of the particular air masses. Continental Arctic (cA) forms over a region that is covered with ice and snow. Moisture content is low. The troposphere is made up of air masses. The transition zone between two air masses is called a front. The interaction of these air masses, along their frontal zones, is responsible for weather changes.


The present day theory, which explains the formation of depressions, or lows, was developed by the Norwegians, and is known as the Polar Front Theory. It is based on the fact that the polar regions are covered by a mass of cold air and the equatorial regions by a mass of warm air. In the temperate zone, the two air masses meet. As it moves south, it is gradually heated. Strong winds set up turbulence and, if it acquires moisture, stratocumulus clouds with light snow will develop. If it moves over open water, such as the Great Lakes, it will be heated and acquire moisture. A steep lapse rate and accompanying instability give rise to cumulus clouds and snow showers. Eddying, gusty winds cause restricted visibility in blowing snow. This usually follows a path from the polar regions across the prairies and into the eastern pan of the continent. It rarely affects the British Columbia coast. Maritime Arctic (mA) forms over Siberia or Alaska and travels across the North Pacific where it becomes moist and unstable. Precipitation occurs in the form of snow showers. Snow and rain loses most of its moisture as precipitation on the western slopes of the Rockies and is dry when it reaches the prairies. Maritime Arctic (mA) that develops over the North Atlantic affects only the East Coast of North America. In summer, mA develops in polar regions and moves south over the lakes of the northern pan of the continent. Daytime heating makes the air unstable. Maritime Polar (mP) reaches the Pacific Coast after a long journey over the ocean. It is more extensively modified than mA and the air mass is somewhat more stable. Orographic lift along the mountains produces extensive cloud formation and considerable rain. It is, therefore, drier when it reaches the prairies.

Maritime Tropical (mT) forms over oceans and water bodies of the Tropics. Very warm and moist, it rarely penetrates north of the Great Lakes in winter. Although it does frequently appear aloft, unstable when it is rifled at a front, giving snow, freezing rain, rain, severe icing and turbulence. Extensive fog often occurs, especially on the East Coast.


Air masses do not usually mix. The transition zone between the two masses is therefore narrow and is called a polar front.

From the discussion of air masses, it is evident that the cold air dome covering the polar region, in fact, encloses two other domes and that frontal systems will be found along the zones separating them. Continental Arctic air makes up the smallest and most northerly dome and its edge, the Continental Arctic front, is the most northerly frontal system to be found on the weather map. A dome of Maritime Arctic air overlies the (cA) air and its edge lies somewhere between the Continental Arctic front and the polar front. The fronts are named for the colder air mass involved in the system. The sloping surface of each dome acts as a lifting surface for the warmer air mass associated with it.  A front, correctly defined, is the transition zone between two air masses as it appears on the surface. The sloping side of the cold air is called a frontal surface.

The discussion below deals with the development of frontal waves and depressions along the polar front. Similar frontal waves and depressions occur along the Continental Arctic and the Maritime Arctic fronts.


The area contained within the bulge of the polar front is the warm sector and contains the warm or tropical air, while the rest of the area is composed of the polar air. The pressure at the peak of the frontal wave falls and the low-pressure area deepens. The surface winds become stronger and the fronts begin to move. Both fronts are curved in the direction toward which they are moving. The peak of the wave is called the crest. At the cold front, the cold flow of air from the northwest is undercutting the warm southwesterly flow. At the warm front, the southwesterly flow of warm air is overrunning the retreating flow of cold air.

The air on the northern side (considering the Northern hemisphere) of this surface of separation is termed arctic and polar air. It is nominally cold, and dry. The air on the southern side is termed tropical air. It is normally warm and moist. Due to the difference in the properties of these two air masses, the polar front is known as a surface of discontinuity. Depressions form along this surface of discontinuity (the polar front) and are the means whereby interchange takes place between the warm and cold air masses.

Along the polar front, the cold polar air flows from the northeast towards the southwest on the north side, while the warm air flows from southwest towards the northeast on the south side. The arrangement is not a stable one but is subject to continual disturbances due to the warm air bulging north and cold air bulging southward. This northward bulge, once having started, continues to develop, the cold air on the northward side swinging round at the back, setting up a counterclockwise circulation and emphasizing the bulge. This bulge is the new depression (low-pressure area) just born. It normally travels northeastward along the polar front. The polar front has now been bent or broken into two sectors, the warm front and the cold front. This deformation is called a frontal wave, is moving over the ground in a direction parallel to the isobars in the warm.

The cold front moves faster than the warm front and soon catches up with it. The two fronts merge causing an occluded front. As the occlusion increases in dimension, the low-pressure area weakens and the movement of the fronts slows down. Sometimes a new frontal wave may begin to form on the westward portion of the polar front. In the final stage, the two fronts have become a single stationary front again.

A COLD FRONT is that part (or parts) of a frontal system along which cold air is advancing and is colored blue on the weather map.

A WARM FRONT is that part (or parts) of a frontal system along which cold air is retreating and is colored red on the weather map.



As a mass of warm air advances on a retreating mass of cold air, the warm air, being lighter, ascends over the cold air in a long gentle slope. As a result, the cloud formation associated with the warm frontal system may extend for 500 or more nautical miles in advance of it. Warm fronts usually move at relatively slow speeds and therefore affect a vast area for a considerable length of time.

If the warm air is moist and stable, stratiform clouds develop in a distinctive sequence. The first signs of an approaching warm front are high cirrus clouds which thicken to cirrostratus and altostratus as the warm front approaches. The ceiling gradually falls and there follows a long belt of steady rain falling from heavy nimbostratus cloud. Precipitation may lead the frontal surface by as much as 250 nautical miles.

If the warm air is moist and somewhat unstable, cumulonimbus and thunderstorms may be embedded in the stratiform layers. Heavy showers in advance of the surface front can then be expected.

Very low stratus clouds and fog throughout the frontal zone are typical characteristics of warm fronts.

The passing of the warm front is marked by a rise of temperature, due to the entry of the warm air, and the sky becomes relatively clear.


When a mass of cold air overtakes a mass of warm air, the cold air being denser, stays on the surface and undercuts the warm air violently. Surface friction tends to slowdown the surface air while a sharp fall in temperature, a rise in pressure and rapid clearing usually occur with the passage of the cold front.

Sometimes, an advancing cold front will be relatively slow moving. Because it does not undercut the warm air so violently, a rather broad band of clouds develops extending a fair distance behind the frontal surface. If the warm air is stable, these clouds will be stratiform; if the warm air is unstable, they are cumuliform and possibly thunderstorms. With passage of the frontal surface, clearing is more gradual.


There is generally some part of a front along which the colder air is neither advancing nor retreating. There is no motion to cause the front to move because the opposing air masses are of equal pressure. The surface wind tends to blow parallel to the front and the weather conditions are similar to those associated with a warm front although generally less intense and not so extensive. Usually a stationary front will weaken and eventually dissipate. Sometimes, however, after several days, it will begin to move and then it becomes either a warm front or a cold front.


When the progress of time as a depression advances, the cold front gradually overtakes the warm front and lifts the warm sector entirely from the ground. It is simply a case of the cold air catching up with itself as it flows around the depression. Thus only one front remains, which is called an occluded front or occlusion. An occluded depression soon commences to fill up and die away.

The cold air, in the distance it has traveled, may have undergone considerable change. Therefore it may not be as cold as the air it is overtaking. In this case (cool air advancing on colder air), the front is known as an occluded warm front or a warm occlusion and has the characteristics of a warm front, with low cloud and continuous rain and drizzle. It the warm air is unstable, heavy cumulus or cumulonimbus cloud may be embedded in the stratiform cloud bank.

It the cold air is colder than the air it is overtaking (cold air advancing on cool air), the front is known as an occluded cold front or a cold occlusion. A cold occlusion has much the same characteristics as a warm front, with low cloud and continuous rain. If the warm air is unstable, cumulonimbus and thunderstorms are likely to occur, with the violent turbulence, lightning and icing conditions associated with these clouds.

It will be noted that in the case of either a warm or cold occlusion, three air masses are present, a cool air mass advancing on a cold air mass, or a cold air mass advancing on a cool air mass, with, in either case, a warm air mass lying wedge shaped over the colder air. This wedge shaped mass of warm air is known as a trowel in Canada. (In some other countries, such as in the US, it is called an upper front.)


In Canada, the term upper front refers to a non-occlusion situation.  Sometimes, cold air advancing across the country may encounter a shallow layer of colder air resting on the surface or trapped in a topographical depression. The advancing cold air rides up over the colder, heavier air. The cold front which is the leading edge of the advancing cold air, therefore, leaves the ground and moves along the top of the colder air. It is then known as an upper cold front.

Sometimes, the structure of the advancing cold front is such that the cold air forms a shallow layer for some distance along the ground in advance of the main body of cold air. The frontal surface If the main mass of cold air, in this situation, will usually be very steep. The line along which the frontal surface steepens is also known as an upper cold front.

On occasion, an advancing warm front rides up over a pool or layer of cold air trapped on the ground. A station on the ground does not experience a change of air mass because the front passes overhead. This is known as an upper warm front.

Sometimes, the surface of the cold air that is retreating ahead of an advancing warm front is almost flat for some distance ahead of the surface front and then steepens abruptly. The line along which the surface of the retreating cold air steepens sharply is also called an upper warm front.


The theory of the polar front, which for the sake of simplicity has been described in the form of its original conception, might leave the impression that depressions form only along some well defined line Iying somewhere midway between the poles and the equator. Air masses are in a constant state of formation over all the land and water areas of the world. Once formed, they tend to move away from the source regions over which they form. The same frontal processes and phenomena occur whenever a mass of warm air and a mass of cold air come in contact.

There is a widespread impression among pilots that fronts always bring bad weather and that all bad weather is frontal. Actually some fronts have little or no weather associated with them. A slight change of temperature and a windshift may be the only evidence that the front has gone through. And, of course, bad weather can develop without the passage of a front. Fog, for example, generally occurs when no fronts are present and severe thunderstorms may develop in an air mass, which has no frontal characteristics.

Another common misconception is that the front is a thin wall of weather. This false idea is perhaps occasioned by the line that indicates a front on a weather map. The line on the map only shows the surface location at which the pressure change, windshift and temperature change occur. The actual weather associated with the front may extend over an area many miles in width, both well ahead and also for many miles behind the actual line on the weather map.

A front itself is actually a transition zone between two large air masses with different properties of temperature and moisture. Each individual air mass may extend over hundreds of thousands of square miles. Everywhere along the boundary of an air mass, where it overrides or undercuts the air mass upon which it is advancing and for a considerable height upward from the surface as well, there is a frontal zone. The frontal weather associated with the front, therefore, can be expected to extend for hundreds of miles along the boundary of the air mass.

Frontogenesis means a front, which is increasing in intensity.

Frontolysis means a front, which is decreasing in intensity.

If you examine the diagrams showing fronts on a weather map, you will notice that all fronts lie in regions of lower pressure. The isobars are bent sharply at a front. These two factors are characteristic of all fronts.


Cold fronts are not all the same. The weather associated with a cold front may vary from a minor windshift to severe thunderstorms, low ceilings, restricted visibility and violent gusty winds. The severity of the weather is determined by the moisture content and stability of the warm air mass that the cold air mass is undercutting and the speed of the advancing cold front.

Fast moving cold fronts may travel across the country with a speed of 30 knots or more.  If the warm air that is being undercut by the cold air mass is very moist and unstable, towering cumulus clouds and thunderstorms are likely to develop. Heavy rain or hail may be associated with the front. A slower moving cold front advancing on more stable and drier air in the warm sector will produce less severe weather conditions, stratus or altocumulus clouds with light or no precipitation.

A long line of cumulus clouds on the western horizon is usually an indication of an approaching cold front. Sometimes a deck of altocumulus cloud or decks of stratus and stratocumulus extending ahead of the front will mask the main frontal cloud from the view of the high flying or low flying pilot respectively.


Surface Wind: The wind direction will always veer as the front passes. Gustiness may be associated with the windshift.

In flying through a cold front, the windshift may be quite abrupt and occurs at the frontal surface rather than at the front. The windshift is always such that an alteration in course to starboard is required, no matter which way you are flying through the front.

Temperature: On the ground, the temperature may drop sharply as the front passes, but usually it drops gradually. The air immediately behind the front has been warmed in passing over the warm ground. Therefore, it may be several hours before the temperature drops to the true value of the cold air mass. In flying through a cold front, there will be a noticeable temperature change when passing through the frontal surface.

Visibility: Visibility usually improves after passage of a cold front. If the front is moving fairly rapidly, the width of frontal weather generally is less than 50 miles. If the front is moving slowly, however, flight operations may be affected for many hours.

Pressure: The approach of a cold front is accompanied by a decrease in pressure. A marked rise will be noticed when the front has passed.

Turbulence: Turbulence may be associated with the cold front if it is active, although thunderstorms are not always present. Even in cases where there are no clouds, turbulence may be a problem. As a rule, flight through an active cold front can be expected to be rough.

Precipitation: The frontal rain or snow is usually narrow, especially if it is showery in character. Icing in the turbulent cumulus clouds can be severe.


A long line of squalls and thunderstorms which sometimes accompanies the passage of a cold front is called a line squall (or squall line). It is usually associated with a fast moving cold front that is undercutting an unstable warm air mass. It may form anywhere from 50 to 300 nautical miles in advance of the front itself. The line squall is a long line of low black, roller like cloud, which often stretches in a straight line for several hundred miles, and from which heavy rain or hail falls for a short time. Thunder and lightning frequently occur. The squall is also accompanied by a sudden wind change from southerly or south-westerly to north or north-westerly, together with a sudden drop in temperature and a rise in barometric pressure. The actual wind squall lasts only for a few minutes but is often extremely violent, constituting a serious menace both to shipping and to airplanes. The signs indicating the approach of a line squall are unmistakable. Airplanes on the ground should be immediately hangared. Those in the air should at all costs avoid this violent weather phenomenon.


Warm front changes are usually less pronounced than cold front changes. The change is also generally very gradual. However, the weather at a warm front is usually more extensive and may cover thousands of square miles. A wide variety of weather characterizes warm fronts. The weather may even vary along a given front.

The degree of overrunning and the moisture content and stability of the overrunning warm air determine the seventy of the weather. If the warm air is very moist, the cloud deck forming in the overrunning air may extend for hundreds of miles up the slope of the retreating cold air. It the warm air is unstable, thunderstorms may be embedded in the cloud deck.

High cirrus cloud is the first sign of the approach of an active warm front. Cirrostratus soon follows (the high thin cloud which causes a halo around the sun or moon). The cloud gradually thickens and the base lowers until a solid deck of altostratus/altocumulus covers the area. Low nimbostratus moves in, merging with the altostratus. With the result that a solid deck of cloud extending from near the surface to 25,000 feet or more covers the whole area. Precipitation is usually heavy.


Windshift: With the passage of a warm front, the wind will veer, but the change will be much more gradual than in the case of a cold front.

When flying through a warm front, the windshift will occur at the frontal surface and will be more noticeable at lower levels. When flying through a warm front, the windshift is such that a course alteration to starboard is necessary.

Temperature: The warm front brings a gradual rise in temperature. A pilot flying through the frontal surface will notice a more abrupt temperature rise.

Visibility: Low ceilings and restricted visibility are associated with warm fronts and, because warm fronts usually move quite slowly, these conditions persist for considerable time.

When rain falls from the overrunning warm air, masses of irregular cloud with very low bases form in the cold air. Fog is frequently a condition 50 nautical miles ahead of an advancing warm front.

Turbulence: Cumulonimbus clouds are frequently embedded in the main cloud deck and these storms are responsible for the most severe turbulence associated with a warm front. However, these storms and the turbulence they occasion are less severe than those associated with cold fronts. The principal problem with these storms is that they cannot be located by sight since they are embedded in the main cloud cover.

Precipitation: The first precipitation begins in the region where the altostratus layer of cloud is from 8000 to 12,000 feet above the ground. As the front approaches, the precipitation becomes heavier. Occasional very heavy precipitation is an indication of the presence of thunderstorms.

Winter Warm Fronts

In winter, when temperatures in the cold air are below freezing and temperatures in the lower levels of the warm air are above freezing, snow and freezing rain can be expected.

Snow falls from that part of the warm air cloud that is high and therefore below freezing in temperature. From the lower cloud, where temperatures are above freezing, rain falls. However, as the rain falls through the cold air (of the cold air mass that the warm air is overrunning), it becomes supercooled and will freeze on contact with any cold object. This is known as freezing rain (ZR).

In the area ahead of the freezing rain, there is a region where the rain falling through the cold air becomes sufficiently supercooled to freeze and falls to the ground as ice pellets (IP). A pilot approaching the frontal surface at higher altitudes may not encounter the ice pellets, but the pilot flying at quite low altitudes can expect to encounter snow, ice pellets and then freezing rain.

Icing is a problem associated with warm fronts in winter. Snow is not responsible for icing, unless it is very wet when it can stick to an airplane and form ice. Freezing rain, however, causes a rapid build up of ice. Icing will also be a problem in the cloud layers.


The weather that occurs with a trowel is a combination of cold and warm front conditions. The cloud pattern ahead of the approaching trowel is similar to that of a warm front. Cold front cloud formations will exist behind it. Cumulus buildups and thunderstorms are likely to be interspersed with stratiform clouds, continuous precipitation and widespread low ceilings. In winter months, freezing rain and severe icing conditions are likely hazards as the rain aloft in the occluded warm air falls through the freezing temperatures of the ground based cold sectors. The maximum precipitation, convective activity and icing conditions usually occur in the northeast sector of the low and extend some 50 to 100 miles ahead of the occluded front.


Click here to go to the next section of this chapter: Ceiling and Visibility

The basis for this section is the Flight Training Manual by Transport Canada. However, the text was modified for US users and readers by Dr. Claudius Carnegie of the ALLSTAR website. Any questions should be directed to Dr. Carnegie at directly.

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Updated: 12 March, 2004