the atmosphere
atmospheric weight
cloud formation
hazards of thunder storms
prevailing winds
temperature and humidity
weather fronts
understand jetstreams
weather glossary

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air masses

Air masses are parcels of air that bring distinctive weather features to the country. An air mass is a body or 'mass'of air in which the horizontal gradients or changes in temperature and humidity are relatively slight. That is to say that the air making up the mass is very uniform in temperature and humidity.
An air mass is separated from an adjacent body of air by a transition that may be more sharply defined. This transition zone or boundary is called a front. An air mass may cover several millions of square kilometres and extend vertically throughout the troposphere.

Weather Phenomenon

Prior to the Passing of the Front

Contact with the Front

After the Passing of the Front

Temperature Warm Cooling suddenly Cold and getting colder
Atmospheric Pressure Decreasing steadily Levelling off then increasing Increasing steadily
Winds South to southeast Variable and gusty West to northwest
Precipitation Showers Heavy rain or snow, hail sometimes Showers then clearing
Clouds Cirrus and cirrostratus changing later to cumulus and cumulonimbus Cumulus and cumulonimbus Cumulus

The temperature of an air mass will depend largely on its point of origin, and its subsequent journey over the land or sea. This might lead to warming or cooling by the prolonged contact with a warm or cool surface. The processes that warm or cool the air mass take place only slowly, for example it may take a week or more for an air mass to warm up by 10 C right through the troposphere. For this to take place, an air mass must lie virtually in a stagnant state over the influencing region. Hence, those parts of the Earth's surface where air masses can stagnate and gradually attain the properties of the underlying surface are called source regions.

The main source regions are the high pressure belts in the subtropics, which produce tropical air masses, and around the poles, that are the source of polar air masses.

Polar and tropical source regions: The blue and red arrows show the polar and tropical regions respectively.

modification of air masses

As we have seen, it is in the source regions that the air mass acquires distinctive properties that are the characteristics of the underlying surface. The air mass may be cool or warm, or dry or moist. The stability of the air within the mass can also be deducted. Tropical air is unstable because it is heated from below, while polar air is stable because it is cooled from below.

As an air mass moves away from its source region towards the British Isles, the air is further modified due to variations in the type or nature of the surface over which it passes. Two processes act independently, or together, to modify an air mass.

An air mass that has a maritime track, i.e. a track predominantly over the sea, will increase its moisture content, particularly in its lower layers. This happens through evaporation of water from the sea surface. An air mass with a long land or continental track will remain dry.

Fig 2:
Modification of air mass by land and ocean surfaces

A cold air mass flowing away from its source region over a warmer surface will be warmed from below making the air more unstable in the lowest layers. A warm air mass moving over a cooler surface is cooled from below and becomes stable in the lowest layers.

Fig 3:
Modification of air mass due to surface temperature

If we look at the temperature profiles of the previous example, the effects of warming and cooling on the respective air masses are very different.

Fig 4:
Modified vertical temperature profiles (----- line) typical of: a) tropical air cooled from below and b) polar air heated from below on its way south. Note that where the air is heated from below the effect is spread to a greater depth of the atmosphere.

weather in an air mass

Five basic types of air masses determine the weather. They can bring anything from scorching heat to bone-chilling cold depending on the type of air mass.

These air masses are:

Long sea track
Short sea
Very warm or hot
Very cold
Near sea
Rather cold
than Pm)
than Pm)
Relatively dry
Rather moist
Moist in lowest layers
Very dry
Very moist
Fairly moist (not as
moist as Pm)
Fairly moist (not as
moist as Pm)
Change of lapse rate
Little change
Cooled from below
Heated from below
Little change
Cooled from below
Warmed in summer
Heated from below
Heated from below
Heated from below
Generally stable
Stable aloft
Clear, occasional thundery showers
Rain or snow showers
Low cloud, drizzle
Broken cloud, dry
Variable cloud, showers
Showers (mainly coastal)
Showers (mainly coastal)
Moderate or poor
Moderate of poor
Moderate or poor
Often poor with coastal fog
Very good
Very good

polar front

Several fronts and semipermanent high and low pressure systems characterize the Arctic. The "polar front" marks the boundary between cold polar air masses and warm tropical air masses. The polar front is intermittent rather than continuous around the globe. The strength of the polar front depends on the magnitude of the horizontal temperature gradient across the front. Where the temperature gradient is steep, the front is strong and is a potential site for cyclone or low pressure system development. Where temperature contrast is small, the polar front is weak.

Like the polar front, the "arctic front" is discontinuous and depends on the temperature contrast between two air masses. The arctic front is the boundary between polar and arctic air masses and lies to the north of the polar front. The arctic front can be as strong as the polar front. It is particularly prominent during summer in northern Eurasia.

Semipermanent high and low pressure systems ("highs" and "lows") are identified with particular regions and have seasonal characteristics. In winter, the Icelandic Low extends from near Iceland north into the Barents Sea, and is associated with frequent cyclone activity. The Aleutian Low is present in the Gulf of Alaska. The Beaufort-Chukchi Sea region is dominated by a ridge of high pressure linking the Siberian High and high pressure over the Yukon of Canada. In April and May arctic pressure gradients decrease. The Icelandic and Aleutian lows weaken. The Siberian High disappears, and is replaced by a wide but shallow low. The Arctic High is centred over the Canadian Arctic Archipelago. In summer, pressure gradients are generally weak. Intermittently, however, cyclones enter the Arctic from northern Eurasia and the north Atlantic, and tend to persist over the Canadian Basin. By October the pattern has almost returned to the winter configuration. The Icelandic and Aleutian lows strengthen, as does the Siberian High.

Semi permanent Highs and Lows

The Arctic is characterized by "semi permanent" patterns of high and low pressure. These patterns are semi permanent because they appear in charts of long-term average surface pressure. They can be considered to largely represent the statistical signature of where transitory high and low systems that appear on synoptic charts tend to be most common.

Aleutian Low

This semi permanent low pressure centre is located near the Aleutian Islands. Most intense in winter, the Aleutian Low is characterized by many strong cyclones. Travelling cyclones formed in the sub polar latitudes in the North Pacific usually slow down and reach maximum intensity in the area of the Aleutian Low.

Icelandic Low

This low pressure centre is located near Iceland, usually between Iceland and southern Greenland. Most intense during winter, in summer, it weakens and splits into two centres, one near Davis Strait and the other west of Iceland. Like its counterpart the Aleutian Low, it reflects the high frequency of cyclones and the tendency for these systems to be strong. In general, migratory lows slow down and intensify in the vicinity of the Icelandic Low.

Siberian High

The Siberian High is an intense, cold anticyclone that forms over eastern Siberia in winter. Prevailing from late November to early March, it is associated with frequent cold air outbreaks over east Asia.

Beaufort High

The Beaufort High is a high pressure centre or ridge over the Beaufort Sea present mainly in winter.

North American High

The North American High is a relatively weak area of high pressure that covers most of North America during winter. This pressure system tends to be centred over the Yukon, but is not as well-defined as its continental counterpart, the Siberian High.

Polar Lows

Small cyclones forming over open sea during the cold season within polar or arctic air masses are called "polar lows." Typically several hundred kilometers in diameter, and often possessing strong winds, polar lows tend to form beneath cold upper-level troughs or lows when frigid arctic air flows southward over a warm body of water.

Polar lows last on average only a day or two. They can develop rapidly, reaching maximum strength within 12 to 24 hours of the time of formation. They often dissipate just as quickly, especially upon making landfall. In some instances several may exist in a region at the same time or develop in rapid succession.

In satellite imagery polar lows show characteristic spiral or comma shaped patterns of deep clouds, sometimes with an inner "eye" similar to those seen in tropical cyclones. Convective cloud bands occupy the surroundings (see figure below). Analysis of aircraft and radiosonde data collected during field experiments reveals that polar lows may possess warm cores. This finding, coupled with their appearance in satellite imagery, has prompted some investigators to refer to polar lows as "arctic hurricanes," although they seldom, if ever, possess hurricane strength winds.

Polar lows are difficult to predict even with current high resolution and high performing operational numerical models, because they usually occur in remote oceanic regions where data are too sparse to define the model initial state on a sufficiently fine scale. However, present-day models can depict synoptic-scale patterns favourable to the development of the smaller scale systems, allowing forecasters to use the predictions in conjunction with satellite imagery and conventional observations to make subjective forecasts of their occurrence.

A NOAA-9 polar orbiter satellite image (visible band) of a polar low over the Barents Sea on 27 February 1987. The southern tip of Spitsbergen is visible at the top of the image. The polar low is centred just north of the Norwegian coast. Image contributed by S. Businger, Department of Meteorology, University of Hawaii.

The Polar Vortex

The polar vortex is a persistent large-scale cyclonic circulation pattern in the middle and upper troposphere and the stratosphere, centred generally in the polar regions of each hemisphere. In the Arctic, the vortex is asymmetric and typically features a trough (an elongated area of low pressure) over eastern North America. It is important to note that the polar vortex is not a surface pattern. It tends to be well expressed at upper levels of the atmosphere (that is, above about five kilometres).


A front is defined as the transition zone between two air masses of different density. Fronts extend not only in the horizontal direction, but in the vertical as well. Therefore, when referring to the frontal surface (or frontal zone), we referring to both the horizontal and vertical components of the front.


A cold front is that part (or parts) of a frontal system along which cold air is advancing and is coloured 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 coloured red on the weather map.

types of front

the 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.

Symbolically, a warm front is represented by a solid line with semicircles pointing towards the colder air and in the direction of movement. On coloured weather maps, a warm front is drawn with a solid red line.

There is typically a noticeable temperature change from one side of the warm front to the other. In the map of surface temperatures below, the station north of the front reported a temperature of 53 degrees Fahrenheit while a short distance behind the front, the temperature increased to 71 degrees. An abrupt temperature change over a short distance is a good indication that a front is located somewhere in between.

If warmer air is replacing colder air, then the front should be analyzed as a warm front. If colder air is replacing warmer air, then the front should be analyzed as a cold front. Common characteristics associated with warm fronts have been listed in the table below.

    Before Passing  While Passing   After Passing
Winds  south-southeast  variable   south-southwest
Temperature  cool-cold, slow warming  steady rise   warmer, then steady
Pressure  usually falling  levelling off   slight rise, followed by fall
Clouds  in this order: Ci, Cs, As, Ns, St, and fog; occasionally Cb in summer  stratus-type   clearing with scattered Sc; occasionally Cb in summer
Precipitation  light-to-moderate rain, snow, sleet, or drizzle  drizzle or none   usually none, sometimes light rain or showers
Visibility  poor  poor, but improving   fair in haze
Dew Point  steady rise  steady   rise, then steady

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.

the cold front

A cold front is defined as the transition zone where a cold air mass is replacing a warmer air mass. Cold fronts generally move from northwest to southeast. 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.

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 coloured weather maps, a cold front is drawn with a solid blue line.

There is typically a noticeable temperature change from one side of a cold front to the other. In the map of surface temperatures below, the station east of the front reported a temperature of 55 degrees Fahrenheit while a short distance behind the front, the temperature decreased to 38 degrees. An abrupt temperature change over a short distance is a good indicator that a front is located somewhere in between.

If colder air is replacing warmer air, then the front should be analyzed as a cold front. On the other hand, if warmer air is replacing cold air, then the front should be analyzed as a warm front. Common characteristics associated with cold fronts have been listed in the table below.

    Before Passing   While Passing   After Passing
Winds   south-southwest   gusty; shifting   west-northwest
Temperature   warm   sudden drop   steadily dropping
Pressure   falling steadily   minimum, then sharp rise   rising steadily
Clouds   increasing: Ci, Cs and Cb   Cb   Cu
Precipitation   short period of showers   heavy rains, sometimes with hail, thunder and lightning   showers then clearing
Visibility   fair to poor in haze   poor, followed by improving   good, except in showers
Dew Point   high; remains steady   sharp drop   lowering

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.

the stationary front

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.

A noticeable temperature change and/or shift in wind direction is commonly observed when crossing from one side of a stationary front to the other.

In the map above, temperatures south of the stationary front were in the 50's and 60's with winds generally from the southeast. However, north of the stationary front, temperatures were in the 40's while the winds had shifted around to the northeast. Cyclones migrating along a stationary front can dump heavy amounts of precipitation, resulting in significant flooding along the front

occluded fronts

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.

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. This 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 the direction the front is moving. On coloured 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 occurred 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. Common characteristics associated with occluded fronts have been listed in the table below.

    Before Passing  While Passing   After Passing
Winds  southeast-south  variable   west to northwest
Cold Type
Warm Type
Pressure  usually falling  low point   usually rising
Clouds  in order: Ci, Cs, As, Ns  Ns, sometimes Tcu and Cb   Ns, As or scattered Cu
Precipitation  light, moderate or heavy precipitation  light, moderate or heavy continuous precipitation or showers   light-to-moderate precipitation followed by general clearing
Visibility  poor in precipitation  poor in precipitation   improving
Dew Point  steady  usually slight drop, especially if cold-occluded  

slight drop, although may rise a bit if warm-occluded

The cold air, in the distance it has travelled, 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 the US, it is called an upper front.)

upper fronts

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.

frontal weather

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 lying 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.

weather at the cold front

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.

weather changes

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

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 passing 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.

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.

Precipitation: The frontal rain or snow is usually narrow, especially if it is showery in character.

line squalls

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.

weather at the warm front

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.

weather changes

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.

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).

weather at trowals and upper fronts

The weather that occurs with a trowal 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 build-ups 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.