40 AIR MASSES, FRONTS, DEPRESSIONS AND HIGHS

148. AIR MASSES

The horizontal distribution of air temperature in a certain area is obtained by plotting on synoptic chart values of temperatures measured simultaneously at many points within the area, and drawing in isotherms (lines joining places with the same temperature). The cold and warm areas will be immediately apparent, as will also those places where the temperature gradient is greatest (this is the rate at which the temperature varies in a horizontal direction perpendicular to the isotherms) where the isotherms are close together, the gradient, or rate of change in the temperature, is large. On charts covering a large area several thousands of miles across, we can usually find two or more air masses, each several hundreds of square miles in area, and each characterised by its own temperature (varying by only a degree or so).

At the boundaries of these air masses the temperature gradient is much greater where there is a rapid change in temperature from one air mass to its neighbour. These boundaries are known as frontal zones. An air mass may be defined as a mass of air with dimensions of the order of several hundreds of miles across, and with little or no horizontal variation of any of its properties, especially temperature, and is formed by prolonged contact with the Earth’s surface.

A warm air mass is produced by prolonged contact with a warm surface and, conversely, a cold air mass with a cold surface and after such contact the surface air over many hundreds of square miles becomes homogeneous and assumes the characteristic temperature and humidity of the region in which it was formed. The region in which an air mass is formed is called the air mass source, and these are usually areas where there are large, slow-moving, anti-cyclones in which there is little or no wind and conditions are ideal for the formation of an air mass, such as the polar and sub-tropical high-pressure belts. Hence, source regions are usually either polar or sub-tropical, producing corresponding polar or tropical air masses. Air flows outward from these high-pressure areas, and cold polar air masses are constantly meeting warm tropical air masses in the temperate latitudes. Weather forecasting in these latitudes is, therefore, largely a study of the air masses present in or near the area for which a forecast is required, since with each air mass is associated its own characteristic weather, dependent on the source of the air mass and the route of its movement from that source; moreover, disturbed weather is associated with the interaction of the two streams of air of different temperature and humidity, and in the frontal zones, or fronts, between two air masses there is invariably a belt of bad, or relatively bad, weather.

An air mass which is migrating from its source region is progressively modified. An air mass having a maritime track (i.e. one mostly over the sea) will tend to become saturated, especially in its lowest layers; one with a continental track (i.e. one mostly over land) will retain a dew point similar to its original value because little water is available over land for evaporation. These factors are used in classifying air masses, and in Meteorology the five basic air masses, typed according to their source and track are: Arctic Maritime (Am), Polar Maritime (Pm), Polar Continental (Pc), Tropical s Maritime (Tm), and Tropical Continental (Tc). Thus, we may say, for example, that at a certain time a Tropical Maritime air mass covers Britain, by which we mean that the air mass has had a sub-tropical source and a maritime track before reaching Britain are shown in fig. 40-1.

Most air mass sources are several thousand miles away so that air masses take several days to reach Britain and are considerably modified along their track; thus, the contrast between polar and tropical air masses is lessened and weather changes accompanying an air mass change are less abrupt. Places of similar latitude on the eastern sides of large continents (e.g. the New England States of the U.S.A., and Japan) experience far greater extremes because the air masses crossing them are relatively less modified. The following Section refers to the Eastern North Atlantic, with particular reference to the British Isles and Western Europe. The principles apply in all temperate latitudes, with modifications due to geography.

149. PROPERTIES OF AIR MASSES TROPICAL MARITIME (Tm)

• originates in the oceanic area between the Azores and the West Indies.
•  travels N.E.—wards over the sea which is getting steadily colder with increased latitude.
•  the lower layers cool and the relative humidity increases near the surface.

The weather to be expected in a Tm air mass is, therefore:

(I) wind usually from S.W.              iv precipitation if any, in drizzle form((ii)clouds of stratiform type           v  air temp. higher than sea temp.

(iii)tendency for sea fog                  vi relative humidity high.

POLAR MARITIME (Pm) –  

(a) originates in polar region of North America.

(a) travels S.E.-wards over sea which is getting warmer as latitude decreases.

(c)  the lower layers are warmed and the relative humidity increases (but was low to start with).

The weather to be expected in a Pm air mass is, therefore:

(i) wind between S.W. & N., gusting    iv precipitation as showers

(ii) clouds of cumuliform type               v air temp. lower than sea temp.

(iii) visibility generally good                  vi relative humidity lower than Tm air.

ARCTIC MARITIME (Am) – When Polar Maritime air flows from the North it has a short sea track from its source region around Spitzbergen or the Arctic Ocean and is, therefore, colder when it reaches Britain, so much colder (often near 0°C.) that the air mass is sometimes designated Arctic Maritime (Am). Its properties are similar to those of Polar Air except that it is colder and very unstable, the resulting showers being frequent and heavy and usually of snow.

RETURNING POLAR MARITIME (rPm) – A slow-moving depression west of Britain forces Pm air into latitudes well south of 50°N.,  so that when it approaches from the S.W., it has been greatly modified – it is warmer and moister than Pm. air which comes from the N.W., and is called returning Polar Maritime air. As it approaches Britain (fig. 40-2c), it crosses progressively cooler water which cools and stabilises its lower layers and prevents the formation of convective clouds, although the air mass is still unstable aloft and showers may develop as the air mass flows over high ground  especial in the west.

POLAR CONTINENTAL (Pc) – Air originates in Siberia and Eastern Europe in winter and reaches Britain as the result of an anti-cyclone over Scandinavia. Although very cold, the lower layers are warmed and moistened as it crosses the North Sea, so that large amounts of cumulus and cumulonimbus clouds form over the East coat of Britain, with showers of rain or snow. If it persists for several days, the most severe winter weather results. In summer, Polar Continental air resembles a rather cooler form of Tc air.

TROPICAL CONTINENTAL (Tc) –  Tropical air from a source region over North Africa rarely reaches Britain because the necessary prolonged south to south-east winds are uncommon, but in summer a similar air mass formed over S.E. Europe or S. Russia can reach Britain under the influence of an anti—cyclone over N.E. Europe. It is hot, dry and hazy as the result of both dust and smoke from industrial areas on the Continent, and is the cause of so-called “heat waves” over Britain in summer.

150. TYPES OF FRONTS

An air mass is certain, sooner or later, to meet another air mass which started from a different source and followed a different track, and whose properties are different. Air masses do not readily mix, and we have already defined the area between two air masses as a frontal zone, in which the temperature gradient will be large, in comparison with the gradients within the masses. A frontal zone slopes upwards above the colder air mass, which thus lies in the form of a wedge below the warmer air. A front is the intersection of this frontal zone with the Earth’s surface, or the line along which the warmer air leaves the surface on its upward climb above the colder air.

If the warm air is replacing the cold air, the front is called a warm front, (fig. 40-3a); the warm air, being lighter, ascends over the colder, heavier air. If the cold air is replacing the warm air, the front is called a cold front, (fig.  40-3b); in this event, the cold air undercuts the warm air and forces it upwards. It should be noted that the slope of the frontal zone in a warm front is about 1 in 100 to 1 in 150, whereas the slope in a cold front is somewhere near l in 50, at least twice as steep, so that in a cold front the warm air ascends much more rapidly. On synoptic charts it is found that where the isobars cross a front there is a distinct discontinuity, or ‘kink’, and the kink always points towards the higher pressure, so that the wind, which blows nearly along the isobars, always veers (i.e. shifts to the right) at a front (whether it be a warm front or a cold front) as that front passes over an observer.

A front is in motion when either warm air or cold air are replacing each other, in the case of a warm front at the speed and direction of the surface wind, and in the case of a cold front at a faster speed near that of the geostrophic wind. When a front forms the boundary between two air masses moving in the same or opposite directions so that neither is replacing the other, its movement is small and irregular and it is called a quasi—stationary front (see fig. 40-4).

Because of its greater speed, a cold front often overtakes a warm front, and an occluded front or occlusion results. Fig. 40-6 illustrates an occlusion (a) shortly before the cold front overtakes the warm front, (b) a cold front-type occlusion, where the overtaking cold air mass is colder than the air ahead of the occluded front and undercuts it, and (c) a warm front-type occlusion, where the overtaking cold air is not as cold as the air mass ahead of the front and rises above it. Fig. 40-7 shows the synoptic chart plan presentation of the formation of an occlusion. There is the same characteristic kink in the isobars and consequent veer of wind on passage of an occlusion as with other fronts, but the very nature of an occlusion is a weakening process – the temperature gradient across the front becomes weaker as the occlusion develops and eventually the front disappears altogether. The different types of fronts are represented on synoptic charts by the conventional symbols:

The boundary between Polar and Tropical Air Masses in the North Atlantic is known as the Polar  Front and is of course, a quasi-stationary front an usually characterised by some cloud and possibly showers or light rain. The mean position of the Polar front in winter and in summer is shown in fig. 40-8, although the actual position varies considerably from the mean from day to day. The Polar air stream and the Tropical air stream flow parallel to each other, either in the same or in opposite directions, and since there is no wind component at right-angles to the front, the Polar Front is more or less station. Owing to the difference in density between the two air masses and to the rotation of the Earth, the Polar Front is not vertical but inclined at a slope varying between 1 in 50 and 1 in 20 towards the cold air.

151. FRONTAL DEPRESSIONS

As the cause of frequent changes of weather in temperate latitudes is largely attributable to depressions with their associated fronts travelling in an easterly direction across the oceans, it is wise for the navigator to understand something about their development and constitution.

We referred above to a stationary front formed by the boundary between two air masses moving in the same or opposite directions. Fig. 40-8(a) shows the Polar Front in the North Atlantic lying in an east-west direction, separating Pm air to the north and Tm air to the south, with both airstreams flowing to the east. Often this arrangement is locally distorted by a surge of tropical air northwards, and this bulge of warm air, called a frontal wave, moves along the front with the velocity of the warm air stream (fig. 40-9(b)). The leading edge of this frontal wave is a warm front, and the rear edge a cold front. Some waves may die, whereas others increase in size and low-pressure centre, with a complete circulation of wind around it, forms at the crest of the wave (fig. 40-9(c)). This condition would occur about one day later than fig. 40-9(b), and it will be seen that the warm bulge is larger, and the initial stationary front distorted further. The area between the two fronts is called the warm sector of the depression, and in it the isobars are a little closer together than initially and so the warm air moves faster, and because the isobars have backed a little, slightly to the left of its original course.

Once a Low has reached the warm sector stage, it usually continues to develop, or “deepen” as this is called: pressure continues to fall and the pressure gradients become large so that strong winds are produced. The warm sector increases in amplitude and at the same time becomes narrower, and at this stage the depression is said to be fully-developed. The cold front steadily catches up on the warm front until an occlusion occurs, first near the centre of the Low, but later progressively outwards as the development continues (fig.,40-10(d)). The final stage in the life—cycle of a depression is the rapid occluding of its warm sector, weakening of the temperature gradients across the occlusion (i.e. the front becomes weaker with less—disturbed conditions, rain and squalls), and a transfer of the Low’s centre from the tip of » the occlusion to the cold air (fig. 40-10) illustrates this condition in a decaying depression). After this, the depression’s movement becomes slow and irregular and it slowly dies or “fills up”, i.e. the pressure within it rises.

152. SECONDARY DEPRESSIONS

When a depression is occluded, its speed of advance is reduced and it may even stop. When this occurs, secondary disturbances or waves are liable to form on the trailing cold front.

Sometimes the only effect is that the barometer ceases to rise and there is a temporary reversion to an overcast sky with little or no rain. A secondary depression may, however, develop to a considerable extent as shown in fig. 40-11. Secondary depressions travel round the primary depression in an anti-clockwise direction in the northern hemisphere and sometimes become deep and absorb the parent depression. A secondary, especially in winter, may give rise to severe gales on the side away from the primary.

A section of the Polar Front lying across the North Atlantic sometimes shows two or more Lows simultaneously, each evolving to a large extent independently and each at a different stage of development, with the easternmost usually being the most advanced. The members of such a family of depressions move along similar tracks, but the cold polar air usually pushes farther south behind each Low so that the track of any given member is often farther south than that of its fore-runner. The series is ended when polar air breaks through to low latitudes and the Polar
Front degenerates, only to reform again at higher latitudes. Fig. 40-12 shows a situation with a family of three Lows, each of a different type. Low A is an old occluded depression, Low B is a cold front wave, and Low C is a slow-moving Low at low latitudes. High A is part of the sub-tropical high-pressure belt and High B is a slow-moving blocking High, while the shaded areas represent rain areas.

153. WEATHER ASSOCIATED WITH DEPRESSIONS

Usually, the most active regions of a depression are near its fronts, with which are associated most of the cloud and precipitation in a new Low. The part of the warm front which usually gives the most cloud and precipitation lies near the Low’s centre, whereas with the cold front it is its trailing part which does this. In the later stages of the occlusion process, the remaining part of the warm front often gives a weak cloud and precipitation area, while the cold front is more active. A useful rule-of-thumb is: an active warm front passing an observer is followed by an inactive cold front, and vice-versa.

Fig. 40-13(a) shows a plan of a depression or Low, and fig. 40-13(b) a vertical section along the line AB in fig.(a), which lies to the south of the Low’s centre (a typical situation over Britain). The passage of such a Low from west to east over an observer situated at A would most likely give rise to the following sequence of weather:-

In advance of the warm front: as the warm Tm air of the depression’s warm sector is rising over the colder Pm air in which the observer is situated, cloud is formed anything up to 500 miles ahead of the front. Since this ascent is more or less uniform throughout the area affected, the cloud is all of the stratiform or sheet type. Furthest ahead of the warm front, and about l5 hours before passage of that front over the observer, wisps of cirrus, followed by cirrostratus cloud will occur at heights exceeding 6,000 metres (20,000ft.), gradually increasing and obscuring the blue sky to give it a milky appearance.

At this stage, the barometric pressure will begin to fall very slowly, and the relative humidity to rise slowly. As the warm front approaches, the cloud becomes lower and denser, the cirrostratus giving way gradually to altostratus and later, nimbostratus. The wind will be from between S. and S E., and increasing in strength. Some 4 to 6 hours before passage of the surface front, when it is between 100 and 200 miles away, rain, which becomes continuous, will begin. The cloud continues to get lower until within 50 miles or so of the warm surface front it is almost at sea level and surface visibility becomes poor (this is largely due to the evaporation of the falling rain into the colder air through which it is falling, and this also causes the relative humidity to increase more rapidly). As the front approaches, the barometric pressure falls more quickly, at the rate of about one or two millibars an hour, and the wind continues to increase in strength.

On Passage of the Warm Front: the barometer will cease to fall, or fall more slowly, and may even rise slowly. The wind will veer quite suddenly, usually to between S.W. and W., and may continue to with squalls as the surface passes. The main cloud mass away and is replaced by the or stratocumulus typical of air mass in the warm sector freshen, front moves stratus the Tm of the depression, and the rain will change to drizzle. The change from polar to tropical air causes a temperature rise of about 5°C., concentrated over a period of l to 2 hours as the front passes.

In the Warm Sector: the barometer remains fairly steady but temperature and humidity remain high (often called “muggy” weather). The wind will remain steady in direction fairly strong if near the centre of the depression and possibly freshening a little. The sky will remain overcast with low stratus or stratocumulus giving either intermittent or continuous drizzle and moderate to poor visibility. 

On Passage of the Cold Front: – because of the steeper slope of the cold front, the speed of ascent of the warm air is about twice as fast as that at the warm front, altocumulus and finally cirrostratus and cirrus with patches of blue sky. The rear edge of the cloud mass, found at high levels, is often very clear-cut and extends as an almost straight line across the sky. The barometer will rise sharply as the cold front passes, and the wind will veer to N. or N.W. with possible squalls. The temperature will fall sharply as the front passes and the relative humidity will begin to decrease.

In the Rear of the Cold Front: – there is a gradual improvement, with clearer skies and the characteristic cumulus or cumulonimbus clouds of the cooler air mass giving brighter, showery weather. The barometer will continue to rise, but progressively more slowly, and the wind will probably decrease in strength and continue to veer for a time. The temperature will continue to fall slowly and the visibility will become good.

If the Front is Occluded: – the cloud systems of the warm and cold fronts have combined, and an approaching occlusion is heralded by conditions very similar to those so condensation is more rapid and there is a short period of heavier rain. As the front passes, the cloud may include nimbostratus, but this quickly gives way to altostratus before a warm front, although the period of rain ahead of the front is usually shorter. The difference from a warm front is noticed as soon as the front passes, because instead of the warm sector Tm air, there is a direct change to another polar air mass. The passage of the occlusion and the subsequent conditions, therefore, resemble that of the cold front, although there is perhaps less chance of showers in the clearer weather after the occlusion has passed.

Depression Centre to South of the Observer: – no front is experienced, the barometer will fall steadily for a while then rise steadily, with no abrupt change, and the wind will back slowly from S.E. to N.E. There will be a period of stratiform cloud and rain, probably prolonged.

154. ANTICYCLONES

An anticyclone or High is an area of relatively high atmospheric pressure. It appears on a weather map as a fairly large area of high pressure around which are roughly concentric isobars (see fig. 40-14). A well-developed anticyclone may have a central pressure of over 103Ombs.

Winds circulate around an anticyclone in a clockwise direction in the Northern Hemisphere and in an anticlockwise direction in the Southern Hemisphere. In general, Highs are larger, slower-moving and more persistent than Lows, and their pressure gradients make the winds characteristically light and variable near the centre; stronger breezes are experienced further away, and at the periphery, where the isobars may be squeezed by the passage of a depression, strong winds and gales are not uncommon, even though the barometer is high and steady.

As the wind in an anticyclone blows slightly outwards from the centre across the isobars, air must be leaving the system at the surface, and if the pressure is being maintained, this means that the air is being replaced, and this can only be from aloft. Probably the most important characteristic of a developing anticyclone, and one which largely determines the weather found in it, is the widespread slow descent of air, known as subsidence. This subsidence has two effects: –

(i) When descending air sinks into layers of higher pressure it is warmed (just as air is when it is forced through a bicycle tyre connection by a pump), and 

(ii) As the air temperature rises its relative humidity decreases and any cloud present tends to dry out.

Thus, anticyclones may be expected to be associated with warm, dry, clear conditions, and this is usually true in summer when temperatures generally are relatively high and cloud layers tend to burn away during the day. Occasionally, however, moist air drifts inland from over the sea ﷯and turbulence may cause this air to be lifted above the level at which condensation takes place, giving rise to cumulus clouds. Frequently these clouds spread out horizontally to form a layer of stratocumulus clouds which tend to disappear or reduce in thickness by night, but re-forming or thickening after sunrise. During the winter months, this type of cloud is slow to clear since the sun is low in the sky and has little strength, so that in this case, an anticyclone can produce dull, gloomy weather.

Anticyclones in temperate latitudes do not normally move along a reasonably- predictable path as do depressions, but usually drift about in an irregular way. Sometimes they block the path of depressions and give settled weather for a few days. This occurs when one or more large, warm anticyclones form with their centres about 50°N. and 60°N., and so prevent the Polar Front from lying in its normal position so that frontal depressions must either move into very high latitudes (70°N. or more), or into low latitudes (30° to 40°N.) The normal tracks of these depressions are blocked and this has led to the use of the term blocking high to describe such an anticyclone. Typical situations when weather over Britain is dominated by a blocking high to describe such an anticyclone. Typical situations when weather over Britain is dominated by a blocking anticyclone are shown in fig. 40-15.