From Observations to Forecasts Part 4. Air masses a view from space

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1 From Observations to Forecasts Part 4. Air masses a view from space J.F.P. Galvin Met Office, RAF Akrotiri This article aims to show satellite images of the clouds (or lack of them) typically associated with the air masses that affect the British Isles. Each air mass generally has particular characteristics, notably stability, typical temp erature and humidity all related to its source. There are some difficulties using this approach, however, not least that they only apply in the zone of the mid-latitude Westerlies, close to ocean areas. Air masses are significantly modified as they flow over land or sea. In particular, moist air masses are dried as they flow inland and differences between air masses are likely in different places. Other approaches have been used by authors to describe weather differences, notably Lamb (1972) who used a more complex synoptic system. However, air masses, as described by Pedgley (1962), remain the most commonly used descriptive form, since certain types of weather generally accompany them. Most air masses have their origins in anticyclonic regions, but the cloud patterns associated with them and the weather they bring are determined by their moisture content and whether they are warmed or cooled as they approach the British Isles. As they originate from anticyclones, all are initially stable and not associated with bad-weather systems, which form as the air mass interacts with other air masses or its surroundings. The reader should not necessarily expect exactly the weather described for a particular air mass in this article to affect a certain location; mesoclimatology may significantly alter the local weather associated with an air mass. For instance, scattered showers are relatively rare in polar maritime air masses in the Teign basin of Devon, sheltered to the west as it is by Dartmoor. Where possible, I have tried to suggest how mesoscale features, such as hills or seas, have modified the weather seen in the satellite imagery I have selected. Necessarily, however, these images are just a single snapshot on a particular day and the same image will never be repeated exactly. We will proceed through each of the air masses: arctic maritime, polar maritime, returning polar maritime, tropical maritime, tropical continental and polar continental all as seen by satellite. In each case, a description of the weather that occurred in each illustrated example is given together with the typical characteristics of the air mass in summer and winter (Davies et al., 1997; Kelly et al., 1997) and its persistence (Abercromby, 1883). Arctic maritime (Am) Figure 1 shows an air mass that formed from the Arctic anticyclone, close to the North Pole. When there is an anticyclone over the central-north Atlantic and low pressure over the North Sea or near-continental Europe, northerly winds bring this arctic maritime air across the British Isles. This air mass is always comparatively dry, whatever the season. In the early autumn case shown in this image, surface dew points were between about 2 o C and 5 o C in much of Britain, reflecting the cold source of the air. However, near the east coast there were Figure 1. High resolution satellite image of the British Isles under the influence of an Am air mass at 1204 UTC on 24 September 2004 (courtesy B.J. Burton, obtained from higher values of 6 8 o C, due to the longer sea track over the North Sea. Temperatures are usually several degrees below normal, whatever the season, and it is often cold at night, especially away from windward coasts. The low average temperature of Am air is reflected in its low tropopause, which is usually around 8km in winter, but may reach 10km in summer. The warming of the boundary layer of this air mass as it crosses the warm seas north of the British Isles makes it unstable. It is also moistened at low levels, although the upper troposphere stays dry. As a result, showers are often well scattered; they rarely form where surface dew points remain low. Thus, in this case, there were slight showers only over the North Sea, in northeast Scotland and eastern England in particular, Norfolk, which is prone to showers in northerly winds. Because of the low water-vapour content and remote source of the air, visibility is often excellent in this air mass and rain showers rarely bring visibility below 10km; it is generally only the snow, sleet or small hail showers of winter that bring poor visibility. The rather cloudy weather over eastern coastal areas of England was due to a combination of factors. Convection in air moistened over the North Sea was aided by convergence near the coast, as northnorthwesterly winds over land met northerlies over the sea. A temperature inversion at the top of the boundary layer a remnant of the Arctic anticyclone helped cloud to spread out into extensive stratocumulus. This developing inversion, associated with a ridge of high pressure over western parts of the British Isles, is also evident in the series of wave clouds over many central parts of England and northern Scotland. Areas downwind of high ground, such as central and southern Scotland, much of Wales and many parts of southern England, had a fine, almost cloudless, day. A line of convective cloud forming downwind of the North Channel, through west Wales and southwestern England, is also notable and is a common feature in this air mass. Deep convection can also be seen over the hills of Shropshire and Herefordshire. The dryness of the air gives it a cool feel even in summer, when temperatures may Weather March 2010, Vol. 65, No. 3 59

2 60 Weather March 2010, Vol. 65, No. 3 Air masses - a view from space struggle to rise to near normal by late afternoon. In windier locations, particularly on the east coast, it may feel very cold. In this case, the surface wind speed reached 10ms 1 with gusts to 15ms 1 typical values for a well-developed Arctic maritime airstream. In winter, snow showers are often associated with this air mass. Most often, these fall on higher ground, where accumulations may be enough to cause disruption. Sleet or rain is usually seen at lower levels, but in some cases snow may fall, in particular in heavier showers (Pike, 2001). Polar maritime (Pm) The polar maritime air mass is characterized by scattered showers, as indicated by the patches of convective cloud shown in Figure 2. The showers are most frequent and heaviest close to the area of low pressure that can be seen to the north of Ireland. Pm air brings initially dry cold air from ocean areas (in western Europe, it arrives on westerly or northwesterly winds). The low mean temperature of the air mass is associated with a low tropopause: typically at around 9km in winter and 10km in summer. As the air moves over the warm ocean, it becomes unstable as warmth and moisture are added to the boundary layer. Dependent on the source of the air, instability may reach the upper troposphere, so heavy showers can form. In winter, these showers may be of snow, especially on higher ground, or over northwestern continental Europe. However, only a few showers are likely to reach inland areas in winter, even early in the afternoon, Figure 2. False-colour high resolution polarorbiting satellite image showing Pm air over northern and western parts of the British Isles at 1447 UTC on 20 December 2004 (courtesy B.J. Burton, obtained from com/~wokingham.weather/wwp2.html). when instability reaches a maximum. This generally restricts showers to western parts, whilst well inland, especially to the east of high ground, the cloud may remain well broken. Except in the coldest air, or on high ground, most showers are of rain, although small hail may fall in winter. In summer, showers are more likely to form as the air spreads inland across relatively warm ground, although the dryness of the air generally keeps them isolated. However, if the air mass has very little wind shear and a long sea track, lines of rain may form. These are typically seen downwind of Liverpool, sometimes reaching parts of southeast England. As a rule, visibility is good or excellent except in showers. Heavy rain showers may bring periods of moderate visibility and sleet or small hail showers may reduce visibility below fog limits (1000m). Pm air usually brings temperatures somewhat below the seasonal normal in summer, in particular at night, but often close to normal in winter. This was the case on 20 December In many ways, this winter image is typical. A vigorous cold front can be seen as a relatively narrow band of cloud across eastern and many southern parts of the British Isles; Pm air is following it from the west. Over the eastern Atlantic, there is characteristic open-cell convection: rings of cloud formed by the tops of large convective clouds. Lines of large cumulus or small cumulonimbus clouds can be seen over the Bristol Channel and southwest England and convection can be seen over the Cambrian Mountains. Isolated deep cumulonimbus clouds are apparent over the Lake District and uplands of Lancashire, and vigorous deep convection, spreading out into large cumulonimbus tops, can be seen over Northern Ireland and the Hebridean Sea. There is also the complicating factor of a trough, however, which can be seen over southwest Scotland and central Ireland in this image. Trough lines may bring more or less continuous bands of showers, many of them heavy. Frequently, these troughs form below an upper trough, which has developed in the polar air behind a cold front. In this case, the cold front has occluded in the north, so that there are polar air masses on either side of it. As can be seen in this image, the other characteristic of Pm air is that it often contains a low pressure area that initially formed on the tip of a wave on the polar front, but has become detached as it deepened and the upper trough advanced over it. This low is formed mainly by upper tropospheric dynamics, rather than surface heating as is more often the case with arctic maritime air. Returning polar maritime (rpm) As suggested by its name, this air mass is modified from Pm air, which has an origin in the polar anticyclone. As defined over the British Isles, returning polar maritime air must return from the southwest, from south of 50 o N. The main difference between rpm air and Pm air is the length of the sea track across the warmth of the North Atlantic Drift. rpm air is modified so much that it has very different weather characteristics from Pm, justifying its separate identity. As seen with Pm, this air mass is potentially unstable. However, it is much moister at low levels than its progenitor; it is also warmer, such that both maximum and minimum temperatures are usually near or somewhat above normal. In winter, the air mass is often cooled as it crosses land, so that it becomes stable close to the surface. This modification gives rpm air some of the characteristics of tropical maritime air, although it has a relatively low tropopause, at around 10km. Its character is thus between those of Pm and tropical maritime air masses. The high moisture content of rpm and its northward motion can bring extensive cloud with outbreaks of slight rain or drizzle, especially in southwestern areas. Instability remains important, however, and can always be seen in radiosonde profiles above the boundary layer. The convergence along troughs, or the uplift over high ground, is frequently able to release the potential instability in rpm air. With wet-bulb potential temperature characteristically decreasing with and the high moisture content of this air mass, heavy showers are often seen; these may be accompanied by thunder. Although most showers are of rain, hail sometimes occurs. Wintry precipitation rarely occurs in this air mass, except over the highest mountains in winter. Figure 3 shows rpm air over the British Isles at 1307 UTC on 24 March An occluded front lies over northern England, southern Scotland and much of Ireland; rather cloudy skies can be seen across most other parts of the British Isles, most of it having developed as a result of capped convection within the boundary layer. Figure 4 shows the radiosonde profile from Herstmonceux, East Sussex, at 1200 UTC on 24 March Near sea level, the temperature reached o C during the afternoon over southern England and south Wales. Although these maxima are a little above the seasonal normal, they were insufficient in themselves to initiate deep convection. Nevertheless, deep shower clouds are evident in Figure 3 over south Wales and parts of southwest England. Occurring as a small area, these showers some of which were heavy developed close to a trough, convergence along which was able to release instability in the air mass. The convergence along a trough line, uplift over high ground and cold advection at, provided sufficient forcing to give additional lift to pockets of air which were able to rise above about 5000m. The cool

3 Figure 3. False-colour polar-orbiting satellite image at 1307 UTC on 24 March 2006, showing rpm air across much of the British Isles (courtesy B.J.Burton: obtained from Figure 4. Radiosonde profile for Herstmonceux, East Sussex, at 1200 UTC on 24 March Free convection is limited to about 2900m, given a maximum temperature of 13 o C, showing the need for forcing to produce heavy showers. Cold advection is evident in winds backing with above 400hPa (courtesy University of Wyoming, Department of Atmospheric Science). origin of this air mass is evident in the tropopause of 10km, although this is somewhat higher than that of the original Pm air, since the air has warmed over the North Atlantic. Although the tropopause is usually somewhat higher in summer than in winter, it remains somewhat lower than that of subtropical air. In summer, skies are usually clearer in this air mass than in the early spring case shown here, except near troughs. In the warmer months, however, instability is released more readily than in winter and may bring heavy thundery showers. This air mass is the one that most commonly brings thunderstorms not associated with a frontal system. Although visibility is often good in rpm air, in particular in summer, it is rarely much more than 15km. During winter, when this air mass passes over cool ground, moderate visibility is more common even locally poor in drizzle or over hills. Heavy showers will also usually reduce visibility, although the onset of heavy rain after drizzle may bring an improvement from moderate levels. Tropical maritime (Tm) Figure 5 shows a tropical maritime air mass making gradual progress eastwards across the British Isles on 21 November Tm air is characteristically warm and moist, having its origin over the subtropical oceans. In western Europe, its motion is from the southwest. During the winter, this air mass often brings the warmest weather to the western fringes of the continent. As a wave on the polar front develops, this air is preceded by a warm front and followed by a cold front. Initially very warm, the air passes over increasingly cold water as it moves northeast, so its stability increases. Due to its warmth and moisture, however, its density is relatively low, so it ascends above the air masses ahead of and behind it, producing thickening cloud and precipitation (Collier, 2003). This falls mainly as rain, although sometimes as snow in winter over high ground, as well as at low levels from central Europe eastwards. Tm air is the main source of precipitation in winter in the middle latitudes. Between the warm and cold fronts, the characteristic cloud is stratocumulus, which may have extensive stratus below it, particularly when the stratocumulus is thick enough to produce drizzle (Overton and Galvin, 2005). As can be seen in this image, however, mountain ranges may break up the stratocumulus. In this case, holes developed in the cloud over east Wales, the Marches and, in Figure 5. High resolution false-colour polarorbiting satellite image at 1130 UTC on 21 November 2004 over western Europe (courtesy University of Dundee Satellite Receiving Station). particular, over many parts of Ireland. Above the stratocumulus there is often cirrus or cirrostratus, especially in the air near the polar front. This forms in the jet stream, which forms in the warm Tm air just below the tropopause. These strong winds are associated with the dynamics that deepen the depression and the cloud of the frontal wave. The high moisture content of Tm air often brings poor visibility at low levels, in particular when surface temperatures were low prior to the arrival of the warm sector. In winter, the visibility may not rise above 10km in the boundary layer and in places there may be fog, especially on higher ground. Looking to the west of the British Isles, a supplementary feature can be seen. It is a so-called split cold front. When the westerly component of the wind at higher levels is stronger than that near the surface, much of the thick frontal cloud, some of it developing in convective currents just behind the upper cold front, moves ahead of the surface position of the cold front, as originally described by Miles (1962). This results in much of the cold-frontal precipitation falling within the warm sector, some of it as showers. In this case, the upper cold front can be seen arcing north and south of 55 o N, 12 o W, whilst the surface front arcs north and south almost 3 o (170km) further west, indicated by a marked edge to the stratocumulus cloud. Although the weather situation in this case was typical in many respects, it was atypical in that the tropical maritime air was preceded by an arctic maritime air mass that had settled over the British Isles during the preceding two days. This meant that, following overnight cooling under clear skies in eastern parts of the United Kingdom, the warm moist tropical air made very slow progress at the surface, resulting in an almost isothermal layer near the surface with a temperature close to 0 o C, just ahead of the warm front, over northern and eastern Scotland and northeast England. Whilst some snow fell on higher mountains, many areas of lower ground had freezing rain, resulting in very slippery surfaces. This situation is very unusual in Britain, particularly at the beginning of the winter season. The advection of warm air was dramatic, once it finally made progress into eastern parts. Surface air temperatures reached 11 o C or 12 o C within the warm air over western areas and temperature rises of around 5 degc in an hour were noted in eastern Scotland. As in this case, the surface wind may be very light before the tropical air arrives, but freshens in the warm air, particularly close to the cold front, where a conveyor belt may bring the warmest, but most moist, air of all (Young, 1994). Indeed, this air mass is the one most likely to bring gales across the British Isles. The higher the average temperature of the troposphere, the deeper the air mass; the Air masses - a view from space Weather March 2010, Vol. 65, No. 3 61

4 62 Weather March 2010, Vol. 65, No. 3 Air masses - a view from space of the tropopause is usually between about 12km and 13km in Tm air. Tropical continental (Tc) Figure 6 shows an air mass that formed from an anticyclone over central Europe, bringing warm, comparatively dry, air across the British Isles. If we confine our consideration to typical surface weather conditions, this air mass is relatively rare, since it originates from anticyclones that have a comparatively short life, of the order of a few days to a few weeks at most. As a result, I will only discuss the air mass from a summer case, even though tropical continental air also occurs in winter. With the short nights of winter, the characteristics of Tc air, close to the surface, are similar to those of polar continental air (although the air aloft is generally much warmer than that of polar air and so the air mass is more stable). In many cases, this air mass is seen only for a short time, before frontal systems move in from the west; this is seen in Figure 6, where there is a waving cold front through Cornwall and across the Irish Sea and western Scotland. The air mass is warm and, although initially dry near the surface, where the air travels a long distance over the sea cooling by the relatively low sea temperatures of spring or early summer can cause saturation in the boundary layer. This is often the case close to the east coasts of England and Scotland and low cloud may spread well inland (Galvin, 2003). Some low clouds, formed over the North Sea, can be seen over eastern-central Scotland in Figure 6. Figure 6. High resolution satellite image of the British Isles under the influence of a Tc air mass at 1250 UTC on 4 May A weak cold front can be seen across western Britain, with a wave over the northern Irish Sea (courtesy B.J. Burton, obtained from com/~wokingham.weather/wwp2.html). Overnight, under mainly clear skies, fog or low cloud may form inland. The fog forms over low-lying parts when wind speeds are low, whilst stratus may settle onto the tops of ranges of hills, especially over England, when the southeasterly winds are a little stronger (Mansfield, 1988). There are often layers of higher relative humidity above the surface in Tc air and altocumulus castellanus is a characteristic cloud of this air mass. In Figure 6, a band of these medium-level clouds, ascending in unstable conditions, stretches across Wales, Morecambe Bay and southern Scotland. A marked difference in stability occurs near the surface. Near coasts, the air mass remains stable, frequently bringing almost unbroken sunshine, whilst inland, cumulus clouds usually form by afternoon, although their vertical extent is generally limited by an inversion at the top of the mixing layer. These elements can be seen in the radiosonde profile shown as Figure 7. In some cases, convection from the surface may be sufficiently vigorous to allow deep convection, bringing thunderstorms, in particular in late summer. These storms mainly occur in the evening or overnight, as cooling occurs at the cloud tops, thus increasing instability through the middle layers of the troposphere. Indeed, this is the most common air mass associated with thunderstorms in some parts of the British Isles. Thunderstorms developed ahead of, and in association with, the front shown over Ireland in Figure 6, and were violent over Northern Ireland and southwest Scotland in the evening. In the spring case shown in this satellite image, surface dew points rose to between 12 o C and 14 o C in much of inland Britain, reflecting the high values of θ w of this air. Temperatures are usually several degrees above normal in summer and, on 4 May 2006, maxima of o C were widely seen across inland parts of southeastern England. Figure 7. Radiosonde profile from Watnall, Nottinghamshire at 1200 UTC on 4 May Medium-level instability can be seen between about 2700m and 4000m (courtesy Department of Meteorology, University of Wyoming, sourced from html). The daytime warmth is often aided by the relatively light east to southeasterly winds of this air mass, which has spent most of its time over land although in this example, a tightening pressure gradient and increasing instability led to increasingly blustery conditions during the day. Visibility is often good away from windward coasts as convection becomes established. Where the air mass is stable, near these coasts, visibility may be moderate or poor. This is due to a combination of the moistening of the air at the surface as it crosses the English Channel or North Sea and pollution from industrial sources in continental Europe. Where advection fog forms over seas and coasts, visibility may fall to between 500m and 1000m, and on affected coasts maximum temperatures are usually several degrees below normal. Polar continental (Pc) This air mass has its origin in the semi-permanent anticyclone that forms over northeastern Europe. In winter, this anticyclone is typically well developed and may extend south into central Asia at low levels, intensifying over cold snow-covered ground. The air is always comparatively dry, whatever the season, and is usually drier at low levels than Am air. In the late winter case shown in Figure 8, surface dew points were between about 4 o C and 1 o C across much of Britain, reflecting the cold dry source of the air. Dew points, however, also reflected the relatively long sea track of this air across the North Sea, so that, typically, they were lower in southern England than across eastern Scotland. In this case, the surface temperature of the North Sea was between 6 o C and 7 o C close to the east coasts of England and Scotland. Having a cold origin, Pc air has a low tropopause in winter, typically 9km, although there is some variability and Pc air in summer may have a tropopause above 10km. In this case, it was at 9300m. A significant feature of Pc air is the difference in overnight minimum temperature between eastern coastal districts and areas further west. On 22 February 2005, there was an air frost westward of about 10km to 30km inland in rural areas, while air temperatures remained above freezing in the coastal zone. The anticyclonic origin of this air mass can be seen in the radiosonde profile for Albemarle, Northumberland (Figure 9), which shows instability to about 4km. However, the moistening of the air near the surface was sufficient to enable cumulonimbus clouds to form with a cloud-top temperature of 23 o C. The warming and moistening of the boundary layer of this air mass makes it unstable, although showers are often

5 Figure 8. High resolution satellite image of the British Isles under the influence of a Pc air mass at 1416 UTC on 22 February 2005 (courtesy B.J. Burton, obtained from com/~wokingham.weather/wwp2.html). isolated since the air mass is dry in depth. The potential temperature of the air is low when it leaves continental Europe; temperatures rise slowly and may only reach 4 to 6 o C along the east coast, as was the case here. Thus, in this case, there were slight sleet and snow showers affecting mainly eastern and central England, eastern Scotland and the central lowlands of Scotland. Many western areas remained dry. Upland eastern Figure 9. Radiosonde profile for Albemarle, Northumberland at 1200 UTC on 22 February The temperature inversion at 4km was a cap to convection (courtesy University of Wyoming). areas saw considerable accumulations of snow from these showers, although there were large variations from place to place, even over small distances. Close inspection of Figure 8 shows significant snow lying only on parts of the Grampian Mountains, Southern Uplands and the Pennines. Although it was rather cloudy over many eastern areas of England and Scotland, there was some sunshine. Further west, overnight cooling typically brought broken low cloud early in the day, associated with very slight flurries of snow or ice-pellets. The layer cloud cleared late in the morning, allowing sunshine to develop which led to cumulus and cumulonimbus clouds developing further west as surface temperatures rose. Isolated showers reached many central-western parts of England during the afternoon, although to the lee of high ground and in the far west, it typically remained dry. This transition is marked by the change from deep convection to lines ( streets ) of cumulus clouds in Figure 8. The moistening of this air, combined with its passage over industrial areas, often brings moderate, locally poor, visibility to many eastern areas and in this way it differs markedly from Am air. However, recent legislation to restrict pollution in Europe has brought a notable improvement of visibility in this air mass over the past 20 to 30 years. With winds from such a dry source, it will often feel cold in Pc air, even if the temperature and humidity have risen markedly as the air has crossed the North Sea. This cool feel is present even in summer (although this air mass is relatively uncommon in the summer season and may be substantially modified). Summary This article has given a description of each of the weather types to be expected from the air masses that affect the British Isles. To summarize, Tables 1 and 2 have been prepared to provide an easy reference to these characteristics. In each case, the description is general and it should be remembered that local variations are likely. The stability is given as that which may be expected on windward coasts, since the diurnal temperature cycle affects all air masses as they cross land. In each case, the description is for a deep air mass (such that fronts are not bringing another air mass above the one described). The relative temperature anomalies are for mean temperature: cloudy airstreams (e.g. Tm) have a low temperature range, so that maximum temperatures are near normal, but minima are usually well above normal. The anomalies vary with distance from the sea and sea temperature, compared with the potential temperature of the air mass. Air masses - a view from space Weather March 2010, Vol. 65, No. 3 Table 1 The characteristics of air masses affecting the British Isles in summer. Air mass Visibility Temperature anomaly Arctic maritime Excellent or good (locally Cool or very cool moderate in precipitation) Polar continental Polar maritime Returning polar maritime Tropical continental Tropical maritime Good (locally moderate in rain) Good or excellent (locally moderate in hail showers) Moderate to good (locally poor in drizzle or thundery showers; local fog) Moderate to good (locally poor in thundery rain; Iocal fog) Moderate (locally good; poor in drizzle; local fog) Near normal Near normal or cool Near normal or warm Very warm or hot Near normal Weather type Scattered showers of rain or small hail (snow possible on high ground) Usually dry; possible smoke haze Scattered rain or hail showers, especially in west Rain or drizzle in places; heavy thundery showers locally; fog on SW coasts Dry for most; fog may affect coasts; thundery showers; smoke haze Slight rain or drizzle; fog on high ground and coasts Stability (at the coast) Stable Stable, but potentially unstable at Stable, but potentially unstable at Stable Tropospheric 9 10km 10 12km 10 12km 11 12km 12 14km 12 14km 63

6 Table 2 The characteristics of air masses affecting the British Isles in winter. Air mass Visibility Temperature anomaly Arctic maritime Excellent or good Cold (locally moderate or poor in precipitation) Weather March 2010, Vol. 65, No. 3 Air masses - a view from space Polar continental Good (locally moderate in showers or haze; poor in snow) Cold Polar maritime Good or excellent Near normal or cool Returning polar maritime Tropical continental Tropical maritime Moderate to good (locally poor in drizzle or thundery showers; local fog) Moderate to good (Iocal fog) Moderate (poor in drizzle; local fog) Warm or near normal Cool Warm Weather type Scattered showers of rain, small hail or snow Wintry showers near NE coast; otherwise dry; possible smoke haze Scattered rain, hail or snow showers, especially in west Rain or drizzle in places, thundery showers locally; fog on hills Dry for most; radiation fog; smoke haze. Slight rain or drizzle (may be persistent); fog on high ground Stability (at the coast) Unstable to the mid troposphere Stable, but potentially unstable at Unstable at low levels Neutral or stable Tropospheric 8 9km 9 11km 9 11km 10 11km 11 13km 11 13km References Abercromby R On certain types of British weather. Q. J. R. Meteorol. Soc. 9: Collier CG On the formation of stratiform and convective cloud. Weather 58: Davies T, Kelly PM, Osborn T Explaining the climate of the British Isles, in Climate of the British Isles. Hulme M, Barrow E (eds). pp Galvin JFP Weather image: North Sea stratus. Weather 58: Kelly PM, Jones P, Briffa K Classifying the winds and weather, in Climate of the British Isles. Hulme M, Barrow E (eds). pp Lamb HH British Isles weather types and a register of the daily sequence of circulation patterns Meteorol. Off. Geophys. Mem. No HMSO: London. Mansfield DA An investigation into stratus distribution over the United Kingdom, Meteorol. Mag. 117: Miles MK Fronts. Weather 17: Schools Supplement No.12, Overton AK, Galvin JFP Readers forum: precipitation from various clouds. Weather 60: Pedgley DE A course in elementary meteorology (Meteorological Office): 9:3. HMSO: London. Pike WS Meteorologist s profile Frank Emerson Lumb. Weather 56: Young MV Back to basics: Depressions and anticyclones: Part 2 Life cycles and weather characteristics. Weather 49: Correspondence to: J.F.P. Galvin Met Office, FitzRoy Road, Exeter, EX1 3PB, UK. jim.galvin@metoffice.gov.uk Crown Copyright, 2010, published with the permission of the Controller of HMSO and the Queen s Printer for Scotland DOI: /wea.452 Book review 64 By Howard B. Bluestein Oxford University Press 2006 xii + 180pp Paperback ISBN13: Tornado Alley For everyone with an interest in all forms of severe weather! This autobiographical account of Howard B. Bluestein s long and distinguished experience in tornado and thunderstorm research gives readers a basic scientific understanding of thunderstorms (in their many varieties), tornadoes, and the instruments developed to track, measure and forecast these life-threatening phenomena. It includes gripping accounts of the trials and rewards of dedicated storm chasing, from the elation of finding that elusive vortex to the more frequent sight of an empty sky. The book begins in the early 1950s, when tornado research was in its infancy. The autobiographical accounts start in the late 1970s, and Howard tells us of his very first tornado. Without losing the sense of excitement, he then proceeds to describe all the new technologies devised for tornado research over the next 20 years. He has included numerous sketches and photographs (including one of a cauliflower) in his descriptions of most aspects of severe convection. Unsurprisingly, this book concentrates on the great plains of America, with some references to other American tornadoes; there is no mention of other tornado-prone areas of the world, although worldwide experts on this subject are included. The 180 pages are full of pictures, diagrams and descriptions of the things Howard has seen and experienced in his lifetime of storm chasing. And there s not a single mathematical equation! Stephen Moseley DOI: /wea.535