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Sources: BBC, National Weather Services, Intellicast, Space Science and Engineering Center, the Met Office, Yahoo! Weather

TODAY’S HEADLINES

• Severe weather in the British Isles (see BBC News, Pictures, Video, Audio, Yahoo News, UK Gales, Power cuts)
• Extremely cold in Edmonton
• Hot and sunny in Singapore and Kampala
• Hot and humid again in Jakarta
• Snow in Sweden, Finland and Nevada
• Sunny in Constanta and Toulouse
• Warm in North Carolina
• Cool in California

TODAY’S EXTREMES

Hottest: Jakarta International School (Indonesia) and Two Boats School (Ascension), both 32°C

Coldest: James Gibbons School (Edmonton, Canada), -28.0°C

Windiest: Beck Junior High School, Katy, Texas, USA, 95 km/h

Driest: Pasadena City College, California, USA, relative humidity 45%

Wettest: Jakarta International School (Indonesia), 91.8 mm of rain

FEATURE OF THE DAY

To find this feature, which is intended for secondary schools, click here. A feature for primary schools will appear tomorrow.

TODAY’S REVIEW OF METLINK OBSERVATIONS

Have you looked at today’s weathercams? If not, click here.

To view today’s observations in the database, click here.

Unfortunately, the database was not available for much of the weekend and did not become available again until about 09:30 GMT today. The problem lay not with the database but with the host server. We are sorry for the break in service. Despite the problem, quite a number of schools have entered data for Saturday 26 January and Sunday 27 January and these observations can be viewed in the database. To do so, go first to the database and then type in the search box 26-01 if you wish to view Saturday’s data, 27-01 if you require Sunday’s.

To find out how your observations compare with the average for January, click here.

EASTERN ASIA

Hot and humid again in Jakarta, Indonesia. The temperature 24°C and the relative humidity 100% at 00:00 GMT (7 am Local Time). Minimum temperature 23°C. Maximum 32°C. Not raining at the time of the observation but there had obviously been a downpour, the measured rainfall amount being 91.8 mm. "A fine, hot, sunny day with lots of blue sky" at the American School, Singapore, where the temperature at 06:30 GMT (2.30 pm Local Time) was 29.0°C and the wind a light north-easterly (9 km/h).

AFRICA AND ASCENSION

"Hot and sunny" in Kampala, where the temperature at 10:00 GMT (12 noon Local Time) was 24°C and the wind a 15 km/h breeze from the south-east. Evidently a very pleasant day, with only three oktas of cumulus cloud. Pleasant, too, at Pretoria, at 10:05 GMT (12.05 pm Local Time), when the sky was only partly cloudy (4 oktas of stratocumulus), the wind very light (only 3 km/h) and the temperature 26.6°C. Not quite so pleasant at 19:10 GMT, however, when slight drizzle was falling. Cloud amount then 5 oktas of cumulus and cirrocumulus and the temperature 22.8°C. Maximum temperature 28.7°C. "Overcast and warm" at Highbury Preparatory School, KwaZulu Natal, where the temperature was 21°C at 05:00 GMT (7 am Local Time), the relative humidity 91% and the cloud amount 7 oktas (of altostratus cloud).

Warm and breezy on Ascension, where the temperature at 12:45 GMT was 27°C, the wind speed 15 km/h and the cloud amount 4 oktas (of cumulus cloud). Slight rain falling at the time of the observation and 15 mm of rainfall measured. Maximum temperature 32°C. "Sunny with building cloud cover" at Peterhouse in Zimbabwe, where, at 06:00 GMT (8 am Local Time), the temperature was 22.6°C, the wind calm and the cloud amount 3 oktas (of cumulus cloud).

EUROPE

A wet and windy night over many parts of England and Wales, followed by a day of severe weather over Scotland, northern England and the northern half of Ireland. Trees blown down. Many trucks (lorries) blown over. Bridges closed. Road, rail and sea transport disrupted. Power failures in some places. Flood alerts along rivers and coasts in many parts of the British Isles. Gusts of wind of more than 120 km/h in many parts of Ireland, southern Scotland and northern England. A gust of more than 200 km/h on a mountain in Scotland this afternoon. Several people dead as a result of the stormy weather. For a chart showing weather observations over the British Isles at 06:00 GMT today, click here.

At Wells Cathedral School in Somerset, "a sunny and breezy day after a very, very wet and windy night". Rainfall 26.1 mm, which is quite a large amount for a period of 24 hours at a station in the United Kingdom. Thunder and lightning reported from Milford Haven School in Pembrokeshire, where heavy showers occurred and the wind was strong (44 km/h at 08:56 GMT). The Medina Valley Field Centre on the Isle of Wight reported that "very blustery weather over the weekend" continued today "causing choppy seas". Here, the wind speed was 33 km/h at 09:00 GMT. Much better at Westbourne House School in West Sussex, where the day was windy but clear, with only 2 oktas of cumulus cloud at 10:30 GMT. Windy but sunny at the Royal Hospital School in Suffolk, where there was only one okta of cloud at 14:30 GMT (wind speed 33 km/h). "Heavy wind and rain through the night, slackened off now" was the report from Alleyne’s High School in Staffordshire. Rainfall here only 11 mm. Quite breezy at 13:00 GMT, with a wind speed of 24 km/h. "A dull and blustery day with patches of heavy rain" at Altrincham Girls’ Grammar School in Lancashire, where rain of medium intensity was falling at 13:00 GMT and the wind speed was 33 km/h. Rainfall amount as much as 40 mm.

Stronger winds occurred elsewhere in the British Isles, especially farther north: 67 km/h at Edgbaston High School for Girls in Birmingham, St Patrick’s Primary School in Lancashire, St Oswald’s Primary School in Sheffield, Roose Primary School in Cumbria, St Theresa’s School in County Galway, George Watson’s College in Edinburgh and Queensferry Primary school in Edinburgh; 81 km/h at Portgordon Primary School in Morayshire, where the weather was "dull, wet and windy". Heavy rain fell in most parts of Scotland. Lionel School on the Isle of Lewis measured 84 mm and the students of Farr High School on the north coast of Scotland said the weather there at 09:00 GMT was "cold, cloudy, rainy, wet and foggy". The temperature at the time was 6.9°C, which was a little higher than at Lionel School, where it was only 4°C at 12:33 GMT. Elsewhere in the British Isles, temperatures were generally in the range 8-11°C, but the wind made the weather feel much colder than that. Over almost all parts of the British Isles, winds blew from a westerly point. The only exception was Farr High School, where the wind blew from the north-east.

Temperatures again low in Scandinavia, with the coldest place Mikkeli in Finland, where the minimum temperature was —21.6°C and the snow depth reported to be 65 cm. Cold, too, at Vörå-Oravais-Maxmo Högstadieskola, where the minimum was -16.2°C. "Wet and dull" at Bor School, near Värnamo in southern Sweden, where, at 07:00 GMT (8 am Local Time), the temperature was only +0.4°C and snow of medium intensity was falling. For a chart of weather observations over Scandinavia at 06:00 GMT today, click here.

"A wonderful day" with lots of sunshine was experienced by the students of Carmen Sylva High School in Romania. Maximum temperature 10.7°C. "The weather is getting warmer every day", said the students. Not so nice at Bozen in northern Italy today, where, at 09:00 GMT (10 am Local Time), the sky was overcast and the temperature only +2°C. There was a "beautiful clear fresh day with light breezes" at the International School of Toulouse in south-west France, where, at 11:00 GMT (noon Local Time), the temperature was 13°C and the lightest of easterly winds was blowing (3 km/h). The weather was mild at Strasbourg in eastern France, too, but rather cloudy. A westerly wind of 15 km/h was blowing during the day and the maximum temperature was 14.0°C.

Charts showing temperatures, significant weather and distributions of barometric pressure over Europe at 00:00 GMT and 12:00 GMT today show snow over Sweden at 12:00 GMT, warm weather over southern Europe and very strong winds over the British Isles.

NORTH AMERICA

Extremely cold in Edmonton, Alberta, where the temperature fell to —33.4°C over the weekend. Slightly less cold than that today at 16:30 GMT (9.30 am Local Time), when the temperature was —25.0°C. A sunny day, though, with a light wind. Much warmer than of late and quite sunny at Raleigh in North Carolina, with a maximum temperature of 24°C and only 3 oktas of cloud at 18:00 GMT (1 pm Local Time). Warm and very windy at Katy in Texas, where the temperature at 20:15 GMT (2.15 pm Local Time) was 25.7°C and the wind speed 95 km/h. The students, however, considered their weather "cloudy and cool". "Very cold and cloudy" at Fernley in Nevada, where snow of medium intensity was falling from a completely overcast sky at 16:38 GMT (8.38 am Local Time) and the temperature was —3°C. "Sunny and cold" at Moses Lake in Washington State, where the temperature was +0.6°C at 19:00 GMT (11 am Local Time) and the cloud amount small (only 2 oktas of stratus, cirrus and cumulus). "Cold and sunny" at Cantara Street Elementary School in Reseda, California, where the temperature was 9.5°C at 18:39 GMT (10.39 am Local Time). Not much warmer in Pasadena, where the temperature was 11°C at 20:20 GMT (12.20 pm Local Time). The students of both schools were expecting rain during the afternoon. Slight drizzle falling at Mendenhall Middle School (Livermore, California) at 22:00 GMT (2 pm Local Time). Temperature only 6.7°C. For a chart showing weather observations over the south-west of the USA at 18:00 GMT today, click here.

TODAY’S ANALYSIS

NB On the so-called ‘visible images’ from satellites, what you see is more or less what you would see with your own eyes if you were in the satellite and looking in black and white. On the so-called ‘infra-red images’, the whitest areas are the coldest and the blackest the hottest. Thus, high clouds show as white areas. Medium-level clouds and very cold land areas show as grey. For information about obtaining satellite images of the highest quality, see the Notes section below.

AUSTRALASIA

A vigorous depression brought severe weather to Japan over the weekend. This depression can be seen on the visible satellite image for 03:00 GMT today; and the chart of observations made at 06:00 GMT today shows that the northerly and north-westerly winds on the western side of the depression were cold.

Today’s weather chart for Australia and adjacent regions shows that barometric pressure was low over most of Australia and also to the south of Tasmania. It was high over the Great Australian Bight and over the Indian Ocean to the west of Perth. The infra-red satellite image for 05:32 GMT today shows the clouds associated with the Low south of Tasmania, the cold front south of the Great Australian Bight and the eastward-moving trough over eastern Australia. Some scattered outbreaks of cumulonimbus activity over northern Australia can also be seen.

For information about the weather over Singapore, Indonesia and other equatorial regions, click here.

AFRICA

Barometric pressure was low today over South Africa and the waters immediately to the south of South Africa (1007 mb off Port Elizabeth). Pressure was high to the south-west of Africa (1025 mb near Gough Island) and near 40°S 50°E (also 1025 mb). A ridge of high pressure extended northwards over the South Atlantic as far as Ascension Island.

EUROPE

Today’s weather charts for 00:00 GMT and 06:00 GMT show a deep depression (952 mb at 00:00 GMT, 957 mb at 06:00 GMT) north of Ireland. This depression was moving north-eastwards, accompanied by very strong westerly winds on its southern flanks. At the automatic weather station on the buoy at 53°50’N 19°50’W, the wind speed reached 26.3 m/s (Force 10) at 15:00 GMT yesterday. For winds over Ireland at 11:00 GMT today, click here. The fronts of the depression that brought the gales which caused such havoc over the British Isles brought precipitation to southern Scandinavia and northern Germany (for a satellite image that shows them, click here). The dominant weather influence over southern Europe was high pressure.

To obtain the latest information about flood watches and flood warnings in Great Britain, click here for England and Wales (from the Environment Agency), here for Scotland (from the Scottish Environment Protection Agency).

NORTH AMERICA

Today’s weather chart and satellite images show fronts affecting central parts of the USA, with low pressure over the Rockies and the Midwest. Cold winds from the north flowed across California, Nevada and Oregon. Winds over the south-east of the USA were warm south-westerlies between the Low over the Midwest and a High over Florida and Georgia. Satellite images show an extensive area of cumulus and cumulonimbus activity over the eastern North Pacific, where cold air was passing over warmer water and thus being heated from below to become convectively unstable. Skies were clear over many southern and eastern parts of the USA today. High pressure was the dominant influence over western and central parts of Canada, with centres of high pressure over southern Manitoba (1033 mb) and British Columbia (1031 mb).

FEATURE OF THE DAY

To return to Today’s Review of MetLink Observations, click here.

To return to Today’s Analysis, click here.

THE INTERTROPICAL CONVERGENCE ZONE

Turn to many books that cover climate and you will find that the near-equatorial belt of cloud and rain known as the Intertropical Convergence Zone (ITCZ) is shown as a continuous wavy line around the globe.

In such books, climatic charts are usually provided for the months of January and July. Charts for January usually show the ITCZ as a line dipping south into latitudes 10 to 20°S over Australia, South America and southern Africa. Elsewhere, it is shown as a feature that lies close to the equator. Charts for July usually show the ITCZ as a feature extending into the sub-tropics over eastern Asia, which questions the definition of the ITCZ. Is the ITCZ a zone? Indeed it is, wherever it occurs, but is it inter-tropical? In latitudes 30-35°??? Certainly not!

Meteorologists do not now consider the term ‘Intertropical Convergence Zone’ appropriate for the climatic feature that lies over southern and eastern Asia from June to September. Satellite images during the northern hemisphere’s summer show that, over the Indian Ocean, a belt of cloud and rain often lies just to the south of the equator. This is really all that exists of the ITCZ from about 50°E to the west Pacific during the period June to September. The low pressure and heavy rain that occur over northern parts of the Indian subcontinent, Bangla Desh, Myanmar (Burma) and south-west China are associated with the monsoon of southern Asia, which is now recognised as an atmospheric feature that is essentially different in structure and formation from the ITCZ.

Is the ITCZ continuous? No, if clouds and rain are the defining features. See, for example, today’s global composite from the University of Wisconsin-Madison. This shows that the activity of the ITCZ varies greatly from region to region. In some parts, the zone appears to be almost non-existent. In others, it is quite active. Today, it was quite active over the Indian Ocean and Indonesia, as the visible, infra-red and water-vapour satellite images show. Remember that visible images show you what you would see if you were in the satellite and looking with your own eyes at the view below (but looking in black and white). Water-vapour images show distributions of water vapour in the upper troposphere. The wetter the atmosphere, the whiter it appears, and the drier it is, the darker it appears. Infra-red images show you temperatures. The hotter the surface below, the darker it appears. The colder the surface below, the whiter it appears. The tallest rain clouds of the ITCZ reach altitudes of 12-15 km, where the temperature is typically in the range —65 to -80°C. No wonder the tops of the clouds are composed of ice crystals.

Satellite images show you the tops of cloud systems; that is, in the case of ITCZ cloud systems, the tops of the cumulonimbus clouds that produce rain. As we have seen from the MetLink observations received from Jakarta and Medan during the past week, rainfall amounts from ITCZ cloud systems can be considerable (75-100 mm in a day). The cumulonimbus clouds of the ITCZ are grouped in so-called ‘cloud clusters’, each of which typically has a diameter of several hundred kilometres. The anvils of individual clouds spread horizontally below the tropopause and in some places almost merge, which is why cloud clusters seen from above show up as extensive sheets of cirrus cloud covering hundreds of square kilometres. Within clusters, the parts of individual cumulonimbus clouds that lie below anvil level are typically no more than about 10 km in diameter.

We can now see why rainfall amounts in the parts of the tropics affected by the ITCZ vary so much from place to place and day to day. Places that find themselves under cumulonimbus clouds receive quite large amounts of rainfall. Places not far away may escape rain altogether. And some places may be in regions where the ITCZ is absent or comparatively inactive. Fronts of the kind associated with depressions of middle latitudes are not found in near-equatorial latitudes. The occurrence of rain within the ITCZ is more determined by local factors such as orographic lifting than by weather systems like those that bring clouds, precipitation and strong winds to middle latitudes. However …

Disturbances sometimes develop within the ITCZ. When they do, they start life as well-organized cloud clusters. They then become tropical depressions, appearing on weather charts as distinct areas of low pressure, the winds associated with them not exceeding Force 7. If such disturbances develop, they become tropical storms, when, by definition, the winds averaged over a period of one minute are in the range Force 8 to Force 11. Gusts may, of course, be much greater than 117 km/h, which is the maximum speed in the Force 11 category. When a tropical storm intensifies, it becomes a tropical cyclone, its mean winds now 118 or more km/h (Force 12).

The farther the ITCZ lies from the equator, the more likely disturbance development becomes. Thus, in the period December to March/April, cyclones form over the Indian Ocean and western parts of the South Pacific. In the period June to September, when the ITCZ reaches its farthest north, tropical cyclones form over the North Pacific and North Atlantic Oceans. They also form over the Arabian Sea and Bay of Bengal, but only in the month or so before the south-west monsoon begins and the two months after it retreats. Wherever cyclones occur, they do not develop if the sea-surface temperature is less than 26.5°C. For this reason, tropical cyclones do not occur over the South Atlantic or eastern parts of the South Pacific.

What are the seasonal migrations of the ITCZ? To find out, click here for an animated sequence of monthly rainfall distributions around the world. Note that the colour convention is unusual, in that dry areas are shown in blue, wet areas in orange and brown! Click here for an animated sequence of monthly temperature distributions around the world.

To return to Today’s Review of MetLink Observations, click here.

To return to Today’s Analysis, click here.

NOTES

SATELLITE IMAGES FROM DUNDEE

You can obtain satellite images of the highest quality from the Dundee Satellite Receiving Station, Dundee University, Scotland. For further information, see the ‘Notes’ sections of the MetLink weather reports for 22 and 23 January.

To return to Today’s Analysis, click here.

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