Weather Data

Weather data come chiefly from weather observing stations on land, and from ships at sea (Figure 15.1). Measurements are made of pressure (Section 1.5), temperature (Section 3.1), humidity (Section 6.3), rainfall (Note 10.B) and wind (Section 14.1). Other readings are taken of visibility, lightning and thunder, and cloud cover at three levels of the troposphere. In addition, there were about 600 buoys drifting at sea by 1992, and another 200 or so moored, measuring air and sea-surface temperatures and pressure, and then transmitting the data to passing satellites for later relay to ground receivers. Weather radar is now widely used, especially near cities, to monitor rainfall, wind and temperature (Note 15.A).

The number of places where weather data are collected has increased steadily (Note 15.B). There were over 13,000 weather stations worldwide by 1994, where observers were taking readings at the same moment, each three or six hours. But only 17 per cent of the places were in the southern hemisphere, where the network of observations remains inadequate because of the vast areas of ocean, ice and desert, and lack of finance (Table 15.1).

Weather forecasting requires information on winds and temperature aloft; so wind, temperature and humidity profiles are measured daily at about a thousand weather stations. Wind and temperature are also reported by most large civil aircraft, producing several thousand soundings of the troposphere every day in the vicinity of airports.

Increasing use is made of satellites to fill the large gaps in observations over the oceans and unpopulated land (Figure 15.1). Instruments on the satellites measure cloudiness (Section 8.8), rainfall (Note 10.C), upper-level winds (Section 12.3) and surface winds over the ocean (Section 14.1). Such measurements now outnumber those from weather stations, but the latter remain important: (i) to calibrate satellite instruments, (ii) to provide more accurate data, and (iii) for obtaining guidance in interpreting satellite data.

Measurements are also taken at climate stations, where instruments indicate the daily extreme temperatures, twice-daily humidity, daily rainfall and, in some cases, hours of sunshine (Section 2.2), soil temperatures (Section 3.5), pan evaporation (Section 4.5) and dew (Section 4.7). These data are mailed to headquarters at the end of the month for eventual

Figure 15.1 Typical daily coverage of surface observations at weather stations and ships.

Table 15. 1 Numbers of daily weather stations active in 1994. These stations measure eighteen elements, such as daily mean temperature, extreme temperatures, dewpoint, mean wind speed, maximum wind speed, mean pressure, visibility, total precipitation and snow depth

Area Number

South Africa, Namibia, Botswana and 168 Lesotho

Argentina 129

Brazil 342

New Zealand (plus islands) 319

Australia (plus islands) 666

Southern hemisphere 2,369

Northern hemisphere 11,225

analysis, whereas information from weather stations is sent immediately for use in forecasting, and subsequently is passed on as climate data.

In Australia, for instance, there are about sixty weather stations staffed by the Bureau of Meteorology, 125 automatic weather stations (eighteen of which are offshore, on reefs and islands), 560 co-operative stations and hundreds of nearby ships sending data. Upper atmosphere soundings are made from about fifty places regularly. Ozone is measured at five stations, solar radiation at nineteen, and rainfall-intensity measurements are taken at 600 places. In addition, there are over 6,000 voluntary observers of daily rainfall (Note 10.B).

Data Sets

The longest continuous set of daily data comes from Kew near London, begun in 1773. The longest sets in Australia began at Parramatta in Sydney in 1821 and Perth in 1830, in South Africa at Cape Town in 1841, in New Zealand at Dunedin in 1853, in Brazil at Rio de Janeiro in 1851. Daily measurements have been taken at the Argentine base at Orcadas in Antarctica (at 61°S) since 1903, and at the South Pole since 1957. Unfortunately, almost half the 610 southern-hemisphere stations in 1991 had records for less than twenty years, mostly in South America, and many stations in Africa have closed or record only intermittently.

Likewise, there are only about 60-70 places in the southern hemisphere where upper-atmosphere measurements are taken regularly, as against more than 600 in the northern hemisphere.

westerly to south-westerly. A series of daily synoptic charts like this shows the trends common in the region, and was the best tool available until the 1970s for forecasting tomorrow's weather.

Handling Weather Data

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