Measuring very small changes

Meteorologists record temperatures for purposes of weather forecasting. They need to be able to inform people of the temperature they may expect over the next few days, and in particular whether the temperature will present any hazards, such as gales, snow, ice, or frost. They also need to track the way the temperature changes from day to day, because this is part of the forecasting procedure. Meteorological records are precise, of course, but they are not intended primarily to help climatologists to track temperature changes over decades. They do not need to take account of changes in methods, provided that all observers make the same change at the same time. Nor do they need to record slow changes, such as the gradual encroachment of a city into the formerly rural area where a particular

Central England temperature record

It is impossible to detect any change in climate unless there is a record of past conditions against which recent trends can be checked. Such long-term records are extremely scarce. Many professional and amateur meteorologists have measured the temperature, pressure, and rainfall and have kept weather diaries, but their records are difficult to interpret. The observers were not using standard instruments, calibrated in the same way, they were not siting their instruments in the same types of location, and they were not taking readings at the same time of day— or always at the same time each day. Lack of standardization can introduce huge discrepancies.

Professor Gordon Manley, a British geographer and climatologist, compiled one of the best sets of long-term temperature records. It covers an area of central England bounded by Preston, Bristol, and London—the map shows the area. The record is of mean monthly temperatures from 1659 and a con tinuous record of mean daily temperatures from 1772 until the present. Professor Manley published the record as a paper, "Central England Temperatures: monthly means 1659 to 1973" (Quarterly Journal of the Royal Meteorological Society, vol. 100, pp. 389-405, 1974). Manley spent most of his working life at Bedford College, University of London, where he was professor of geography He was also president of the Royal Meteorological Society.

Mean daily temperatures are calculated as the mean of the minimum and maximum temperatures for the 24-hour period. All temperatures are in degrees Celsius.

Since Professor Manley died, the record has been kept up to date at the Climate Data Monitoring section of the Hadley Centre of the United Kingdom Meteorological Office. That is where the Central England temperature record is held. It is not available to students or to the general public.



Central England climate record. The record of temperature from 1659 covers an area bounded by Bristol, Preston, and London.

Central England climate record. The record of temperature from 1659 covers an area bounded by Bristol, Preston, and London.

weather station is located. A change of this kind occurs too slowly to affect the day-to-day comparisons of temperature.

Climate change occurs when there is a sustained rise or fall in the average temperature over the entire world. Detecting such a change is difficult, because it is gradual. Between 1860 and 2000 the average global temperature is believed to have risen by 0.72-1.44°F (0.4-0.8°C). This is an average rise of 0.005-0.01°F (0.003-0.006°C) each year. The temperature has not risen steadily, however. There was a fairly rapid rise between about 1905 and 1945, followed by a fall in temperature that continued until the late 1970s, and then a resumption of the rise. The change each year is well

Weather balloons and satellites

within the limits of natural variability. Cool and warm summers, cold and mild winters, vary by more than the change scientists are trying to detect.

Air temperature is measured in four different ways. Surface weather stations measure the temperature at surface level. Weather balloons measure the temperature above the surface. Balloons also measure air pressure at known heights. Air pressure and temperature are closely linked by the universal gas equation. This is pV = nR*T, where p is the pressure, Vis the volume, n is the amount of gas in moles, R* is the gas constant (8.31434 joules per kelvin per mole), and T is the temperature in kelvins. The air pressure at a specified altitude can be compared with the surface pressure and the temperature at that height calculated very accurately. Finally, weather satellites monitor atmospheric temperature.

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