Recent climate change may be studied by analysing instrumental records of common climate elements such as temperature, precipitation (rain, snow and hail), humidity, wind, sunshine and atmospheric pressure, which have been obtained with standard equipment. According to internationally recognised procedures, the measuring instruments must be properly installed in suitable places, carefully maintained and conscientiously observed. Although it is not possible to measure the climate per se, the records of individual climate elements taken together can be used to specify the physical state of the climate at a given place, for a particular period of time. Such records of climate elements collected over time are known as "time series".
The most commonly measured element is temperature. Temperature is a valuable climate element in climate observation because it directly provides a measure of the energy contained within the system under inspection. For example, a global average temperature reveals information about the energy content of the Earth's climate system, and most significantly the global energy balance. A higher temperature would indicate that more energy is stored within the Earth's climate. Changes in global temperature indicate changes in the global energy balance.
Many surface air temperature records extend back to the middle part of the 19th century. The oldest record comes from central England and is over 300 years in length. The measurement of surface air temperature is essentially the same now as it was then, using a mercury-in-glass thermometer, which can be used down to -39C, the freezing point of mercury. For lower temperatures, mercury is usually substituted by alcohol. More recently, observations of surface ocean temperature have been collected to provide a more accurate picture of the global average surface temperature. Maximum and minimum temperatures measured during specified time periods, usually 24 hours, provide useful information for the construction and analysis of temperature time series. Analysis involves the calculation of averages and the identification, using various mathematical techniques, of periodic variations and trends, which may reveal evidence of climate change
Rainfall is measured most simply by noting periodically how much has been collected in an exposed vessel since the time of the last observation. Care must be taken to avoid underestimating rainfall due to evaporation of the collected water and the effects of wind. Time series can be constructed and analysis performed in a similar manner to those of temperature. Rainfall, however, varies much more widely than temperature over relatively small geographical areas, and over short periods of time. Typically, a sharp thunderstorm may affect one locality, but not another perhaps only 10 miles away. Consequently, analysis of precipitation time series is more complex than for temperature.
The measurement of global rainfall offers an indirect assessment of the energy of the Earth's climate system. Increased heat storage within the atmosphere and surface oceans will increase the rate of evaporation, cloud formation and ultimately precipitation.