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Climate entails the statistical characteristics of weather conditions in a given area. Climate zones are characterized by different combinations of climate variables, such as temperature, precipitation, humidity, and wind. When assessing the impacts of climate change on a region, it is important to consider the statistical characteristics of each climate variable, and the indices that are most useful for measuring that variable.
For example, in a given season daily temperature measures are often distributed normally around a mean temperature (Figure 4.1), meaning that very cold days are just as likely as very warm days. However, in many places rain occurs on only a fraction of the days in a season, meaning that the number of days with little no rainfall is very high, while the number of days with heavy rainfall is very low (Figure 4.1). This difference in the distributions of temperature and rainfall in one characteristic that influences the indices most useful for talking about each variable, as well as our ability to make predictions about each variable. Additional characteristics of each variable, and indices useful for assessing each variable, are described below.
Figure 4.1 Probability distribution function of precipitation and temperature. Source: Jha et. al.
Precipitation
Annual rainfall in 2015, Source: NASA Earth Observatory
In situ measures of precipitation are typically collected using a rain gauge. The frequency with which gauges are checked can determine what information may be available. At some stations, gauges are checked only once a month, providing information on total rainfall but no information on the intensity, length, or frequency of individual rainfall episodes. Stations that check rain gauges daily or hourly may be able to provide additional information important to end-users, such as the intensity of individual rainfall episodes and the length of time between rainfall episodes. Some precipitation indices of interest may include:
Greatest total precipitation over a 1-day period
Greatest total precipitation over a 5-day period
Average rainfall amount on rainy days (also called Simple Precipitation Daily Index)
Number of days when Precipitation exceeds 10mm
Number of days when precipitation exceeds 20mm
Maximum length of a dry spell
Maximum length of a wet spell
The precipitation amount when the year is in the 95th percentile
The precipitation amount when the year is in the 99th percentile
While station data on precipitation can provide very accurate information for the location from which it was collected, it may not provide much information about even very close locations without weather stations. While temperature fluctuations are relatively homogeneous across space, high precipitation in one location may not indicate an increase in rainfall in neighboring locations. For this reason, interpolating precipitation measures across space can prove challenging. Where there is considerable topographic variation, interpolation may be less reliable over even smaller distances.
Temperature
Temperature Measures from March 31-April 3, 2015. Source: NASA JPL
Unlike precipitation, temperature is more easily interpolated across larger geographic areas. Even where topography varies, an increase in temperature in one location typically signals an increase in neighboring locations, even if the temperature in one area tends to be cooler than the other. For this reason, it is easier to interpolate temperature across a region.
Depending on the topic of study, different temperature indices may be more useful than others. Those concerned with whether temperatures surpass a particular threshold may consider the number of days in a season surpassing a given temperature, or the the temperature on the coldest or warmest day of the year. Those concerned more with seasonal averages may consider the mean daily high and low temperatures over a three-month period.
Some temperature indices of interest may include:
Highest high temperature in a year
Lowest low temperature in a year
Average daily high temperature in a month
Average daily low temperature in a month
Number of days in a season when temperature exceeds 30 °C
Number of days in a season when temperature is below 0 °C
Average length of heat waves
The temperature at which a day is in 95th percentile
The temperature at which a day is in the 99th percentile
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