Higher Global Temperatures = More Rain

 Recently, I discussed the 'biblical' rainfall experienced in Hereford in September 2024. As global (and local) temperatures increase as a result of climate change, we expect precipitation levels to increase commensurately. This is just basic physics - warmer air contains more moisture so more water will precipitate when it rains/snows/hails. Not only does the extra water vapour in the air enhance the greenhouse effect (a feedback that speeds up the rate of temperature rise), it also increases the probability of intense rainfall events. According to the IPPC, rainfall is expected to increase in both intensity and frequency.

"At the global scale, the intensification of heavy precipitation will follow the rate of increase in the maximum amount of moisture that the atmosphere can hold as it warms (high confidence), of about 7% per 1°C of global warming." (Chapter 11, IPPC AR6)

I was curious as to whether this global prediction of a roughly 7% increase per ℃ in precipitation levels was borne out locally and nationally. Table 1 lists temperature and rainfall data from the Credenhill Weather Station, situated 3-4 miles away. I have used 30-year climate averages (1961-1990, 1971-2000, 1981-2010, and 1991-2020) to smooth the (considerable) year-to-year variations in precipitation.

TABLE 1: Credenhill: Mean Annual Temperatures and Rainfall
Climate Period	Mean Annual Temperature (oC)	Mean Annual Rainfall (mm)
1961-1990	9.44	656.8
1971-2000	9.76	673.2
1981-2010	9.95	664.9
1991-2020	10.25	695.5

The associated scatter plot (Figure 1) is shown below (Figure 1) ...

Using the regression equation (Rainfall = 42.8*Temperature + 251. R² = 0.76), a 1 ℃ rise in temperature produces a 42.8 mm rise in rainfall; equivalent to an average increase of 6.4 % per ℃ for the period covered by the data.

Repeating the process for another local weather station (approximately 15 miles away) at Ross-on-Wye; the data is presented in Table 2 and Figure 2.

TABLE 2: Ross-on-Wye: Mean Annual Temperatures and Rainfall
Climate Period	Mean Annual Temperature (oC)	Mean Annual Rainfall (mm)
1961-1990	9.86	686.3
1971-2000	10.19	706.2
1981-2010	10.49	733.5
1991-2020	10.80	764.3

Again, from the regression equation (Rainfall = 83.6*Temperature - 141, R² = 0.988), rainfall in Ross-on-Wye has increased at an average rate of 8.6% per ℃ during the 1961-2020 period.

Data for another local weather station at Shobdon Airfield, approximately 15 miles away, are summarised in Table 3 and Figure 3.

TABLE 3: Shobdon Airfield: Mean Annual Temperatures and Rainfall
Climate Period	Mean Annual Temperature (oC)	Mean Annual Rainfall (mm)
1961-1990	9.17	783.32
1971-2000	9.48	798.92
1981-2010	9.77	793.78
1991-2020	9.96	797.29

The slope of the regression line indicates a much smaller relative increase in rainfall with temperature. From the regression equation (Rainfall = 14.4*Temperature + 656, R² = 0.5), rainfall at Shobdon Airfield has increased at an average rate of 1.8% per ℃ during the 1961-2020 period.

In the final 3 examples, we look at much wider geographical areas, starting with the (English) Midlands. Table 4 and Figure 4 summarise the data. 

TABLE 4: Midlands, England: Mean Annual Temperatures and Rainfall
Climate Period	Mean Annual Temperature (oC)	Mean Annual Rainfall (mm)
1961-1990	8.90	768.05
1971-2000	9.20	777.53
1981-2010	9.53	792.70
1991-2020	9.84	809.77

From the regression equation (Rainfall = 44.6*Temperature + 369, R² = 0.987), rainfall in the English Midlands has increased at an average rate of 5.7% per ℃ during the 1961-2020 period.

Temperature and rainfall data for England can be found here and is summarised in Table 5 and Figure 5.

TABLE 5: England: Mean Annual Temperatures and Rainfall
Climate Period	Mean Annual Temperature (oC)	Mean Annual Rainfall (mm)
1961-1990	9.05	820.31
1971-2000	9.35	830.95
1981-2010	9.66	849.78
1991-2020	9.97	869.55

From the regression equation in Figure 5 (Rainfall = 54.3*Temperature + 326, R² = 0.985), rainfall in the England has increased at an average rate of 6.4% per ℃ during the 1961-2020 period.

And, finally, Table 6 and Figure 6 look at the temperature and rainfall relationship for the whole of the UK (data here).

TABLE 6: United Kingdom: Mean Annual Temperatures and Rainfall
Climate Period	Mean Annual Temperature (oC)	Mean Annual Rainfall (mm)
1961-1990	8.34	1084.08
1971-2000	8.60	1111.69
1981-2010	8.88	1141.95
1991-2020	9.16	1162.70

From the regression equation in Figure 6 (Rainfall = 97*Temperature + 277, R² = 0.993), rainfall in the United Kingdom (UK) has increased at an average rate of 8.6% per ℃ during the 1961-2020 period.

Analysing real world data on a local, regional and countrywide basis, we can confirm the IPPC prediction of an expected 7% increase in precipitation with every 1 ℃ rise in temperature is correct. Some localities will be less (e.g. Shobdon Airfield), some will be more (e.g. Ross-on-Wye), but, in the end, it will be the physics that decides the global average value of 7%.