Nights warm faster

NOAA’s Global Historical Climatology Network (GHCN) publishes 3 different monthly temperature sets for each of the ~7000 weather stations. 1) Average Temperatures 2) Minimum Temperatures 3) Maximum Temperatures. Seasonal variations are averaged out over a 30 year normalisation period to derive so-called ‘temperature anomalies’. This is done by subtracting the 30-year monthly averages from each temperature value which causes temperature trends to zero at the normalisation period. Normally only the average temperature (Tav) anomalies are presented, but I decided to analyse all three. These are the results for global temperatures (including oceans).

Fig 1. Comparison of temperature anomalies normalised to 1961-1990 for a)Tmax b)Tmin c)Tav. All curves pass through zero around 1975 due to the chosen normalisation period. The difference Tmax-Tmin is shown by the blue curve which is plotted versus the right hand Y-scale.

A reduction in Tmax-Tmin of about 0.1C is observed since 1950. Minimum temperatures always occur at night over land areas. This means that nights have been warming faster than days since 1950. The effect is actually much larger than 0.1C because nearly 70% of the earth’s surface is ocean with just single monthly average ‘anomalies’. So nights over land areas have on average warmed ~ 0.3C more than daytime temperatures.

If we assume that average land temperatures have risen by ~1C since 1900, then maximum temperatures have really risen only by 0.85C while minimum temperatures have risen by 1.15C.

This effect may also be apparent in equatorial regions where the night/day and winter/summer temperature differences are much smaller than at high latitudes.

Figure 2. All 117 meridional temperature anomaly profiles from 1990 to 2016. They are coloured blue if the annual global anomalies < -0.2C, Blue,-0.2<grey<0.2, 0.2<yellow<0.4, red > 0.4. Traces are 80% transparent to view them all.

Radiative cooling of the land surface mostly occurs at night. It is much greater when the air is dry such as over desert regions and at the poles. During the day convection and evaporation dominate heat loss. Enhanced CO2 reduces slightly night time cooling efficiency. UHI is also larger at night.

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