The frequency of hot May days has declined sharply in the US over the past century, with this year being just about average.

However, the average temperature this month has been in the top five.

Afternoon temperatures have been much cooler than 1934.

Maximum temperatures are decreasing, minimum temperatures are increasing – so the daily range in temperatures has plummeted.

One of the main reasons for the warmer nights and cooler days is the latent heat of water.  The US is getting wetter and having fewer droughts, so we get more evaporation during the day and more condensation at night. This moderates both daytime and nighttime temperatures.

Climate at a Glance | National Centers for Environmental Information (NCEI)

The hottest May was also the driest May in the US. During May, 1934 most of the US was in severe or extreme drought.  That was also the hottest May, and the one with the largest diurnal temperature range.

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