Story Highlights

STEAMIER SUMMERS

As summers get hotter from the increase in greenhouse gases, they are also getting stickier. More evaporation occurs in a warming atmosphere, and on a world where water covers nearly three-quarters of the surface, it means an increase in water vapor in the air.

During hot summer days, additional moisture in the air stresses the body by making it harder to cool itself through perspiration. This effect is not just irritating and uncomfortable, it can also raise the risk of heatstroke and heat exhaustion and in some cases, death. According to the Centers for Disease Control, more than 7,800 people died from heat-related illnesses from 1999-2009.

More moisture in the air also keeps the nights warmer, as humid air does not cool as easily as dry air. The warmer and more sultry nights mean that the body has less time to recover after a hot and steamy day in the sun, which further increases the risk of heat-related illnesses.

As both temperature and dewpoint rise with climate change, the number of days in which they combine to raise the risk for heat-related illness is also expected to climb. The number of danger days — days when the heat index (the combination of heat and humidity) is at least 105°F — will likely increase substantially in much of the country. We’ll be doing a special update on danger days soon so stay tuned.

DOG DAYS

For a large part of the country, the statistically hottest few weeks of the year are upon us. Often referred to as the dog days, the term relates to Sirius, known as the Dog Star. As Sirius is the brightest star in the sky (and fifth closest to the sun), the legend states that on days when Sirius rises and sets at the same time as the sun, the combination of the bright star and the sun makes those days especially hot. In modern times, that astronomical pairing occurs between July 3 and August 11.

METHODOLOGY

Dewpoint data was converted from vapor pressure. Vapor pressure data was obtained from the Daymet dataset at the Oak Ridge National Laboratory.