It’s the curse of summer weather: just as you start to relax outside, you feel a telltale tickle on your ankle. You swat the air but it’s already too late—another mosquito bite. While most are just an itchy nuisance, mosquito-borne diseases are an increasing risk in the United States. And as the climate warms, mosquito seasons are getting longer in much of the country.
According to studies from the National Institutes of Health, mosquitoes survive best at temperatures between 50-95°F and a relative humidity of 42% or more. Climate Central counted these “mosquito days” each year for 239 locations in the contiguous U.S., updating our 2016 analysis with a newer dataset. We found that 64% of sites recorded an increase from the 1980s to the 2010s—outnumbering those with a decrease by about 2 to 1.
More than 100 places now suffer a week or more of additional mosquito days. Increases are highest along the Pacific Coast, Ohio Valley, and Northeast—including an extra month in San Francisco, Santa Maria (Calif.) and Seattle. As the climate warms, mosquitoes can emerge earlier in the spring and survive longer in the fall. Though many sites do see fewer days, most are in southern areas—where more of the summer is getting too hot for mosquitoes. (Lest we forget, humans don’t fare well above 95°F either).
With longer mosquito seasons, the risk goes up for mosquito-borne diseases. According to Mordecai et al. (2017), the diseases of Zika, chikungunya, and dengue are transmitted between 61-93°F—a range that is getting more common in 94% of cities analyzed. These diseases are limited in the U.S., but West Nile virus has reached nearly every state and is projected to spread with climate change. Annual disease cases from mosquito, tick, and flea bites have more than doubled since 2004.
According to the National Association of County and City Health Officials (NACCHO), 84% of mosquito control efforts lack critical capabilities. Educating the public, tracking disease rates, and strengthening public health services are all key actions. Newer research hopes to predict outbreaks from the weather—or even convert females to non-biting males. But unless we curb climate change, the rise of mosquito days will continue to be a summer buzzkill.
Daily minimums and maximums for temperature and relative humidity were obtained for the contiguous United States from the gridMET dataset (via Google’s Climate Engine). Based on the linked studies by the National Institutes of Health, mosquito-suitable days are defined as having temperatures between 50-95°F and an average relative humidity of 42% or more. We compared the average annual number of days in the first ten years and last ten years of the study period (1980-2019).
This analysis examines conditions on individual days rather than multi-day heat events which can affect mosquito populations. Our previous analysis of the mosquito season was conducted using the Daymet dataset, which required calculating relative humidity using vapor pressure and saturation (and may cause variations in the calculated results).