How the Mosquito Season Dates Are Worked Out
The rules
- Active: monthly mean temperature at or above 50°F. Means are mid-month values, interpolated to the day.
- Start: the day the mean rises through 50°F, or the usual last 28°F hard freeze if that is later.
- End: the day the mean falls through 50°F, or the usual first 28°F hard freeze if that is earlier.
- Peak: the warmest month; the peak window is every month with a mean of 70°F or more.
- Act by: 14 days before the start, roughly one egg-to-adult cycle.
Freeze dates are NOAA's 50% dates: half of years see the freeze earlier, half later.
From stations to counties
NOAA publishes 1991-2020 normals for about 7,000 US stations. For each county we take its Census internal point and blend the best three nearby stations, weighting by distance and by the height gap, because temperature falls with elevation. Each county is graded on how well its stations match: close 2,544, fair 526, distant 70. 7 counties have no usable freeze date and are dated on temperature alone.
How long seasons run
Across counties, the season runs 151 days at the short end (10th percentile), 206 at the median and 282 at the long end (90th percentile). Each county page ranks its season against the rest of its state.
Known weak spots
One point stands for a whole county, and in a county that spans a valley and a mountain range it can land in either. King County's point sits in the Cascade foothills, so its season reads shorter than Seattle's. Puerto Rico and four remote Alaska boroughs are left out because no station falls within 120 km. Temperature is not the only driver: rain and standing water decide how many mosquitoes there are once it is warm enough. And every date is a normal year, not this year.
Where to go next
See the range: Hennepin County, MN · Cook County, IL · Harris County, TX · Miami-Dade County, FL.
When Does Mosquito Season Start? Dates Across the US · When Do Mosquitoes Go Away? The End of the Season