Right outsideWeather
Morning dew and the dew point: why grass is soaked without rain
When: April to October
When surfaces cool below the dew point, water vapor condenses into droplets on grass, cars and webs.
Why this time of year
Late-summer nights in eastern Massachusetts pair humid air with lengthening darkness, so a clear, calm night cools lawns below the dew point and soaks them by sunrise without a drop of rain.
How to notice it
- The evening before, check for clear skies, light wind, and humid air; that combination lets the ground cool fastest.
- At dawn, compare surfaces: open grass against pavement, a car roof against soil under a tree.
- Check under a tree; sheltered grass often stays drier because the canopy blocks the open sky the ground needs to cool to.
Try this with kids: Can you write your name in the dew on a car window?
the dew point is the temperature air must cool to before its moisture starts condensing
Safety
- Dew-wet grass, wooden steps, and metal ramps are slippery.
Sources
- forecast.weather.govDew is moisture condensed on objects near the ground whose temperatures have fallen below the dew point; dew point is the temperature air must be cooled to for saturation.
- weather.govClear skies maximize overnight longwave heat loss and light winds keep moist surface air from mixing with drier air aloft, the recipe that cools surfaces to the dew point; soil moisture supply matters too.
In the guide for April, May, June, July, August, September, October