Why Isn't 100 Percent Humidity Always Rain?

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On particularly swampy days, the meteorologist might tell you that it's 100 percent humidity out there. While that might seem to suggest there should be pure rain showers outside, there may not even be a hint of drizzle. So what does it mean to reach peak humidity?

The percentage of humidity is not measured by the ratio of water to air. Instead, it looks at how much water vapor the air can hold at a certain temperature. When the humidity is at 100 percent, it's holding the absolute maximum; that means no more water can evaporate because the atmosphere has reached its limit.

Humid days feel terrible in part because our sweat can't evaporate, making it feel much hotter outside. Things like temperature and pressure affect the air's capacity to hold water vapor; the hotter it is, the more vapor the air can hold. If the temperature or pressure change, then rain is possibly in the forecast.

But in order for rain showers to occur, humidity has to be at 100 percent up where the clouds are (at ground-level, meanwhile, it can be at any percentage).