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Why isn't California as humid as other parts of the country?

A look into the science of muggy weather, and why California is generally spared from the oppressive humidity.

SACRAMENTO, Calif. — Summers in the Sacramento area can be brutal. However, if you complain about the heat to someone who resides east of the rocky mountains, they are likely to respond like this:

"At least it’s a dry heat. "

How come it feels like you’re nearly swimming through the air during summer in the eastern U.S., but not here in California?

The answer is connected to ocean currents.

The ocean currents in the northern hemisphere rotate in a clockwise manner. For the Pacific coast, this means that the cold waters from the far northern pacific are drawn to our coasts. 

In the Atlantic, warmer tropical waters are transported up the east coast. 

Credit: NOAA
The effects of the clockwise ocean currents can be seen on this July 28 sea surface temperature map

Warmer water has higher rates of evaporation, so the bathtub-like waters of the Gulf of Mexico produce abundant moisture for the atmosphere. This moisture is pumped northwards when winds blow from the south, suffocating the eastern U.S. under a stifling air mass. 

The Pacific waters don't warm up nearly as much as those in the Gulf or the Atlantic, saving California from experiencing the humid conditions experienced in the eastern US.

   

The dew point, the temperature at which water droplets condense, is the preferred metric in deciphering how muggy it will feel outside. 

Meteorologists prefer dew point over relative humidity because the relative humidity is dependent on temperature while dew point relates only to the amount of water vapor in the air.

For example, say it is 75 degrees with 50% humidity. This combination produces a dew point of 55, which falls in the comfortable range. However, 95 degrees paired with 50% humidity equates to a sultry dew of 74, which is nearly oppressive. This is because warm air has the ability to hold more water than cooler air.

Credit: NWS Chicago
This handy chart links dew points to how muggy the air feels

Dew points greater than 60 are generally where it starts to feel “sticky”. The Sacramento valley rarely experiences dews higher than 65, and combing through meteorological records I could only find 70-degree dewpoints recorded once in the Sacramento area.

Could California get more humid due to climate change?

Climate researchers at UC San Diego believe that as the climate continues to warm, we can expect more "humid heatwaves". 

Humid heatwaves draw moisture from the southwest during monsoon season, causing uncomfortable conditions and warmer overnight temperatures. Water vapor absorbs the suns radiation efficiently and tends to trap that radiation overnight, keeping minimum temperatures warmer than they would be with lower levels of moisture. 

Humans have a much more difficult time cooling down in humid conditions. When it's muggy, sweat has a more difficult time evaporating from the skin, making it seem hotter. 

The greatest example of a "humid heatwave" was in July of 2006. Elevated moisture levels paired with an intense heatwave killed 600 people in the state.