Why causes this place to have such high humidity?

My daughter Chloe is attending the U.S. Navy’s Nuclear Power School at Goose Creek, South Carolina. She recently sent me a text, asking, “What causes this place to have such high humidity?”

As a native Californian, I can totally relate. I was stationed at Naval Station Mayport, Florida, near the mouth of the St. Johns River on the Atlantic side of the state.

For people that live in the southeastern United States, dealing with humidity is a way of life.

After a while, you just seem to submit to it. The only relief in Mayport was the development of afternoon thunderstorms, which brought some cooling.

I remember taking a shower and walking toward the white sands of Jacksonville Beach and being as wet as when I was still in the shower.

The sweat would lie on your skin and never seemed to evaporate. The reason the sweat did not evaporate was the high relative humidity levels and hot temperatures.

The relative humidity is dependent upon moisture content and temperature of the air.

The keyword here is “relative.”

Along the California coastline, we often experience 100% relative humidity levels in the morning as the fog and mist roll in from the Pacific Ocean, but we feel entirely comfortable, even cold. It’s when you have high relative humidity levels combined with hot temperatures that you feel like you’re in a hot tub next to an outdoor wood-fired pizza oven.

So why is it so humid east of the Rocky Mountains in the summer?

At any given season, and at the same latitude no less, the weather along the East Coast can be so unlike that found along the West Coast. The areas of low and high pressure and the winds they produce can have radically different effects on the atmospheric conditions along each coastline.

The predominant weather feature along the Western Seaboard is the Eastern Pacific High — an area of high pressure over the northeastern Pacific Ocean.

The clockwise circulation around this high brings cooler air from the Gulf of Alaska, often producing a persistent marine layer during summer along the West Coast that reduces the need for air conditioners.

During the winter months, this onshore flow generally keeps temperatures mild.

On the other side of the country, the predominant weather feature during the summer is the Bermuda High — a semi-permanent area of high pressure over the Atlantic Ocean near the Bermuda Islands.

The clockwise circulation around this high brings warm, humid air from the Gulf of Mexico and the Caribbean Sea and produces the three Hs — heat, humidity and haze.

Often, the southeastern part of our country gets little or no relief from the hot and humid conditions.

In winter, frigid storms that often originate in Canada make their way through the eastern part of the country, driven in by the upper-level winds.

Not only do similar areas of high pressure off the West and East coasts have very different effects, but winds with the same directions on each coastline also can contribute to totally different local weather.

Along the California coast, northeasterly winds — referred to as Santa Anas in Southern California, Santa Lucia winds along the Central Coast and diablo winds in Northern California — blow from the land out to sea. They are lovingly described by surfers as offshore winds.

Along most of the Pacific coastline, these northeasterly winds are technically called katabatic winds, from the Greek word katabatikos, which means “going downhill.”

As the air mass descends the side of the mountain range, it warms at the rate of about 5.5 degrees per 1,000 feet of descent.

Meteorologists refer to this rate of warming as the dry adiabatic lapse rate. If the air is warm at the top of the mountain range, it can be sizzling hot and bone dry by the time it reaches the valleys below.

It is an entirely different story along the East Coast. Northeasterly winds, especially during the winter months, can drive in massive storms off the Atlantic Ocean toward the northeastern part of the United States.

These cyclones can rapidly intensify as relatively warm air from the oceanic Gulf Stream clashes with cold air from the northwest. These intersecting air masses can liberate unfathomable amounts of latent heat as water vapor condenses into clouds and precipitation.

This heat causes surface air to expand and quickly rise into the sky, decreasing air pressure. When pressure drops fast enough, meteorologists refer to this explosive development as a bomb.

These storms are referred to as nor’easters, and they can produce torrential rains, blizzards and even tornadoes.

I have lived on both coasts, and I can honestly say that the Central Coast has the best weather found anywhere.

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This story was originally published June 22, 2020 at 11:55 AM.