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Dew point makes difference in comfort

It gets hot in San Luis Obispo County, but it’s a dry heat and it usually cools off at night. Unlike the record-high dew-point temperatures that are morphing the country east of the Rockies into an enormous sauna, San Luis Obispo County resides in a meteorological Goldilocks zone, made possible by our normally dry summer Mediterranean climate.

When I was stationed in Jacksonville, Fla., with the U.S. Navy, I learned firsthand what high dew-point temperatures felt like. It was even worse in the Persian Gulf. The sweat would simply lie on your skin and never seemed to evaporate when the dew point got to 70-plus degrees.

The dew-point temperature is the temperature at which air must be cooled for it to become saturated. At that point, the air can no longer hold all of its water vapor, some of which condenses into water, as dew or clouds. Dew point is simply the temperature when dew forms.

When the dew-point temperature and air temperature are the same, the relative humidity is at 100 percent. Along the coastal regions of the Central Coast, dew-point temperatures usually average in the 40s during the winter and the 50s in the summer.

When the dew point climbs to more than 70 degrees, the air becomes uncomfortable and sticky. As it reaches 80 degrees, it can take your breath away.

The highest dew point ever recorded in the world was in Dhahran, Saudi Arabia, on the Persian Gulf when the dew point reached 95 degrees July 8, 2003.

When it comes to what makes you most uncomfortable, the dew-point temperature is an excellent indicator. The hypothalamus gland in your brain tries to regulate your body temperature by setting off millions of sweat glands when the atmosphere heats up.

Sweating is an essential function that helps your body stay cool. On a typically hot day, it is possible to lose more than seven quarts of water in the form of sweat evaporating off your skin.

Evaporation is a cooling process. Just go outside on a windy day with wet clothes on, and you’ll probably get chilled as water changes from a liquid to a gas.

If the air is saturated with water vapor, it can’t hold any additional vapor. So, when the dew point is high, less perspiration on your skin can evaporate into the air and our bodies just can’t cool as effectively. In essence, the higher the dew-point temperature, the more moisture there is in the air.

Last Tuesday night, the Hector International Airport in Fargo, N.D, reported a record-breaking 83 degree dew-point temperature.

That’s also when Dave Congalton and Charlotte Alexander happened to be visiting friends north of Fargo who live in a farmhouse that was built in 1913. The farmhouse didn’t have any air conditioning, so every hour on the hour they would go out to their car and turn on the air conditioner to cool off.

Later that night, thunderstorms moved through the area and produced cooler temperatures. Dave told me he was struck by the vast amounts of standing water throughout the state. That probably contributed to the high dew-point temperatures.

During winter, dew-point temperatures can average less than 0 degrees Fahrenheit in Fargo, N.D. That’s very dry indeed.

Today’s forecast

A classic Central Coast summer weather pattern will continue with night and morning marine low clouds, fog and areas of drizzle along the beaches and coastal valleys and near normal temperatures (low 90s) at the Mid-State Fair.

Today’s temperatures will range from the low-60s along the northwesterly facing beaches (Nipomo, Montaña de Oro, Los Osos and Morro Bay) under mostly overcast skies through the day, while the southwesterly facing beaches (Cayucos and Avila Beach) will range from the low to mid-70s under sunny afternoon skies.

The coastal valleys (San Luis Obispo) will range in the mid-70s, and the Mid-State Fair will warm to the low 90s.

Monday afternoon’s moderate to fresh (13- to 25-mph) northwesterly winds will increase to strong to gale force (25- to 38-mph) levels along the coastline on Tuesday. This condition will produce clearing at the beaches and warmer temperatures at Cayucos and Avila Beach.

The wind fields will shift northward off Cape Mendocino on Wednesday, leaving behind a warming trend. Temperatures will increase a few degrees each day at the Mid-State Fair through the end of July.

The remnants of hurricane Dora, the fourth named storm in the Pacific this season, may produce mid- to high-level clouds and higher dew-point temperatures on Thursday and Friday.

A return to normal to slightly above normal temperatures is expected by next weekend, with less night and morning low clouds along the coast.

Surf report

Moderate to fresh (13- to 24-mph) northwesterly winds this afternoon will generate 3- to 4-foot northwesterly (320-degree deep-water) sea and swell (with a 5- to 9-second period) this afternoon through Monday morning.

The northwesterly (onshore) winds will further build to fresh to strong (19- to 31-mph) levels and will drive up the northwesterly sea and swell to a 4- to 5-foot height on Monday afternoon and night.

Strong to gale force (25- to 38-mph) northwesterly winds along the Central Coast will generate 4- to 6-foot northwesterly (320-degree deep-water) sea and swell with a 5- to 8-second period) on Tuesday.

The wind fields will shift northward off Cape Mendocino on Wednesday, leaving behind a 4- to 6-foot northwesterly (290-degree deep-water) swell (with an 8- to 13-second period) on Wednesday through Saturday.

Arriving from the Southern Hemisphere: Today’s 1-foot Southern Hemisphere (200-degree deep-water) swell (with a 14- to 16-second period) will continue at this height and period through Tuesday.

A 1- to 2-foot Southern Hemisphere (190-degree deep-water) swell (with a 16- to 18-second period) will arrive along our coastline on Thursday and Friday.

Seawater temperatures will range from 56 to 58 degrees through Monday, decreasing to 53 to 56 degrees on Tuesday through Wednesday.

Seawater temperatures will rise to between 56 and 58 degrees on Thursday through Friday.

Conservation tip

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John Lindsey is a media relations representative for PG&E. He is also a local weather expert and has lived along the Central Coast for nearly 25 years. To subscribe to his daily weather forecast or ask him a question, email