Weather Watch

Hot and humid? Cool and dry? The heat index tells you how it feels when you step outside

It gets hot along the Central Coast, but it’s typically a dry heat, and it usually cools off at night. We live in a meteorological Goldilocks zone, made possible by our ordinarily-dry summer Mediterranean climate.

However, last week, a low-pressure system off the coastline brought plenty of monsoonal moisture from the south and produced high dew-point temperatures that morph the Central Coast into an enormous sauna with convective rain showers and thunderstorms.

Stationed in Jacksonville, Florida, at the U.S. Navy Search and Rescue Swimmer School and later at Mayport, Florida, with HSL-36, an SH-2F Seasprite helicopter squadron near the mouth of the St. Johns River, I learned firsthand about heat and humidity. It was even worse in the Persian Gulf. The sweat would lie on your skin and never seemed to evaporate.

The reason sweat didn’t evaporate was due to high-relative humidity levels. Relative humidity is dependent upon moisture content and temperature of the air. The key word here is relative. We often experience 100 percent relative humidity levels in the morning as the fog and mist rolls in from the Pacific. But we feel entirely comfortable, even cold. It’s when you have high relative humidity levels combined with hot temperatures that you feel scratchy.

When it comes to what makes you most uncomfortable, the dew-point temperature is an excellent indicator. When the dew point climbs to more than 70 degrees, the air becomes thick and sticky. As it reaches 80 degrees, it can take your breath away. The dew-point temperature is the temperature at which air must be cooled for it to become saturated or achieve 100 percent humidity. At that point, the air can no longer hold all of its water vapor, some of which condenses into water, ice or snow or clouds. Dew point is simply the temperature when dew forms.

When the dew-point temperature increases, 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 liquid to gas. When a gas, like water vapor, changes phases to a liquid, it’s called condensation, and that’s a warming process. That’s one of the reasons it often gets warmer when it rains.

When the air can’t hold any additional water vapor, it’s saturated. Consequently, when the dew point is high, less perspiration on your skin can evaporate, and our bodies just can’t cool as efficiently.

A few years ago in July, the city of Bandar Mahshahr at the far northern part of the Persian Gulf in Iran reached an unimaginable dew-point temperature of 90 degrees. At the same time, the air temperature was 115 degrees, which produced a heat index level of 165 degrees. That 165-degree index temperature would have had an apparent relative humidity of 47 percent. I’ll take Death Valley any day over that type of heat.

So what is the heat index? This index is an “apparent or perceived temperature.” In other words, it’s a measure of how hot air feels against your skin due to lack of evaporation; it can be much higher than the actual air temperature. We may say perceived temperature, but the risk is genuine, such as heat exhaustion and stroke and other heat-related illness. Above is the National Weather Service Heat Index table. It can be a lifesaver.

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Battling the heat and humidity, more than 100 PG&E men and women are working with Florida Light and Power and other utilities from throughout the country to restore power to millions of Floridians recovering from Hurricane Irma. To learn about their stories, please visit

John Lindsey’s column is special to The Tribune. He is PG&E’s Diablo Canyon marine meteorologist and a media relations representative. Email him at or follow him on Twitter @PGE_John.

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