Weather Watch

Knowing dew point can save your life

Over the last week, I received a few questions about dew point temperature.

As any experienced aviator or mariner will tell you, knowing the temperature at which dew forms is critical when it comes to forecasting fog. Flying an aircraft, sailing a boat or driving a car in fog can prove challenging.

I once served in an air crew on a U.S. Navy H-2 Seasprite helicopter during a moonless night over the Atlantic Ocean on a mission to find and rescue a sailor who had fallen overboard. After a long search, we found him and deployed our search-and-rescue swimmer. As we were hovering about 40 feet above the ocean, a fog bank moved over our area and made it difficult to keep our swimmer and survivor in sight. Without any visual references, I developed vertigo, or the leans, while hauling the swimmer and survivor back into helicopter with the rescue hoist. I had the overwhelming feeling that the helicopter was leaning over and moving toward the water. One of pilots had the same sensation, but thankfully he kept his eyes on the artificial horizon and gauges in the cockpit.

Since that night, I’ve always paid close attention to the dew point temperature spread before heading out on the water.

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 fog. Dew point is simply the temperature when dew forms.

Fog can develop when air temperature and the dew point are within three degrees Fahrenheit of each other. Dew point temperature can also help to determine the heights of cloud ceilings, such as the height of the base of cumulus clouds. When the dew point temperature and air temperature are the same, the relative humidity is at 100 percent.

High dew point temperatures indicate high humidity levels. Depending on the air temperature, dew point temperatures over 60 degrees usually indicate it’s going to be “sticky,” while dew point temperatures of less than 30 degrees usually indicate very dry conditions.

Along the Central Coast, dew point temperatures usually average in the 40s during the winter and the 50s in the summer. Dew point temperatures can average less than 0 degrees Fahrenheit in Fargo, N.D., during winter, very dry indeed; while in New Orleans it can average over 70 degrees during the summer months.

Weather forecast

Saturday’s sustained northwesterly winds reached over 40 mph with gusts nearing 60 mph at Diablo Canyon nuclear power plant.

These gale force northwesterly winds over the last few days have produced a great amount upwelling along the coastline and very cold seawater temperatures. In fact, seawater temperatures have dropped to a bone-chilling 48 degrees along the Pecho Coast near the power plant.

A cold late season upper-level low pressure system will move southeastward over the Central Coast toward Southern California today. This system will produce partly to mostly cloudy conditions along with below normal seasonal temperatures.

The system has little moisture, but a few rain showers may develop in the interior of San Luis Obispo County this morning.

Today’s high temperatures will range from the mid- to high 60s in the interior to the low to mid-60s in the coastal valleys.

Temperatures along the coastline will range from the low 60s along the southerly facing (Avila and Cayucos) beaches and only the mid- to high 50s along the northerly (Los Osos) facing beaches.

Near the Earth’s surface, a steep pressure gradient will continue to produce strong to gale force (25 to 38 mph) northwesterly (onshore) winds with gusts reaching 50 mph along the coastline today.

The northwesterly (onshore) winds will finally decrease Monday, allowing a slight warming trend to develop along with the return of coastal night and morning clouds and fog.

Another late season cold front will produce gentle southerly winds and increasing clouds Tuesday as it moves down the California coastline. Frontal passage should occur Tuesday night into Wednesday morning, producing periods of rain. Rainfall amounts will be light, ranging between 0.05 and 0.25 inches.

Another round of increasing northwesterly (onshore) winds will develop later Thursday and will continue through the Memorial Day weekend. At this point it appears that it will remain mostly dry with with warmer temperatures, especially in the interior.

Surf report

Strong to gale force northwesterly winds will continue to generate an 8- to 10-foot northwesterly (310-degree deep-water) sea and swell (with a 5- to 11-second period) along our coastline through tonight.

This northwesterly sea and swell will decrease to 6- to 8-feet Monday morning, further lowering to 4- to 5-feet (with an 8- to 14-second period) by Monday night and will remain at this height through Tuesday.

A 5- to 7-foot northwesterly (290-degree deep-water) swell (with an 8- to 13-second period) is forecast along our coastline Wednesday through Thursday morning.

Increasing northwesterly winds will produce a 4- to 6-foot northwesterly (310-degree deep-water) sea and swell (with a 5- to11-second period) Friday, increasing to 7 to 9 feet Saturday.

Arriving from the Southern Hemisphere:

A 3- to 4-foot Southern Hemisphere (210-degree deep-water) swell (with a 17- to 19-second period) will arrive along our coastline today and will remain at this height and period through Monday.

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 more than 23 years. If you have a question, send him an e-mail at pgeweather@pge.com.

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