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 the helicopter with the rescue hoist. I had the overwhelming feeling that the helicopter was leaning over and moving toward the water. One of the 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.
In the Hanford area, dew point temperatures usually average in the high 30s to low 40s during the winter and the low 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.