A pilot tunes in an automated weather observation while inbound for an early-evening arrival at the destination airport. It’s been a long flight on a muggy day, and although the temperature has begun to drop, there remains a comfortably wide gap between the surface temperature and the dew point—relieving any concerns about ground fog forming before the flight concludes.
That’s good news—and all pilots learn the importance of monitoring the temperature/dew point spread as a flight proceeds. But keeping tabs on the possibility of ground fog isn’t the only reason a pilot should know how many degrees the temperature must fall before it reaches the value at which moisture in the air mass will condense, causing clouds to form. Another reason is that a temperature/dew point spread of 10 degrees has different implications when the temperature is 80 degrees Fahrenheit than it does when the temperature is, for example, 55 degrees Fahrenheit.
Which of those two scenarios might be of more concern to a pilot?
"The higher the dew point, the more moisture in the air. As we move into the warmer months of the year, keep an eye on dew points. Dew points above, say, 60 degrees F (15.5 degrees C) mean that not much cooling is needed to create fog or clouds. Increasing dew point temperatures also can confirm that frontal passages are imminent, as fronts push moisture-laden air masses ahead of them. Conversely, low dew points indicate dry air masses," explains the article "Wx Watch: Dew Point Review."
It is also important to understand that high dew points "correlate well with the probability and severity of thunderstorms. Once thermals, fronts, and other lifting forces go to work on air masses with high dew points, towering cumulus or cumulonimbus clouds can soon follow. Cooling takes place as moisture-laden air is lifted higher and higher in unstable air masses. Most severe thunderstorms (those with 50-plus-knot surface winds, three-quarter-inch hail, or tornadoes) happen in air masses with dew points above 70 degrees F (21 degrees C)."
Lifting forces, and the cooling of an air mass after sunset, are two of four methods by which air may reach its saturation point. What are the others?
Chapter 11 of the Pilot’s Handbook of Aeronautical Knowledge describes them all, noting that "no matter what causes the air to reach its saturation point, saturated air brings clouds, rain, and other critical weather situations."