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.

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