Is dew point the actual minimum overnight temperature?

Question

Is dew point the actual minimum overnight temperature?

Answer 1

No.

Answer 2

No, dew point is the temp at which the moisture in the air becomes apparent (Fog, mist etc)

Dew point is a result of the tempreature and relative humidity.

Cheers, Witty.

Answer 3

Yep, that's what the dew point is. It's the temperature at which the water vapour condenses, if the air is cooled at the same pressure.

But, the question was, is that the minimum overnight temperature.

The answer is No, not necessarily.

Ambient air temperature is measured at a standard height of 1.25 metres above the ground. As the ground cools, the air next to it cools. Eventually, by conduction and whatever, a deeper and deeper layer of cool air forms. But at the ground the temperature is lowest. When the temperature there reaches the dew point, dew forms on the ground lowering the RH (because water vapour has been removed from the air) and lowering the dew point. The resulting moisture gradient allows the water vapour to diffuse downwards, and lowers the RH and the dew point at the level that the ambient air temperature is measured, and allows the temperature to decreace below the original dew point.

Now, and this is the intersting bit. If the deposition of dew is not fast enough to remove the water vapour diffusing down, the temperature will cool to the dew point and fog will form. That's it; no more cooling until the fog clears next morning.

BUT, if the initial temperature is low enuogh and cooling is rapid enough that a frost forms on the ground, and since the saturation vapour pressure over ice is lower than over water, water vapour will be removed from the air much faster. In this situation the fog will be shallow or intermittent - form, then clear, form again and clear.

This is why fog forecasting for airport operations is so difficult to get right.

Of course, once it has formed, that's only half the problem. When will it finally clear so that aircraft can land?