# What is the relationship between temperature, dew point and the formation of clouds?

What can you infer from the temperature / dew point values in this METAR?

EGJG 121630Z 09012KT 9999 SCT006 15/14 Q1015

I understand the temperature is 15 degrees and the dew point is 14, and I found this:

The temperature / dew point spread is low, so the risk of cloud forming is high especially if the temperature falls.

Why is the risk of clouds forming high, especially if the temperature is falling?

• because it means that the air is almost saturated with humidity. a small drop in temperature and it is fully saturated, forcing condensation (cloud/fog) to take place.
– Federico
Dec 14, 2016 at 11:46
• As well as looking at one single METAR, it's useful to look at a series of METARs over time. If you see that the temperature-dew point spread (difference) is getting smaller over a few hours, then you can usually assume that the weather is getting worse, i.e. clouds and rain are likely. Dec 14, 2016 at 14:19

The dew point is the temperature at which condensation (a cloud) forms.

If the dew point is the same as the temperature on the ground, then there will be fog.

The temperature usually decreases as the altitude increases. This is called the lapse rate. In general, the closer the dew point is to the ground temperature, the lower the cloud ceiling will be.

• Your definition of dew point is not far from the mark, but could use improvement. A more accurate definition would include the statement that dewpoint is the point where condensation occurs at the same rate as evaporation, which is an important difference from what you sate. It is inaccurate to state that if temperature and dewpoint are the same there will be fog. Such conditions are conducive to fog, but will not necessarily result in fog. Dec 14, 2016 at 15:15
• @JonathanWalters Well, there is always some moisture in the air, so there will always be some "fog", even if it is a tiny amount. Dec 14, 2016 at 15:48
• I don't understand your statement; given there will a be moisture content, there will always be some fog? Dec 14, 2016 at 16:01
• This is not strictly correct, supersaturation can have an effect upon cloud formation - as can the presence of cloud condensation nuclei.
– os1
Dec 15, 2016 at 14:00
• While not 100% accurate from a meteorological perspective this is basically the weather knowledge that's imparted in your typical private pilot ground school. It is quite literally "Close Enough For Government Work" :) Dec 16, 2016 at 21:50