Dew Point

[donnlass]

How the Dew Point affects the weather reporting for aircraft?

I know on their weather reports the Dew Point is given but how does this affect aircraft directly?

Apart from weather affecting flight etc.

[Conventional Gear]

Indication of likelihood of fog formation (when dew point temp = air temp) and for GA pilots risk of carb icing, also can be used for rough cloud base calculations.

But as you want 'apart from weather directly affecting the flight' I'm not sure of the answer.

[donnlass]

Thats great, thanks for your answer, just wasnt sure how dew point fitted in with the general weather checks etc

[bingofuel]

Essentially the dew point is the temperature at which a parcel of air can no longer hold all the water vapour in the air and some of the water has to condense out which can form fog/mist.
So if the air temp is higher than the dew point then the air can hold all the water vapour in the air parcel,and it is clear, if the air temp is lower than the dew point vapour condenses out and becomes visible moisture (fog)

[mixture]

Indication of likelihood of fog formation (when dew point temp = air temp) and for GA pilots risk of carb icing, also can be used for rough cloud base calculations.

.... and for any pilot tells you that your ability to use your choice of destination may be curtailed by the RVR and cloud base issues associated with fog. But then in the commercial world your company SOPs, aircraft capability and runway CAT class will largely make the final decision for you. In the GA world, it basically tells you to be more ready than normal for a diversion !

[Dont Hang Up]

Here is a related question that noone has ever answered for me.

Dew point is calculated by comparing the temperatures of a dry-bulb thermometer and a wet-bulb thermometer. The idea being that evaporation cools the wet bulb thermometer by an amount related to relative humidity.

I've never been quite sure if dew point requires some calculation or whether it is a direct reading of the wet bulb temperature. However , the question is - how does that work when (as often happens) the temperatures are below freezing?

[2 sheds]

Don't hang up...

1. A calculation - either an automatic computer programe, a set of tables or a slide-rule.

2. Exactly the same way, except that the wet bulb is now an ice bulb, and a different table or scale is used.

Cheers

2 s