Humidity

[Jeffrey]

Carb Ice is the insidious heart-stopper for normally aspirated PPL's, and yet despite the informative Safety Sense Leaflets there does not appear to be any met info on humidity, and so no way to relate it to temperature on the carb ice incidence graph.
Or am I missing something?

[fatboy slim]

To calculate the humidity you require 'wet bulb' temperature readings as well as the (dry bulb) temp given on METARs so there is no way of calculating the % humidity, save calling the tower after the readings have been taken and asking for the figures. A table is then needed to calculate humidity.

NB It is important not to confuse 'wet bulb temp' with dew point, the dew point is the lowest point at which saturated air can be cooled without condensation to water. It is a theoretical calculation based on 100% Relative Humidity.

[bluskis]

Humidity will vary very locally, take cloud, showers, vicinity of ocean, lakes, rivers.

It is also what happens in your particular airplane venturi which matters, so the advice covers the whole wide range of possible ice forming scenarios.

Expect carb icing in the range of temperatures given in the pamphlets. Your engine instruments should warn you when it is happening.

[lunkenheimer]

In response to one specific aspect of your question, if one has temperature and dewpoint information, RH can be calculated.

Here is a calculator that claims to make the conversion:
http://www.mcwar.org/humid.html
but I cannot vouch for its accuracy.

Still, as Bluskis points out, the information is on little value for carburetor ice purposes.

[vancouv]

Personally, I do carb ice checks regularly and don't worry too much about the actual probability.

So long as you're checking frequently enough it shouldn't really matter to you, should it?

[distaff_beancounter]

When flying in the UK, I have a simple rule:-

ALWAYS assume that it is carb icing weather!

Last summer it was proved to me, just how local carb icing conditions can be:-

Flying across the English Channel in almost CAVOK, when in mid channel, port engine showed definite signs of carb icing, that cleared with carb air to hot. Starboard engine showed no signs of carb icing whatsoever. This was in a type that is not very prone to carb icing.

[Jeffrey]

Thanks for the information but the point I was trying to make was that humidity info is not readily available and wondered if there was sufficient need for it to be included in TAFS or METARS? Only then could the carb ice graph be practically applied.

[Aussie Andy]

The UK five day forecasts on the Flying High Aviation Weather website here include forecasts of humidity at various altitudes.

Also, as others have said, METARs give temp / dew point - if these two numbers are close together, humidity is high. If they are the same, then it is 100%!

Andy

[Flyin'Dutch']

Am I missing something?

Jeffrey wrote:

Thanks for the information but the point I was trying to make was that humidity info is not readily available and wondered if there was sufficient need for it to be included in TAFS or METARS

How about temperature and dewpoint in the METARS?

Seems a good starter, dont you think?



FD

[Don D Cake]

Temperature and dewpoint are stated in METARS and ATIS (if available). On the horizontal axis of the Carb Icing chart is temperature, on the vertical axis dewpoint.

Therefore using temperature, dewpoint and the chart you can determine the risk of carb icing and the relative humidity.

Or am I missing something....?