Long Range Weather Planning: Rel Humidity

I'm looking at a XC for Tuesday night (5 days out) and had a semi-interesting question pop up and wonder if any weather smart people can help.

Looking at a 190km or 1:15 XC flight over Indiana and Illinois possibly finishing after dark with NO ability to change launch time. And weather.com is calling for some showers and a relative humidity of 60%.

Considering ceilings ONLY for this question, is there a direct conversion from relative humidity to ceilings. At first glance 60% seems like it ought to mean fairly high ceilings (I'd be comfortable with 3000 feet over flat terrain but higher would of course be just fine)

So smart people educate away. Is relative humidity linear like temp 10 dew point 6 would be 60%. If so could I extrapolate that in this case with a overnight low of 10 C then the DP is 6, 2 degrees/1000 ft to cloud base would be ceilings of 2000 ft.

My go vs no go is completely an option as it always should be in GA, can and would drive if needed. So is there a way to guess ceilings based on temp and RH?

Thanks as always

Grant

Can't answer but 1.5 hours out you have TAFs, METARs, radar and satellite info.

((temp - dew point) / 4) * 1000 = ceiling

90F - 60F = 30F
30F / 4 = 7.5
7.5 * 1000 = 7,500 feet agl

Hope this helps

I like this link for...

....a long range Probabilty of Precipitation - which gives a general idea of the expected weather...

http://www.hpc.ncep.noaa.gov/medr/poploop12hr.html

But with 5 days on, it sort of depends on how well the speed of the moving fronts is predicted - one is fast approaching from the West in your time frame.

Added -----------------

For Indianapolis - 8 pm EDT Tuesday - prediction 59 temp 46 dewpoint - 80% approx. sky cover

With the formula above - 13/4 = 3.25 = 3,250 ft cloudbase

This is an "educated guess" - but you can keep checking each day as the flight gets closer.

I used the NWS graphical point forecast page --

http://www.crh.noaa.gov/ndfd/graphical/sectors/ind.php#tabs

Just scroll forward to your flight day.

I too would like to know the answer to your question. How do you relate relative humidity to dew point?

That one is easy...

Norman CYYJ said:

I too would like to know the answer to your question. How do you relate relative humidity to dew point?

Click to expand...

...I did every day for years in High School in the Meteorogical Society...

You take the dry bulb temperature and the wet bulb temperature and look it up on a chart -

http://www.engineeringtoolbox.com/docs/documents/816/psychrometric_chart_29inHg.pdf

Or, if you prefer an on-line application for the calculation, here is one from the NWS -

http://www.hpc.ncep.noaa.gov/html/dewrh.shtml

So the short answer is NO, RH is not linear between temp/dp but thanks to Gil we have some fine tools to work with.

The NOAA calculator can turn the very basic weather.com info (temp/humidity) into a guessed dew point for cloud base guesstimates.

thanks

Thanks Gil. I had never really looked into it before and the calculator certainly shows the relationship. Lots of data stored in RVer's heads on here.