Steerpike

(post for weather geeks only ...)

So I'm here in Scottsdale, AZ, and it's "relatively cool" for the time of year - 95. I checked on the weather back home - Walnut Creek, CA - and it was 98 - warmer than normal. So then I paid attention to the humidity and dew point figures. This is what has me puzzled...

Walnut Creek:
Temp: 98F
Humidity: 20% (which is 'low' by most standards!)
Dew point: 50F

Scottsdale:
Temp: 95F
Humidity: 11% (dry as a bone!)
Dew point: 32F

Now, weather buffs say, 'Dew Point' is a better measure of the feeling of 'humidity' than the 'humidity' measure itself. So does it make sense that the Dew Point in Scottsdale is so much lower than Walnut Creek? That is - can a 9% diff in humidity give an 18 degree diff. in Dew Point? I can tell you that 98F in Walnut Creek is pretty 'hot' feeling, while being outside here in Scottsdale at 95F is pretty 'easy'.

For comparison, New York city today:
Temp: 90F
Humidity: 54%
Dew Point: 71F

and Houston:
Temp: 86F
Humidity: 75%
Dew Point: 77F

nethead

It has been a scorcher here today near Harrisburg PA.
Currently 10.20pm
81.6f
humidity 62%
dew point 68f

paddingtongreen

If you used the more correct "Relative Humidity" it might make more sense, it is not a linear graph.

Relative humidity = actual vapor density/saturation vapor density.

The vapor load required to produce saturation, increases with air temperature. Therefor, if you reduce the temperature of a given amount of air, the relative humidity will increase until the saturation point is reached. That temperature, where the relative humidity is 100% is the dew point; lower it further and the moisture must precipitate.

There is no straight forward way to calculate relative humidity because air pressure and elevation both add more wrinkles to the curve, so to speak.

Brit3964

Try the Carolinas...50%+ humidity anytime of year! Yesterday in the car the temperature gauge said 101F while traveling at 60+ mph! HOT.

English Muffin

I hear you!!!

Steerpike

So ... Scottsdale had a Dewpoint of 32F - is that the absolute minimum, since 32F is freezing? At 10am today, it's 87F with humidity 15%, Dewpoint 34F.

CelticRover

The high in Scottsdale today is likely to be around 107F with overnight lows in the mid eighties. This is down from 116F a couple of weeks ago.
The weather folks here also use a 'heat index' which combines humidity etc to produce a temperature more akin to what it actually feels like, the higher the humidity the higher the index.

paddingtongreen

I don't know, I just have the general principles, but it sounds reasonable that a hoar frost precipitates all the vapor below that temperature.

I went looking for a site that I looked at when I wanted to know about this stuff; it sent meto this one where they have a downloadable calculator.

I checked into this because I have emphysema and I wanted to find out why I don't get as much oxygen from a breath in summer as I do in spring fall and winter.

fatbrit

This year's monsoon doesn't seem to have developed yet! I'm blaming this new bloody Englishman who's just moved in for bringing the bloody weather with him. When I moved here in October of 2000, it rained for 2 weeks solid -- so there is evidence to back up this theory.

CelticRover

Air density perhaps?

CelticRover

It's that well documented 'Sasanach Syndrome' we were all taught about in primary school.

The Horticulturalist

It's like a sauna down here in MS.

I went out to the garden with the binoculars to look at a hummingbird in the tree, I couldn't figure out why they were blurry despite my attempts to focus, I realised after a while that the lenses were all steamed up!

CelticRover

.........especially when they have been sitting in the air conditioning beforehand.

Steerpike

Guess not - at 12:20pm, Scottsdale is at 91F, with 12% humidity and a Dewpoint of 31F !

cindyabs

I wish I could remember my dad's saying about dew and whether it would or wouldn't rain.


HA! Found it- Dew indicates a good day ahead; a dry morning is sign of showers.