You Know its dry when the RH is in the negatives

[hurricanedude]

Observation From Luke AFB..Arizona

Updated: 4:55 PM MST on July 06, 2004
Observed at Glendale Luke AFB, Arizona (History)
Elevation: 1089 ft / 332 m
108 °F / 42 °C
Mostly Cloudy
Humidity: -1%
Dew Point: 12 °F / -11 °C
Wind: 10 mph / 17 km/h from the SW
Pressure: 29.78 in / 1008 hPa (Falling)
Visibility: 20.0 miles / 32.2 kilometers
UV: 2 out of 12
Clouds (AGL): Mostly Cloudy 28999 ft / 8839 m

[Anonymous]

Wow! Is that really possible? I would think u cant have -1% himidity any better than u cant have 101% humidity.

[Guest]

DOes anyone know the equation for figuring the Relative humidity percentage using the air and dew point temp? That is certainly strange to a NEGATIVE percent anything.

[Wnghs2007]

I dont think that is possible. But maybe. It just seems like It would not be.

[Stormsfury]

Observation From Luke AFB..Arizona

Updated: 4:55 PM MST on July 06, 2004
Observed at Glendale Luke AFB, Arizona (History)
Elevation: 1089 ft / 332 m
108 °F / 42 °C
Mostly Cloudy
Humidity: -1%
Dew Point: 12 °F / -11 °C
Wind: 10 mph / 17 km/h from the SW
Pressure: 29.78 in / 1008 hPa (Falling)
Visibility: 20.0 miles / 32.2 kilometers
UV: 2 out of 12
Clouds (AGL): Mostly Cloudy 28999 ft / 8839 m

No, I think that's an automated conversion error ... especially when talking about very low humidity and extreme temp/TD's spreads this severe. The lowest ever humidity I've seen was 3% in California during a Santa Ana event.

SF

[vbhoutex]

I believe ASLKahuna posted about RH in the 1%-2% range last year. I'm too lazy to look for the posting here on the site.

[Aslkahuna]

That is an error in the observation since RH can not be a negative value. With a dewpoint of 12F the RH is probably in the 3-5% range. The lowest dewpoint I experienced was 2% with a temperature of 77F and a dewpoint around -18F (not C)-at the same time a nearby station was recording 1% with 77F and a dewpoint of -22F (it was seeing that reading that sent me out to measure my local dewpoint). And to think that when the monsoon starts, that very same Luke AFB could have dewpoints in the 70's.

Steve

[Wnghs2007]

That is an error in the observation since RH can not be a negative value. With a dewpoint of 12F the RH is probably in the 3-5% range. The lowest dewpoint I experienced was 2% with a temperature of 77F and a dewpoint around -18F (not C)-at the same time a nearby station was recording 1% with 77F and a dewpoint of -22F (it was seeing that reading that sent me out to measure my local dewpoint). And to think that when the monsoon starts, that very same Luke AFB could have dewpoints in the 70's.

Steve

WOW!!!!!Those are some low Humidity and Dewpoint Values.

[weatherlover427]

I've seen a humidity of 5% or lower here (it caused my digital gauge to stop registering because it was so low). Was during a Santa Ana of course.

[Wnghs2007]

I've seen a humidity of 5% or lower here (it caused my digital gauge to stop registering because it was so low). Was during a Santa Ana of course.

Shesh I would go insane if I saw that here in Georgia. We usually dont have RH values under 20%. And I dont remeber when dewpoints were under -5 degrees farenhiet. I just dont. Thank god for the gulf of Mexico

[Dan]

I've seen a humidity of 5% or lower here (it caused my digital gauge to stop registering because it was so low). Was during a Santa Ana of course.

Shesh I would go insane if I saw that here in Georgia. We usually dont have RH values under 20%. And I dont remeber when dewpoints were under -5 degrees farenhiet. I just dont. Thank god for the gulf of Mexico

yeah I feel the same. I hate dry heat just because my skin and my eyes get irritated. But on the other hand, evaporative cooling is at a maximum at that condition so you don't have to worry as bad about heat sickness, like we do here in the Southeast US.

[Aslkahuna]

WHOA! Dry Heat is just as dangerous if not more so than humid heat as can be attested to by those who attempt the cross the Desert on foot during the Summer before the Monsoon (or those years ago who went on the Jornada del Muerte [Journey of the Dead] in NM)-how do you think Death Valley got it's name. Under high dry heat conditions, you may feel comfortable (or reasonably so) while all of the while you are dehydrating so fast that in a short period of time you are in serious trouble (if you wait until you feel thirsty, you are already in bad shape). Additionally, here in AZ the dry heat tends to be with the highest temperatures (except around Yuma where dry and wet heat are both very hot) and if the air temperature exceeds the body temperature then any wind will no longer provide any cooling. There's nothing worse than getting hit by a wind blast of 115-120F air when you walk outside. Once the monsoon starts in AZ, we then exchange our blowtorch heat for a Sauna.

Steve

[Wthrman13]

Wow! Is that really possible? I would think u cant have -1% himidity any better than u cant have 101% humidity.

You can't have negative values of relative humidity, but technically you can have values over 100% (called supersaturation), and this occurs in the atmosphere in regions of strong rising motion in deep cumulus convection. Rarely in these updrafts, the relative humidity can actually reach values of 104%. The reason this can happen is because relative humidity is calculated with respect to a *plane* surface of water (or sometimes ice, depending on the application). However, in these updrafts, the only way water vapor can condense is in the form of droplets on cloud condensation nuclei, and this requires energy to overcome the surface tension of the developing cloud droplets. Depending on the amount and type of cloud condensation nuclei available (i.e. dust motes, aerosols, etc.), condensation will occur at greater or lesser values of supersaturation, or may even occur with the relative humidity less than 100%. See, for example,

http://amsglossary.allenpress.com/gloss ... aturation1

[USAwx1]

Wow! Is that really possible? I would think u cant have -1% himidity any better than u cant have 101% humidity.

You can't have negative values of relative humidity, but technically you can have values over 100% (called supersaturation), and this occurs in the atmosphere in regions of strong rising motion in deep cumulus convection. Rarely in these updrafts, the relative humidity can actually reach values of 104%. The reason this can happen is because relative humidity is calculated with respect to a *plane* surface of water (or sometimes ice, depending on the application). However, in these updrafts, the only way water vapor can condense is in the form of droplets on cloud condensation nuclei, and this requires energy to overcome the surface tension of the developing cloud droplets. Depending on the amount and type of cloud condensation nuclei available (i.e. dust motes, aerosols, etc.), condensation will occur at greater or lesser values of supersaturation, or may even occur with the relative humidity less than 100%. See, for example,

http://amsglossary.allenpress.com/gloss ... aturation1

Nice Post.

[Aquawind]

Great Thread..nice verbage on the extremes of humidity people..