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abrowne
07-02-2018, 04:48 AM
Hello,

Does anyone know where to find a formula (or approximation, or even just some examples) of how quickly air cools with altitude? I am particularly interested in how air behaves over a hot desert. How high would you have to go before the air was cool again?

Also, if anyone knows: are there significant updrafts due to the extreme temperatures, or is the air fairly stable over a large, hot desert? (I would so love to hear from a small-craft pilot, hang glider, etc. who's flown over a hot desert.)

Thank you!!

Helix
07-02-2018, 05:12 AM
I'll be flying over a hot desert in a very small 'plane in the second half of the month. Unfortunately, it's mid-winter here so it will be bloody freezing (5 -- 20C). But if you're still stuck for answers* by the time I leave, I'll ask the pilot for info.

*You won't be. Someone will know the answer, for sure.

Siri Kirpal
07-02-2018, 06:09 AM
Sat Nam! (Literally "Truth Name"-a Sikh greeting)

Not sure about the temps, the condors and vultures love the updrafts over the Sonora Desert.

Blessings,

Siri Kirpal

Enlightened
07-02-2018, 06:16 AM
I live at 7000 ft above sea level. We can get into the 100s in summer. Air is thinner the higher one goes in altitude. This means there is less light reflection and refraction and more radiant heat. You are also closer to the sun. A simple Google search yields Mt. Everest averages -2F in July. Everest is 29000 ft in elevation. Where I live, averages: Jan: 43F; July: 85F. -76F (average temp at the summit of Everest) and -2F (average July temp of Everest),

Even with these comparisons, I would not make assumptions (as it is probably different in winter).

Also, I live at a different latitude than Everest. 39 degrees N for me and 28 for Everest.

Because of the aforementioned, I do not believe there is such an equation (due to variables). I could be wrong though. Other things like desert weather phenomena affecting the atmosphere (like haboobs) may play into it as well.

Albedo
07-02-2018, 06:40 AM
https://www.ventusky.com/ allows you to see the current temperature at the surface as well as many different elevations above the ground, for any point on the Earth's surface. Give it a whirl. (Just play with the altitude setting at the left)

Maryn
07-02-2018, 04:03 PM
I grew up in southern Arizona. About an hour from Tucson, in the Santa Catalina Mountains, is Mount Lemmon, a summer retreat where many Tucson area locals spend summer weekends and/or vacations. It's less than 10,000 feet--and around 30 degrees cooler than the desert floor below.

abrowne
07-02-2018, 10:04 PM
Thank you guys so much!!! This was exactly what I needed. Incredibly helpful. Thank you!!

blacbird
07-02-2018, 10:09 PM
Air above a major desert region tends to be descending, high-pressure air (which is why a desert is there in the first place), and very stable. Major updrafts would only occur during periods of instability, and create thunderstorms (relatively rare in a desert . . . which is why it's a desert).

caw

Rob40
07-03-2018, 12:26 AM
pilot here.
Look up the "adiabatic lapse rate"

https://en.wikipedia.org/wiki/Lapse_rate

chart to use:
click for the graph used in such a thing (https://upload.wikimedia.org/wikipedia/commons/1/13/Emagram.GIF) because it's huge and too big for the forum sanity.



its generally 3 deg C. per 1,000 feet altitude as a general rule. The rest of the Wiki text there should give you as much exactness as you wish to calculate. So as an example, at 36,000 feet, yes the air really is somewhere around -60 F.

As a footnote for fun use in your daily lives ,if you ever wanted to know how high up the bottom of the clouds were:

you can take the temperature and dewpoint difference, known as the "spread" and divide it by 4.4, then multiply by 1,000 and that will give you the cloud bases. (Which really is about 227 feet per degree so multiply 200 by the spread and add a 10% for something close enough.) So you have fog on the ground or low clouds when the temp/dewpoint is equal or close and enough humidity.

Al X.
07-03-2018, 12:29 AM
Edit: ^posted at the same time. His explanation is more through.

Rob40
07-03-2018, 12:33 AM
Also this may be of interest: If you know the surface temperature, and the dew point, you can calculate the height of the bottom of a cloud deck. Temperature decreases with altitude, and the dew point increases. The adiabatic lapse rate is -2c per 1,000 feet.

Jinks! That's too funny. We're on the same page.

Bolero
07-03-2018, 08:16 PM
Curiosity - does lack of rivers also play a part in a desert being a desert? All the water flowed the other way from any nearby higher ground?

Al X.
07-03-2018, 08:43 PM
Curiosity - does lack of rivers also play a part in a desert being a desert? All the water flowed the other way from any nearby higher ground?

It's the lack of rain that causes a desert to be a desert. Deserts usually have lots of rivers, mostly which are dry and subject to flash flooding when it does rain. A year round flowing river in a desert is only going to cause vegetation of its banks at most.

blacbird
07-03-2018, 09:10 PM
It's the lack of rain that causes a desert to be a desert. Deserts usually have lots of rivers, mostly which are dry and subject to flash flooding when it does rain. A year round flowing river in a desert is only going to cause vegetation of its banks at most.

E.g., the Nile, which is about the only free-flowing large river through a desert that I can think of. In a desert climate, surface evaporation exceeds water supply by a big margin.

caw

Siri Kirpal
07-03-2018, 11:19 PM
Sat Nam! (literally "Truth Name"--a Sikh greeting)

The Colorado River mostly goes through desert too.

Blessings,

Siri Kirpal

Helix
07-04-2018, 02:31 AM
Finke River, which cuts through some old geology in central Australia, runs through desert. It doesn't flow at the surface all the time, but there are substantial permanent waterholes at the surface.

If cyclones drop enough water in the right place, the mostly dry Cooper Creek, the Diamantina, and Georgina/Eyre River start flowing. They empty into Lake Eyre and it takes many months for the water to get that far. The area they run through (SW Queensland) is called the Channel Country because when there's water around, the dry plains turn into thousands of square kilometres of braided channels. Completely impassable.

Thomas Vail
07-11-2018, 12:32 AM
Curiosity - does lack of rivers also play a part in a desert being a desert? All the water flowed the other way from any nearby higher ground?

It's lack of rainfall that makes a desert a desert. Such as locations in the rain shadow of mountains, where the prevailing atmospheric currents causes air to dump most of its moisture on the other side, meaning what reaches the mountains is very dry, or is in a region to subjected to descending high altitude air currents (also dry).

Rivers will provide moisture along their banks (the Nile, as mentioned previously is a good example of this) but they don't supply much saturation to anything beyond that.

cbenoi1
07-11-2018, 11:03 PM
its generally 3 deg C. per 1,000 feet altitude as a general rule.
Actually, this is the equation when a dry air mass is rising. A wet air mass would lose 1.5 deg C per 1000' altitude. In a "standard atmosphere", an observer (ex: an outside temperature gauge attached to a plane) would measure a drop of 1.98 deg C per 1000' altitude as it climbs.

If you want to know - roughly - what's the temperature above you, subtracting 2 deg C per 1000' of altitude is a good approximation. The temperature levels off at about -56 deg C at the top of the Troposphere ( ~ 18,000' ), then there are some weird temperature gradients in effect going higher.

As for "how does flying over a desert in a glider looks like", you might want to check this:

https://www.faa.gov/regulations_policies/handbooks_manuals/aircraft/glider_handbook/media/gfh_ch09.pdf


-cb

writbeyondmeasure
07-14-2018, 09:09 PM
Atmospheric scientist here. The answers above are pretty good, but I would just add a couple of points. In a desert, it's safe to assume the dry adiabatic lapse rate (-9.8 deg C per 1km - or 3 deg C per 1000 feet in American) If you were flying over the equator, use the moist adiabatic lapse rate which is about -5 deg C per 1 km, and anywhere else use about -7.5 deg C per 1km).

Temperature decreases with height in the Troposphere. The top of the troposphere (called the 'tropopause') varies with latitude and is around 7km near the poles and 12-15km (up to 20km if it's super stormy) at the equator. 10km is not a bad estimate for desert regions. In the stratosphere, the layer above the troposphere, the temperature actually increases with height at the dry adiabatic lapse rate (the stratosphere is very dry).

If you want daily measurements of the vertical profile of the atmosphere, which will tell you the temperature at different heights all over the world, the University of Wyoming provides this data online for free to anyone. Google 'University of Wyoming Soundings' select your location and the plot you want is called a 'skew T plot'

The air is pretty stable over a desert. As mentioned above, deserts owe their existence to high-pressure systems that tend to sit above them. This means air tends to descend over deserts. You can still get storms and therefore updrafts in desert regions, so having an updraft in your story is possible - just uncommon. You may also get local, small-scale updrafts if you have topography to channel the air in such a way.

Wesley_S_Lewis
07-17-2018, 03:22 AM
It looks like the answers you've received so far are much more scientific than what I have to offer. I will just add that, as a former skydiving instructor, I was always taught to estimate three degrees Fahrenheit for every 1,000 feet above ground level, and that pretty much jibes with what I saw when jumping into the Arizona desert and the West Texas Permian Basin.

blacbird
07-17-2018, 05:28 AM
It's also worth mentioning that, although the standard image of a desert is hot and dry, not all deserts are actually hot. Heat is not a prerequisite; dry is. The driest desert in the world, by far, is the Atacama in Chile. It really isn't particularly hot, as a consequence of altitude. Similarly the Taklamakan and Gobi in China.

caw