06102020, 03:42 AM



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I find this on wikipedia dew point article :
"This means that, if the pressure increases, the mass of water vapor in the air must be reduced in order to maintain the same dew point. For example, consider New York (33 ft or 10 m elevation) and Denver (5,280 ft or 1,610 m elevation[9]). Because Denver is at a higher elevation than New York, it will tend to have a lower barometric pressure. This means that if the dew point and temperature in both cities are the same, the amount of water vapor in the air will be greater in Denver."
https://en.wikipedia.org/wiki/Dew_point
Interesting, does anybody else already know that ?

06102020, 12:56 PM



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Please, anybody to discuss about this ?

06102020, 01:33 PM

Status:
"Finally in subtropical Fort Worth Texas for a whole month"
(set 3 days ago)


Location: In climate zone Cfa/hardiness zone 8a /zip code 76131
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Quote:
Originally Posted by Hartfordd
Please, anybody to discuss about this ?

Nonsense. New York City is humid subtropical; Denver is semi arid continental in climate, end of story.

06102020, 01:52 PM



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Quote:
Originally Posted by Isleofpalms85
Nonsense. New York City is humid subtropical; Denver is semi arid continental in climate, end of story.

Why nonsense ? It’s actually proven scientifically. And New York City is humid continental, Denver is more a kind of steppic continental climate than a true semi arid climate (although it has more xeric characteristics than humid ones). But it’s not actually the topic here...
Last edited by Hartfordd; 06102020 at 02:38 PM..

06102020, 02:29 PM



Location: Downtown Phoenix, AZ
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Quote:
Originally Posted by Hartfordd
Why nonsense ? It’s actually proven scientifically. And New York City is humid continental, Denver is more a kind of steppic continental climate than a true semi arid climate. But it’s not actually the topic here...

I agree it doesn't make sense either. We're true desert with dewpoints in the 20s&30s outside of monsoon season when they creep up to 60 for 2 months, and I would venture we have higher pressure than the East Coast almost all the time

06102020, 02:44 PM



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Quote:
Originally Posted by FirebirdCamaro1220
I agree it doesn't make sense either. We're true desert with dewpoints in the 20s&30s outside of monsoon season when they creep up to 60 for 2 months, and I would venture we have higher pressure than the East Coast almost all the time

https://www.vaisala.com/sites/defaul...NBLOWv1.pdf
"Increasing the pressure of a gas increases the dew point temperature of the gas. Consider an example of air at atmospheric pressure of 1013.3 mbar with a dew point temperature of 10 °C (14 °F). From the table above,
the partial pressure of water vapor (designated by the symbol “e”) is 2.8 mbar. If this air is compressed and the total pressure is doubled to 2026.6 mbar, then according to Dalton’s law, the partial pressure of water vapor, e,
is also doubled to the value of 5.6 mbar. The dew point temperature corresponding to 5.6 mbar is approximately 1 °C (30 °F), so it is clear that increasing the pressure
of the air has also increased the dew point temperature of the air. Conversely, expanding a compressed gas to atmospheric pressure decreases the partial pressures of all of the component gases, including water vapor, and therefore decreases the dew point temperature of the gas. The relationship of total pressure to the partial pressure of water vapor, e, can be expressed as follows:
P1/P2 = e1/e2
By converting dew point temperature to the corresponding saturation vapor pressure, it is easy to calculate the
effect of changing total pressure on the saturation vapor pressure. The new saturation vapor pressure value can then be converted back to the corresponding dew point temperature. These calculations can be done manually using tables, or performed by various kinds of software."

06102020, 02:54 PM



Location: NYC
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If the barometric pressure is fixed at 1 atm, and if we know airtemp and RH, then I use the following simple formula in my head to compute the approximate dew point:
DP = T + 20*log_2(RH).
Here DP is dew point temperature in degrees F, T is airtemp in degrees F, RH is the rel humidity as a decimal (not percent), and log_2 is the base 2 logarithm.
Basically the formula just says that for a fixed airtemp, every time you cut RH in half, you subtract 20F from the dew point.
So if it's 75F with 100% RH, then the DP is 75F. If it's 75F with 50% RH, then the DP is 55F. If it's 75F with 25% RH, then the dp is 35F. If it's 75F with 12.5% RH, the dp is 15F, and so on.
The above formula can be inverted to give RH = 0.5^((T  DP)/20), as a decimal. Multiply that by 100 to get the %RH.
The above formula becomes inaccurate at cold temps and extremely low RH. The more precise version (called the Magnus Formula) is given in the Wiki article, and has more parameters than just T/DP/RH.
Last edited by Shalop; 06102020 at 03:07 PM..

06102020, 03:38 PM



Location: ATL > HOU > DAL > ATL
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SkewT anyone? Been a while but off the top of my head:
Take an unsaturated parcel at ground level (let's just say 1000 mb) and lift it.
To find T, follow the dry adiabat (I know 90 with a dew point of 75 is muggy, but it's still "dry").
To find Td of the lifted parcel, you follow the mixing ratio line.
Eventually the two lines intersect and T=Td. The air is saturated and you've formed a cloud base (aka the LCL  lifted condensation level). Now at this point, the Td will be slightly lower than the Td on the ground. The mixing ratio lines on skew Ts is not parallel to, well, the T lines.
So at least in terms of thinking about Skew Ts and convection, it makes sense.

06122020, 11:55 AM



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Quote:
Originally Posted by FirebirdCamaro1220
I agree it doesn't make sense either. We're true desert with dewpoints in the 20s&30s outside of monsoon season when they creep up to 60 for 2 months, and I would venture we have higher pressure than the East Coast almost all the time

I read your post again and I think you don't really read this :
"This means that, if the pressure increases, the mass of water vapor in the air must be reduced in order to maintain the same dew point. For example, consider New York (33 ft or 10 m elevation) and Denver (5,280 ft or 1,610 m elevation[9]). Because Denver is at a higher elevation than New York, it will tend to have a lower barometric pressure. This means that if the dew point and temperature in both cities are the same, the amount of water vapor in the air will be greater in Denver."
https://en.wikipedia.org/wiki/Dew_point
And the altitude of Phoenix is about 1086 ft / 331 m while NYC is about 33 ft / 10 m so Phoenix has a lower barometric pressure than NYC.
QED
Last edited by Hartfordd; 06122020 at 12:58 PM..

06132020, 07:33 AM



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Really nobody to talk about this ???

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