Wbd
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General Meteorology
Hi there "Wrong way up"
This is from "General Meteorology" Page 33 Chap 6.15 (except the spelling errors)
" If the air passing over the wet bulb thermometer is not saturated, evaporation of water from the muslin takes place. As the faster moving water molecules esacpe into the air, they take heat energy with them.
The temp. of the water which remains on the muslin therefore falls. The latent heat of the vaporisation is carried away by the molecules that have been convereted to water vapour. If evapration occurs, the wet bulb temp. will therfore be lower than the dry bulb reading.
The difference between the temp. of the dry bulb and the wet bulb thermometer depends on the rate of evaporation. it is known as the wet bulb depression.wet bulb depression
And there is a bit more. The gen and Aviation met books are available at most avaition or tech book stores. I highly reccomend them.
If you get a chance go to your local met station, the guys there are usually very keen to show you the workings of a met station. Of relevance to this question is the location of the two therometers (wet and dry) in the white louvered box (the name of whice escapes me for the moment, any takers) which is located above the ground about 1.2 m. The box allows the therometers to read onlt the temp of the air passing through the box. The two therometers are essentialy the same as those you see hanging on thewall, ie they use mercury in a glass tube. The wet buld thermo. has a small pool of water just beneth it and a muslin ( no not the ones we are nearly at war with) or cloth suspended in the water and over the mercury resiviour of the therometer.
Of course technology is catching up, and there may some new fangled way of doing it nowdays, go and ask and let us all know.
I must admit that your threat of beers had me running to the book shelf to get the answer. But to no avail, as I don't know you and you don't (i hope) know me.
So enjoy the beers.
PS If you are real kind, just send the carton to EASTERNS care of the pilots crew room and I may get one, NOT.
This is from "General Meteorology" Page 33 Chap 6.15 (except the spelling errors)
" If the air passing over the wet bulb thermometer is not saturated, evaporation of water from the muslin takes place. As the faster moving water molecules esacpe into the air, they take heat energy with them.
The temp. of the water which remains on the muslin therefore falls. The latent heat of the vaporisation is carried away by the molecules that have been convereted to water vapour. If evapration occurs, the wet bulb temp. will therfore be lower than the dry bulb reading.
The difference between the temp. of the dry bulb and the wet bulb thermometer depends on the rate of evaporation. it is known as the wet bulb depression.wet bulb depression
And there is a bit more. The gen and Aviation met books are available at most avaition or tech book stores. I highly reccomend them.
If you get a chance go to your local met station, the guys there are usually very keen to show you the workings of a met station. Of relevance to this question is the location of the two therometers (wet and dry) in the white louvered box (the name of whice escapes me for the moment, any takers) which is located above the ground about 1.2 m. The box allows the therometers to read onlt the temp of the air passing through the box. The two therometers are essentialy the same as those you see hanging on thewall, ie they use mercury in a glass tube. The wet buld thermo. has a small pool of water just beneth it and a muslin ( no not the ones we are nearly at war with) or cloth suspended in the water and over the mercury resiviour of the therometer.
Of course technology is catching up, and there may some new fangled way of doing it nowdays, go and ask and let us all know.
I must admit that your threat of beers had me running to the book shelf to get the answer. But to no avail, as I don't know you and you don't (i hope) know me.
So enjoy the beers.
PS If you are real kind, just send the carton to EASTERNS care of the pilots crew room and I may get one, NOT.
Last edited by AMRAAM; 26th Feb 2003 at 08:15.
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Short and sweet
Thanks R
or are you saying mine is a bit long. Pun intended.
Invert - has this provided the answer, or are you after what the temp actualy means to us in the air?
or are you saying mine is a bit long. Pun intended.
Invert - has this provided the answer, or are you after what the temp actualy means to us in the air?
Join Date: May 2001
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Guys. The latest.
The white box is now a "shelter", Stevensons patent ran out some time back so we don't have to pay it anymore.
These days there is a hell of a lot more in them than a couple of thermometers, depending on the station.
Please do arrange to go and have a look at what happens at an observing station, but don't just turn up at the gate. With all the security changes now in vogue, arrange it through the public relations section at any Bureau Regional Office, they will be very helpfull. You may also be very surprised at just what you can get to help you miss the really bad lumpy bits.
The white box is now a "shelter", Stevensons patent ran out some time back so we don't have to pay it anymore.
These days there is a hell of a lot more in them than a couple of thermometers, depending on the station.
Please do arrange to go and have a look at what happens at an observing station, but don't just turn up at the gate. With all the security changes now in vogue, arrange it through the public relations section at any Bureau Regional Office, they will be very helpfull. You may also be very surprised at just what you can get to help you miss the really bad lumpy bits.
Bugsmasherdriverandjediknite
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Hey pharcanal, are the met boys that worried about some one pinching their thermometers that they employ strict security measures? or is it them big balloons that are getting pinched?.
did we have a few ales together in DN about november last year.... just before I went OS?
did we have a few ales together in DN about november last year.... just before I went OS?
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Wiz
Nah mate. I was exiled to the freezing south when you were here. Luckily I'm back and warm again. I'll try to be here when you are next.
The security aint ours, it's because most serious observing stations are INSIDE the sterile area and the watchers get all twitchy about just anyone wandering in and out, so the gate is normally locked.
The security aint ours, it's because most serious observing stations are INSIDE the sterile area and the watchers get all twitchy about just anyone wandering in and out, so the gate is normally locked.
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AMRAAM,
Thanks for all the info on WBD- it comes up fairly regurlary on the TTF's (especiall into YPPH) and it wasn't in any of the TTF decodes in the WWT or AIP so I was interested- I have the Gen and Aviation books somewhere, moving 73 times ( I was in GA) Ihave been unable to find them. One of my mates is an Eastern's Captain so I was going to give the Crownies to him to take in but knowing him he'll drink them before they get to you so IOU lager.. Cheers!
Thanks for all the info on WBD- it comes up fairly regurlary on the TTF's (especiall into YPPH) and it wasn't in any of the TTF decodes in the WWT or AIP so I was interested- I have the Gen and Aviation books somewhere, moving 73 times ( I was in GA) Ihave been unable to find them. One of my mates is an Eastern's Captain so I was going to give the Crownies to him to take in but knowing him he'll drink them before they get to you so IOU lager.. Cheers!
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Definitions
Absolute humidity: The mass of water vapor in a given volume of air( i.e., density of water vapor in a given parcel, usually expressed in grams per cubic meter
Actual vapor pressure: The partial pressure exerted by the water vapor present in a parcel. Water in a gaseous state (i.e. water vapor) exerts a pressure just like the atmospheric air. Vapor pressure is also measured in millibars.
Condensation: The phase change of a gas to a liquid. In the atmosphere, the change of water vapor to liquid water.
Dewpoint: the temperature air would have to be cooled to in order for saturation to occur. The dewpoint temperature assumes there is no change in air pressure or moisture content of the air.
Dry bulb temperature: The actual air temperature. See wet bulb temperature below.
Freezing: The phase change of liquid water into ice.
Evaporation: The phase change of liquid water into water vapor.
Melting: The phase change of ice into liquid water.
Mixing ratio: The mass of water vapor in a parcel divided by the mass of the dry air in the parcel (not including water vapor)
Relative humidity: The amount of water vapor actually in the air divided by the amount of water vapor the air can hold. Relative humidity is expressed as a percentage and can be computed in a variety of ways. One way is to divide the actual vapor pressure by the saturation vapor pressure and then multiply by 100 to convert to a percent.
Saturation of air: The condition under which the amount of water vapor in the air is the maximum possible at the existing temperature and pressure. Condensation or sublimation will begin if the temperature falls or water vapor is added to the air.
Saturation vapor pressure: The maximum partial pressure that water vapor molecules would exert if the air were saturated with vapor at a given temperature. Saturation vapor pressure is directly proportional to the temperature.
Specific humidity: The mass of water vapor in a parcel divided by the total mass of the air in the parcel (including water vapor)
Sublimation: In U.S. meteorology, the phase change of water vapor in the air directly into ice or the chance of ice directly into water vapor. Chemists, and sometimes meteorologists, refer to the vapor to solid phase change as "deposition."
Wet bulb temperature: The lowest temperature that can be obtained by evaporating water into the air at constant pressure. The name comes from the technique of putting a wet cloth over the bulb of a mercury thermometer and then blowing air over the cloth until the water evaporates. Since evaporation takes up heat, the thermometer will cool to a lower temperature than a thermometer with a dry bulb at the same time and place. Wet bulb temperatures can be used along with the dry bulb temperature to calculate dew point or relative humidity
Absolute humidity: The mass of water vapor in a given volume of air( i.e., density of water vapor in a given parcel, usually expressed in grams per cubic meter
Actual vapor pressure: The partial pressure exerted by the water vapor present in a parcel. Water in a gaseous state (i.e. water vapor) exerts a pressure just like the atmospheric air. Vapor pressure is also measured in millibars.
Condensation: The phase change of a gas to a liquid. In the atmosphere, the change of water vapor to liquid water.
Dewpoint: the temperature air would have to be cooled to in order for saturation to occur. The dewpoint temperature assumes there is no change in air pressure or moisture content of the air.
Dry bulb temperature: The actual air temperature. See wet bulb temperature below.
Freezing: The phase change of liquid water into ice.
Evaporation: The phase change of liquid water into water vapor.
Melting: The phase change of ice into liquid water.
Mixing ratio: The mass of water vapor in a parcel divided by the mass of the dry air in the parcel (not including water vapor)
Relative humidity: The amount of water vapor actually in the air divided by the amount of water vapor the air can hold. Relative humidity is expressed as a percentage and can be computed in a variety of ways. One way is to divide the actual vapor pressure by the saturation vapor pressure and then multiply by 100 to convert to a percent.
Saturation of air: The condition under which the amount of water vapor in the air is the maximum possible at the existing temperature and pressure. Condensation or sublimation will begin if the temperature falls or water vapor is added to the air.
Saturation vapor pressure: The maximum partial pressure that water vapor molecules would exert if the air were saturated with vapor at a given temperature. Saturation vapor pressure is directly proportional to the temperature.
Specific humidity: The mass of water vapor in a parcel divided by the total mass of the air in the parcel (including water vapor)
Sublimation: In U.S. meteorology, the phase change of water vapor in the air directly into ice or the chance of ice directly into water vapor. Chemists, and sometimes meteorologists, refer to the vapor to solid phase change as "deposition."
Wet bulb temperature: The lowest temperature that can be obtained by evaporating water into the air at constant pressure. The name comes from the technique of putting a wet cloth over the bulb of a mercury thermometer and then blowing air over the cloth until the water evaporates. Since evaporation takes up heat, the thermometer will cool to a lower temperature than a thermometer with a dry bulb at the same time and place. Wet bulb temperatures can be used along with the dry bulb temperature to calculate dew point or relative humidity
