ATOC 181 Lecture Notes - Lecture 10: Vapor Pressure, Fluid Parcel, Humidity
7 Jun 2018
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LESSON 10
ATMOSPHERIC MOISTURE
**Midterm: from lecture on seasons included to the lecture on Friday (lesson 11) included
*** instead of 13th April, the final quiz will be on April 11th
• Count each molecules of water vapour in atmosphere: not most efficient method
• Saturation: in order to know relative humidity, need to know how much water vapour is needed to
saturate the air
• The water vapour content or humidity present in the air (air parcel) can be expressed in a number of
ways
• Air parcel does not really exist, but represent really certain part of the atmosphere: contain certain
amount of air with it main components as well as water vapour
• Same pressure as the air around, same temp, no exchange of heat and substance with air around
Measure of Humidity
• Water vapour density
• Specific humidity
• Vapour pressure
• Relative humidity: most often hear about this one: measure of how much water vapour in the air
• Dew point temperature: used especially in US
Water Vapour Density
• Density: mass of one unit of volume (1m3). If the mass of 1m3 of air is 1g, density = 1g/m3
• Slight problem: suppose that this air parcel of 1m3increases in size, but you did not add or remove any
water vapour inside, there is the same amount of molecules
o Thus, the mass is the same and volume increases = 2m3. Density = 0.5g/m3.
o If pressure decreases, volume will increase. Kind of like a balloon. Can inflate without the
air/water vapour inside changing
• Not really use, not representative, it changes
• Water vapour changes when volume changes
Specific Humidity
• Mass of water vapour contained in 1kg of air (dry air)
• When use specific = per unit of mass (1kg)
• Amount of water vapour is very low as compared to air, use g/kg instead kg/kg
• Mass of water vapour: 1g, mass of air = 1kg.
o specific humidity = 1g/kg
• If air parcel increases in volume without adding or removing water vapour molecules, the mass does not
change.
• The specific humidity does not change when volume changes.
o Important feature when want to count for water vapour. Naturally, air expands or shrinks. As
goes up, expands. When goes down, shrinks.
find more resources at oneclass.com
find more resources at oneclass.com
o Will use specific humidity
Water Vapour Pressure
• Part of total air pressure due to water vapour
• Dalton law: the total pressure is the sum of the individual pressure of each component
• Container with liquid water and dry air: in that case, no water vapour and no pressure due to water
vapour
• If leave this container for a while, liquid water will evaporate. As does so, water vapour forms as liquid
water evaporates. As water vapour amount increases, the partial pressure due to water pressure will
also increase
• The more water vapour, the higher the pressure, the more humidity
• Water vapour pressure increases when humidity increases
Saturation Vapour Pressure
• Evaporation puts more water vapour molecules in the air.
• After a certain time, the number of molecules that will evaporate will be the same amount that will
condense: some sort of equilibrium or after a while, no more evaporation
• At certain temperature, there is a limit on the quantity of water vapour that can evaporate
• When this limit is reached, there are as many molecules that condense as molecules that evaporate
• A state of equilibrium is reached
• Limit: saturation vapour pressure
• At 20g = limit 23.4hpa
• Normal pressure near the surface:1000hpa
• If increases the temp to 30oC, the saturation vapour pressure is increasing fairly fast (42.5hPa), almost
twice for 10 oC
• Saturation vapour pressure increases with temperature.
• For a given temperature, the saturation vapour is fixed. Not depend on anything else except temperature
of air. There is a limit on the quantity of water vapour that can evaporate
• Can be described as the amount of water vapour that is necessary to saturate the air at any given
temperature
• The curve of saturation vapour pressure: saturation
vapour pressure (hPa) in function of temperature:
increases very fast when temperature increases. Know
when look at this curve the amount of water vapour
needed to saturate
• At normal temp and pressure: give saturation or
specific humidity (g/kg)
• Describes how much water vapour is necessary to
saturate the air at any given temperature
• Saturation vapour pressure is independent of
atmospheric pressure and depends only on
temperature
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
**midterm: from lecture on seasons included to the lecture on friday (lesson 11) included. Measure of humidity: water vapour density, specific humidity, vapour pressure, relative humidity: most often hear about this one: measure of how much water vapour in the air, dew point temperature: used especially in us. Water vapour density: density: mass of one unit of volume (1m3). Can inflate without the air/water vapour inside changing: not really use, not representative, it changes, water vapour changes when volume changes. If air parcel increases in volume without adding or removing water vapour molecules, the mass does not change: the specific humidity does not change when volume changes. Important feature when want to count for water vapour. When goes down, shrinks: will use specific humidity. If leave this container for a while, liquid water will evaporate. As does so, water vapour forms as liquid water evaporates.
