Water in the Atmosphere Lab 5 October 5, 2009
Jan 19, 2016
Water in the Atmosphere
Lab 5October 5, 2009
Water Is Important!!!
Properties of Water• Physical States
– Gas (Water Vapor)• Molecules move freely and
mix well with other molecules
– Liquid• Molecules are close together
and constantly bump one another
– Solid• In ice, molecules are arranged
in a hexagonal crystal
– Only natural substance that occurs naturally in all three states on Earth’s surface
Ice Molecule
Properties of Water Heat Capacity
Highest of all common solids and liquids Compressibility
Virtually incompressible as a liquid Density
Density of seawater is controlled by temperature, salinity, and pressure
Liquid water has maximum density at +4°C Solid phase has lower density since it must form crystal
structure What would happen if ice was more dense than water???
Radiative Properties Transparent to visible; Absorbs infrared
Phases of Water
• Condensation• Evaporation• Melting• Freezing• Sublimation
– Molecules have enough energy to escape from the surface of ice into air above and directly into the vapor phase
• Deposition– Water vapor molecule
attaches itself to an ice crystal and changes to ice
Evaporation• Water has a very high surface
tension– Takes energy to break the
hydrogen bonds on a water surface in order to evaporate
• What can enhance evaporation from the surface of water?– When temperatures are increases,
molecules move faster (gain energy) and can break the surface tension more easily
– Wind also enhances evaporation
Saturation If we evaporate water in a closed
container, eventually the evaporated water vapor will condense back into the liquid.
The air above the water is said to be saturated with water vapor when the evaporation and condensation rates reach equilibrium.
If this set up is heated, more water will have to be evaporated, and the amount of water vapor saturating the air will be greater.
How would elevation affect this?
Condensation
Depends on temperature For condensation to be really effective,
water vapor needs something to condense onto. We call these things in air Condensation Nuclei.
• Dust, smoke, salts, other particles… When air is warm and molecules move fast, water
vapor may bounce off the Condensation Nuclei. When air is cold and molecules move more slowly,
water vapor is more likely to stick. This shows, again, that you are more likely to have more
water in the vapor form in warm air than in cold air.
Measuring Water Vapor
Water vapor is clearly important in the atmosphere Greenhouse effect, latent heat
How do we measure water vapor Absolute humidity, relative humidity, mixing ratio,
vapor pressure
Absolute Humidity
If we were able to remove all of the water vapor in a parcel of air with a known volume and measure its mass,
It’s like water vapor density (mass/volume) Usually measured in g m-3
But, since air moves up and down a lot in the atmosphere, its volume changes, too. This makes absolute humidity variable.
Absolute Humidity mass of water vapor
volume of air
Absolute Humidity
The actual amount of water vapor is the same, but the absolute humidity changes.
Specific Humidity (q)
Specific Humidity mass of water vapor
total mass of air
Surface Specific Humidity
Zonally Averaged Specific Humidity
Mixing Ratio (r)
1 g kg-1 = For every one kilogram of dry air, there is an additional one gram of water vapor in it
Very similar to specific humidity Uses only dry air, where specific humidity
uses the dry air PLUS the water vapor itself
Mixing Ratiomass of water vapor
mass of dry air
Vapor Pressure (e)
• The air’s moisture content may also be described by measuring the pressure exerted by the water vapor in the air.
• Dalton’s Law– The total pressure exerted by the gases in a mixture
is equal to the sum of the partial pressures of each individual component in a gas mixture.
– For 1000 mb of air:• 78% N2 = 780 mb• 21% O2 = 210 mb• 1% H2O(v) = 10 mb ---> actual vapor pressure
– More air = more pressure– Higher vapor pressure = Larger # of water vapor
molecules
Saturation Vapor Pressure (es)
• Recall: when evaporation and condensation are in equilibrium, the air is saturated with water vapor.
• Saturation vapor pressure describes how much water vapor is necessary to make the air saturated at any given temperature.– It is the pressure that that amount of
vapor would exert.
• Saturation vapor pressure depends primarily on the air temperature.– Exponential
relationship
• When water and ice both exist below freezing at the same temperature, the saturation vapor pressure just above water is greater than the saturation vapor pressure over ice.
Relative Humidity (RH)
RH is not the actual amount of water vapor in the air.
100% = saturated >100% = supersaturated
RH water vapor content
water vapor capacity
RH actual vapor pressure
saturation vapor pressure100%
RH actual mixing ratio
saturation mixing ratio100%
Changing RH
Increase vapor content Higher RH at same Temp
Increase Temperature Lower RH for same vapor content Hot = fast = less likely to condense = lower RH
Dew Point Temperature
This is a measure of moisture content. Temperature to which the air must cool
to reach saturation with respect to water.
Frost Point Temperature to which the air must cool to
reach saturation with respect to ice.
Representing Atmospheric Conditions
As with station plots and contouring, it's favorable to be able to represent vertical atmospheric conditions in a simple manner Skew-t diagrams
Let's first discuss
Skew T Diagrams•Why are skew T diagrams
useful?– Forecasting applications:
• Temperature and dew point profile of atmosphere
• Daily maximum temperature• Level of cloud formation• Stable vs. unstable air• Precipitation type (icing forecasting)• Level of tropopause• CAPE (Convective Available Potential Energy)• Microburst forecasting• And many more…
Isobars (pressure)
Isotherms (temperature)
In Celsius
Dry Adiabats
Saturation Adiabats
Saturation Mixing Ratio
Skew-T/Log-P Diagram
@ 950 mbT=15CTd=0C
Td T
Td T
Finding mixing ratio (w)
Td T
Finding saturationmixing ratio (ws)
Skew-T
Uses Locations and magnitudes of inversions
Stable/unstable layers
Cloud base heights
Precipitation types
Severe weather potential
First, we must understand how an air parcel travels vertically in the atmosphere
Air Parcel
When talking about the movement of air we usually refer to a “parcel” of air
Think of a small blob of air An air parcel always has
uniform properties throughout
Dry Adiabatic Process Adiabatic refers to a process in which there is no
energy exchanged between an air parcel and its environment Rising air expands and cools Descending air compresses and warms Warming/cooling occurs at the
Dry Adiabatic Lapse Rate ( 9.8 K km-1 )
The dew point also decreases as a parcel is raised “Dry Adiabatically” by 2 K per km
Dew point Lapse Rate ( 2 K km-1 )
Moist Adiabatic Process When water vapor condenses in the parcel as it’s
rising, latent heat is added and it cools slower The parcel then cools at the
Moist Adiabatic Lapse Rate ( 6.5 K km-1 )
Lifting a Parcel
Recall that a parcel’s mixing ratio stays constant as it moves vertically
Initially, a parcel being lifted will cool at the
Dry Adiabatic Lapse Rate At some level the parcel will have cooled enough so that it’s
mixing ratio is equal to the saturation mixing ratio When the dry adiabat from the surface temperature meets the
saturating mixing ratio line from the surface dew point, the parcel will have reached saturation and condensation can occur
This is called the Lifted Condensation Level (LCL)
Lifting a Parcel Once a Parcel has reached the LCL, it will continue to rise,
cooling at the Moist Adiabatic Lapse Rate Often the temperature of the parcel at the LCL is still cooler
than the temperature of the environment If the parcel is lifted further it will reach its
Level of Free Convection (LFC), the point at which the parcel becomes warmer than the environment and will be accelerated upward by buoyancy
As it continues to rise it will eventually reach a point where it is cooler than the environment again.
This is the Equilibrium Level (EL)
Lifting a Parcel