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