ATS 351 - Lecture 4Water in the Atmosphere

Water is very important!Hydrologic Cycle

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

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

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.

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.

• With the same number of water vapor molecules in the air, saturation is more likely to occur in cool air than warm air.

So, we have all this really important water vapor in the

air all of the time.

It would be really helpful if we could keep track of it.

Let’s review how we measure water vapor in the

atmosphere.

Absolute Humidity• Absolute humidity tells us the mass

of water vapor in a fixed volume of air - or water vapor density

Absolute Humidity = Mv/ M

• When a volume of air fluctuates, the absolute humidity changes even though the vapor content has remained constant– Therefore, absolute humidity is not

commonly used in atmospheric studies.

Specific Humidity (q)• When the mass of the water vapor

in the parcel is compared with the mass of all air in the parcel (vapor included)

Specific Humidity = rv/ (1 + rv)

• This measurement does not change as a parcel rises and descends.

Zonally Averaged Specific Humidity

Mixing Ratio• Compares the mass of the water

vapor in the parcel to the mass of the remaining dry air.

R = 0.622 e / (P – e)

• Very similar to specific humidity– Uses only dry air, where specific

humidity uses the dry air PLUS the water vapor itself

• Mixing ratio (and specific humidity) stay constant as long as water vapor is not added to or removed from the parcel.

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 does not indicate the actual

amount of water vapor in the air, but instead tells us how close the air is to becoming saturated

RH = (e/es) * 100

• RH = 100% is saturated air• RH > 100% is supersaturated air

Changing Relative Humidity

• How do we alter a location’s relative humidity?– Change the water vapor content

• Increase w.v. content raise actual vapor pressure relative humidity increases

– Change the air temperature• Increase temperature increase saturation

vapor pressure relative humidity decreases

• Warm = faster molecules = less likely to condense = lower RHReminder: RH = (e/es)*100

Relative Humidity• Since water vapor content

generally does not vary much during an entire day, changing air temperature primarily regulates the daily variation in relative humidity

Dew Point• Temperature to which air would

have to be cooled for saturation to occur (with respect to water).

• It is a good indicator of air’s actual water vapor content– Higher dew point = higher water vapor

content– Adding w.v. to the air increases the

dew point• Frost point: when dew point is

determined with respect to a flat surface of ice

Dew Point & RH• Relative humidity can be

misleading in indicating areas with high water vapor content.

• Dew point is important to look at, along with RH, in order to determine the water vapor content of a location.

• One location has a RH of 100% and a dew point of 0F while a second location has a RH of only 35% but a dew point of 45F– Which location has more water vapor

in the air?• Dry air can have high relative humidity.

July Dew Point Averages

Skew T Diagrams• Since the advent of rawinsonde

observations, thermodynamic diagrams have been used to plot sounding data and to assess atmospheric stability.

• Despite numerous advancements in technology and forecast techniques, the thermodynamic diagram remains an essential tool of today's weather forecaster.

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

@ 950 mbT=15CTd=0C

Td T

Td T

Finding mixing ratio (w)

Td T

Finding saturationmixing ratio (ws)