1. Water in the Atmosphere
ATS Lecture 4
Water in the Atmosphere

2. Water is very important! Hydrologic Cycle

3. Properties of Water Physical States Gas (Water Vapor)‏ Liquid Solid
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

4. 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

5. 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
Wind enhances: by blowing the vapor molecules already in the air away. Therefore, this prevents saturation from occurring which would allow for a greater amount of evaporation.

6. 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.

7. 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.

8. 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.

9. Absolute Humidity = Mv/ M
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.
The volume of an air parcel is constantly changing because of it rising and sinking - so therefore even though the water vapor content has remained constant, abs. humidity changes. So, since we are trying to measure the water vapor content in the atmosphere - this type of measurement is not useful

10. Specific Humidity = rv/ (1 + rv)
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.

11. Zonally Averaged Specific Humidity
The specific humidity is highest in warm, muggy tropics and as we move away from the tropics, it decreases. Major deserts of the world are at around 30 degrees. This figure shows that at that latitude, the average air contains nearly twice the water vapor than the air at 50 degrees. Therefore, the air of the desert is not “dry” and the water vapor content is not very low.

12. Mixing Ratio
Compares the mass of the water vapor in the parcel to the mass of the remaining dry air.
R = 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.

13. 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
Last bullet: more water vapor molecules = more vapor pressure just like when you have a balloon and put more AIR molecules into it, it’s total pressure increases.

14. 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.

15. 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.
The statement that water has a higher saturation vapor pressure than ice means that at any temperature below freezing, it takes more vapor molecules to saturate air directly above water than it does to saturate air above ice. This occurs because it is harder for molecules to escape from ice than water. (more air = more pressure?)‏

16. 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
Most common and familiar way of determining atmospheric moister but it is also highly misunderstood.
It is the ratio of the amount of water vapor actually in the air to the maximum amount of water vapor required for saturation at that particular temperature (and pressure).

17. 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 RH
Reminder: RH = (e/es)*100

18. 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

19. 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

20. 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 0F while a second location has a RH of only 35% but a dew point of 45F
Which location has more water vapor in the air?
Dry air can have high relative humidity.

21. July Dew Point Averages
Dew points are very high during the summer. The highest dew point area (south) receives humid air from the warm Gulf of Mexico. The lowest dew points are in Nevada where it is surrounded by mountains and is shielded from moisture moving in from the southwest and northwest.

22. 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.

23. 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…

25. Isobars (pressure)‏

26. Isotherms (temperature)‏
In Celsius

27. Dry Adiabats

28. Saturation Adiabats

29. Saturation Mixing Ratio

31. @ 950 mb T=15C Td=0C Td T

32. Finding mixing
ratio (w)‏ Td T

33. Finding saturation
mixing ratio (ws)‏ Td T