Water in the Atmosphere ATS 351 - Lecture 4 Water in the Atmosphere

Water is very important! Hydrologic Cycle

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

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 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.

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 = 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

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.

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.

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 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.

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. 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?)‏

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).

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

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 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.

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.

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 mb T=15C Td=0C Td T

Finding mixing ratio (w)‏ Td T

Finding saturation mixing ratio (ws)‏ Td T