Chapter 4 - Moisture & Atmospheric Stability

Water vapor is a form of water - it is an odorless colorless gas that mixes freely with the other gases of the atmosphere.

  1. Hydrologic cycle - water is continually cycled between the atmosphere, land & seas
  2. Powered by the Sun
  3. Plants absorb water through their roots, and the release it into the atmosphere - called transpiration
  4. Water balance - precipitation is exactly matched by evaporation & transpiration - 380,000 cubic km per year. (would cover Earth to depth of 1 m.)
Box 4-1 Water - A unique substance
  1. Readily converted from state to state (solid, liquid, gas)
  2. Solid phase (ice) is less dense than the liquid phase (water)
  3. Water expands as it cools through 4C (everything else contracts as it cools)
  4. Water has an unusually high specific heat
  5. Unusual properties mostly due to water's ability to form hydrogen bonds
Water's Changes of State
  1. One calorie - the amount of heat required to raise the temperature of one gram of water by 1 degree C
  2. Latent Heat - heat energy that is transferred to or from a body, thereby changing its state, but not its temperature. (solid to liquid, liquid to gas - ice to water, water to steam). Latent means hidden.
  3. Evaporation - the process of converting a liquid to a gas (vapor). Latent heat of vaporization must be supplied. Most energetic molecules escape. Liquid cools.
  4. Condensation - the process in which water vapor changes to the liquid state. Latent heat of condensation is released.
  5. Melting - the process in which a solid changes to a liquid. Requires heat.
  6. Freezing - reverse of melting - releases latent heat of fusion
  7. Sublimation - conversion of a solid directly to a gas
  8. Deposition - reverse of sublimation. Vapor converted directly to solid. (Frost)

SO, what is Freezer Burn? WHY does it occur?

Humidity - Water Vapor in the Atmosphere
  1. Humidity - describes the amount of water vapor in the air
  2. Absolute humidity - mass of water vapor in a given volume of air - mixed units - mass of water vapor (gm) / volume of air (cubic m)
  3. Mixing ratio - mass of water vapor in a unit of air, compared to the mass of the dry air - mass of water vapor (gm) / mass of dry air (kg)
Vapor Pressure & Saturation
  1. Pressure - caused by change of momentum of molecules as they hit a surface
  2. Vapor pressure - that part of the total atmospheric pressure that is due to its water vapor content
  3. Saturation - number of molecules evaporating = number condensing
  4. Saturated vapor pressure - pressure when the air is saturated (cannot hold any more water)
  5. For every 10C change in temperature, amount of water needed for saturation doubles
  6. Rate of evaporation depends on temperature and vapor pressure in the air
Relative Humidity
  1. Relative humidity is the ratio of the air's actual water content to the amount of water vapor required for saturation at that temp & pressure
  2. Same thing as RH = actual mixing ratio / saturation mixing ratio
  3. Relative humidity can be changed by adding water or by changing the temperature
  4. If the water vapor content remains constant, decreasing the temperature increases the relative humidity
How to Change Relative Humidity (again)
  1. Add or subtract moisture
  2. Increase or decrease the temperature
Box 4-2 Dry Air at 100% Relative Humidity?
  1. The RH is a measure of how much water the air has in it, relative to the amount that it could actually hold at that temperature.
  2. Cold air with 100% humidity (all the water it can hold) can actually contain less water than hotter air with a lower RH
Humidifiers & Dehumidifiers
  1. In winter, the air is cold and cannot hold a lot of water vapor. Water evaporates form the skin, leading to dry skin. The solution is to add moisture, with an humidifier.
  2. In summer, the air can be very humid (high RH), preventing the body from cooling by sweating. The solution is to remove moisture from the air (dehumidifier).
Natural Changes in Relative Humidity
  1. Daily changes in temperature
  2. Temperature changes that result as air moves horizontally from one location to another
  3. Temperature changes as air moves vertically
How Relative Humidity Changes
  1. RH = (actual mixing ratio / saturated mixing ratio) x 100%
  2. Suppose mixing ratio at 20C is 7 g/kg
  3. RH = (7 g/kg / 14 g/kg) x 100 = 50% (see Table 4-1)
  4. Suppose temperature rises to 20C.
  5. RH = (7 g/kg / 26.5g/kg) x 100 = 26% - decreases
  6. Suppose temperature falls to 1C.
  7. RH = (7 g/kg / 3.9g/kg) x 100 - increases, but is > 100%!!!
  8. What actually happens is that 7 - 3.9 = 3.1 gm condenses to form droplets
Dew Point Temperature
  1. Temperature to which a parcel of air would need to be cooled to reach saturation
  2. Dew point temperatures are shown on weather maps
  3. Example - Air at 23C contains 5 g vapor/kg of air
  4. Cooling air to 5C (Table 4-1) will cause saturation & condensation
  5. Thus dewpoint is 5C.

SO, Is frost just frozen dew?

Measuring Humidity
  1. Instruments are called hygrometers
  2. Psychrometer - wet & dry bulbs, observe temperature, look up tables
  3. Hair hygrometer - not very useful
  4. Electric hygrometer - electrical resistance as relative humidity changes
Box 4-4 Atmospheric Hazard: Humidity and Heat Stress
  1. The body sweats in order to cool off (by the sweat evaporating).
  2. If the air is humid, the sweat can evaporate, but water from the atmosphere returns to the skin (dynamic equilibrium)
  3. Net effect is very little sweating.
  4. Physical exertion should be avoided on hot humid days
Adiabatic Temperature Changes
  1. Adiabatic cooling is the basis of cloud formation
  2. Adiabatic - no heat lost or gained
  3. A rising parcel of air expands because the atmospheric pressure decreases, and the parcel cools adiabatically (the expanding parcel has to do work)
  4. Unsaturated air cools at 10C per km of ascent - this rate is called the dry adiabatic rate .
  5. If the parcel of air rises enough, it will cool to its dewpoint, and condensation occurs. The altitude at which this occurs is the lifting condensation level.
  6. After this level, latent heat is liberated and slows the rate of cooling. Rate is called the wet adiabatic rate . 5C per km (high moisture content), 9C per km (low moisture content)
Lifting Processes
  1. Orographic lifting - air is forced to rise over a mountainous barrier
  2. Frontal wedging - warmer, less dense, air is forced over cooler, denser air.
  3. Convergence - a pileup of horizontal air flow results in upwards movement
  4. Localized convective lifting - unequal surface heating heats localized pockets of air
Orographic Lifting
  1. Lifting causes adiabatic cooling, clouds, rain on windward side of mountains
  2. Often get rain shadow deserts on leeward side of mountains.
  3. See Box 4-6.
Box 4-6 Orographic Effects
  1. Causes of rain shadows.
  2. Water is extracted form the air on the windward side
  3. Air on the leeward size is warmer than on the windward side and therefore has a lower RH
Frontal Wedging

Warmer, less dense, air is forced over cooler, denser air.

Convergence
  1. Collision of air masses
  2. Obstacle slows or restricts horizontal air flow (wind)
  3. Air moving from a relatively smooth surface to a rougher surface
Localized Convective Lifting
  1. On warm summer days, unequal heating of the Earth's surface may cause pockets of air to be warmed more than the surrounding air
  2. Warm parcels of air (thermals) rise above the lifting condensation level, and clouds form
Rain Shadow Deserts and Chinooks
  1. Orographic lifting by mountains leads to rain shadows
  2. Air being pushed down the mountain has lost a lot of water (as rain), and is adiabatically heated
  3. Relative Humidity drops
  4. Chinooks move down the eastern slope of the Rockies - snow eater
Atmospheric Stability
  1. A rising air parcel cooler than surrounding air - stable air - parcel tends to sink
  2. A rising air parcel warmer than surrounding air - unstable air - parcel tends to keep rising
  3. Absolute Stability - environmental lapse rate is less than wet adiabatic rate
  4. Absolute Instability - environmental lapse rate is greater than dry adiabatic rate
  5. Conditional Stability - moist air has an environmental lapse rate between the dry and wet adiabatic rates. Air is stable wrt unsaturated parcel of air, but unstable wrt a saturated parcel of air.
  6. "Conditional" because air must be forced upward before it reaches the level where it is unstable and rises on its own
Stability & Daily Weather

Instability is enhanced by:

  1. Intense solar heating warming the lowermost layer of the atmosphere
  2. The heating of an air mass from below as it passes over a hot surface
  3. General upward movement of air caused by processes such as orographic lifting, frontal wedging, convergence
  4. Radiation cooling from cloud tops

Stability is enhanced by:

  1. Radiation cooling of Earth's surface after sunset
  2. The cooling of an air mass from below as it passes over a cool surface
  3. General subsidence within an air column
Problem 4, Part c
  1. As the air rises, more condensation occurs because the cooling air cannot hold any more water vapor.
  2. The dewpoint temperature is equal to the local air temperature.
  3. So, at the top of the mountain, the dewpoint temperature is 0C.