Download Chapter 4: Moisture and Atmospheric Stability The hydrologic cycle

Chapter 4: Moisture and Atmospheric Stability
The hydrologic cycle
from: USGS http://water.usgs.gov/edu/watercycle.html
Evaporation: enough water to cover the entire surface of Earth to 1 meter cycles through the
atmosphere each year
Most evaporation occurs from the oceans
A much smaller amount from lakes, streams, and runoff
Infiltration
Less rain falls on oceans than evaporates from the surface of the oceans
More rain falls on land surface than evaporates from the land
Most water runs off the land surface but some infiltrates the ground
Even water the infiltrates the land surface eventually makes its way back to the ocean
Transpiration
Plants use water for photosynthesis but they also put water into the atmosphere by
transpiration
Water and change of state
Under the conditions existing on Earth, water exists in three states: solid, liquid, and gas
From Wikipedia. http://en.wikipedia.org/wiki/State_of_matter
Heating water in the liquid state requires 1 calorie per degree per gram
Latent heat of fusion: melting ice requires 80 calories per gram
Note the temperature does not change during the melting process
Note that when water freezes it releases 80 calories per gram
The ‘hidden’ or latent heat is breaking some of the bonds in the ice allowing the molecules to
flow
Latent heat of vaporization: evaporating water requires 540 to 600 calories per gram
When water vapor condenses it releases the same amount of energy per gram
The range occurs because it requires less heat to vaporize water at 100°C than at 0°C
The latent heat is breaking all the remaining bonds: the molecules to become gaseous
Sublimation – ice can go directly to the gaseous phase
The energy required for this process is equal to the sum of the energy involved in melting
and vaporization combined
Deposition is the opposite process and releases the same
amount of energy required during sublimation
Humidity – water vapor in the air
Humidity – the general term for the amount of water vapor
in the air
Absolute humidity – the mass of water vapor in a given
volume of air
Absolute humidity =
Mixing ratio – the amount of water vapor in a unit of air
compared to the remaining mass of dry air
Mixing ratio =
Vapor pressure – that part of the total atmospheric pressure
attributable to its water-vapor content
Saturation – an equilibrium condition in which the
number of water molecules leaving the surface of a
liquid water reservoir is equal to the number of water
molecules returning to the liquid reservoir
Saturation vapor pressure – the pressure exerted by the
motion of water-vapor molecules in a sample of
saturated air
Figure 1: The picture shows the particle
transition, as a result of their vapor
pressure, from the liquid phase to the gas
phase and converse.
(from:
https://en.wikipedia.org/wiki/Vapor_pressure)
The saturation vapor pressure is an exponential function
of the temperature of the water (or air assuming
equilibrium temperature) See Figure 2
Relative humidity – the ratio of the air’s actual water-vapor
content compared with the amount of water vapor required
for saturation at that temperature (and pressure)
Example: Using Table 4–1 on page 102, calculate the
relative humidity for air that has a mixing ratio of 10 g of
water vapor per kilogram at 25°C
From the table, saturation occurs at 20 g per kg at a
temperature of 25°C, therefore:
/
/
100%
50% relative humidity
Figure 2: A graph showing the
temperature dependence of water-vapor
pressure.
(from:
https://en.wikipedia.org/wiki/Vapor_pressure)
Changes in relative humidity
Add or subtract moisture
Evaporation from oceans, lakes, bodies of water
Transpiration from plants
Change the temperature
Heating air causes relative humidity to drop
Cooling air causes relative humidity to rise
A 10°C change in air temperature doubles the amount of water required to reach
saturation
Natural changes in relative humidity
1. Daily changes in temperature (day / night)
2. Temperature changes as air moves horizontally
3. Temperature changes as air moves vertically
Dew point – (or dew point temperature) the temperature to which a parcel of air would need to
be cooled to reach saturation
Advantages of dew point
Unlike relative humidity (how near air is to being saturated), dew point is a measure of
actual moisture content
As long as moisture is not being added or subtracted from the air, the dew point is constant
as the temperature changes (unlike relative humidity which changes with changes in the
temperature)
Humidity measurement
Hygrometer – an instrument used to measure humidity
Psychrometer – two identical thermometers mounted side-by-side, one with a thin muslin
wick tied around the bulb is called the wet bulb
Air is passed over the two bulbs either by fanning or by swinging the thermometers
causing evaporation from muslin wick so that the temperature of the wet bulb
thermometer is lower than the dry bulb thermometer
The higher the humidity is in the air, the smaller the temperature difference between the
thermometers
Tables (like those in Appendix C-1 and C-2) are consulted to determine the relative
humidity or the dew point
Example: A swing psychrometer is used to measure the humidity on a day where the dry
bulb measures 24ºC and the wet bulb measures 20ºC. Find the relative humidity and
the dew point
Find the difference in wet and dry bulb temperatures
24ºC - 20ºC = 6ºC
From the tables
C-1: relative humidity = 55%
C-2: dew point temperature = 14°C
Hair hygrometer – hair changes length in proportion to humidity
The limitation is that this hygrometer is very slow
Electric hygrometer – has an electrical conductor coated with a moisture-absorbing
chemical which causes the current to vary with humidity
Many weather stations have converted to electrical hygrometers
Adiabatic temperature changes – changes in temperature of the air in which heat is neither added
nor subtracted from the air
Causes of adiabatic temperature changes
Expansion causes air to cool
Compression causes air to warm
Adiabatic cooling and condensation
Parcel – an imaginary volume of air that acts independently of the surrounding air in which it
is assumed no heat enters or leaves
Entrainment – when surrounding air does infiltrate a vertically moving column of air
No entrainment occurs in a parcel
Dry adiabatic rate – the rate of heating or cooling of a vertically moving column of
unsaturated air (air less than 100% relative humidity)
Wet adiabatic rate – the rate of heating or cooling of a vertically moving column of
saturated air (air at 100% relative humidity)
Note that cloud formation occurs during wet adiabatic cooling (rising air)
Lifting condensation level – the altitude at which a parcel reaches saturation and cloud
formation begins
Processes that lift air
1. Orographic lifting – air is forced to rise over a mountainous barrier
Rain shadow desert – air that reaches the leeward side of a mountain warms as it descends
causing the relative humidity to drop making condensation or precipitation very unlikely
2. Frontal wedging – warmer, less dense air is forced over cooler, more dense air
Front – masses of warm and cold air collide producing a front
3. Convergence – a pile up of horizontal air flow results in upward movement
4. Localized convective lifting – unequal surface heating causes pockets of air to rise
because of their buoyancy
The critical weathermaker: atmospheric stability
Stable air – if a parcel that is forced to rise cools fast enough that it is cooler than the
surrounding air it will be more dense than the surrounding air and if allowed to do so will
tend to sink back to its original position
Unstable air – if a parcel that is forced to rise cools slowly enough that it is warmer than the
surrounding air it will be less dense than the surrounding air and will rise until its
temperature cools enough to equal the temperature of the surrounding air
Absolute stability – occurs when the environmental lapse rate is less than the wet adiabatic rate
Example: suppose the environmental lapse rate is 5°C per 1000 m, the dry adiabatic rate is
10°C per m, the wet adiabatic rate is 6°C per m, the lifting condensation level is 2000 m,
and the air temperature at the surface is 25°C. Show that the air exhibits absolute stability.
Height
Air Temp
Parcel Temp
5000 m
–5°C
–13°C
↑
4000 m
5°C
–7°C
wet rate: 6°C per 1000 m
3000 m
10°C
–1°C
↓
2000 m
15°C
5°C
↑
clouds form
1000 m
20°C
15°C
dry rate: 10°C per 1000 m
Surface
25°C
25°C
↓
Note that the rising parcel is always cooler than the surrounding air (stable air)
Absolute instability – occurs when the environmental lapse rate is greater than the dry
adiabatic rate
Example: suppose the environmental lapse rate is 12°C per 1000 m, the dry adiabatic rate is
10°C per m, the wet adiabatic rate is 6°C per m, the lifting condensation level is 2000 m,
and the air temperature at the surface is 25°C. Show that the air exhibits absolute stability.
Height
Air Temp
Parcel Temp
5000 m
–35°C
–13°C
↑
4000 m
–23°C
–7°C
wet rate: 6°C per 1000 m
3000 m
–11°C
–1°C
↓
2000 m
1°C
5°C
↑
clouds form
1000 m
13°C
15°C
dry rate: 10°C per 1000 m
Surface
25°C
25°C
↓
Note that the rising parcel is always warmer than the surrounding air (unstable air)
Conditional instability – occurs when moist air has an environmental lapse rate is between the
dry and wet adiabatic rates
Example: suppose the environmental lapse rate is 9°C per 1000 m, the dry adiabatic rate is
10°C per m, the wet adiabatic rate is 6°C per m, the lifting condensation level is 2000 m,
and the air temperature at the surface is 40°C. Show that the air exhibits conditional
instability.
Height
Air Temp
Parcel Temp
5000 m
–5°C
2°C
↑
4000 m
4°C
8°C
wet rate: 6°C per 1000 m
3000 m
13°C
14°C
↓
2000 m
22°C
20°C
↑
clouds form
1000 m
31°C
30°C
dry rate: 10°C per 1000 m
Surface
40°C
40°C
↓
Note that the rising parcel is always cooler than the surrounding air below the condensation
level (stable air) but is warmer than the surrounding air above the condensation level
(unstable air)
How stability changes
Factors that enhance instability
1. Intense solar heating that warms the lowermost layer of the atmosphere
2. Heating of an air mass from below as it passes over a warm surface
3. General upward movement of air (caused by any of the lifting processes)
4. Radiation cooling from cloud tops
Factors that enhance stability
1. Radiative cooling of Earth’s surface after sunset
2. Cooling of an air mass from below as it passes over a cold surface
3. General subsidence within an air column