Here is the second installment of the notes for chapter 7.
Properties of water
dipole molecule: the water molecule has an electric charge imbalance so that one end is slightly negative and the other end is slightly positive
hydrogen bonding: the opposite charges attract water molecules to each other
surface tension: because of hydrogen bonding water drops don't spread out into an extremely thin layer
universal solvent: because of its dipole nature ionic solids like sodium chloride readily dissolve in water
phases: water can exist at Earth surface conditions as solid (ice), liquid (water), and gas (water vapor) phases (solid-liquid-gas)
latent heat of freezing-latent heat of melting
water molecules absorb 80 calories per gram to change from the solid (ice) to the liquid (water) state at 0 °C
water molecules release 80 calories per gram to change from the liquid (water) state to thesolid (ice) at 0 °C
latent heat of evaporation-latent heat of condensation
water molecules absorb 540 calories per gram to change from the the liquid (water) state to the gaseous (water vapor) state at 100 °C
water molecules release 540 calories per gram to change from the gaseous state (water vapor) to the liquid (water) state at 100 °C
latent heat of sublimation
water molecules absorb 680 calories per gram to change from the the solid (ice) state to the gaseous (water vapor) state
water molecules release 680 calories per gram to change from the gaseous state (water vapor) to the solid (ice) state
Humidity
relative humidity = (the actual water vapor content of the air) divided by (the maximum water capacity for that temperature)
Warm air has the capacity to hold more water vapor than cool air so relative humidity changes with changing temperature
even if the actual moisture content doesn't change.
saturation: the condition of air that is at 100% relative humidity
the air can't hold any more moisture - its actual moisture content is equal to its moisture capacity for that temperature
dew point: the temperature at which the air is saturated for a given actual moisture content
daily humidity pattern:
as temperature rises during the day, relative humidity goes down because the moisture capacity of the air increases
as temperature falls overnight, relative humidty goes up because the moisture capcity of the air decreases
this pattern holds for all seasons
absolute humidity measures
specific humidity: the weight of water vapor in air compared to the weight of that air
(e.g., 5 grams of water vapor in every kilogram of air)
vapor pressure: the partial pressure exerted by just the water vapor in the air
(remember air pressure is a measure of the weight or pressure exerted by all the molecules in the air)
(e.g., 24 milibars of pressure exerted by water vapor compared to about 1000 milibars exerted by the atmosphere as a whole)
tools for measuring (relative) humidity
hair hygrometer, sling psychrometer
Atmospheric Stability
air is stable if it is at the same density as surrounding air, so it will not tend to rise or fall but will tend to remain in place
Air is cooler at higher elevations because it is farther from the warmth radiated from the Earth's surface...
normal lapse rate is a global average rate of temperature reduction with elevation (6.4 °C/1000 m)
environmental lapse rate is the actual lapse rate on a given day at a specific location
Adiabatic Processes: An adiabatic process is one in which no heat is exchanged with the surrounding air.
Rising air expands as the pressure of the surrounding air decreases (there is less air above it).
The expansion results in cooling because energy is used to separate the molecules.
Sinking air compresses as the pressure of the surrounding air increases (there is more air above it).
The compression results in warming - the energy used to separate the molecules is "given back."
The adiabatic cooling (or warming) rate of rising (or sinking) depends on whether or not condensation occurs.
Dry Adiabatic Rate (DAR): if adiabatic cooling does not lower the temperature below the dew point for the moisture content of the rising parcel of air, then the rate of cooling with rise and expansion is 10 °C/1000 m.
Moist Adiabatic Rate (MAR): if the rising air has reached the dew point, then condensation will continue as the air rises. Since condensation liberates latent heat of condensation, the cooling rate is not as fast, only 6 °C/1000 m.
stable and unstable atmospheric conditions: If a parcel of air is moved upward due to local convergence, convection, orographic lifting, or frontal lifting (see Chapter 8) we will want to know if that air is likely to continue upward and expand and cool until it reaches the vapor point resulting in condensation.
Stable: If rising air cools faster (DAR or MAR) than the surrounding air (Environmental Lapse Rate), then it will be cooler and denser than the surrounding air and will therefore tend to sink back down to its original elevation.
Unstable: If rising air cools more slowly (DAR or MAR) than the surrounding air (Environmental Lapse Rate), then is will be warmer and less dense than the surrounding air and will continue to rise, expand, and cool resulting in continued condensation.
Clouds
cloud formation: cloud-condensation nuclei, cloud droplets
For condensation to occur at the dew point water vapor needs a surface to condense on. In the atmosphere water vapor starts to condense on microscopic particles suspended in the air: dust, soot, volcanic ash, salt spray, etc. These are called cloud-condensation-nuclei. Once microscopic cloud droplets form, they grow larger as water vapor continues to condense on them and droplets merge by collisions. Eventually they grow large enough that they are visible and heavy enough that they may fall as rain (or snow if condensation occurred below freezing).
cloud forms
stratiform: flat or layered
cumuliform: puffy or globular
cirroform: wispy, made of ice crystals
low level clouds
stratus (unbroken layer of cloud), nimbostratus (dark, unbroken layer, rain cloud)
cumulus (puffy, irregular clouds), stratocumulus
(cumulus clouds arranged in a broad layer)
clouds of vertical development
cumulonimbus (thunderhead) forms by vertical growth (upward convection) of cumulus cloud
strong upward convection carries growing cloud droplets to high elevations
(where they may freeze to form hail)
strong downdrafts
produce strong winds associated with thunderstorms
midlevel clouds
altostratus (midlevel flat, layerd cloud)
a corona closely circles the sun or moon shining through altostratus
altocumulus (midlevel puffy clouds)
often form in a patchwork or in rolls
high clouds
cirrus (wispy, "mare's tails")
cirrostratus (high altitude flat, layerd cloud)
a large halo closely circles the sun or moon shining through cirrostratus
cirrocumulus (high altitude puffy clouds)
often form in a patchwork or ripples, but the clouds are/look smaller than altocumulus