Lecture 4: Moisture in the atmosphere Flashcards
Why is water a potent GHG?
it can absorb a significant amount of longwave radiation.
Where is the concentration of water vapour in the atmosphere highest?
Near surface, and at tropics (0.25%) declining towards poles (0.1%)
How were the oceans formed?
Water vapour produced by volcanic degassing during first billion years, due to the bombardment of comets. The atm could not hold all of this, so condensed to form oceans.
Describe the measures of water vapour in the atmosphere based on the mass of water.
Mixing ratio = mass of water vapour / mass of dry air
Specific humidity = mass of water vapour / total mass of air
Absolute humidity = mass of water vapour / volume of air
Describe the measures of water vapour in the atmosphere that are based on vapour pressure.
Relative Humidity = actual vapour pressure / saturated vapour pressure X 100
or, the actual mixing ration / saturation mixing ration
Describe the measures of atmospheric water vapour based on temperature.
Dew point temperature, or wet bulb temperature depression.
When / Where is specific humidity highest? (The grams of water vapour in one kg of air)
Highest at tropics, lowest at poles and decreases with altitude. Highest in summer as temperature sets the upper limit on vapour pressure.
What does Daltons law of pressure state?
That total pressure = the individual pressures exerted by gases on the wall of an air parcel.
What is water vapour pressure?
The partial pressure due to water molecules (measured in milibars)
Put water vapour, nitrogen and oxygen in order of their vapour pressures.
Nitrogen = 780mb
Oxygen = 210 mb
Water Vapour = 10mb
On a molecular level, how does condensation occur?
Water vapour molecules randomly collide with water surface and bond with adjacent molecules.
Define the saturation vapour pressure.
How much water is needed to make air saturated at a given temperature. Rises exponentially and is primarily determined by air temperature.
Relative humidity is the % of moisture content relative to saturated moisture content, but what does this depend on, and what does it mean for its distribution?
Depends on the vapour content of air and temperature. It is high near the equator as actual vapour content is high, and high near the poles as saturation vapour pressure is low.
Why does air cool more slowly as it rises?
When water changes from liquid to gas, it releases latent heat, which warms the surrounding air and counteracts the cooling from expansion.
Evaporation acts to cool a wet thermometer, what does this depend on, and what measurements can it be inferred from?
Depends on the surrounding humidity of the air, and can be inferred from the wet-dry temperature difference.
What two factors give a high wet bulb temperature?
High dry bulb temperature and high humidity
Which condition can lead to heat stress, and even fatality above 35 degrees.
high wet bulb temperature
Once the air is saturated, which three things cause condensation?
- Adding more water
- cooling the air
- mix cold air with warm moist air
What causes dew?
Moisture from the atmosphere condensing on a surface. Surface cools due to infrared heat loss results in air near surface becoming saturated (the dew point).
Frost formation is similar to dew formation, but what causes fog to form? (a surface cloud).
When air cools to dew point and has moisture added, or cold air is mixed with warm moist air.
What is the precondition for cloud formation?
Relative humidity is 100%
What are the four basic cloud types that can combine to form ten different cloud types?
- Cirrus - thin, wispy clouds of ice
- Stratus - layered cloud
- Cumulus - vertical development
- Nimbus - rain producing
Why do clouds form?
Moist air is lifted and cooled adiabatically
The Environmental Lapse Rate (ELR) is usually 6.5 but can vary up to 9.8. How is it calculated?
The rate of temperature decrease with height in the troposphere. (in degrees celsius per km)
What conditions might cause the air to rise with temperature for a small distance?
Still and anticyclonic
What causes high and low ELR?
High = strong surface heating Low = moist, turbulent air
Describe an adiabatic process.
A thermodynamic process in which no heat is exchanged with the atmosphere (or working fluid).
Describe why an adiabatic air parcel cools as it rises.
As it rises, pressure decreases so the air parcel expands. As it is adiabatic, the energy must come from kinetic energy within the air parcel - therefore thermal energy is lost.
Atmospheric stability depends on the ELR - for a DALR of 10, what would be the effect of air masses greater than this, or lower?
ELR less than 10 = stable
More than 10 = unstable
How does an unstable air parcel move relative to a stable one?
Unstable air parcels continue to rise and cool, whereas stable air parcels rise, cool and sink again.
What is the DALR?
The constant rate at which air cools with height as the result of expansion - 9.8 degrees/km. Rises until temperature and density are equal to surroundings.
When does absolute instability occur?
When ELR cools faster than DALR - air bubbles will be cooling more slowly than environment. (Rare except close to ground, where the convection it causes transports heat upwards and reduces ELR).
When does absolute stability and conditional instability occur?
Absolute stability - if ELR is cooling more slowly than SALR
Conditional instability - if ELR is between SLR and DALR
Why is the SALR not constant?
Depends on heat released by condensation.
Latent heat is an efficient way of transporting heat globally, but what is it?
The energy released when one unit mass of water changes phase.
What is caused by latent heat release of condensation?
ALR decreases, as moisture tends to make the atmosphere less stable.
What does the type of cloud formation depend on?
The degree of instability, initial temperature and water vapour content of the air parcel.
What is needed for clouds to form?
Saturated air + something for the drops to condense on = cloud condensation nucleus.
What turns cloud droplets into rain?
- Condensation
- Collision and Coalescence in warm clouds
- The bergeron process in cold clouds
Describe the Bergeron process which occurs in the portion of most mid latitude clouds where temperatures are below the melting point of ice.
- Saturation Vapour Pressure of Ice is lower then supercooled water or water vapour.
- During coexistence, water will sublimate directly onto the surface of ice.
- Ice crystals grow at the expense of super cooled drops.
- Collisions that occur between drops and falling crystals cause growth by riming and aggregation.
