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)
