Hydrology Flashcards
(11 cards)
How and why does the amount of water in the atmosphere change?
It is because water is constantly transported in the atmosphere, as ice, liquid and gas. Although only small quantities (roughly 25mm or 0-4%volume) is transported at a time, huge quantities are cycled.
A water particle can remain in the atmosphere for between <1 hour and 10 years.
What are the relations between water vapour content and temperature?
The greater the temperature, the greater the energy the air has. The greater the energy the air has, the more water vapour (more mass) that it is able to suspend.
If the temperature remains constant, an equilibrium between evaporation and condensation will occur and thus the amount (mass) of water vapour in the air will be stable.
When ice or water is sublimated and evaporated, respectively, to produce water vapour, latent heat is released into the environment, and vice versa.
Why is there a finite capacity of the atmosphere to hold water vapour?
The amount of water vapour that the atmosphere can hold is determined by THE AMOUNT OF WATER AVAILABLE (the water source) and TEMPERATURE (energy required to suspend water).
What are the three processes that lead to water to condensing in the atmosphere?
- Increased water content
Unsaturated air advected over moist surface
This can be due to cold air moving over warm water (advection). It can also be due to the mixing of air masses (in NZ, particularly mixing of Australian/north and Antarctic/southern air masses) to cool the combined air of increase the relative humidity. - Air mass mixing
- Vertical mixing . . . frontal
- Horizontal mixing
It requires the combined air masses to be above the absolute humidity curve in order for condensation to occur; that is, the combined air mass must be over saturated (hold more water than the atmosphere’s temperature can). - Cooling
Land surface changes:
- Warm (moist) air over cool surface
- Diurnal temperature change - radiation cooling
Changes in height of air mass (adiabatic process):
Adiabatic temperature change is “The cooling of warming of air caused when air is allowed to expand or is compressed, not because heat is added or subtracted”. No heat is added or subtracted.
Vertical displacement -> Lower air pressure -> Air Expansion - > Cooling
Due to i)forced or mechanical uplift (orographic) or ii) Convective processes
When does condensation occur?
The condensation level (dew point) is reached when relative humidity is at 100%, however, this is not universally true.
Below RH 100% = when water droplets condense onto particles or other droplets (called cloud condensation nuclei) in the atmosphere.
Above RH 100% = when the air is supersaturated, occurs if the air is very clean.
What are the different measurement techniques?
- Absorption methods (change in dimensions due to absorption)
- Chemical and electrical methods (change in properties due to absorption)
- Condensation methods determine the dew point by cooling
- Psychometric methods use wet and dry bulb temperatures
Scientific method requires measurements to be repeatable, accurate and precise. The accuracy of measurements depends on the research question and purpose for carrying out the measurements.
Humidity is . . .
A general term to refer to amount of water vapour in the atmosphere
Absolute humidity is . . .
The mass of water vapour in a given volume of air. It is affected by changes in temperature and pressure because the volume of the atmosphere is dependent on these things.
Relative humidity (RH) is . . .
The percentage of water vapour in the air relative to the maximum amount that could be held at a given temperature. It is affected by changes in temperature or pressure.
Specific humidity is . . .
The ratio of water vapour in the air compared to a saturated air parcel. It is NOT affected by changes in temperature and pressure. Similar to mixing ratio
Dew point is . . .
The temperature at which the air parcel is cooled for condensation to occur. It is NOT affected by changes in temperature and pressure.
