Ch 5 - Humidity Flashcards
Water Cycle Key Points
- Water Evaporates
- Cools
- Eventually Reaches dew point
- If cools past the dew point it condenses, forms clouds or fog
- When it gets too heavy, the moisture leaves the clouds as rain
- Lands on the land, drains back into rivers and oceans and repeats
Saturation Content
Warm air can hold more water vapour than cold air
The amount of water vapour that can be contained in the air
This is because warmer air is:
- Higher energy - Hydrogen bonds can be broken easier so more get broken which leads to more water vapour in the air
Water Vapour Pressure
Water vapour molecules press down on the surface -> water vapour pressure
When the water pressure increases so much,no more can evaporate
- Instantly Condenses -> 100% Humidity (Saturation Vapour Pressure)
High Temp= Higher saturation vapour pressure point (dew)
Lower Temp= Lower saturation vapour pressure point
Saturation Vapour pressure will always be less over ice than over water at the same temperature
Water below 0C = Supercooled water cooled water droplets
- Much easier to change into ice
Absolute Humidity
Mass of water vapour in a given volume of dry air (g/m^3)
AH depends on:
- Volume: Rising air, AH decreases as the air expands
Sinking air, AH decreases as air contracts
Humidity Mixing Ratio (HMR / Mixing Ratio)
HMR is the mass of water vapour contained in a unit mass of dry air (g/Kg)
HMR= Actual Mass of Water Vapour/ Mass of Dry Air
Remains constant with Altitude (if you keep the moisture content constant)
Saturation Mixing Ratio (SMR)
SMR is the mass of water vapour constrained in a unit mass of SATURATED air (g/Kg) at a certain temp
SMR = Mass of water vapour required for Saturation (g) / Mass of Saturated Air (kg)
SMR will decrease as altitude increases as temperature is decreasing.
Relative Humidity
RH = (Actual Humidity (or AVP or HMR)/ Max humidity (or SVP or SMR)) * 100
RH (%) = (100-5)*(T-Td)
T= OAT
Td= Dew Temp
If temp decreases, Relatively humidity increases
- If temp decreases, SMR decreases because it is linked to temp
Cold air holds less moisture than warm
Absolute H decreases if temp decreases
Diurnal Variation of RH
Highest RH = Coldest part of the day (30 minutes after sun rise)
Lowest RH = Hottest part of the day (2-3 hours after relative noon)
Condensation Level
When the temperature is cooled slightly lower than the dew point temperature, condensation occurs (sublimation - cooling) turning into water droplets and ice crystals
There must be a nuclei for this process to occur as the moisture needs something to condensate onto
- The closer together the DP and the OAT is, the lower the cloud base will be
Psychometor (Spelling)
A device to measure humidity:
2 thermometers:
- Dry Bulb which gives us the dry bulb temperature (OAT)
- Wet Bulb (wrapped in a cloth and connected to a water bottle) which shows the dew point
The drier the air, the lower the dew point which will show a large gap between the two readings
The relative humidity is the difference between the two readings
If close together - Increased RH
- If they are the same temp, 100% RH = fully saturated
Found in a Stevenson screen
Hygrometer
Uses a strand of human hair to measure relative humidity
Change in the hair length (due to absorbing atmospheric moisture) will show (through a series of pivot) the relative humidity
