Humidity Flashcards
(14 cards)
1
Q
Atmospheric Water Vapor
A
> A component of variable mixture
Three expressions of humidity: Absolute humidity, Saturation Humidity, and Relative Humidity.
2
Q
Absolute Humidity (AH)
A
- Also known as real humidity
- The actual amount of water vapor present in the air
- Measured in grains per cubic foot (gr/ft3)
3
Q
Saturation Humidity (SH)
A
- The maximum amount of water vapor the air parcel can hold
- The air is at the brink of condensation
- Air has a maximum capacity like a sponge
- Measured in grains per cubic foot (gr/ft3)
- Depends on the air’s temperature
- Listed in a table
- Also referred to as the air’s capacity to hold water vapor and is temperature-dependent
- Increases with the temperature at an increasing rate
- Warmer air has a much greater capacity to hold water vapor
4
Q
Relative Humidity (RH)
A
- The ratio of absolute humidity to saturation humidity
- Calculated as: (AH / SH) x 100%
- Indicates how humid it feels
- Feels dry when RH < 33%
- Feels moderate when RH is 33-67%
- Feels humid when RH > 67%
5
Q
Relationship between SH and Temperature
A
- SH increases with temperature
- This increase occurs at an increasing rate
- Warmer air has a much greater capacity to hold water vapor
- SH can be found on a table using the air temperature (T
6
Q
Calculating Relative Humidity (RH)
A
- RH = (AH / SH) x 100%
- To calculate RH, you need:
◦ Absolute Humidity (AH), which is measured or reported
◦ Saturation Humidity (SH), which can be found on a table based on the air temperature (T)
7
Q
Example RH Calculation
A
- If T = 70°F and AH = 2.0 gr/ft3
- From the provided tables, at 70°F, SH = 8 gr/ft3
- Therefore, RH = (2 gr/ft3 / 8 gr/ft3) x 100% = 25%
8
Q
How RH Changes
A
- RH changes with SH
- SH changes with temperature (T)
- Absolute humidity (AH) is generally constant in a given air mass unless water vapor is added or removed
9
Q
Dew Point Temperature (DPT)
A
- The temperature (T) to which the air would have to cool to first arrive at RH = 100%
- The temperature at which AH would saturate the air
- At DPT, AH = SH and RH = 100%
10
Q
Finding DPT
A
- DPT can be found from a known AH
- Use the SH table in reverse: read the DPT for the given AH
- The temperature corresponding to a given AH value in the SH table is the DPT for that AH
11
Q
Relative Humidity of 100%
A
- Means that AH = SH
- The air is fully saturated
- The air is at its Dew Point Temperature (DPT)
12
Q
Cooling air below DPT
A
- Causes condensation (clouds)
- May lead to precipitation
- Decreases AH
13
Q
Example: Cooling Air Below DPT
A
- If AH = 2 gr/ft3 and the air cools to T = 20°F
- At 20°F, SH becomes 1 gr/ft3 (from the table)
- Since the air can only hold 1 gr/ft3 at 20°F, AH decreases to 1 gr/ft3
- RH stays at 100%
- This process can lead to the creation of clouds and precipitation due to the drop in AH
14
Q
Conditions for Condensation and Precipitation
A
- We need cooling air for both condensation (clouds) and precipitation
