ATMO 9/9 Evaporation, Condensation, Relative Humidity, & Dew Point

Current Weather:

Hurricane Ike:
- Forecasts not exact
- Moved over Cuba and weakened due to high mountain ranges and being over land

Evaporation & Condensation
- Rate of evaporation
- Rate of condensation
- Competing processes happening simultaneously

*Homework assigned between Today & Thursday*

Closed System Evaporation and Condensation Example:
- 20C constant temp
- Water in the bottom evaporates, as the vapor pressure increases the rate of condensation increases until the two process are at equilibrium (Saturated air, constant vapor pressure = saturation vapor pressure)
- If temp is increased, the rate of evaporation increases and the vapor pressure rises, causing the rate of condensation to increase. This is also true in the opposite direction (net condensation).
- Saturation: the maximum amount of water vapor that can exist in the air (capacity for water vapor). As temperature increases, the saturation vapor pressure increases sharply.
- "Warm air can hold more water vapor than cold air." Translation: The warmer the air, the higher the capacity for water vapor in the air.
- Dynamic Equilibrium means evaporation and condensation are happening at the same rate.

Water Vapor Pressure Graph:
- Water Vapor (Y)
- Temperature (X)
- What amount of water vapor can be in the air at certain temperatures?
- As temperature increases, the maximum amount of water vapor pressure increases exponentially. (50F & 12mb as to 86F & 42mb)
- When water boils the saturation vapor pressure is 1013mb (equal to the surrounding air pressure) and the water can evaporate to vapor
- Water boils at lower temperatures higher in the atmosphere because the air pressure is lower and the saturation vapor pressure is lower.

Water Vapor in the Atmosphere:
- Air Parcel concept

Relative Humidity:
= water vapor content / water vapor capacity
- 100% the air is saturated
- 50% the air contains half of the amount of water vapor as it can contain

Mixing Ratio / Saturation Mixing Ratio:
- Mixing Ratio = U = actual (measured) mass of water vapor (in a parcel) in grams / mass of dry (remaining) air (in parcel) in kilograms
- Saturation Mixing Ratio = Us = mass of water vapor that the parcel would contain if saturated in grams / mass of dry (remaining) air in parcel in kilograms
- Mixing ratio very close to vapor pressure
- Saturation ratio very close to saturation vapor pressure
- Using grams/kilogram the numbers aren't very small fractions or decimals

Relative Humidity:
- RH = actual (measured) water vapor in parcel / mass of water vapor required for parcel to be saturated
- Can also think of this as (water vapor content / maximum possible water vapor content)
- RH in terms of mixing ratios = U / Us
- Us: get it from table of saturation mixing ratios and it depends on the temperature
- You cannot use RH to tell the absolute amount of water vapor in the air. It only tells you how close the air is to being saturated. U is the absolute amount of water vapor in the air

Two ways to change the amount of RH:
- Change the amount of water vapor (U)
- Change the air temperature (Us)

Dew Point Temperature (Td):
= The temperature to which you would need to cool an air parcel in order for it to become saturated or RH = 100%
- The dew point temperature is determined by the amount of water vapor (U)
- As U increases, the dew point temperature increases
- Dew Point is directly related to the amount of water vapor in the air. When the temperature and dew point temperature are equal, the air is saturated and RH is 100%

Use of Saturation Mixing Ratio Table:
- Given an air temp, T, the corresponding entry in the table is the Saturation Mixing Ratio, Us
- There is also a correspondence between dew point temp, Td, and the mixing ratio, U
- T <-> Us
- Td <-> YOU

- If T = 10C and RH =50%, what is Td?
- Us = 7.6 g/kg
- Manipulate RH = U/Us
- U = RH*Us
- U = .5*7.6
- U = 3.8 k/kg
- Td = U = 3.8 k/kg = 0C = Td
- Td = 0C