terminology (water) Flashcards
Saturation Mixing Ratio (w_s)
-Ratio of the mass of water vapor (M_v) to the mass of dry air (M_d) in a parcel of air at saturation = M_v/M_d
- Maximum amount of water vapor a parcel can hold without condensation
- Expressed in parts per thousand, usually grams of water vapor/kg dry air
- Function of state -> mixing ratio of saturated air at a given T, P
-W/o specification, refers to saturation wrt a plane surface of pure water. May also be specified wrt to a plane ice surface
Mixing Ratio (w)
-Ratio of the mass of water vapor (M-V) to the mass of dry air (M_v) in a sample of moist air = M_v/M_d
- Differs from saturation mixing ratio in that it measures the actual amount of water vapor present, while the saturation mixing ratio measures the amount of water vapor that would be present at saturation
- For precise physical computations, specific humidity is preferable quantity to use
-Expressed in parts per thousand, usually grams of water vapor/kg dry air
Specific Humidity (q)
-Mass of water vapor per mass of moist air
q = M_v/(M_v + M_d)
-more precise than mixing ratio, but often mixing ratio is used because it is easier to evaluate
Relative Humidity (RH)
- Ratio (expressed as a percent) of the amount of water vapor in a given volume of air to the amount of that volume would hold if the air were saturated
- RH = 100*(w/w_s)
Dewpoint Depression
- Difference between the temperature and the dewpoint temperature
- Saturated conditions have a dewpoint depression of zero, while dry conditions have a large dewpoint
Saturation Vapor Pressure (e_s)
- The part of total atmospheric pressure attributable to water vapor if the air were saturated
- Because the air is saturated, this represents a maximum vapor pressure possible for a given pressure and temperature
- The vapor pressure of a system, at a given temperature, for which the vapor of a substance is in equilibrium with a plane surface of that substance’s pure liquid or solid phase
- The saturation vapor pressure of any pure substance, wrt a specified parent phase, is an intrinsic property of that substance, and is a function of temperature alone (see Clasius-Claperyon equation)
- For water vapor, the saturation pressure over supercooled liquid differs appreciably from that over ice
Vapor Pressure (e)
-Part of the total atmospheric pressure attributable to water vapor (its partial pressure)
Virtual Temperature (T_v)
- T_v of a moist parcel of air is the temperature at which a theoretical dry air parcel would have a total pressure and density equal to the moist parcel of air
- In other words, two air samples with the same virtual temperature have the same density, regardless of their actual temperature or relative humidity
- T_v > T actual.
- Because the saturation mixing ratio increases exponentially with temperature (roughly doubling with every increase of about 10 deg C), the virtual temperature correction becomes increasingly important for higher dewpoints
Potential Temperature (Theta)
-Temperature that a sample of air would have if it were brought dry-adiabatically to a pressure of 1000hPa (or another standard pressure p_0)
- Theta = T(p_0/p)^K
- where K is Poisson constant, often assumed ot be 2/7, the ratio of the gas constant to the specific heat capacity at constant pressure for an ideal diatomic gas
Lifting Condensation Level (LCL)
-The height at which a pacelof air becomes saturated when it is lifted dry-adiabitically
Equivalent Temperature (T_e)
-Temperature that a sample of air would have it all its moisture were condensed out by a pseudo-adiabatic process (whereby all the condensed moisture is immediately removed from the air sample). The latent heat of condensation then heats the air sample. This equivalent temperature is sometimes called the “adiabatic equivalent temperature.”
Equivalent Potential Temperature (Theta_e)
- The temperature at a level that a sample of air would have if all of its moisture were condensed out by a pseudo-adiabatic process (with the latent heat of condensation heating the air sample), and then the sample is brought dry-adiabatically brought to 1000 hPa (or other reference pressure p_0)
- Equivalent potential temperature is identical to the equivalent temperature, except that the sample is brought dry adiabatically from the equivalent temperature temperature at the initial level to the equivalent potential temperature at the 1000 hPa level
Wet-Bulb Temperature (T_w)
- Temperature to which a parcel of air at a constant pressure cools adiabatically to saturation by evaporation of water into it, all latent heat being supplied by the parcel
- In other words, take a parcel of air, which is not at saturation. Then, at constant pressure (with no vertical motion), evaporate water into the parcel. Evaporative cooling will occur until the parcel reaches saturation. The wet-bulb temperature is reached when the air parcel achieves saturation.
- The wet bulb temperature will always fall between the dewpoint and the temperature, unless the air is saturated. At saturation, the temperature, dew point, and wet-bulb temperature are equal.
- In the real atmosphere T_w often provides a good estimate of what the surface temperature will become after the onset of precipitation and once conditions become saturated
Dewpoint
-Temperature to which a given air parcel must be cooled at constant pressure and constant water vapor content in order for saturation to occur
- When this temperature is below 0 deg C, called frost point.
- Temperature at which the saturation vapor pressure of the parcel = actual vapor pressure of the contained water vapor
- nonconservative wrt vertical adiabatic motions of air in the atmosphere
Wet-Bulb Potential Temperature (Theta_w)
- Wet-bulb temperature a sample of air would have if it were brought along a saturation adiabat to a pressure of 1000hPa
- Same as wet-bulb temperature, except the sample is brought moist adiabatically from wet-bulb temperature at initial level to wet-bulb potential temperature at the 1000-hPa level
