Air parcels A parcel is a “blob” of air A parcel is a “blob” of air Small enough to have only one value of T, p, ρ, etc. Small enough to have only one value of T, p, ρ, etc. Large enough to contain a significant number of molecules. (Are there enough particles to talk about temperature as average kinetic energy, for example?) Large enough to contain a significant number of molecules. (Are there enough particles to talk about temperature as average kinetic energy, for example?)

Lapse Rates Parcel lapse rate – the rate at which temperature changes as the parcel is lifted to a higher altitude Parcel lapse rate – the rate at which temperature changes as the parcel is lifted to a higher altitude Environmental lapse rate – the rate at which the air surrounding the parcel changes as altitude increases Environmental lapse rate – the rate at which the air surrounding the parcel changes as altitude increases

The Adiabatic Lapse Rate An adiabatic process is one during which no heat is exchanged between the substance in question and its surroundings An adiabatic process is one during which no heat is exchanged between the substance in question and its surroundings Many atmospheric motions occur rapidly enough that parcels do not exchange a significant amount of heat with the environment Many atmospheric motions occur rapidly enough that parcels do not exchange a significant amount of heat with the environment Examples: Examples: rising air in a thunderstorm rising air in a thunderstorm Air rising over a topographic barrier (like a mountain) Air rising over a topographic barrier (like a mountain)

The Adiabatic Lapse Rate The adiabatic lapse rate for DRY air on Earth is Γ d = g/c p Γ d = 9.81 m s -2 / 1004 J kg -1 C -1 Γ d = C m -1 Γ d = 9.77 C km -1

The Adiabatic Lapse Rate This means that a rising(sinking) air parcel will cool(warm) at a rate of about 10 o C per km of ascent(descent) unless: It exhanges significant mass or heat with the environment It exhanges significant mass or heat with the environment It becomes saturated with respect to water vapor It becomes saturated with respect to water vapor It rises(sinks) so slowly that radiation heat transfer is possible It rises(sinks) so slowly that radiation heat transfer is possible

Humidity, Condensation and Clouds Circulation of water in the atmosphere Circulation of water in the atmosphere Evaporation, condensation and saturation Evaporation, condensation and saturation Humidity Humidity Dew and frost Dew and frost Fog Fog Clouds Clouds

Circulation of Water in the Atmosphere evaporation evaporation condensation condensation precipitation precipitation hydrologic cycle hydrologic cycle The total amount of water vapor stored in the atmosphere amounts to only one week’s supply of precipitation for the planet.The total amount of water vapor stored in the atmosphere amounts to only one week’s supply of precipitation for the planet.

Fig. 4-1, p. 80 Stepped Art

Evaporation, Condensation and Saturation saturation saturation condensation nuclei condensation nuclei In very clean air, about 10,000 condensation nuclei are typically found in one cubic centimeter of air, a volume approximately the size of your fingertip.In very clean air, about 10,000 condensation nuclei are typically found in one cubic centimeter of air, a volume approximately the size of your fingertip.

Humidity Mixing Ratio (w) Humidity Mixing Ratio (w) The ratio of the mass of water vapor in air to the mass of dry air: w = m v / m d  Usually expressed in g kg -1  Some typical values: Tropical marine boundary layer air: w ≈ 18 g kg -1 Tropical marine boundary layer air: w ≈ 18 g kg -1 Polar air: w ≈ 1 g kg -1 Polar air: w ≈ 1 g kg -1 Stratospheric air: w ≈ 0.1 g kg -1 Stratospheric air: w ≈ 0.1 g kg -1

Specific Humidity The ratio of the mass of water vapor in air to the total mass of the air (dry air plus water vapor): SH = m v / (m d + m v ) w = SH / (1 – SH) SH = w / (1 + w)

Vapor Pressure actual vapor pressure actual vapor pressure saturation vapor pressure saturation vapor pressure “Saturation” describes a condition of equilibrium: liquid water is evaporating at exactly the same rate that water vapor is condensing.“Saturation” describes a condition of equilibrium: liquid water is evaporating at exactly the same rate that water vapor is condensing.

Vapor Pressure Saturation vapor pressure depends only on temperature… Formula: Saturation vapor pressure Saturation vapor pressure at 273 K = 6.11 mb Latent heat of vaporization = 2.5x10 6 J kg -1 Gas constant for water vapor = 461 J kg -1 K K Temperature

Vapor Pressure Saturation vapor pressure depends only on temperature… Formula:

Vapor Pressure Saturation vapor pressure depends only on temperature… Graph:

Relative Humidity definition of relative humidity definition of relative humidity saturation and supersaturation saturation and supersaturation condensation condensation relative humidity and temperature relative humidity and temperature When the general public uses the term “humidity”, they mean “relative humidity.”When the general public uses the term “humidity”, they mean “relative humidity.”

Relative Humidity The ratio of the actual vapor pressure to the saturation vapor pressure. f = e / e s Since e s depends on temperature, the relative humidity measures closeness to saturation, not actual water vapor content.

Fig. 4-5, p. 83

Fig. 4-7, p. 85

Relative Humidity and Dew Point dew point temperature: the temperature to which air must be lowered to reach 100% relative humidity dew point temperature: the temperature to which air must be lowered to reach 100% relative humidity dew point depression and relative humidity dew point depression and relative humidity The dew point temperature is useful for forecasting heat index, precipitation probabilities, and the chance of frost.The dew point temperature is useful for forecasting heat index, precipitation probabilities, and the chance of frost.

Measuring Humidity psychrometers psychrometers hygrometers hygrometers

Topography and Clouds orographic uplift orographic uplift rain shadow rain shadow The rain shadow works for snow too. Due to frequent westerly winds, the western slope of the Rocky Mountains receives much more precipitation than the eastern slope.The rain shadow works for snow too. Due to frequent westerly winds, the western slope of the Rocky Mountains receives much more precipitation than the eastern slope.

Collision and Coalescence Process terminal velocity terminal velocity coalescence coalescence warm clouds warm clouds A typical cloud droplet falls at a rate of 1 centimeter per second. At this rate it would take 46 hours to fall one mile.A typical cloud droplet falls at a rate of 1 centimeter per second. At this rate it would take 46 hours to fall one mile.

Instruments standard rain gauge standard rain gauge tipping bucket rain gauge tipping bucket rain gauge It is difficult to capture rain in a bucket when the wind is blowing strongly.It is difficult to capture rain in a bucket when the wind is blowing strongly.

Doppler Radar and Precipitation radar radar Doppler radar Doppler radar

Fig. 5-39, p. 135 Stepped Art