aos 101-304
DESCRIPTION
AOS 101-304. Moisture and Stability. March 22. Humidity. The amount of water vapor in the atmosphere. Mixing ratio , w : ratio of mass of WV to mass of dry air (g/kg). grams of water vapor. w =. kilograms of dry air. Vapor Pressure. - PowerPoint PPT PresentationTRANSCRIPT
AOS 101-304
March 22
Moisture and Stability
Humidity
• The amount of water vapor in the atmosphere.
• Mixing ratio, w: ratio of mass of WV to mass of dry air (g/kg)
w =
grams of water vaporkilograms of dry air
Vapor Pressure
• Vapor pressure, e: partial pressure of water vapor (Pa)
• The total air pressure of the atm. equals the partial pressures of each constituent gas
Pair = PN2 + PO2
+ PAr + e + PCO2 + ...
• Typical air pressures are around 1000 hPa, vapor pressure is typically 1-25 hPa.
Saturation Vapor Pressure, es
• Definition: For a given temperature, the amount of WV required so that the rates of condensation and evaporation are equal.
• Warmer air has a higher saturation vapor pressure than cooler air.
• Exponential relationship between es and temperature, i.e. es = f(T)
0
5
10
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20
25
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40
45
50
-10 0 10 20 30 40 50 60 70 80 90
Temperature (oF)
Sat
. V
apo
r P
ress
ure
(h
Pa)
• Relative Humidity: ratio of observed vapor pressure and the saturation vapor pressure (for a given temperature)
• Dewpoint (Td): Temperature of air to which a parcel must be cooled to reach saturation.
• Completely relies on vapor pressure.
e = f(Td)
RH =
ef(T)
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20
25
30
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40
45
50
-10 0 10 20 30 40 50 60 70 80 90
Temperature (oF)
Sat
. V
apo
r P
ress
ure
(h
Pa)
Given T = 70oF and RH = 20%, what are es, e and Td?
0
5
10
15
20
25
30
35
40
45
50
-10 0 10 20 30 40 50 60 70 80 90
Temperature (oF)
Sat
. V
apo
r P
ress
ure
(h
Pa)
Given T = 70oF and RH = 20%, what are es, e and Td?
T = 70oF → es = 25 hPa
RH = .2 = e/25 → e = 5 hPa
e = 5 hPa → Td = 27oF
• If you know 2, you know all 5.
T ↔ es
Td ↔ e
RH ↔ e / es
T = 70oF → es = 25 hPa
RH = .2 = e/25 → e = 5 hPa
e = 5 hPa → Td = 27oF
Stability
• Refers to an equilibrium position
• Stable equilibrium: perturbing an object will result in the object returning to original position
• Unstable equilibrium: perturbing an object will result in the object continuing to move away.
In the atmosphere…
• Perturbed objects are parcels (bubbles of air that can’t mix with surroundings).
• Atmospheric stability has to do with buoyancy– A parcel warmer (less dense) than its
surroundings will rise.– A parcel cooler (more dense) than its
surroundings will sink.
• Stability assessed by moving parcels vertically and comparing parcel temperature with the temperature of its surroundings.
However…
• As a parcel rises it does not keep the same temperature, instead it cools ADIABATICALLY.
• Adiabatic: reversible process which results in no loss/gain of heat energy by the parcel
• Instead, during ascent the parcel expands resulting in cooling (or warming by compressing during sinking)
• Dry Adiabatic Lapse Rate (DALR): The rate at which an unsaturated parcel cools as it rises (or warms as it sinks) is constant:
Γd = 9.8oC / km
• If a parcel is saturated, latent heat is released as water vapor condenses slightly counteracting the cooling during ascent. This is the Moist Adiabatic Lapse Rate (MALR):
Γm ~ 6.0oC / km (varies 4-7oC/km)
Note: if condensed water leaves the parcel the process is no longer reversible.
Assessing stability
• On a given day, the atmosphere will have a certain environmental lapse rate (Γe).
• Negative sign needed so that if temperature decreases with height the lapse rate is positive.
Γe =T(z2) – T(z1)z2 – z1
―
For a certain atmospheric layer…
• If Γe > Γd , atmospheric layer is absolutely unstable.
• If Γe < Γm , atmospheric layer is absolutely stable.
• If Γd > Γe > Γm , atm. layer is conditionally unstable, atm. only unstable if saturated.
Absolutely UnstableΓe > Γd (9.8oC/km)
1 km
25201510
DALR MALR
Γe
T(z=0 km) = 25oC
T(z=1 km) = 10oC
Absolutely StableΓe < Γm (6.0oC/km)
1 km
25201510
DALR MALR
Γe
T(z=0 km) = 25oC
T(z=1 km) = 25oC
Conditionally Unstable(9.8) Γd > Γe > Γm (6.0)
1 km
25201510
DALR MALR
Γe
T(z=0 km) = 25oC
T(z=1 km) = 17oC
On average…
• The environmental lapse rate is 6.5oC/km, meaning the atmosphere is usually conditionally unstable or barely stable.
• The closeness to saturation determines if the atmosphere is unstable or not
Atmospheric Soundings
• Radiosondes measure the temperature and moisture profiles of the atmosphere
• These profiles can be used to assess the stability of the atmosphere
Next Week…
• Severe thunderstorms usually result from instability in the atmosphere.
• The summer sun warms parcels near the ground, allowing them to become unstable and rise to the tropopause