Presentation on theme: "Chapter 4 Moisture and Atmospheric Stability"— Presentation transcript:
1 Chapter 4 Moisture and Atmospheric Stability This chapter covers:Waterstates of matterheat capacity and latent heatHumidity and dew pointAdiabatic temperature changes in the atmosphereAtmospheric stability
11 SaturationCondition in which the air is holding the maximum amount of water vapor possibleAmount of water vapor present at saturation depends onTemperature; more vapor at higher temp. Very strong effectPressure; more vapor at higher pressure.
12 Absolute Humidity Amount of water vapor present in air Given as grams water vapor per cubic meter of airValue is affected by air pressure
13 Mixing RatioAmount of water vapor present in air, but more useful than absolute humidityGiven as grams water vapor per kilogram of airTypically ranges from 0 to 4%Value is not affected by air pressure
14 SaturationAir is limited in how much water vapor it can hold without water droplets formingSaturation is the point at which air can’t hold more water vaporMixing ratio at saturation depends on temperature, and somewhat on pressure
15 Contrail: engine exhaust contains water vapor, exhaust cools, becomes saturated with water vapor and condensation occurs
16 Contrail: engine exhaust contains water vapor, exhaust cools, becomes saturated with water vapor and condensation occurs
17 Saturation Mixing-Ratio: How much water vapor can be present in air at different temperatures
18 Relative Humidity the humidity we feel Amount of water vapor in air relative to maximum possible amount (saturation mixing ratio)ExampleTemperature: 20oCSaturation mixing ratio=14g vapor per 1 kg airActual vapor content = 7 g per 1 kg airRelative humidity = 7 g / 14 g x 100% = 50%
19 Dew Point Temperature to which air must be cooled to become saturated Assumes no change in mixing ratioRelative humidity is 100% in air that’s at its dew pointStating air’s dew point is essentially the same as stating its mixing ratio
20 Air’s saturation mixing ratio and relative humidity change with temperature
21 Which has larger mixing ratio? Which has higher relative humidity?Death ValleyAntarctica
28 Adiabatic Temperature Changes Air cools when it expands, warms when its compressedRising air expands and coolsSinking air is compressed and warmsAdiabatic refers to temperature changes w/o heat transferVery important!
30 Dry & Wet Adiabatic Rates Saturated air cools less as it rises because condensation of water releases heatDry adiabatic rate = 10oC / 1000m = 5.5oF / 1000 feetWet adiabatic rate = 5 to 9oC / 1000m (2.75 to 5oF/1000ft)
38 Cause of Rain Shadow Desert Rising air cools at wet adiabatic ratesinking air warms at dry adiabatic rateCause of Rain Shadow Desert
39 Atmospheric Stability Stable Air = Air that tends to not riseUnstable Air = Air that tends to keep rising (regardless of orographics, fronts, etc.)Importance – rising air cools, makes clouds, precipitation, even tornados
40 What Controls Stability Depends on adiabatic cooling rate (dry and wet) vs. Environmental Lapse RateEnvironmental Lapse Rate = the actual, existing decrease in air temperature with altitude
41 Atmospheric Stability, cont. Three types of stability:Absolute stabilityAbsolute instabilityConditional instability
42 Absolute StabilityEnvironmental Lapse rate is less than wet adiabatic rateAs air rises, it cools so much (even if its saturated) that it becomes cooler than surrounding air so it stops rising
44 Absolute instabilityEnvironmental lapse rate is greater than dry adiabatic rateAs air rises, despite cooling at dry adiabatic rate, it becomes progressively warmer than surrounding air and rises faster
46 Conditional Instability Environmental Lapse rate is greater than wet adiabatic rate, less than dry adiabatic rateAs air rises, if it is unsaturated it tends to not rise, but once its saturated it keeps rising
49 NWS Storm Prediction Center Focuses on dangerous thunderstormsProduces estimates of convective stability for locations across the country twice dailyMain website:Soundings (weather balloon data which provide information on environmental lapse rate and more) with stability analysis (somewhat advanced scientifically):
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