Chapter 4 Moisture and Atmospheric Stability This chapter covers: Water states of matter heat capacity and latent heat Humidity and dew point Adiabatic temperature changes in the atmosphere Atmospheric stability

The Hydrologic Cycle

States of Matter: Solid, Liquid and Vapor

Gas (Vapor) widely spaced molecules no bonding between molecules molecules move at high speeds very compressible

Liquid Closely spaced molecules Moderate bonding between molecules molecules move at medium speeds Slightly compressible

Solid (i.e., ice) closely spaced molecules Strong, rigid bonding between molecules No molecule movement – only vibrations Fairly incompressible

Solid Water: Ice

Liquid Water

Water Vapor

Heat Capacity and Latent Heat of Water

Saturation Condition in which the air is holding the maximum amount of water vapor possible Amount of water vapor present at saturation depends on Temperature; more vapor at higher temp. Very strong effect Pressure; more vapor at higher pressure.

Absolute Humidity Amount of water vapor present in air Given as grams water vapor per cubic meter of air Value is affected by air pressure

Mixing Ratio Amount of water vapor present in air, but more useful than absolute humidity Given as grams water vapor per kilogram of air Typically ranges from 0 to 4% Value is not affected by air pressure

Saturation Air is limited in how much water vapor it can hold without water droplets forming Saturation is the point at which air can’t hold more water vapor Mixing ratio at saturation depends on temperature, and somewhat on pressure

Contrail: engine exhaust contains water vapor, exhaust cools, becomes saturated with water vapor and condensation occurs

Contrail: engine exhaust contains water vapor, exhaust cools, becomes saturated with water vapor and condensation occurs

Saturation Mixing-Ratio: How much water vapor can be present in air at different temperatures

Relative Humidity the humidity we feel Amount of water vapor in air relative to maximum possible amount (saturation mixing ratio) Example Temperature: 20oC Saturation mixing ratio=14g vapor per 1 kg air Actual vapor content = 7 g per 1 kg air Relative humidity = 7 g / 14 g x 100% = 50%

Dew Point Temperature to which air must be cooled to become saturated Assumes no change in mixing ratio Relative humidity is 100% in air that’s at its dew point Stating air’s dew point is essentially the same as stating its mixing ratio

Air’s saturation mixing ratio and relative humidity change with temperature

Which has larger mixing ratio? Which has higher relative humidity? Death Valley Antarctica

Hotter: Higher mixing ratio, Lower relative humidity Colder: Lower mixing ratio Higher relative humidity

Relative Humidity, Mixing Ratio and Air Temperature Hotter air can hold much more water vapor than cold air Hotter air can have more vapor in it than cold air, yet have lower relative humidity

Relative Humidity Changes with Temperature Daily

Air Temp, Dew Pt. & Relative Humidity in Heber

Dew Point Temperatures

Adiabatic Temperature Changes Air cools when it expands, warms when its compressed Rising air expands and cools Sinking air is compressed and warms Adiabatic refers to temperature changes w/o heat transfer Very important!

Adiabatic Temperature Changes

Dry & Wet Adiabatic Rates Saturated air cools less as it rises because condensation of water releases heat Dry adiabatic rate = 10oC / 1000m = 5.5oF / 1000 feet Wet adiabatic rate = 5 to 9oC / 1000m (2.75 to 5oF/1000ft)

Dry & Wet Adiabatic Rates

Lifting Condensation Level As air rises, it expands and cools Level (altitude) at which it is cooled to its dew point is the lifting condensation level Clouds form above this level if air is rising

Causes of Lifting Orographic – wind blows over mountains Frontal wedging – warm air forced over colder air Convergence – winds blowing together Convection – solar heating creates hot air that rises

Important along Wasatch Front, much of Western U.S. Orographic Lifting Important along Wasatch Front, much of Western U.S.

Frontal Wedging “Storm Fronts”

Convergence

Convection

Cause of Rain Shadow Desert Rising air cools at wet adiabatic rate sinking air warms at dry adiabatic rate Cause of Rain Shadow Desert

Atmospheric Stability Stable Air = Air that tends to not rise Unstable Air = Air that tends to keep rising (regardless of orographics, fronts, etc.) Importance – rising air cools, makes clouds, precipitation, even tornados

What Controls Stability Depends on adiabatic cooling rate (dry and wet) vs. Environmental Lapse Rate Environmental Lapse Rate = the actual, existing decrease in air temperature with altitude

Atmospheric Stability, cont. Three types of stability: Absolute stability Absolute instability Conditional instability

Absolute Stability Environmental Lapse rate is less than wet adiabatic rate As air rises, it cools so much (even if its saturated) that it becomes cooler than surrounding air so it stops rising

Absolute stability

Absolute instability Environmental lapse rate is greater than dry adiabatic rate As air rises, despite cooling at dry adiabatic rate, it becomes progressively warmer than surrounding air and rises faster

Absolute instability Absolute Instability

Conditional Instability Environmental Lapse rate is greater than wet adiabatic rate, less than dry adiabatic rate As air rises, if it is unsaturated it tends to not rise, but once its saturated it keeps rising

Conditional Stability Conditional Instability Conditional Stability

NWS Storm Prediction Center Focuses on dangerous thunderstorms Produces estimates of convective stability for locations across the country twice daily Main website: http://www.spc.noaa.gov Soundings (weather balloon data which provide information on environmental lapse rate and more) with stability analysis (somewhat advanced scientifically): http://www.spc.noaa.gov/exper/soundings/

Chapter 4 END