Lab 6: Saturation & Atmospheric Stability

Review Lab 5 – Atm. Saturation Relative humidity? Mixing ratio / saturation mixing ratio? Function of temp.. Clausius-Clapeyron curve Sling psychrometer – what does this give us? Dew point? Looking at RH equation above, when temp is reduced, all else being equal, what happens to the RH of a sample of air? Does RH go up or down? Air is saturated when RH=100% Relative humidity measures how close an air sample is to the saturation point Mixing ratio: ratio of water vapor mass to the mass of dry air (g/kg) Saturation ratio: maximum water vapor a sample of air will hold at a given temperature. Is the temp at which air must be cooled in order to reach saturation

Lab 6 Lab 6: Saturation and Atmospheric Stability processes that influence atmospheric saturation – i.e., cause cooling and/or increase in water vapor content atmospheric processes that change either the temp and/or water vapor content of an air sample In this lab, we’ll focus on atmospheric mixing and adiabatic cooling and some processes that drive these conditions Relative humidity measures how close an air sample is to the saturation point Mixing ratio: ratio of water vapor mass to the mass of dry air (g/kg) Saturation ratio: maximum water vapor a sample of air will hold at a given temperature. Is the temp at which air must be cooled in order to reach saturation

Saturation & Atmospheric Stability Two main ways for air to reach saturation: Cooling to its dew point temperature (most common) Increasing water vapor content Remember Condensation produces: Fog Dew Clouds *ALL require saturated air to form! 1. 3. 2. Condensation: occurs when water vapor is cooled enough to change to a liquid. Cloud formation: typically occurs during the warmest part of the day. Dew: ground loses heat in the evening, dew may condense on the grass (fog may form in the air near the surface); Surface cooling that occurs after sunset accounts for some condensation.

Atmospheric Mixing When two air masses of different temps and water vapor content mix When they mix, the new air mass will change in temp and water vapor resulting in new mixing and saturation mixing ratios Changes relative humidity

The new temp of mixed air mass Assuming the two mixing air masses are the same size and you know the temps and RH find: The new temp of mixed air mass The new mixing ratio of the mixed air mass From the above, you can find the new RH (due to change in temp and water vapor) Mixing Ratio = SMR * RH “Saturated air” 100% RH Need to find mixing ratio “Unsaturated air” 0 – 99.9% RH

Adiabatic temperature changes: Adiabatic Cooling Adiabatic temperature changes: Temperature changes in which heat was neither added nor subtracted (closed system) Average internal energy decreases with expansion – changes in average kinetic energy Compressed air = warm air Expanded air = cooler air NOTE: If a parcel moves ↑, it passes through regions of successively lower pressure: Ascending air: EXPANDS Descending air: COMPRESSES

Saturation & Atmospheric Stability DRY adiabatic rate: unsaturated cools at a constant rate of 10°C/1km of ascent warms at constant rate of 10°C/km of descent WET adiabatic rate: saturated (has RH 100%) Slower rate of cooling caused by the release of latent heat Rates vary between 5°C & 9°C/1km Amount of LH released depends on quantity of moisture in the air Dew Point rate: 2°C/1km to the LCL At the WALR after the LCL Dry adiabatic rate: only applies to unsaturated air. Ascending air is expanded, making it cooler; descending air is compressed, making it warmer. Wet adiabatic rate: once the parcel reaches the LCL, the latent heat that was absorbed by the water vapor when it evaporated is liberated. Although the parcel will continue to cool adiabatically, the release of this latent heat slows the rate of cooling. When a parcel of air ascends above the lifting condensation level, the rate of cooling is reduced because the release of latent heat partially offsets the cooling due to expansion. Because the amount of latent heat released depends on the quantity of moisture present in the air (generally between 0 & 4%), the WAR varies from 5°C per 1,000 meters for air with a high moisture content to 9°C per 1,000 meters for air with a low moisture content LCL = altitude at which a parcel reaches saturation & cloud formation begins

Saturation & Atmospheric Stability DALR = 10°C/1km WALR = 5 – 9°C/1km Parcel A Temperature (°C) Height (km) Parcel B Temperature (°C) 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.0 0.5 28° surface 10° WALR Air decreases by 2.5°C Dry adiabatic rate: only applies to unsaturated air. Ascending air is expanded, making it cooler; descending air is compressed, making it warmer. Wet adiabatic rate: once the parcel reaches the LCL, the latent heat that was absorbed by the water vapor when it evaporated is liberated. Although the parcel will continue to cool adiabatically, the release of this latent heat slows the rate of cooling. When a parcel of air ascends above the lifting condensation level, the rate of cooling is reduced because the release of latent heat partially offsets the cooling due to expansion. Because the amount of latent heat released depends on the quantity of moisture present in the air (generally between 0 & 4%), the WAR varies from 5°C per 1,000 meters for air with a high moisture content to 9°C per 1,000 meters for air with a low moisture content 10.5° LCL 13° 1.5 DALR Air decreases by 5°C 18° 23°

Parcel A Temperature (°C) Parcel B Temperature (°C) Height (km) Parcel B Temperature (°C) 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 28°C surface 10°C – 4.5°C – 2°C 0.5 °C WALR: dependent 3 °C Air is cooling by 2.5° 3 – 2.5 = 0.5°C Air is cooling by 2.5° 5.5 – 2.5 = 3°C Air is cooling by 2.5° 13 – 2.5 = 10.5°C Air is cooling by 2.5° 10.5 – 2.5 = 8°C Air is cooling by 2.5° 8 – 2.5 = 5.5°C Air is cooling by 2.5° 0.5 – 2.5 = -2°C Air is cooling by 2.5° -2 – 2.5 = -4.5°C 5.5 °C 8 °C 10.5 °C LCL 13 °C 18 °C DALR: 10°C/km Air is cooling by 5° 28 – 5 = 23°C Air is cooling by 5° 23 – 5 = 18°C Air is cooling by 5° 18 – 5 = 13°C 23 °C

Lifting Condensation Level (LCL): Reached when ascending air cools to its dew point (saturation = 100% RH) – clouds form If it continues to rise: Cools at the wet adiabatic lapse rate (between 5°& 9°C) Calculated based on: Surface temperature & dew point temperature

Part II

Review What is adiabatic cooling? What is environmental lapse rate? Wet versus dry Thinking about atmospheric saturation, how does this influence cloud formation (hint: think about dew point temperature, etc.) What is environmental lapse rate?

Atmospheric lifting forces: Surface heating (air expansion, less dense, rise, etc..) Two surface air masses colliding (convergence) Contact of dissimilar air masses along warm & cold fronts (convergence) Topographic barriers (e.g. orographic lift) Upper air divergence Rising air doesn’t mix substantially with the surrounding atmosphere. Once the initial lifting force stops, the continued rising of an air parcel depends on atmospheric stability (the state of the atmosphere surrounding the parcel). Rising air doesn’t mix substantially with the surrounding atmosphere. Once the initial lifting force stops, the continued rising of an air parcel depends on atmospheric stability (the state of the atmosphere surrounding the parcel).

Orographic Lifting Illustration: prevailing winds force warm, moist air over a 3,000-meter-high mountain range. As the unsaturated air ascends the windward side of the range, it cools at the rate of 10°C per 1,000 meters (dry adiabatic rate) until it reaches the dew-point temperature of 20°C. Because the dew-point temperature is reached at 1,000 meters, we can say that this represents the LCL & the height of the cloud base. From the cloud base to the top of the mountain, water vapor within the rising air is condensing to form more & more cloud droplets. As a result, the windward side of the mountain range experiences abundant precipitation. We will assume that the air that was forced to the top of the mountain is cooler than the surround air & hence begins to flow down the leeward slop of the mountain. As air descends, it is compressed & heated at the dry adiabatic rate. Upon reaching the base of the mountain range, the temperature of the descending air has risen to 40°C, or 10°C warmer than the temperature at the base of the mountain on the windward side. The higher temperature on the leeward side is the result of the latent heat that was released during condensation as the air ascended the windward slope of the mountain range. *if temperature of air ↑, relative humidity ↓ Air ascends: adiabatic cooling often generates clouds & lots of precipitation Air descends: warms adiabatically, making condensation & precipitation less likely

Td (dew point) cools at: Incorporating Dew Point LCL Td (dew point) cools at: 2°C/km below the LCL The WALR above the LCL LCL – T(°C) – Td(°C)/8 25°C – 13°C = 12°C/8 = 1.5 km

Even if this stable air were forced above the LCL, it would remain cooler than its environment & would have a tendency to return to the surface. LAYERED CLOUDS not much vertical development Absolute stability: Environmental lapse rate is less than the wet adiabatic rate (surrounding air cools slower with height) Stable air resists vertical movement, and doesn’t want to move. If it gets forced above LCL it would remain cooler and return to surface Note: air parcel cools faster than ELR

VERTICAL CLOUDS potential for thunderstorms Absolute instability: Environmental lapse rate is greater than the dry adiabatic rate (surrounding air cools faster w/ height) Unstable air rises because of its buoyancy Parcel of air cools slower than ELR

Conditional stability: Moist air has an environmental lapse rate between the dry & wet adiabatic rates