Geography 1111 Lecture 10 Notes 8-31
Geography 1111 Lecture 10 Notes 8-31 GEOG 1111
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This 5 page Class Notes was uploaded by Bridget Notetaker on Friday September 2, 2016. The Class Notes belongs to GEOG 1111 at University of Georgia taught by Hopkins in Fall 2016. Since its upload, it has received 41 views. For similar materials see Intro to Physical Geography in Geography at University of Georgia.
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Date Created: 09/02/16
Geography 1111 Lecture 10 Notes Adiabatic Processes: o In meteorology the term adiabatic refers to a parcel of air changing temperature without heat energy being added or removed A parcel of air that contains warm air is less dense (lighter) and thus may rise As this parcel of air rises in the atmosphere, it will encounter lower pressure and thus expand o As it expands the molecules in it will start to slow down and thus kinetic energy levels will decrease As this occurs, the temperature will also decrease Conversely, a parcel of cold air is more dense (heavier) and thus sinks As this parcel of air sinks in the atmosphere, it will encounter higher pressure and thus be compressed o As it is compressed the molecules in it will start to speed up and thus kinetic energy levels will increase As this occurs, the temperature will also increase o So, expanding air = temperature decrease; compressing air = temperature increase But, the temperature changes without heat being added or subtracted This is Adiabatic Temperature Change; changing the temperature of the air without adding or subtracting heat Simply the result of compressing the air or allowing it to expand o Result: Rising air in the atmosphere expands and cools Sinking air in the atmosphere is compressed and warms o The Rate of Adiabatic Temperature Change varies with the humidity condition (water vapor content) of the air, is it Dry (unsaturated) or Wet (saturated) Unsaturated air will change temperature at a faster rate than will saturated air Dry Adiabatic Rate: (DAR or DALR) When a parcel of air is unsaturated its air temperature > dew point temperature and thus its RH < 100% The DAR of unsaturated air is at a constant rate of 1°C/100m or 10°C/1000m Thus: rising air cools at 1°C/100m (10°C/1000m) and sinking air warms at 1°C/100m (10°C/1000m) Saturated Adiabatic Rate: (SAR or SALR) When the air parcel is saturated its air temperature = d.p.Tº and thus its RH = 100% The SAR is not a constant rate but a variable rate of .5°-.9°C/100m or 5°-9°C/1000m The SAR is dependent on the moisture content of the air o The more water vapor there is in the air, the slower the rate of decline (closer to 5°C / 1000m) o This is because condensation releases latent heat, thus slowing the rate of cooling (or warming) So, as warm air rises (the process of convection) it cools o If the air temperature reaches the dew point temperature, saturation is reached and thus condensation may begin If there is enough water vapor in the air parcel, then a cloud may form The level of the atmosphere where this occurs is known as the condensation level and will vary temporally and spatially Condensation level: is the height in the atmosphere at which condensation occurs, where cloud formation begins (usually seen as the bottom of a cloud mass) o RH = 100% o Air temperature = d.p.Tº Atmospheric Stability: refers to the tendency of an air parcel, with its water vapor, to either remain in place or to change vertical position by ascending (rising) or descending (falling) o A stable parcel of air resists vertical displacement or, when disturbed, tends to return to its starting place o An unstable parcel of air continues to rise until it reaches an altitude where the surrounding air has a density and temperature similar to its own Rules of Stability: 1. When an air parcel is warmer (less dense) than the surrounding air, the parcel will rise: UNSTABLE air or condition 2. When an air parcel is colder (denser) than the surrounding air, it will tend to stay at the same level or sink: STABLE air 3. The Environmental Lapse Rate (ELR) is the temperature profile of the atmosphere (surrounding air) The actual temperature lapse rate in the lower atmosphere at any particular time under local weather conditions is the ELR o Whether a particular parcel of air will rise (unstable) or not rise (stable) is a function of the temperature inside the parcel of air as compared to the temperature outside the parcel of air, and thus the ELR determines air stability or instability Types of Stability: o Stable conditions: exist when the ELR < DAR Absolutely stable conditions or Absolute Stability This usually results in NO UPLIFT of air, and may bring about subsidence or sinking of the air parcel The most severe example of stable conditions is a temperature inversion when the air temperature is increasing with increasing altitude in the troposphere o Unstable conditions: exist when the ELR > DAR Absolutely unstable conditions or Absolute Instability This usually results in UPLIFT of the air parcel and often leads to cloudy sky conditions o Low pressure cells form unstable atmospheric conditions o Conditionally Unstable conditions exist when the ELR is between the DAR and the SAR SAR < ELR < DAR The atmosphere will vary between Stable and Unstable and usually the atmosphere is more Unstable in the upper portion and more Stable in the lower portion Stability is important in daily weather patterns because it controls whether clouds form or not and the type of clouds that may form o This in turn will affect the potential for precipitation Lifting Mechanisms: o For air masses or parcels of air to cool adiabatically (by expansion) and to reach the dew-point temperature and saturate, condense, and form clouds and perhaps precipitation, they must lift and rise in altitude o There are 4 principal lifting mechanisms, which operate in the atmosphere: Convective Lifting Convergent Lifting Orographic Lifting Frontal Wedging Convective Lifting: is the process of warming and parcel of air at the surface by conduction, then the whole parcel rising into the atmosphere since it is warmer than the surrounding air o The heating of the Earth’s surface at a given location will produce the necessary elements to establish these UNSTABLE conditions o If there is enough moisture in the air parcel, then cloud formation may develop and even precipitation This is commonly how afternoon thundershowers are produced in the summertime o This is also part of the process, in conjunction with convergence, that occurs in Low pressure cells Convergent Lifting: is the process by which winds flow together from opposite directions and are forced to rise due to compression or “squeezing” o Both convection and convergence are at work to form the Inter-Tropical Convergence Zone (ITCZ), and Low pressure cells Orographic Lifting: is the process by which air is forced to rise over a mountain range, or other elevated land barrier, and thus cool adiabatically o The pattern of precipitation is governed by this as rain/snow will occur on the windward side of the mountain, while little or no precipitation occurs on the leeward side o This often forms a Rain Shadow Desert on the leeward side (desert in the shadow of the rain) Frontal Wedging: is the process by which cold, dense air acts similarly to a mountain barrier forcing warmer, less dense air to rise over it o The leading edge of a mass of cold air is known as a cold front and similarly the leading edge of a mass of warm air is known as a warm front So this mechanism is associated with cold/warm fronts and Mid-Latitude Wave Cyclones (frontal systems) o This usually produces clouds & precipitation with often severe storms or thunderstorms along the cold front
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