Humidity Humidity is the amount of water vapor in the atmosphere

Transcription

1 Humidity

2 Humidity Humidity is the amount of water vapor in the atmosphere Water is found in all three phases in the atmosphere: gas (water vapor), water (liquid), ice crystal (solid) Highest heat capacity of all solids and liquids except liquid ammonia Highest heat conduction of all liquids Highest surface tension and dielectric constant of any liquid Highest variable in all of atmospheric constituents (0-4%) Well know Greenhouse Gas Water expands 9% when freezing ice flosts Essential for life and climate

3 Water molecule The partly exposed hydrogen atom of one molecule is attracted to the negative oxygen atom of another molecule.

4 Constant Gas Variable Gas Name % Name % Nitrogen N Water vapor H 2 O 0 4 Oxygen O Carbon dioxide CO Argon Ar 0.93 Ozone O * Neon Ne Carbon monoxide CO * Helium He Sulfur dioxide SO * Methane CH Nitrogen dioxide NO * Hydrogen H Aerosol * Xenon Xe

5 deposition evaporation condensation Phase changes of water sublimation

6 Hydrosphere component/reservoirs Ocean (97.25%), cover 71% of Earth s surface Cryosphere (e.g. ice cap, glaciers (2.05%) Groundwater and soils (0.7%) Atmosphere (0.001%) River (0.0001%) Biosphere ( %)

7 Hydrological Cycle

8 Consider a hypothetical jar containing pure water with a flat surface and an overlying volume that initially contains no water vapor (a). As evaporation begins, water vapor starts to accumulate above the surface of the liquid. With increasing water vapor content, the condensation rate likewise increases (b). Eventually, the amount of water vapor above the surface is enough for the rates of condensation and evaporation to become equal. The resulting equilibrium state is called saturation (c).

9 Humidity refers to the amount of water vapor in the air. The part of the total atmospheric pressure due to water vapor is referred to as the vapor pressure or vapor pressure (e). The vapor pressure of a volume of air depends on both the temperature and the density of water vapor molecules. The saturation vapor pressure (e S ) is an expression of the maximum water vapor that can exist. On the other hand, saturation is the air that contain amount of water greater than a threshold, water vapor tend to condense into liquids faster than its evaporates. The air that contain amount of water less than threshold amount is unsaturated (e<e S ). Supersaturated is the temporary situation when threshold value drops so quickly that condensation does not remove water vapor fast enough.

10 Saturation an air parcel is saturated when it holds the maximum amount of water vapour possible; addition of any extra water vapour would lead to condensation the saturation vapour pressure is the vapour pressure at saturation (big surprise); it depends on temperature: "warmer air can hold more moisture"

11 Williams p62 dry E>C saturate de=c warmed E>C cooled E<C SVP depends on temperature. As temperature increases, more molecules are energetic enough to escape into the air. Concept applies to an ice surface. SVP over ice is lower because water molecules are bonded more tightly to ice. For the temperatures of interest, some water molecules are energetic enough to escape into atmosphere and SVP>0.

12 Humidity measurement ไซโครม เตอร (Psychrometer) ประกอบด วยเทอร โมม เตอร สองอ น ปลายกระเปาะของเทอร โมม เตอร อ นหน งห มด วยผ าม สล นช บน า เร ยกว า กระเปาะเป ยก (wet bulb) และ เทอร โมม เตอร อ กอ นเร ยกว ากระเปาะ แห ง (dry bulb) ถ าม การระเหยมากข นก จะท าให อ ณหภ ม ระหว างเทอร โมม เตอร กระเปาะเป ยกและกระเปาะแห งต างก น มากข น ความต างของอ ณหภม เทอร โมม เตอร กระเปาะเป ยกและแห ง สามารถน ามาค านวนหาค าความช น ส มพ นธ ได

13 Psychrometer

14 Absolute humidity is the density of water vapor, expressed as the number of grams of water vapor contained in a cubic meter of air. Specific humidity expresses the mass of water vapor existing in a given mass of air. Saturation specific humidity is the maximum specific humidity that can exist and is directly analogous to the saturation vapor pressure.

15 The mixing ratio is a measure of the mass of water vapor relative to the mass of the other gases of the atmosphere. The maximum possible mixing ratio is called the saturation mixing ratio.

16 Relative humidity, RH, relates the amount of water vapor in the air to the maximum possible at the current temperature. RH = (specific humidity/saturation specific humidity) X 100% More water vapor can exist in warm air than in cold air, so relative humidity depends on both the actual moisture content and the air temperature. If the air temperature increases, more water vapor can exist, and the ratio of the amount of water vapor in the air relative to saturation decreases.

17 In (a), the temperature of 14?C has a saturation specific humidity of 10 grams of water vapor per kilogram of air. If the actual specific humidity is 6 grams per kilogram, the relative humidity is 60 percent. In (b), the specific humidity is still 6 grams per kilogram, but the higher temperature results in a greater saturation specific humidity. The relative humidity is less than in (a), even though the density of water vapor is the same.

18 The dew point is the temperature to which the air must be cooled to become saturated and is an expression of water vapor content. In (a), the temperature exceeds the dew point and the air is unsaturated. When the air temperature is lowered so that the saturation specific humidity is the same as the actual specific humidity (b), the air temperature and dew point are equal. Further cooling (c) leads to an equal reduction in the air temperature and dew point so that they remain equal to each other. When the temperature at which saturation would occur is below 0?C, we use the term frost point.

19 The value corresponding to the row for the dry bulb temperature and the column for the wet bulb depression yields the dew point temperature.

20 The value corresponding to the row for the dry bulb temperature and the column for the wet bulb depression yields the relative humidity.

21 Vapor pressure or vapor pressure Absolute humidity Specific humidity Mixing ratio Relative humidity Dewpoint temperature

22 Adiabatic processes basic idea... remember pressure decreases with height in the atmosphere and air parcel cools by expansion as it ascends, and warms by compression as it descends this is similar to air feeling cold when escaping from an inflated tire

23 Adiabatic processes now for the details... an adiabatic process is a process which takes place without the addition or removal of heat from external sources as long as the air parcel remains unsaturated and no condensation occurs, it cools at a rate of 10 o C for every km of ascent; this rate of temperature change is the dry adiabatic lapse rate

24 Adiabatic processes should condensation occur as the air parcel is cooled, latent heat is released which partially counteracts the adiabatic cooling; the reduced cooling rate is the moist adiabatic lapse rate, which depends on temperature; an average value is 6oC per km of ascent!!! Almost all clouds result from the rapid expansional cooling of air when it ascends!!!

25 Mechanisms That Lift Air 1. Orographic Lifting: Forcing of Air Above a Mountain (Land) Barrier. 2. Frontal Lifting: Displacement of One Air Mass (Warmer) Over Another Air Mass (Cooler). 3. Convergence: Horizontal Movement of Air Into an Area at Low Levels. 4. Localized Convective Lifting: Buoyancy (Heating).

26 Rainshadow Effect Windward Side (Upwind): Precipitation Greatly Enhanced Leeward(Downwind): Low Precipitation

27 Frontal Lifting Displacement of One Air Mass (Warmer) Over Another Air Mass (Cooler) Warmer Air Approaches Colder Air Warmer Air Wedges Over the Colder Air Warm Front Smooth Slope Colder Air Approaches Warmer Air Colder Air Wedges Under the Warmer Air Cold Front Blunt Slope

28 Horizontal Convergence Horizontal Movement of Air Into an Area at Low Levels Mass of Air Not Evenly Distributed Causes Areas of Higher and Lower Pressure Pressure Difference Cause Wind Horizontal Movement of Air Into a Low Pressure Zone Causes Convergence Lifting

29 Localized Convection Localized as Opposed to Global Free Convection Heating of Earth s Surface in Localized Areas Buoyancy: Lighter, Warmer Air Rises Can Speed Up or Slow Down Other Lifting Mechanisms

30 A diabatic process is one in which energy is added to or removed from a system, such as air that is warmed by conduction when in contact with a warm surface or air that passes over a cool surface and loses energy by conduction. The direction of heat transfer is in accordance with the second law of thermodynamics, which dictates that energy moves from regions of higher to lower temperatures.

31 Processes in which temperature changes but no heat is added to or removed from a substance are said to be adiabatic. The rate at which a rising parcel of unsaturated air cools, called the dry adiabatic lapse rate (DALR), is very nearly 1.0?C/100 m (5.5?F/1000 ft).

32 If a parcel of air rises high enough and cools sufficiently, expansion lowers its temperature to the dew or frost point, and condensation or deposition commences. The altitude at which this occurs is known as the lifting condensation level (LCL). The rate at which saturated air cools is the saturated adiabatic lapse rate (SALR), which is about 0.5?C/100 m (3.3?F/1000 ft).

33 Unlike the DALR, the SALR is not a constant value. If saturated air cools from 30?C to 25?C (a 5? decrease), the specific humidity decreases from 27.7 grams of water vapor per kilogram of air to A 5?C drop in temperature from 5?C to 0?C lowers the specific humidity only 1.7 grams for each kilogram of air. This brings about less warming to offset the cooling by expansion, as well as a greater saturated adiabatic lapse rate.

34 The environmental lapse rate (ELR), applies to the vertical change in temperature through still air. A balloon rising through air with an ELR of 0.5?C/100 m passes through air whose temperature decreases from 10?C at the surface, to 9.5?C at 100 m, and 9.0?C at 200 m. The air within the balloon cools at the dry adiabatic lapse rate of 1.0?C/100 m, faster than the ELR, and therefore attains a temperature of 8?C at the 200-m level.

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