Atmospheric Moisture and Precipitation

Transcription

1 Atmospheric Moisture and Precipitation

2 Water is life! 71% of the earth is covered in water 70% of our body weight is water A human can survive days without food; only 2-3 days without water

3 Characteristics of H20: Pure water is odorless, colourless and tasteless Water is a solvent (pure water rarely exists in nature) Water weighs 1g/cm3 or 1kg/L Water has three states: solid, liquid and gas Although there is evidence of water on other planets (Mars) earth is the only planet known to have water in all three phases

4 The Hydrosphere: the hydrologic reservoir Hydrosphere water in the earth-atmosphere system Earth s hydrosphere contains 1.36 billion cubic km % (1.321 billion km3) is located within the oceans (saltwater) 2.78% is freshwater 77.14% of freshwater and % of surface fresh water is within glaciers and ice sheets Less than 1% of freshwater is within lakes, rivers and streams

5 Water Water is unique in that it exists in all three states (solid, liquid, gas) Consists of 2 H atoms and 1 O atom that bond easily hard to separate Water is a versatile solvent Water is slightly weird as it s greatest density occurs in the liquid phase Water is able to shift between states very easily Water exhibits polarity

6 Polarity The 2 H atoms are positively charged and located on one side of the molecule The 1 O atom is negatively charged This polarity causes water molecules to attract other molecules Hydrogen bonding This is why water tends to stick to things It also leads to surface tension; allows things with greater density to float Capillarity = water will move into other substances b/c one water molecule will draw other molecules into it

7 Heat Properties of Water For water to change from on phase to another it either absorbs or releases heat energy: Heat energy is needed to change from phase to another to break the hydrogen bonds between water molecules

8 Phase Change when water changes from one state to another Melting/freezing describe the change between liquid and solid Condensation/evaporation describe the change between liquid and gas Sublimation/deposition describe the direct change between solid and gas

9 Solid Phase: - Unlike other substances that contract went cooled, water expands as a solid - Ice will increase it s volume up to 9% as it cools to -29 C - Reason for potholes, and major landforms/weathering - Because water expands when it s a solid (increasing volume) its density decreases (fewer water molecules per unit volume) Ice has 0.91 times the density of liquid water - That s why ice floats -

10 Melting/Freezing: - Melting/freezing occurs at 0 C - For water to move from a solid (ice) to a liquid, heat energy must be supplied to increase the motion of water molecules to break the strong bond achieved as a solid - There is no change in sensible temperature between ice and water at 0 C Latent heat of melting: heat energy absorbed when water goes from solid to liquid form Latent heat of fusion: heat energy released when water goes from liquid to solid form

11 Liquid phase - Water attains its greatest density at 4 C Liquid water has no shape water assumes shape of its container Liquid water exists under temperatures between 0 and 100 C

12 Evaporation/Condensation phase - Evaporation/condensation occurs at 100 C - More latent heat energy is absorbed and released when water changes phase between liquid and gas than between solid and liquid - Greater latent heat energy is needed to increase the motion of water molecules to break the surface tension created by hydrogen bonding

13 Gaseous Phase water vapor is an invisible gas and each molecule moves independently Sublimation/Deposition Water can move directly from a solid to a gas and back without entering the liquid phase Latent heat of sublimation heat absorbed when water moves from a solid to gaseous state directly Happens in your freezer When gas moves from a gas to solid phase heat is released; ex frost

14 In Summary: - Energy is absorbed by processes that result in water changing phase from a solid to liquid to gas - Energy is released by processes that result in water changing phase from a gas to liquid to solid

15 Humidity

16 Humidity: the water vapor content of the air The capacity of air to hold water is primarily a function of air temperature Warmer air has a greater capacity to hold water vapor than cooler air

17 Relative Humidity: After air temperature and air pressure, relative humidity is the most common piece of information in local weather broadcasts Relative Humidity - the ratio (expressed as a percentage) of the actual amount of water vapor in the air (content) to the amount of water vapor the air can hold at that temperature (capacity) RH = (water vapor content / maximum water vapor capacity) * 100

18 Relative Humidity: Varies b/c of evaporation/condensation, or temperature changes Condensation and evaporation happen continuously Consider warm air to be a large sponge that soaks up large quantities of water vapor Think of cooler air as a small sponge that can only soak up small quantities of water vapor The amount of water it takes to fill up cooler air may only partially fill up warmer air

19 Saturation - occurs when a particle of air contains all the water vapor it can hold (it is filled to capacity) RH is at 100% (the ratio of water vapor content equal the air s capacity to hold water vapor) The net transfer of water molecules between the surface and the air reaches equlibrium (rates of condensation and evaporation are equal) To go past saturation will result in condensation You can pass saturation by decreasing air temperature (decrease the air s capacity to hold water vapor) It may not rain when RH = 100% but if the air goes past it, precipitation will occur

20 Dew Point Temperature - the temperature at which a given mass of air will become saturated given the amount of water vapor contained within that air parcel Air is saturated when the dew point temperature and the air temperature are equal At this point RH = 100%

21 Relative Humidity Patterns b/c the capacity of air to hold water vapor increases as air temperature increases, the daily values of air temperature and relative humidity are inversely related - air temperature rise from just after sunrise to mid-afternoon - Given a constant water vapor content as air temperature rises, relative humidity will decrease Daily RH: - Relative humidity is highest at dawn when air temperature is lowest - Relative humidity is lowest around mid-afternoon when temperature and the capacity to hold water vapor is highest

22 Vapor Pressure the portion of the air pressure exerted by water molecules within the atmosphere Saturation Vapor Pressure the maximum pressure that water vapor molecules can exert Occurs when air has reached the max capacity to hold water vapor at the given temperature Saturation vapor pressure increases exponentially with increasing temperature higher temp can hold more water vapor Saturation vapor pressure is higher over water than ice

23 Specific Humidity measure of the actual amount of water vapor in the air S.H. = mass of water vapor (g) / mass of air (kg) it takes into account the actual mass of water vapor in the air It is not affected by changes in temperature Maximum specific humidity is the maximum mass of water vapor that a kg of air can hold at any specified temperature

24 Measuring Humidity: Hair hygrometer measures humidity based on the change in length of a piece of human hair Human hair changes by as much as 4% in length between 0 and 100% relative humidity Sling Psychrometer measures the rate at which water evaporates from a wet wick One thermometer measures air temp (dry bulb) The bottom of one thermometer is wrapped in a wet cloth That thermometer measures the temperature relative to the rate of evaporation from the wick Evaporation cools the wick

25 Atmospheric Stability

26 When talking about atmospheric stability, we will use the term parcel of air Parcel term used to describe a body of air with specific temperature and humidity characteristics.

27 Two forces determine the motion of an air parcel: Bouyant force: moves an air parcel up An air parcel of lower density will rise As it rises it will expand as external pressure decreases Gravitational force: moves an air parcel down A parcel of higher density will fall As it falls it compresses as external pressure increases

28 Stability the tendency of a parcel to remain in place or to change vertical position by rising or falling A parcel is stable if it resists vertical motion A parcel is unstable if it continues to rise until it reaches an elevation where the surrounding atmosphere has a density or air temperature similar to that of the parcel Changes in a parcel s vertical position will affect the amount of moisture it can contain

29 Adiabatic Processes: Determining atmospheric stability requires the temperature of the parcel and surrounding air: Ex: a parcel that is warmer than the surrounding air will rise until it is cooled to the same temperature as the surrounding air, at which point it will cease to rise

30 Normal lapse rate average decrease in air temperature with elevation (6.4 C/km) Environmental Lapse Rate actual decrease in air temperature with elevation

31 A rising parcel will cool by expansion resulting from a decrease in pressure with increasing elevation As pressure decreases, a parcel will expand and it s internal temperature and density will decrease Temperature decreases b/c sensible heat is consumed in the process of expansion A falling parcel will warm by compression resulting from an increase in pressure with decreasing elevation As pressure increases, a parcel will compress and its internal temperature and density will increase Temperature increases b/c sensible heat is produced in the process of compression

32 These temperature changes within individual parcels of air occur without any significant heat exchange between surrounding environment and the parcel Temperature changes within a parcel that occur b/c of expansion and compression are Adiabatic

33 Adiabatic processes describe the cooling and warming rates of individual parcels of air due to expansion and compression of the parcel rather than energy exchange between the parcel and surrounding air

34 There are two adiabatic rates for a vertically moving parcel, depending on moisture conditions of the parcel: Dry Adiabatic Lapse Rate (DAR or Γd) the rate at which rising dry air cools by expansion or falling dry warms by compression Used for air that is not saturated (has a RH < 100%) Has an average value of 10 C/km Moist Adiabatic Lapse Rate (WAR/ Γm) rate at which rising moist air cools by expansion or falling moist air warms by compression used for saturated air (RH = 100%) Has an average value of 6 C/km This is lower that the DAR b/c of the latent heat of condensation

35 Rising air will cool and it is possible that it will reach the dew point temperature (a relative humidity of 100%) At that point, or the lifting condensation level, moisture will condense from the air and the air temperature will follow the moist adiabatic lapse rate which ranges from between 4 C and 9 C per kilometer (avg 6 C/km) The wet adiabatic lapse rate will be lower for warmer air since there is more moisture to condense

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37 Stable vs. Unstable Atmospheric Conditions

38 Temperature relationships with rising and falling air parcels causes three atmospheric conditions: 2. Unstable 3. Conditionally stable 4. Stable

39 Unstable atmospheric conditions that occur when an air parcel is warmer than the surrounding air and continues to rise

40 Unstable: the environmental lapse rate is greater than the DAR and MAR The parcel cools at the DAR making it warmer than the surrounding air As long as the parcel is warmer it continues to rise Eventually the parcel will cool until it s temperature is equal to the dew point temperature RH = 100% Water vapor in parcel will condense, forming clouds and possible precipitation

41 Conditionally Unstable atmospheric conditions that occur when a parcel rising according to the DAR will resist upward movement but a parcel rising according to the MAR will continue to rise

42 Conditionally Stable: The environmental lapse rate is greater than the MAR but lower than the DAR Unsaturated air will rise according to the DAR Saturated air will rise according to the MAR The surrounding air cools at a rate of 7 C/km and the parcel cools at 6 C/km At 1000m the parcel will still be warmer than the surrounding air As long as it s warmer it will continue to rise under unstable conditions Stability is dependent on the moisture content of the air parcel

43 Stable atmospheric conditions that occur when a parcel rising according to the DAR and MAR will resist upward motion

44 Stable: The environmental lapse rate is lower than the MAR and the DAR Parcel will not rise and clouds will not form

45 Clouds

46 Clouds and Cloud Formation Cloud an aggregation of tiny moisture droplets and ice crystals that are suspended in air, with great enough volume to be visible Form when air becomes saturated with water Fog a cloud in contact with the ground

47 Clouds form when water vapor within a saturated air parcel condense A parcel will rise until its internal temperature is cooled to the dew point temperature As the condensation occurs it warms the parcel and the parcel continues to rise and cool Condensation continues as the parcel rises Liquid water will condense around cloud condensation nuclei If there is enough condensation and the moisture droplets are large enough, eventually the force of gravity will pull them toward the surfece (precipitation)

48 Cloud Types: Clouds are classified by altitude and shape: Strat horizontal Cumulo puffy Nimbus precipitation

49 Name: Altitude: Shape: Stratus Low altitude Horizontal Cumulus Low altitude Globular Nimbostratus Low altitude Horizontal Precipitation Stratocumulus Low altitude Lumpy Grey Middle altitude Contain water and Many styles ice Altocumulus Vertical Cumulonimbus Cirrus High altitude Wispy Characteristics thunderstorms Frozen water molecules

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60 Cirrostratus When viewed through a layer of cirrostratus, the Moon or Sun has a whitish, milky appearance but a clear outline. A characteristic feature of cirrostratus clouds is the halo, a circular arc around the Sun or Moon formed by the refraction (bending) of light as it passes through the ice crystals.

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75 altocumulus standing lenticularus Lenticular Clouds

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77 Stratiform Clouds Fog stratus cloud at the ground Advection Fog moving air over a colder surface (e.g., the Grand Banks of Newfoundland) Radiation Fog motionless air that cools below the dew point due to longwave radiation loss at night

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79 Radiation Fog in the Central Valley of California

80 Stratiform Clouds Fog stratus cloud at the ground Sea Fog forms when a cool moist air parcel comes in contact with cold ocean Occurs along the coasts of continents where cold currents move toward the equator (e.g. California, Peru)

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