topic 6: atmospheric moisture and global precipitation introduction humidity: -absolute humidity...

Download Topic 6: Atmospheric Moisture and Global Precipitation Introduction Humidity: -Absolute Humidity -Specific Humidity -Relative Humidity  Atmospheric Stability

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  • Slide 1
  • Topic 6: Atmospheric Moisture and Global Precipitation Introduction Humidity: -Absolute Humidity -Specific Humidity -Relative Humidity Atmospheric Stability
  • Slide 2
  • Topic 5: Atmospheric Moisture and Global Precipitation Adiabatic Activities Condensation Process: -Cloud & Cloud Classification -Fog Precipitation: -Ice-Crystal Formation Process -Collision-Coalescence Process
  • Slide 3
  • Topic 5: Atmospheric Moisture and Global Precipitation -Types of Precipitation -Acid Rains Global Pattern of Precipitation
  • Slide 4
  • Atmospheric Moisture: An Introduction Water vapor is one of the atmospheric gases > of all water vapor is found within one mile (1.6 km) of the earth surface Very small fraction occurs above 4 miles (6.4 km)
  • Slide 5
  • Atmospheric Moisture: An Introduction Water vapor is unevenly distributed and may be very low over desert areas and may be up to 4% by volume over equatorial areas Evaporation of liquid water is the main source of water vapor (gas) in the atmosphere
  • Slide 6
  • Atmospheric Moisture: An Introduction Latent heat of vaporization is stored in the vapor during evaporation The rate of evaporation is enhanced by: -water temperature -air temperature -degree of windiness
  • Slide 7
  • Changes in the State of Water in Relation to Energy Absorption and Release
  • Slide 8
  • Atmospheric Moisture: An Introduction Evapotranspiration is another source of vapor to the atmosphere It is the transfer of moisture through plants to the atmosphere The maximum rate of evapotranspiration, when water is not limiting, is called potential evapotranspiration (PE)
  • Slide 9
  • Atmospheric Moisture: An Introduction Where PE exceeds actual precipitation: -no water is available for storage -soils become dry and -vegetation turns brown
  • Slide 10
  • Atmospheric Moisture: Humidity Humidity is the amount of water vapor in the atmosphere Its may be measured and expressed as: -Absolute Humidity (gm/m 3 ) -Specific Humidity (gm/kg) -Relative Humidity (%)
  • Slide 11
  • Atmospheric Moisture: Absolute Humidity Absolute Humidity measures the total water vapor content of air Its expressed as weight of water vapor in a given volume of air (gm/m 3 ) Absolute Humidity is very sensitive to: -temperature changes -changes in air volume
  • Slide 12
  • Atmospheric Moisture: Specific Humidity Specific Humidity measures the mass or weight of water vapor content in a given mass of air Its expressed as mass (gm) of water vapor in one kilogram (kg) of air (gm/kg) For any given temperature, there is a maximum mass of vapor that a kilogram of air can hold (i.e. saturation quantity)
  • Slide 13
  • Atmospheric Moisture: Specific Humidity In cold polar air, specific humidity may be as low as 0.2gm/kg In warm equatorial air, specific humidity may be as high as 16gm/kg Specific humidity progressively increases from the poles to a single peak at the equator
  • Slide 14
  • Pole-To-Pole Values of Specific Humidity (gm/kg)
  • Slide 15
  • Atmospheric Moisture: Specific Humidity Both absolute and specific humidity measure the quantity of precipitable water in the atmosphere
  • Slide 16
  • Atmospheric Moisture: Relative Humidity Relative Humidity (R.H.) measures the percentage of water vapor present in the atmosphere relative to the maximum quantity the air could hold at the given temperature Relative Humidity (R.H.) changes as airs capacity to hold moisture (i.e. its vapor content) changes
  • Slide 17
  • There is an inverse relationship between temperature and relative humidity such that relative humidity is lower during the hot daytime and higher during the cooler night
  • Slide 18
  • Relative Humidity and Temperature Changes
  • Slide 19
  • Atmospheric Moisture: Relative Humidity Relative Humidity (R.H.) increases if: -evaporation increases vapor content of air -cooling reduces the holding capacity of air Relative Humidity decreases if: -moisture is removed by condensation or dispersal -heating increases air holding capacity
  • Slide 20
  • Atmospheric Moisture: Relative Humidity Relative Humidity (R.H.) could be measured using this formula: Vapor pressure R.H. =---------------------------------- X 100 Saturation Vapor Pressure R.H. is 100% at saturation R.H. is 50% if only half of the total vapor is present
  • Slide 21
  • Atmospheric Moisture: Relative Humidity Air that has R.H. of 100% (saturation) at cool temperature might be far from saturation at a warmer temperature even if the actual amount of moisture is unchanged Dew point is the temperature at which air becomes saturated during cooling
  • Slide 22
  • Atmospheric Moisture: Relative Humidity Instrument used for measuring relative humidity is called a hygrometer A homemade hygrometer uses a strand of human hair attached to the end of a pointer to determine R.H. The hair changes in length in response to changes in R.H.
  • Slide 23
  • Atmospheric Moisture: Relative Humidity The pointer falls whenever R.H. is high, or the hair lengthens when R.H. is high Another method for measuring R.H. is through the use of a psychrometer which consists of 2 thermometers: -a dry bulb thermometer -a wet bulb thermometer
  • Slide 24
  • Atmospheric Moisture: Relative Humidity The difference between the dry and wet bulb thermometer readings is called wet bulb depression It involves the calculation of the wet bulb depression The wet bulb depression is entered into a psychrometric table to obtain the R.H.
  • Slide 25
  • Pole-To-Pole Variation of Relative Humidity (%)
  • Slide 26
  • Relative Humidity Values (%) and Temperature in o F
  • Slide 27
  • Relative Humidity Values (%) and Temperature in o C
  • Slide 28
  • Dew-Point Temperature in o F
  • Slide 29
  • Atmospheric Stability Rising air causes condensation and precipitation to occur But the ability of air to rise is closely related to atmospheric stability Air will rise when the air is unstable Air will resist upward movement or sink when the air is stable
  • Slide 30
  • Stable and Unstable Atmosphere
  • Slide 31
  • Atmospheric Stability The air is unstable when the parcel of air is warmer (lighter) than the surrounding air The air is stable when the parcel of air is cooler (heavier) than the surrounding air Atmospheric stability could also be stated in terms of the lapse rate concept
  • Slide 32
  • Adiabatic Cooling and Warming
  • Slide 33
  • Adiabatic Cooling in a Balloon and Cooling of Surrounding Air (Environmental Lapse Rate
  • Slide 34
  • Atmospheric Stability The air is stable when: -lapse rate of the surrounding air is less than the dry adiabatic lapse rate (DALR) of the rising air The air is unstable when: -lapse rate of the surrounding air is greater than the dry adiabatic lapse rate (DALR) of rising air
  • Slide 35
  • Atmospheric Stability in Relation to Lapse Rate
  • Slide 36
  • Stable Air
  • Slide 37
  • Atmospheric Stability The air is conditionally unstable when: -lapse rate of the surrounding air is in-between the dry and wet adiabatic lapse rates Stable air produces no clouds due to lack of adiabatic cooling But when stable air is forced to rise, it cools to produce stratiform clouds and drizzly precipitation
  • Slide 38
  • Atmospheric Stability in Relation to Lapse Rate
  • Slide 39
  • Unstable Air
  • Slide 40
  • Conditionally Unstable Air
  • Slide 41
  • Adiabatic Cooling and Warming of Rising Air When unsaturated air rises it cools at a relatively steady rate of 5 o F/1000 ft (10 o C/1000 m) called dry adiabatic lapse rate (DALR) There is no gain or loss of heat during adiabatic cooling or warming As rising air cools, its capacity to hold water decreases (i.e. R.H. increases)
  • Slide 42
  • Types of Lapse Rates
  • Slide 43
  • Adiabatic Cooling and Warming of Rising Air As rising air continues to cool and its relative humidity continues to increase until it reaches saturation or R.H of 100% The temperature at which relative humidity is 100% is called dew point temperature
  • Slide 44
  • Adiabatic Cooling and Warming of Rising Air The altitude at which dew point temperature is reached is known as the lifting condensation level (LCL) Condensation (or cloud formation) begins at LCL when the rising air becomes saturated or R.H. of 100% Hence, LCL becomes the base of a cloud
  • Slide 45
  • Adiabatic Cooling and Warming of Rising Air Condensation causes latent heat to be released As a result, the addition of latent heat to the atmosphere causes the air rising beyond LCL to cool at a slackened or reduced rate
  • Slide 46
  • Adiabatic Cooling and Warming of Rising Air This diminished cooling rate is called the wet adiabatic lapse rate (3.3 o F/1000ft) (WALR or SALR) adiabatic cooling or w

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