GEO 101: PHYSICAL GEOGRAPHY Chapter 07: Water and Atmospheric Moisture Water on Earth Unique Properties of Water Humidity Atmospheric Stability Clouds and Fog
Water on Earth The origin of water A scientific hypothesis Earth s water originated from icy comets and H- and O-laden debris during the formation of the Earth. Water reaches the Earth s surface by outgassing.
The origin of water A scientific hypothesis Figure 7.1a Outgassing: a process by which water and water vapor emerge from deep below earth s surface. For water to remain on Earth s surface, the land temperature had to drop below 100 C (212 F)
Water on Earth The distribution of water today Figure 7.3
Unique Properties of Water Polarity: positive and negative charges by H(+) and O (-) Heat Properties: Phase change --- Solid (ice), Liquid (water), or Gas (water vapor) Vaporization ( liquid to vapor) Condensation (vapor to liquid) Sublimation (solid to vapor and vapor to solid) [Heat energy must be absorbed or released the process]
Three States of Water Figure 7.5
Latent heat: The quantity of heat absorbed or released by a substance undergoing a change of state (heat of transformation )
Phase Changes and Latent Heat Energy Water has the greatest density at 4 C (not the ice) From 4 C to -29 C, the volume of ice can increase up to 9% This expansion is important in the weathering of rocks and in highway and pavement damage
Phase Changes and latent heat energy Figure 7.7 Notice: Water has the greatest density at 4 C (not the ice) From 4 C to -29 C, the volume of ice can increase up to 9% This expansion is important in the weathering of rocks and in highway and pavement damage
Humidity (water vapor in the air) Energy available for powering weather The capacity of air for water vapor is a function of temperature.
Relative Humidity Relative humidity: a ratio (percentage) of the amount of water vapor that is actually in the air compared to the maximum water vapor possible in the air at a give temperature. Relative Humidity = Actual water vapor in the air X 100 Maximum water vapor possible in the air at that temperature
Relative Humidity Figure 7.8
Relative humidity changes with temperature even though the actual water vapor in the air stay the same during the day.
Vapor pressure: The share of air pressure that is made of water molecules is the Vapor Pressure. Saturation Vapor Pressure: The vapor pressure when the air is saturated.
Saturation and Dew point If the actual water vapor in the air = the maximum water vapor that the air can hold at that temperature the air is saturated (100% humidity) If the air is saturated, any addition of water vapor or decrease of temperature will lead to condensation!
Saturation and Dew point Dew point temperature a given mass of air becomes saturated at that temperature and net condensation starts to form droplets Fig. 7.9a
Humidity Instruments Hair Hygrometer Human hair changes as much as 4% in length between 0% and 100% relative humidity Figure 7.14 Sling psychrometer
Atmospheric Stability Stable tends to remain at its original position Unstable tends to rise upward To determine the degree of Stability or Instability, we need to measure two temperatures: Temperature inside the air parcel Temperature in the air surrounding the parcel
Atmospheric Stability Stability: The tendency of the air parcels to change their position Air parcel: A volume of air that has specific temperature and humidity characteristics Figure 7.15
Atmospheric Stability Buoyancy force: an uplifting force caused by density differences Cooler air higher density sinks Warmer air lower density rises
Clouds A cloud is an aggregation of tiny moisture droplets and ice crystals suspended in air. Clouds are indicators of overall condition, e.g., stability, moisture, weather.
Altocumulus
Cloud Formation Processes Clouds form as air becomes saturated with water. When air parcel rises, it cools and may reach the dew point Water start condensation around the condensation nuclei (dust, sot, ash, etc) When temperature is supercooled, ice-crystals starts to form
Cloud Types and Identification Figure 7.23
Cloud Types and Identification
Fog Fog is a cloud layer on the ground, with visibility restricted to less than one km. Advection fog Evaporation fog Valley fog Radiation fog
Advection Fog (when air in one place migrate to another place where conditions are right for saturation)
Evaporation Fog (water molecules evaporate from the water surface into the cold overlying air)
Valley Fog (cool air settled in the valleys)
Radiation Fog (radiative cooling of the surface chills the air layers directly above that surface)
End of Chapter 07!