Lecture 5

• in ice hydrogen bonds are stable
• in liquid water hydrogen bonds constantly break and re-form
• hydrogen bonds are strong enough for water molecules to stick together
• water behaves anomalously as freezing temperature decreases with increase in pressure (v relevant to subglacial conditions)
• boiling point at the top of Everest is 71 degrees C
• water density varies with temperature
• ice has complete hydrogen bonding
• when ice melts, some hydrogen bonds are broken and therefore the water molecules can crowd (pack) together more closely
• solid water is less dense than liquid water
• broadly as the temperature of water increases above 0 degrees the molecules bounce around (vibrate) more resulting in a decrease in density
• up to 4 degrees the vibrating is overshadowed by the breaking of hydrogen bonds leading to an initial increase in density with warming
• between 0 and 4 liquid water becomes more dense with increasing temperature
• above 4 liquid water becomes less dense with increasing temperature
• melting and boiling points ensure the common presence of liquid water around the planet (essential for life - compare with Mars)
• heat capacity (amount of hear required to raise temperature of a specific substance by 1 degrees) is very high and the highest of any liquid except ammonia
• heat of vaporization (heat required to convert liquid water to water vapor) is one of the highest of all liquids
• surface tension (how strongly the molecules of a liquid are attracted to each other compared to other adjacent molecules) is very high
• absorption of radiation - water is transparent to visible light, except towards the red end of the spectrum, but uv radiation that is damaging to life is absorbed, as is infra-red radiation by water vapour
• water as a solvent (a substance that dissolves a solute - a chemically different solid, liquid or gas - resulting in a solution) - water is an excellent solvent
• water may have come from a number of sources:

  • from earths primordial atmosphere with the retention of liquid water possible due to a high enough atmospheric pressure
  • extra-planetary: impact of comets and other water-rich objects
  • volcanic activity
  • gradual leakage of water from the mantle
• types of fluxes:

  • evaporation
  • transpiration (release of water vapor to the atmosphere from plants, largely through pores (stomata) on leaves)
  • evapotranspiration (the combines effect of evaporation and transpiration)
  • precipitation
  • lateral atmospheric transport
  • runoff from rivers to the ocean
• factors controlling mean annual runoff:

  • R=P-E
  • R is mean annual runoff from drainage basin
  • P is mean annual precipitation across drainage basin
  • E is mean annual evapotranspiration across drainage basin
• evaporation is controlled by the kinetic energy of molecules. direction of movement can be away from the surface. increase of kinetic energy with temperature
• loss of molecules lowers temperature (evaporative cooling - sweating). balance of molecules into and out of water surface produces vapor layer that can be saturated (but wind increases evaporation rate). availability of water to be evaporates clearly impacts evaporation rate
• transpiration rate is controlled by:

  • water supply to plant
  • leaf area
  • density of stomata
  • plant adaptations to reduce transpiration (e.g. fine hairs on leaf)
  • light (affects opening of stomata)
  • temperature
  • relative humidity
  • wind
• potential evapotranspiration (PE): the rate of evapotranspiration that would occur under a specific set of conditions given a limitless supply of water
• actual evapotranspiration (AE): the actial rate of evapotranspiration under a specific set of conditions (i.e. the rate of evapotranspiration that actually occurs at a location)
• rivers remove sediments and solutes from land to the ocean
• the thermohaline circulation controls global energy fluxed and thus climate
• lighter isotopes preferentially evaporate, heavier isotopes preferentially condense
• fractionation enables estimation of past ice volume and sea level
• oceanic evaporation

  patterns controlled by air and ocean temperatures, cloud cover and atmospheric (wind speed) and ocean circulation
• land evapotranspiration


  patterns controlled by precipitation, soil/surface moisture, air temperature, cloud cover and atmospheric circulation (wind speed)