Water – Its Role in Atmospheric Processes Geog 1, Week 5, 26 March 2002 Need to Understand Properties of water Latent heat Cloud formation

Water on Earth Part of the formation of the planet Constantly moving to the surface to balance chemical losses On surface 97.2% salt water in oceans 2.2% freshwater on surface 0.005% soil moisture 0.6% groundwater

Freshwater 99.6% is frozen (glaciers and ice sheets) 0.3% in freshwater lakes 0.03% in clouds and water vapour 0.003% in rivers and streams

So, even though there is lots of water on the Earth’s surface, only a tiny fraction of it is accessible for human use. How does it circulate from oceans to rivers?

86% evap. from oceans 80% rain into oceans 6% moves over land 14% evap. from land 6% runoff 20% rain/snow over land Hydrologic Cycle (average precipitation about 1 metre)

Important Properties of water Exists in three phases on surface sold (ice), liquid (water), gas (water vapour) Expands when it freezes –hence ice floats, important in weathering of rocks Lots of energy absorbed/released when water changes phase, especial liquid/vapour (latent heat of evaporation)

Latent and Sensible Heat To heat one kilogram of water by 1 o C 4.18 kJ (kilojoule = 1000 J) To melt one kilogram of ice 335 kJ To evaporate one kg of water (from 20 o C) 2448 kJ [Christopherson - 1 calorie = Joule]

Latent Heat Small cumulus cloud 500 to 1000 t of water 1.2 to 2.4 TJ (terajoule = J) heat released by burning 30,000 to 60,000L petrol Hurricane Andrew 30 x t water 75 x 10 9 TJ = 180 times more energy than the world uses in one year!

Humidity In addition to understanding the importance of latent heat in moving energy around the globe – The role of humidity must also be understood, because this determines cloud heights, comfort levels, and evaporation rates.

At 0 o C air 1m 3 of air can hold 4.9g water vapour At 10 o C air 1m 3 of air can hold 9.4g water vapour At 20 o C air 1m 3 of air can hold 17g water vapour At 30 o C air 1m 3 of air can hold 30g water vapour At 40 o C air 1m 3 of air can hold 51g water vapour

If warm air that has absorbed water rises (and so cools) it will reach a point where it becomes saturated, it cannot maintain the water as vapour and so condensation will occur. Like steam from a kettle forming as the escaping hot vapour cools to room temperature.

If air is very dry, i.e. can absorb lots of moisture before reaching saturation, it will cause more rapid evaporation. Hence, dry air evaporates perspiration and cools us down, but air with lots of moisture in it results in less evaporation and less cooling. Sling psychrometer demo

Expressing Humidity Relative Humidity –The actual water vapour in a volume of air expressed as a percentage of the maximum the air could hold at that temperature. Specific Humidity –The mass of water vapour (in grams) in one kilogram of air

Lapse Rates Should be understood by anyone contemplating climatology in 2 nd year Week 2, I said air cools 6.4 o C every 1000m BUT NOW Dry air cools at 10 o C for every 1000m called ‘dry adiabatic rate’ or DAR Saturated air cools at 6 o C for every 1000m called ‘moist adiabatic rate’ MAR Why the difference?

The saturated air cools slower because as it cools some of the water vapour condenses, releasing heat and slowing the cooling rate

Clouds When air rises and cools (or cools by mixing with cold air) it will reach its saturation point and as it cools further tiny water droplets will start to form. These droplets are only 0.02mm in diameter (20 microns or 20µm), too small to see individually but they scatter light and appear as a cloud, mist or fog.

Rain/Snow Each tiny drop forms on still smaller particles called Aitken particles or cloud- condensation nuclei. The tiny 20µm droplets remain suspended in the air, but as they bounce around they coalesce, gradually increasing in size until 1 million or more form a rain drop (~2mm) which is heavy enough to fall.

Next Week Easter Break Tuesday 9 April – Weather Chapter 8 of Christopherson