EG1204: Earth Systems: an introduction

Meteorology and Climate

Lecture 3Evaporation and humidity

Topics we will cover• Phases of water• Hydrologic (water) cycle• Absolute humidity• Specific humidity• Vapour pressure• Relative humidity• Dew Point• Measuring humidity

Phases of water

• As a solid, water forms hexagonal (6-sided) crystals we call ice

• In freezing air, if enough energy is available, ice can change directly into gas (water vapour). This is called sublimation

• If a water vapour molecule combined with ice crystals it is deposition

Phases of water

• Applying warmth (energy) to an ice crystal means that the molecules vibrate faster - so much so that they can vibrate out of their hexagonal crystal structure - the ice melts

• At the surface of water, some molecules have just enough energy to break free from the rest - called evaporation

Phases of water

• Some water vapour molecules with very little energy can combine with other water molecules on the surface of water - called condensing

• If a cover is placed over a beaker of water, eventually an equilibrium between escaping and returning water molecules is reached.

Phases of water

• When this state of equilibrium is reached the air in the beaker is said to be saturated with water vapour

• Removing the cover from the beaker would allow some molecules to be blown away - so the air would no longer be saturated and more would have to evaporate to take their place

Phases of water

• Higher temperatures lead to enhanced evaporation

• Conversely, condensation is more likely to occur when the temperature is lowered

Linacre et al, 1997

Hydrological (water) cycle

• Evaporation and transpiration over terrestrial areas accounts for only about 15% if the 1.5 billion billion gallons that annually evaporate. The other 85% evaporate over the oceans

• If all atmospheric water vapour were to condense and fall as rain it would cover the globe 2.5 centimetres thick

• The mass of water vapour in a given volume (parcel) of air

• Represents water vapour density - usually expressed as g/m3

Absolute humidity

AH = mass of water vapour / volume of air

• Mass of water vapour compared to the total mass of the parcel of air (including water vapour)

• usually expressed as g/kg

Specific humidity

SH = mass of water vapour / total mass of air

Vapour pressure

• The air’s moisture content may also be expressed in terms of the pressure exerted by the water molecules within it

• Air pressure at sea level is the result of pressure exerted by all gas molecules (nitrogen and oxygen included). The total pressure is equal to the sum of all pressures from all gases - known as Dalton’s law of partial pressure

Vapour pressure

• An increase in the number of water vapour molecules will tend to increase the total vapour pressure

• Actual vapour pressure indicates the air’s total water vapour content. Saturation vapour pressure describes how much water vapour is necessary in order to make the air saturated at any given temperature (remember the beaker hypothesis).

Vapour pressure

• Saturation vapour pressure is the pressure that the water vapour molecules would exert if the air were saturated with vapour at a given temperature

Relative humidity

• A common measure that is often misunderstood

• It tells us how close the air is to being saturated

• It is the ratio of the amount of water vapour actually in the air compared to the maximum amount of water vapour required for saturation at that particular temperature and pressure

Relative humidity• Put more simply, it is the ratio of the air’s

water vapour content to its capacity:

RH = water vapour content / water vapour capacity

RH(%) = (actual VP / SVP) * 100

Dew point

• The dew point is the temperature to which air must be cooled (with no change in air pressure or moisture content) for saturation to occur

• When the dew point temperature is reached on a surface, dew, frost or fog forms

• Lifting condensation level for air aloft

Measuring humidity

• Humidity is measured using a psychrometer or hygrometer (whirling)

• Wet and dry bulb thermometers based in a Stevenson screen use the same principle

• Difference between wet and dry bulb temperatures indicates water vapour content of the air

ABOVE: field hygrometer

RIGHT: Assman instrument