MET 003
Heat, temperature and water
Heat and temperature
Heat and temperature are not the same phenomena.
TemperatureAn expression of the heat condition of a body. Determined by the kinetic energy, the average speed, of the body’s molecules.
HeatThe transfer of thermal energy. Heat flow is the transfer of energy because of a temperature difference alone.
Heat capacityThe amount of energy required to change the temperature of a substance by 1 degree. Different materials can have different heat capacities.
How heat moves
Three ways: conduction, convection and radiation. In the atmosphere they work together.
1Conduction
Heat transfer between two objects that are in contact. The better the conductor, the more rapidly the heat transfers.
2Convection
Heat transport in the atmosphere by upward currents of air. Warm air is lighter than cold air, and starts to rise on its own.
The dominant heat transfer mechanism in gases and liquids.
3Radiation
Heat transfer by the emission of electromagnetic waves. The solid and liquid surfaces of the earth are good absorbers.
The earth receives all its heat from the sun as radiation, and loses heat to space as radiation too.
Solar and terrestrial radiation
What comes in from the sun, and what the earth sends back out.
In from the sun
Solar radiation is ultraviolet, visible and infrared light.
UltravioletVisible lightInfrared
Where it goes
Back out from the earth
The earth sends heat back out, as long-wave radiation.
Long-wave radiationEmitted from the earth.
Some of it is trappedIn the atmosphere, by water and water vapour, carbon dioxide (CO₂) and methane.
The greenhouse effectEssential to life on earth.
Temperature inversion
The exception to the rule that it gets colder higher up.
NormallyTemperature decreases with altitude.
InversionThe temperature rises with altitude: warm air over cold air.
For exampleCold fronts.
Water in three states
Solid (ice), liquid (water) and gas (water vapour). Every change of state takes in or gives out latent heat.
Latent heat
Latent means hiddenThe heat changes the state of the water, not its temperature. A thermometer does not show it.
Towards the gasMelting, evaporation and sublimation take the heat from the surroundings, which cool.
Towards the solidFreezing, condensation and deposition give the heat back, and the surroundings warm.
In rising airCondensation in a cloud gives out latent heat. That is why saturated air cools more slowly than unsaturated air as it rises: the saturated adiabatic lapse rate, in MET 004.
Water vapour
Warm air holds moreWarm air can hold more water vapour than cold air.
InvisibleGenerated by evaporation or sublimation.
RemovedBy condensation or deposition.
Lighter than airWhich helps trigger convection.
Humidity, saturation and dew point
How much water vapour the air holds, against how much it could.
HumidityThe amount of water vapour in the air.
Relative humidityThe degree of saturation of the air, measured in per cent.
SaturationThe air is incapable of holding more water vapour.
A state of saturation occurs when
The air is cooledTo a temperature at which it can hold no more water vapour.
Or water vapour is addedBy evaporation, until the air is saturated.
Dew pointThe temperature a parcel of air must be cooled to for its water vapour to condense into water: the saturation temperature. Important for calculating the likelihood of fog, carburettor icing and so on.
Ice and supercooling
Ice, and why water does not always freeze at 0 °C.
Ice, the solid state
Seen asIce crystals in clouds, and frozen precipitation.
Commonly formsWhen liquid water is cooled below 0 °C at standard atmospheric pressure.
Or straight from vapourIt can also deposit from water vapour, with no liquid phase in between.
Below 0 °C, still liquid
SupercoolingLowering the temperature of a liquid below its freezing point without it becoming solid.
Why it stays liquidA liquid needs a nucleus to crystallise. Without one, water stays in the liquid phase.
On impactSupercooled droplets freeze instantly when they hit an object.
Adiabatic heating and cooling
A change in the temperature of air because of a change in its pressure. No heat is added or taken away.
Why it mattersClouds, rain and snow are all created because of it.
Dew and fogDew and most fog form another way: the air is cooled from below, by a cold surface. See MET 004.
Not the ISA lapse rateThe ISA’s 1.98 °C per 1 000 ft is how temperature falls with height in the standard atmosphere, not how rising air cools.
How fast, and whyThe rates, and why rising air cools at all, are in MET 004.
Temperature and dew point in aviation
What the two mean for flying.
Mist and fogThe difference between temperature and dew point indicates mist or fog forming: the smaller it is, the nearer the air is to saturation.
DensityTemperature has an inverse relationship with density. See the density explorer in MET 002.
InversionsA temperature inversion indicates reduced performance.
IcingFreezing temperatures affect the possibility of icing when liquid water droplets are present.
Reading material
Meteorology
MET 003 Heat, Temperature and Water PowerPoint / PDF
EPN Group Training Document Meteorology, chapter 3 Reading list, course material
Ahrens, C. Donald, Meteorology Today: An Introduction to Weather, Climate, and the Environment, 8th edition, 2007 Reference, paid