AVIATION 120
Meteorology
Today’s Agenda
– Advective cooling
– Adiabatic processes
• Expansion/compression
• Adiabatic lapse rates
• Expansion cooling processes
– Temperature and human comfort
Advective Cooling
• Lower levels in the atmosphere can be cooled if
the air moves over a surface that is cooler than
itself
• The air cools due to turbulent mixing and
conduction in the lowest levels of the atmosphere
• You can recognize the occurrence of advective
cooling by looking for isobars that cross isotherms
on an analysis chart
Adiabatic Processes
• Adiabatic Processes refer to the heating or cooling of air by
expansion or compression where no other factors are
influencing the temperature
– Known as adiabatic heating or adiabatic cooling
• Air that is forced to rise will expand as it encounters lower
pressure and will cool adiabatically
• Air that is forced to descend will contract as it encounters
higher pressure and will heat adiabatically
• Unsaturated air will warm or cool at about 3°C per 1000’ or
about 10°C per 1000 m
– This is known as the dry adiabatic lapse rate (DALR)
– Descending air will usually warm at this rate
• Saturated air will warm or cool at an average rate of 1.5°C
per 1000’ or 6°C per 1000 m
– This is known as the moist or saturated adiabatic lapse rate (SALR)
– Is quite variable due to the variable amount of released latent heat
between warm (moist) air and cold (dry) air
– Varies between 1.1°C for warm air to 2.8°C per 1000’ for cold air
Examples of Adiabatic Processes
• Convection
• Orographic and upslope lift
• Frontal lift
• Mechanical turbulence
• Large scale ascent (low pressure areas)
• Large scale descent (high pressure areas)
Temperature and Human Comfort
• Wind chill
– On a calm day, our bodies insulate us somewhat from the outside
temperature by warming up a thin layer of air close to our skin, known as
the boundary layer. When the wind blows, it takes this protective layer
away, exposing our skin to the outside air. It takes energy for our bodies to
warm up a new layer, and if each one keeps getting blown away, our skin
temperature will drop, and we will feel colder.
– Wind also makes you feel colder by evaporating any moisture on your skin,
a process that draws more heat away from your body
– Studies show that when your skin is wet, it loses heat much faster than
when it is dry
– Used to be expressed as the loss of body heat (W/m2)
– In Canada, wind chill index is now expressed as what the temperature
would feel like on human skin on a calm day
– Since it isn’t an actual air temperature, the degree sign is not used with
wind chill values
– E.G. if the air temperature is -10°C and the wind chill is -20, human skin
would feel as cold as it would on a calm day with a temperature of -20°C
Wind Chill Conversion Chart
Old Wind Chill (watts/metre2) New Wind Chill
1400 -21.8
1450 -22.4
1500 -23.1
1550 -23.7
1600 -24.4
1650 -25.0
1700 -25.7
1750 -26.4
1800 -27.1
1850 -27.9
1900 -28.9
2000 -34.0
2100 -35.9
2200 -40.8
2300 -42.7
2400 -47.7
2500 -49.6
2600 -54.6
Temperature and Human Comfort
• Humidex
– Describes how hot, humid weather feels to the average person
– Humidex combines the temperature and humidity into one number
to reflect the perceived temperature
– Because hot, humid air doesn’t readily evaporate moisture from the
skin like hot, dry air, the temperature feels warmer on humid days
– Highest recorded humidex in Canada was 52.1 in Windsor on June
20, 1953
Range of humidex: Degree of comfort
Less than 29 : No discomfort
30 to 39 : Some discomfort
40 to 45 : Great discomfort; avoid exertion
Above 45 : Dangerous
Above 54 : Heat stroke imminent