Download dry adiabatic lapse rate

THERMALS
What are thermals ?
Thermals are columns of rising warm air
What is required for a thermal ?
•Solar radiation – to warm the ground and surface features.
What is required for a thermal ?
•Solar radiation – to warm the ground and surface features.
•Surface differences – to provide differential warming of the air.
What is required for a thermal ?
•Solar radiation – to warm the ground and surface features.
•Surface differences – to provide differential warming of the air.
•Sufficient warming of the air – to start it rising – Trigger temperature.
What is required for a thermal ?
•Solar radiation – to warm the ground and surface features.
•Surface differences – to provide differential warming of the air.
•Sufficient warming of the air – to start it rising – Trigger temperature.
•Remain warmer than its surroundings – Instability.
What is required for a thermal ?
•Solar radiation – to warm the ground and surface features.
•Surface differences – to provide differential warming of the air.
•Sufficient warming of the air – to start it rising – Trigger temperature.
•Remain warmer than its surroundings – Instability.
•Dry air – to give a reasonable cloudbase – Dew point.
What is required for a thermal ?
•Solar radiation – to warm the ground and surface features.
•Surface differences – to provide differential warming of the air.
•Sufficient warming of the air – to start it rising – Trigger temperature.
•Remain warmer than its surroundings – Instability.
•Dry air – to give a reasonable cloudbase – Dew point.
•Limit to growth – to prevent overdevelopment – Stability.
Solar Radiation
•Solar radiation does not warm the air it passes through.
•Solar radiation warms any surface it hits.
•That surface warms anything in contact with it - conduction.
•The air above the surface warms by mixing - convection.
Surface differences
•Angle - south facing slopes receive more sunlight.
•Wind - surfaces in a wind shadow will warm better.
•Hard surfaces – buildings, car parks, dry fields warm better.
•Foliage ‘stores warmth’ for release later in the day.
•Wet surfaces do not warm.
Will the warm air rise ?
•Is the warmed air warmer than its surroundings?
•Will it stay warmer than its surroundings?
•And for how long?
•All this depends on various ‘Lapse Rates’.
•A Lapse Rate is the rate at which temperature changes with altitude.
•Some are physical rules of the atmosphere, and one (the important
one) is what is actually happening out there.
Lapse Rates
There are four lapse rates to consider:
•Dry Adiabatic Lapse Rate (DALR).
•Saturated Adiabatic Lapse Rate (SALR).
•Environmental Lapse Rate (ELR) of a standard atmosphere.
•Environmental Lapse Rate (ELR) of the actual atmosphere.
DALR definition
The dry adiabatic lapse rate (DALR) is the rate of temperature decrease
with height for a parcel of dry or unsaturated air, rising under adiabatic
conditions. Unsaturated air has less than 100% relative humidity; i.e. its
actual temperature is higher than its dew point. The term adiabatic means
that no heat transfer occurs into or out of the parcel. Air has low thermal
conductivity, and the parcels of air involved are very large, so transfer of heat
by conduction is negligibly small.
Under these conditions when the air rises (for instance, by convection) it
expands, because the pressure is lower at higher altitudes. As the air parcel
expands, it pushes on the air around it, doing work. Since the parcel does
work but gains no heat, it loses internal energy so that its temperature
decreases. The rate of temperature decrease is 3.0°C per 1,000 ft. The
reverse occurs for a sinking parcel of air.
DALR demonstration
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The Dry Adiabatic Lapse Rate (DALR) is
the rate of temperature decrease of dry air with height.
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It is 3 C per 1000 feet
SALR definition
When the air is saturated with water vapour, at its dew point, the saturated
adiabatic lapse rate (SALR) applies. This lapse rate varies strongly with
temperature. A typical value is around 1.5°C per 1,000 ft.
The reason for the difference between the dry and saturated adiabatic lapse
rates is that latent heat is released when water condenses, thus decreasing
the rate of temperature drop as altitude increases.
SALR demonstration
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The Saturated Adiabatic Lapse Rate (SALR) is
the rate of temperature decrease of saturated air with height.
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It is 1.5 C per 1000 feet
ELR definition
The environmental lapse rate (ELR), is the rate of decrease of
temperature with altitude in the stationary atmosphere at a given time
and location.
As an average, the International Civil Aviation Organization (ICAO)
defines an international standard atmosphere (ISA) with a temperature
lapse rate of 1.98 °C per 1,000 ft from sea level to 36,090 ft.
From 36,090 ft up to 65,620 ft the temperature is constant at −56.5 °C,
which is the lowest assumed temperature in the ISA. The standard
atmosphere contains no moisture.
Unlike the idealized ISA, the temperature of the actual atmosphere does
not always fall at a uniform rate with height. For example, there can be
an inversion layer in which the temperature increases with height.
ELR demonstration
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Example of an
actual atmosphere
ISA
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The Environmental Lapse Rate (ELR) for the International Standard
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Atmosphere (ISA) is 1.98 C per 1000 feet
The ELR of the actual atmosphere varies dramatically with
meteorological conditions.
ELR an example
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Actual ELR is measured by taking dry and wet bulb thermometers
(and other measuring equipment) to altitude on a balloon ascent.
Dry bulb thermometer measures temperature.
Wet bulb thermometer measures dew point.
Stability
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DALR
SALR
ELR 3
ELR 2
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ELR 1
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ELR1 < SALR = absolutely stable
SALR < ELR2 < DALR = conditionally unstable
ELR3 > DALR = super-adiabatic lapse rate = absolutely unstable
What does all this give us?
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•Air at 18O will rise to about 500 feet and stop – low level inversion
•Air at 20O will rise above the inversion – trigger temperature
•Air at 21O will rise to about 4500 feet and stop – top of thermal, and
......cloudbase. Here things get a bit more complicated – see next slide
•Air at 24O will rise to about 5500 feet and stop – high level inversion
What happens at cloudbase?
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The parcel of air that rose from the surface at 21 has now cooled to about
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7 at 4500’, where it meets the ELR and stops rising. However it also meets
the dew point line, moisture condenses to form cloud. The parcel of air is
now saturated and must follows the SALR, to the inversion at 5200’.
This is a day of short lived shallow clouds, before going blue.
Meteorologists make it difficult !
That fairly simple graph of temperature against altitude is made difficult
by the professionals, and is called a Tephigram or a Skew-T plot.
•The temperature gridlines are rotated through about 45 O clockwise.
•Altitude is plotted as pressure, at lower levels 1mb = 30ft, so 850mb is
about 5000ft.
•Whole families of DALR and SALR are plotted.
•Moisture content lines are added at lower levels.
•A wind speed and direction indication is added.
Meteorologists make it difficult !
A Tephigram or Skew-T plot
Rules of thumb
General rules to spot a good thermal soaring day (but we all know
that rules don’t apply in Devon!):
•The passage of a cold front 24-36 hours earlier - Instability
•A ridge of high pressure, anticyclonic curvature - Stability above
•North westerly wind < 10knots - cold, clear airmass
•Clear night, wet grass - low dew point, high cloudbase
•Cloudbase = (Tmax – Tmin) x 400ft
Further study
For a much deeper, but fairly simply, explanation of the analysis of
Tephigrams visit Weatherjack’s tutorial page at:
www.weatherjackwx.co.uk/tutorials/tut-soundings/tut-snds-01.html