Download 12A Monday

Monday’s lesson
(At the end the lesson you will be able to…)
Describe the changes in temperature with height through the
lower layers of the atmosphere.
Explain what an adiabatic lapse rate is.
Explain why and how the SALR is different from the DALR
(Lapse rates are often shown in diagram form in exam questions)
Environmental lapse rate (ELR)
This is the expected (average) decrease in temperature with
height through the lower atmosphere, approximately 0.65
degrees per 100m.
The ELR varies according to height, time of year, and over
different surfaces.
Adiabatic means the rising and sinking of air.
So an Adiabatic process is anything that occurs as air
rises or sinks.
As air rises and cools, the
water vapour in the air
condenses back into water
droplets
• When a parcel of air holds the maximum amount of moisture it can it
is called saturated.
• The point at which a parcel of air is saturated is known as dew point
• The altitude at which dew point is reached is known as condensation
level
orographic uplift
Adiabatic lapse rate:
Used to explain what occurs as a parcel of air rises, decreases in
pressure and temperature, but increases in volume. The conditions are
reversed if a parcel of air moves towards Earth.
Adiabatic lapse rates can be either dry (air) or saturated (air)
Dry adiabatic lapse rate (DALR):
This is the rate a
parcel of air cools at as
it rises (or warms if
falling) if condensation
does not occur. The
rate is approximately
1 degree per 100m
and is shown on the
diagram
Saturated Adiabatic Lapse Rate (SALR):
The rate at which air cools if it has risen sufficiently to reach dew point
temperature, and condensation occurs.
The rate of cooling is slower than the DALR because of the release of
latent heat. The SALR varies from 4 degrees per 1000m to 9 degrees
per 1000m but the average is 5.5 degrees.
The reason for the variation is because of the ability of warmer air to
hold more moisture and so it release more latent heat after
condensation.
When does a parcel of air stop rising?
Air will continue to rise and cool until it reaches the same temperature
as the surrounding air. (of course)
This marks the top of cloud development.
Stability 1
Stability
This is where a parcel of air rises and cools at a quicker rate than the air
surrounding it.
The parcel of air is colder and denser than its surroundings so cannot
rise further and sinks.
Condensation and then subsequent rainfall does not occur. Stability is
most commonly associated with high pressure and anticyclones.
Stability 2
The diagram
below shows a
graph of stable
air conditions:
(The DALR lies to
the right of the
ELR).
Instability
Unstable conditions (instability) in the atmosphere exist when a rising parcel
of air cools more slowly than the air surrounding it
On warm summer day's high levels of insolation can create high surface
temperatures.
The air above such localised surfaces is then heated by conduction, leading
to a high lapse rate.
The air rises and cools less quickly than its surroundings.
If it remains warmer than its surroundings, the air parcel will continue to
rise.
If dew point is reached, clouds and thunderstorms will happen
We got up to here yesterday!
So………unstable air:
• Air continues to rise even though it has left the (warm) ground
• Vertical currents dominate.
• Cumulonimbus clouds and thunderstorms.
Summary
• SALR:
If rising air is saturated, its temperature falls at about 6 degrees per 1000m.
Latent heat is released due to condensation, so decrease is slower.
• ELR:
Temperature of air decreases with height at an average rate of 6.5 degrees
per 1000m.
• DALR:
When a mass of air rises, its temperature decreases adiabatically. The air
expands and loses heat energy as it rises and if it is unsaturated, it loses 10
degrees for every 1000m of ascent.
FINALLY – 2 MORE
Conditional instability
This occurs where the continuation of air rising to form
condensation, clouds and precipitation, is conditional on
something (for example, mountains and fronts).
The state of instability is based on the air being forced to
rise initially. Then it passes over the mountain and
returns to stability.
Temperature inversions
Temperatures usually decrease with height through the
lower atmosphere (troposphere).
A temperature inversion is when the normal conditions
are reversed and warm air lies above cold air. It acts to
trap air in the lower layers, and can cause the build up of
pollutants and smog as in Los Angeles.
Weather phenomena
associated with local energy
budgets (mist, fog, dew,
temperature inversions, land
and sea breezes).
At the end of our third lesson of the week
1. I will know the three states in which moisture can
be found in the atmosphere and understand the
terms condensation, evaporation, sublimation,
freezing and melting
2. You will be able to explain the formation of
weather phenomena associated with local
energy budgets (mist, fog, dew, temperature
inversions, land and sea breezes)
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Environmental Lapse Rate
SALR
DALR
Stability
Instability
Condensation level
Troposphere
Dew point
Adiabatic
Saturated
Conditional instability*
Temperature inversion*
Environmental Lapse Rate
SALR
DALR
Stability
Instability
Condensation level
Troposphere
Dew point
Adiabatic
Saturated
Conditional instability*
Temperature inversion*
Spring Assessment
1. Draw a labelled diagram to show the ‘night model’ of the radiation
balance in the local energy budget. (4)
2. Describe and explain two ways in which the local energy budget might
be different during the day. (4)
3. What effects does cloud cover have on the earth’s energy budget. (6)