Download HERE

EG1204: Earth Systems: an introduction
Meteorology and Climate
Lecture 5
Atmospheric Instability
Topics
•
•
•
•
Introduction
Stable air
Unstable air
Precipitation processes
• Field Day Reminder
Introduction
• If a parcel of air expands and cools, or
compresses and warms, with no
interchange of heat with its
surroundings, this situation is called an
adiabatic process
• As long as the air in the parcel remains
unsaturated, the rate of adiabatic
cooling or warming remains constant
Introduction
• This rate of heating and cooling is
about 10°C for every 1000m of change
in elevation and applies to unsaturated
air only - it is therefore referred to as
the dry adiabatic lapse rate (DALR)
Introduction
• As rising air cools it will become saturated
when the dew-point is reached. Further lifting
is accompanied by the release of latent
energy into the rising air. Because the latent
heat added offsets the cooling due to
expansion, the air no longer cools at the
DALR - but at a lesser rate called the moist
adiabatic lapse rate (MALR)
Introduction
• Unlike the DALR, the MALR is not constant - it varies
greatly with temperature and hence with moisture
content
• Warm saturated air produces more liquid water than
cold saturated air
• The added condensation in warm saturated air
releases more latent heat - so the MALR is much less
than the DALR when the rising air is warm - but the
two rates are nearly the same when the rising air is
very cold
Introduction
• An average temperature value for the MALR is about
6°C per 1000m
• We can determine the stability of the air by comparing
the temperature of a parcel of air with its surroundings
• Rising air which is cooler than its surroundings will be
more dense and so will tend to sink back to its original
level. In this case the air is stable because it resists
upward displacement
• Rising air that is warmer will continue to rise until it
reaches the same temperature as its environment this is unstable air.
Stable Air
• A subsidence inversion may also occur when a
large layer of unsaturated air sinks (subsides)
and warms by adiabatic compression.
• The sinking layer becomes compressed by the
weight of the atmosphere
• The top of the layer becomes warmer than the
base - thus forming stable conditions
• Such inversions may occur at the surface, but
more frequently aloft associated with high
pressure areas
Stable Air
• It is worth mentioning that sometimes the
lapse rate is exactly equal to the dry
adiabatic rate - this is known as neutral
stability
• Rising or sinking unsaturated air will cool
or warm at the same rate as the air
around it.
Precipitation processes
• We have already seen how important
condensation nuclei are for the formation of
droplets when the air becomes saturated
• To keep a droplet in equilibrium, more water
vapour molecules are needed around it to
replace those that are constantly evaporating
from the surface
Precipitation processes
• Small cloud droplets have a greater curvature
which causes a more rapid rate of
evaporation. As a result of this process
(curvature effect) smaller droplets require
an even greater vapour pressure to keep
them from evaporating away. This requires
the air to be supersaturated - with a
relative humidity greater than 100%. The
smaller the droplet, the greater the
supersaturation needed to keep it in
equilibrium
Precipitation processes
• So - how do droplets with a diameter of <1µm grow
to the size of a cloud droplet?
• The answer lies with the cloud condensation nuclei.
Many of these nuclei are hygroscopic (have an
affinity for water vapour)
• Condensation may begin when the vapour pressure is
much lower than the saturated vapour pressure
• This reduces the equilibrium vapour pressure
required and is known as the solute effect
Precipitation processes
• In warm clouds (tops warmer than -15ºC) the action
of collisions between droplets is important
• Random collisions with already large droplets
mediated by salt particles (hygroscopic condensation
nuclei) produce larger droplets when they collide
• Large droplets begin to reach terminal velocity and
collide with smaller droplets in their wake - merging
together in a process called coalescence
• Falling droplets may evaporate on their way down, or
reach the ground as drizzle if the air below is moist
Precipitation processes
• In very deep convective clouds the ice-crystal process is an
important factor in precipitation
• Ice crystals may form nuclei upon which other ice crystals may
form
• These are deposition nuclei as water vapour changes directly
into ice without passing through the liquid phase
• The constant supply of moisture to an ice crystal allows it to
enlarge rapidly, it becomes heavy enough to overcome updrafts
and begins to fall
• If these crystals stick together (accretion) the icy matter
(rime) that forms is called graupel (or snow pellets).
As part of the EG1204 unit structure, all students
MUST participate in a Meteorology Field Data
Collection Exercise. The data collected during this field
component will comprise part of the analysis required
for Assignment 2.
The fieldwork will take place on:
Thursday 22nd February
The location for the fieldwork will be
All Saints Park
(directly outside the Library and next to All Saints
building off Oxford Road).
In order to discover when YOU are required to attend
throughout the day, you must check the online GROUP
LIST. There are 20 groups in total. Students MUST
attend the slot to which they have been assigned—this
cannot be made flexible as there will not be enough
equipment to support additional numbers in different
slots. Check the website for details of your timeslot
and check your university email account
You will be split into working groups of 6 people and
conduct a meteorological transect across All Saints
Park—collecting data for later analysis in Excel or SPSS
statistical packages.
Health & Safety
• All Saints Park is a PUBLIC SPACE
• Don’t lay measuring tapes across
pathway unmonitored
• Be prepared for rainfall and cold
weather
• DO NOT play with instruments or leave
them on the ground