EG1204: Earth Systems: an introduction

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EG1204 Earth Systems an introduction

• Meteorology and Climate
• Lecture 5
• Atmospheric Instability

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Topics

• Introduction
• Stable air
• Unstable air
• Precipitation processes
• Field Day Reminder

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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

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Introduction

• This rate of heating and cooling is about 10C
  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)

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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)

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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

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Introduction

• An average temperature value for the MALR is
  about 6C 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.

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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

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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.

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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

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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

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Precipitation processes

• So - how do droplets with a diameter of lt1µm 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

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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

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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).

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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).
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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 assignedthis 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 Parkcollecting data for later analysis in
Excel or SPSS statistical packages.
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Health Safety

• All Saints Park is a PUBLIC SPACE
• Dont lay measuring tapes across pathway
  unmonitored
• Be prepared for rainfall and cold weather
• DO NOT play with instruments or leave them on the
  ground