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 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