2 AimTo provide a sufficient understanding of cloud and precipitation
3 Objectives Understand background relevant to cloud formation Describe stability of the atmosphereState how cloud is formedDefine different types of cloudDescribe cloud dissipation processes
4 1. Background Overview on cloud As air rises from the surface, the water vapour it contains is not the only thing is carries with itAlong with water vapour, there are microscopic particles such as salt grains, carbon particles. Also present are pollen, spores of various planes and even virusesThese are called condensation nuclei and are a vital link in the process of cloud formation.When dew point (discussed later) is reached in the rising air due to adiabatic cooling, they provide a great abundance of surfaces onto which condensation can occur.
5 1. Background Moisture in the atmosphere – 3 states of water Clouds are formed from water vapour in the atmosphere condensing into water dropletsThere are 3 states of waterGas (vapour)Liquid (water)Solid (ice)Water vapour is not visible in the atmosphere, but water in its liquid and solid states areExamples of water in its liquid states are cloud, fog, mist, rain, dew.Examples of water in its solid state are ice crystals, snow, hail, ice or frost
6 1. Background Moisture in the atmosphere – 3 states of water To change from one state to another state, there must be a transfer of heat energy.This heat energy is known as latent heat and is vital part of any change of state
7 1. Background Moisture in the atmosphere – relative humidity The amount of water vapour in the atmosphere is one of the most important factors in determining what weather is to be expected.The term relative humidity is a measurement which indicates how close the air is to saturation (ie 100%)RELATIVE HUMIDITY = VAPOUR PRESENT ÷ VAPOUR POSSIBLE X 100%
8 1. Background Moisture in the atmosphere – relative humidity When temperature changes, the amount of vapour possible is reduced as the air cools, therefore relative humidity changes even though the actual amount of water vapour present may be constant.
9 1. Background Moisture in the atmosphere – dewpoint The actual temperature at which saturation occurs depends on the amount of water vapour present in the first placeA parcel of air with a high relative humidity requires only a small amount of cooling to reach saturation, whereas a parcel with a low relative humidity would require a lot of cooling.The temperature at which saturation occurs is called dew point temperature.Dew point temperature is the temperature required to cause the relative humidity of a given sample of air to reach 100%
10 2. Air Stability Adiabatic processes An adiabatic process is when there is a temperature change without there being any heat transfer into or out of the system.When a gas is compressed, no additional heat is added, the heat that is already present is simply squeezed into a smaller volume, causing temperature to rise.
11 2. Air Stability Adiabatic processes When a parcel of air rises through the atmosphere it encounters lower pressure and expandsThis expansion causes adiabatic coolingBecause the rate at which pressure drops is fairly constant, the rate at which the temperature of the rising parcel cools is also constantProvided cloud doesn’t form, a parcel of air rising through the atmosphere will cool by expansion at a rate of 3°C/1000ft. This is called the Dry Adiabatic Lapse Rate (DALR)
12 2. Air Stability Adiabatic processes If the relative humidity is such that cloud forms within the rising parcel, the lapse rate within the rising parcel will now have changedThis is due to the release of latent heat of condensation within the cloudRising saturated air (cloud), cools at the saturated adiabatic lapse rate (SALR) of about 1.5°C/1000ft
13 2. Air Stability Environmental Lapse Rate (ELR) The actual change of temperature with an increase in height is called the Environmental Lapse Rate (ELR).The actual ELR is not uniform and may vary from place to placeThe difference between the DALR, SALR and ELR is the important factor in determining cloud height.A high ELR encourages warm air to keep rising promoting instability, these conditions can cause cloud to form to great heightsA lesser ELR may indicate a stable atmosphere.Stability in the atmosphere depends upon the value of the ELR.
14 2. Air Stability Stability Any parcel of air that is warmer than its surroundings must also be less dense.This parcel of air will rise until it meets air of the same temperatureLikewise a parcel of air that is cooler than its surroundings will be more dense therefore sink until it reaches air of equal temperature.The stability of the atmosphere determines the behaviour of these rising and sinking parcels.
15 2. Air Stability Stability The air is said to be stable when the rising or sinking air stops rising or sinking when it meets air of equal temperature of its own accordThe air is unstable when the rising or sinking air continues to rise or sink
16 2. Air Stability Stability Under ISA, a parcel of air generally changes by 2 degrees per 1000’This is known as Environmental Lapse RateSaturated air (air that is 100% humidity) cools slower at 1.5 degrees per 1000’, due to the presence of water causing it to release less latent heatThis is known as Saturated Adiabatic Lapse Rate (SALR)Unsaturated air (air that is not 100% humidity) cools at 3 degrees per 1000’This is known as Dry Adiabatic Lapse Rate (DALR)
17 2. Air Stability Stability – Dry adiabatic lapse rate Whenever the DALR curve lies below the ELR, dry air is stable
18 2. Air Stability Stability – Dry adiabatic lapse rate Whenever the DALR curve lies above the ELR, dry air is unstable
19 2. Air Stability Stability – Saturated adiabatic lapse rate Whenever the SALR lies below the ELR, saturated air is stable. Ie the SALR is greater than the ELR.
20 2. Air Stability Stability – Saturated adiabatic lapse rate Whenever the SALR lies above the ELR, saturated air is unstable. Ie, the SALR is less than the ELR.
21 2. Air Stability Stability – Absolute stability and instability The shallow ELR produces absolute stabilityThe steep ELR produces absolute instability
22 3. Cloud Formation Clouds formed by convection The process of convection is due to warm air rising as it is less dense than cold air which sinks because it is more denseWhen the sun shines, the ground is warmedThe surface layer which contains moisture is heated so therefore rises.As this parcel of air rises, it coolsWhen it reaches a the dew point temperature, cloud forms.
23 3. Cloud Formation Clouds formed by orrographic uplift When an air mass hits a mountain range or raised ground it is forced to rise.As it rises, it cools.If it rises and cools to the saturation point, condensation occurs and the water contained within the air becomes visible as cloud.
24 3. Cloud Formation Clouds formed by turbulence and mixing When air flows over the surface of the earth, frictional effects from the surface cause variations in wind direction and strengthThe strength of the turbulence is dependent upon wind strength and the roughness of the surface.The stronger the wind, and the rougher the surface, the stronger the turbulenceIf the air in the rising currents cools to the dewpoint temperature, water vapour will condense to form liquid water droplets and cloud will form
25 3. Cloud Formation Clouds formed by frontal uplift When cold air and warm air meet, the heavier cold air remains at the surface forcing the lighter warm air to rise over it.
26 4. Cloud Naming and Identifying Cumuliform and StratiformClouds can be grouped into 2 different types of cloudThese two groups are cumuliform and stratiform and they are classified according to their temperatureWhen the cloud is warmer than the environment it is called cumuliform or cumulusWhen the cloud temperature is equal to, or cooler than the environment air it is called stratiform or stratus.
27 4. Cloud Naming and Identifying Cumuliform identificationA cumulus cloud is warmer than the surrounding airSince cumulus clouds form in a bubble of warm air, the cloud takes on the appearance of a big fluffy cell, usually with a fairly flat base
28 4. Cloud Naming and Identifying Stratiform identificationA stratus cloud is cooler and therefore heavier than the environment airBecause a stratus cloud is colder, therefore heavier, it requires an updraft to keep it in placeBecause of this, no part of the stratus cloud rises further than it needs to therefore has a fairly flat shape.
29 4. Cloud Naming and Identifying Stratiform identification - NimbostratusA nimbostratus cloud is characterised by a formless layer which is almost uniformly grey.“Nimbo” comes from the latin word “nimbus” which means precipitation or rain.Nimbostratus cloud generally forms from altostratus in the middle range then subsides to the lower range during precipitation.Precipitation in Nimbostratus is generally moderate to heavy and can last several days
30 4. Cloud Naming and Identifying Classification according to heightApart from their shape or form, clouds are also classified according to height.This height is the base of the cloud above mean sea level.For the purposes of cloud classification, the atmosphere is divided into 3 layers:LowMiddleHigh
31 4. Cloud Naming and Identifying Classification according to height – low cloudWhen the cloud base is between sea level and 8000ft, the cloud is classified as low cloud.Clouds such as cumulus, stratus, stratocumulus, nimbostratus and cumulonimbus are some examples of low cloud
32 4. Cloud Naming and Identifying Classification according to height – middle cloudWhen the base is between about 8000’ and 18000’, it is classified as middle cloud.The prefix alto is added to distinguish itSome examples of middle level cloud are altocumulus and altostratus
33 4. Cloud Naming and Identifying Classification according to height – high cloudWhen the cloud base is above 18000’ it is classified as high cloudThe prefix Cirro is added to distinguish itCirrocumulus and cirrostratus are some examples of high level cloudThese clouds are composed of ice crystalsThe exhaust gasses from aircraft engines contain water vapour which condense to form cirrus cloud. These are called contrails.
34 4. Cloud Naming and Identifying OktasThe term “Oktas” is used to describe how much cloud is in the skyIf we break the sky into eighths, we can say how much cloud is in the skyOktasDefinitionCategorySky clearFine11/8 of sky covered or less, but not zero22/8 of sky covered33/8 of sky coveredPartly Cloudy44/8 of sky covered55/8 of sky covered66/8 of sky coveredCloudy77/8 of sky covered or more, but not 8/888/8 of sky completely covered, no breaksOvercast
35 4. Cloud dissipation processes SubsidenceWhen air ceases to rise, cloud particles are free to settle into warmer air where they can evaporateIf the air subsides it will be subject to adiabatic warmingThis will also cause the cloud particles to evaporate
36 5. Cloud dissipation processes PrecipitationWe have learnt cloud is composed of waterPrecipitation removes water from inside the cloudEventually, the water content of the cloud reduces to a point in which the cloud becomes unsaturated, and the remaining cloud particles are free to evaporate.Precipitation can be classified into different categories according to the different states in which the water is fallingRain (liquid water drops)Drizzle (fine water droplets)Snow (ice crystals)Hail (small balls of ice)Freezing rain (liquid drops that freeze on contact)Dew or frost
37 5. Cloud dissipation processes VirgaVirga is the name given to precipitation such as rain or snow that evaporates before hitting the ground.A extreme hazard of a microburst can be a result of virga due to the latent heat being absorbed from the air.This absorption of latent heat creates a parcel of air that may sink, or even plummet quite rapidly toward the ground.
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