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Weather & Climate LECTURE 2

Weather & Climate LECTURE 2. Moisture in the Atmosphere. Evaporation and Condensation: accompanied by absorption/liberation of heat evaporation: energy absorbed when water increases in temp, and when it changes from a solid to liquid, and from liquid to a gaseous state

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Weather & Climate LECTURE 2

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  1. Weather & Climate LECTURE 2

  2. Moisture in the Atmosphere • Evaporation and Condensation: • accompanied by absorption/liberation of heat • evaporation: energy absorbed when water increases in temp, and when it changes from a solid to liquid, and from liquid to a gaseous state • condensation: energy lost when water decreases in temp, and when changes from a gaseous to a liquid state, and from a liquid state to a solid state

  3. Moisture in the Atmosphere Today’s lecture emphasis: - cooling and condensation - dependent on the amount of moisture in cooling air - saturated vs unsaturated

  4. Water Content Indices Exam questions based on this are common. Be sure to familiarise yourself with these indices 1. Vapour Pressure 2. Humidity: - absolute - specific - relative 3. Dew point/Condensation Level

  5. Water Content Indices Vapour Pressure - that part of the total atmospheric pressure due to water vapour - max amt of water vapour air can hold at a specific temp = saturation vapour pressure - S.V.P is dependent on temperature - higher temp, more moisture, therefore higher S.V.P

  6. Water Content Indices Humidity: Absolute Humidity - density of water vapour (weight per unit volume of air - g/m³ - changes when air expands or contracts - all things remaining constant, absolute humidity falls when an air parcel expands

  7. Water Content Indices Humidity: Specific Humidity - weight of water per unit mass of air (g/kg) - does not change as air expands or contracts - therefore not temperature dependent (holding all things constant)

  8. Water Content Indices Humidity: Relative Humidity - ratio of water vapour to max possible at the current temperature - (specific humidity/saturation specific humidity) x 100%

  9. Water Content Indices Relative Humidity Short-coming: - confusing to compare RH of air of different temps because: - air in area X with with temp of 30 deg C with a RH of 50% may contain 16g of water - air in area Y with a temp of 4 deg C with a RH of 50% may contain only 2g of water - Therefore, RH not a good measure to compare absolute quantities of moisture in the air between 2 areas - Better way is to use vapour pressure

  10. Water Content Indices Dew Point - temp to which air must be cooled to reach saturation - saturation: point where condensation occurs ie Condensation point/level - if saturation occurs below 0 deg C, it is known as the frost point

  11. Lapse Rates Lapse Rate: Rate at which temperatures decrease with increasing altitude Before moving on to lapse rates, we have to understand 2 concepts: 1) Diabatic Process 2) Adiabatic Process

  12. Lapse Rates Diabatic Process: - involves addition/removal of energy from a system - boiling water - air cooling as it moves over a cold surface

  13. Lapse Rates Adiabatic Process: - where temp changes without addition or removal of heat - according to the gas laws - air cools when it expands, heats up when compressed

  14. Lapse Rates This leads us to 1) Dry Adiabatic Lapse Rate [DALR] 2) Saturated (Wet) Adiabatic Lapse Rate [SALR]

  15. Lapse Rates 1) Dry Adiabatic Lapse Rate [DALR] - Rate at which a RISING parcel of unsaturated air cools - about 10 deg C for every 1000m of ascent

  16. Lapse Rates 2) Saturated Adiabatic Lapse Rate [SALR] - when air reaches the condensation level, it becomes saturated - continues to cool at a slower rate. Why? - some of the heat loss is used to convert water vapour into condensation (clouds/ice) - SALR about 5 deg C per 1000m of ascent

  17. Altitude SALR Condensation Level DALR Temperature Lapse Rates

  18. Lapse Rates Introducing… The Environmental Lapse Rate - vertical change in temperature through still air - 6.5 deg C per increase in 1000m - it is variable: changes from day to day, place to place, altitude to altitude

  19. Lapse Rates The ELR determines a parcel of air’s stability If a parcel of air within an air mass is heated locally (eg forest fire), its static stability is determined by the ELR Static stability: the parcel of air’s susceptibility to uplift

  20. Lapse Rates Static Stability Statically unstable air: continues to rise given an initial upward push - occurs when density of a parcel of air is less than the surrounding environment (imagine a helium-filled balloon) Statically Stable: resists upward displacement, sinks back to original position once heating stops - when density of air parcel is more than that of the surrounding

  21. Lapse Rates and Adiabatic Lapse Rates When looking at rising parcels of air, we need to consider: 1) Dry 2) or Saturated ? - determines the lapse rate at which it will rise 3) The ELR

  22. Lapse Rates and Adiabatic Lapse Rates These combinations will determine air parcels of: 1) Absolute Instability/Unstable Air 2) Absolute Stability/Stable Air 3) Conditional Instability/Conditionally unstable Air

  23. Height SALR ELR DALR Temp Instability/Absolutely Unstable For Instability to occur, ELR> DALR & SALR

  24. Instability/Absolutely unstable When ELR>DALR, - rising parcel cools at a slower rate than surrounding - hence gets progressively warmer in comparison to surrounding Ht SALR ELR C DALR Temp - unstable because: - even when heating of the parcel stops, it continues to rise (due to difference in density) - rise at an increasing rate as temp difference between the air parcel and surrounding increases

  25. Instability/Absolutely Unstable When ELR>SALR - air parcel cools even more slowly (energy used due to condensation) - temperature differences even greater - rate of rise therefore increases at an increasing rate - instability increases Ht SALR ELR C DALR Temp Therefore, when ELR>DALR, SALR, instability occurs and the air parcel continues to rise

  26. Stability/Absolutely Stable When ELR<DALR, SALR Height SALR ELR DALR Temp

  27. Stability/Absolutely Stable Ht When ELR<DALR - rising parcel of unsaturated air cools more rapidly than surrounding air - becomes relatively denser - once heating stops, will sink to original position ELR SALR C DALR Temp When ELR<SALR, - saturated air cools at SALR - still remains colder than surrounding - tends to sink to original position when heating stops

  28. Height SALR Condensation Level ELR DALR Temp Conditionally Unstable When ELR is between DALR and SALR - and dependent on whether there is heating beyond the level of free convection Qn: What happens as air rises at the ELR depicted?

  29. Height SALR Level of free convection Condensation Level ELR DALR Temp Conditionally Unstable Qn: What happens within the yellow section as the air parcel cools at the SALR? - will remain stable, sink to original position - if heating continues will eventually rise to equal ELR

  30. Height SALR Condensation Level ELR DALR Temp Conditionally Unstable What happens when air parcel continues to rise above LFC at SALR? - cools slower (hence warmer) than atmosphere - less dense than atmosphere, so rises readily - easily forms clouds Level of free convection

  31. Factors Affecting ELR The ELR is not constant, but can vary according to: - Time of Day/Amt of Insolation - Advection (Lateral Movement) of Cold/Warm air at different levels - Advection of an air mass with a different ELR

  32. Limits to Rising Unstable Air Does unstable air ever stop rising YES. Otherwise the earth’s atmosphere will be replaced by a vacuum. - unstable air will usually eventually rise to a layer of stable air - if not, mechanism of entrainment will limit the rise

  33. Air Inversions In general, temperatures decrease with elevation in the troposphere - reverse can happen: temperatures can increase with height in troposphere - situation known as ‘inversion’ - extremely stable, rising air experiences negative buoyancy, resists vertical mixing

  34. Air Inversions Ht Less Warm Air Troposphere Warm air Inversion Layer Cool air Temp Ground Conditions: - calm, clear, anti-cyclonic conditions - rapid terrestrial radiation at/near ground level

  35. Air Inversions Air inversions set up conditions for the formation of - dew - frost - frost dew And of special interest, Mists and Fog - Radiation Fog - Advection Fog NB: The third type of fog in your notes, Upslope Fog, is not a result of temp inversion, but more so due to the adiabatic process due to a decrease in pressure

  36. Condensation and Cloud Formation 3 main mechanisms of cloud formation: - Orographic Uplift - Frontal Lifting - Localised Convection Form different types of clouds - high clouds - middle clouds - low clouds - clouds with vertical development

  37. Condensation and Cloud Formation HOMEWORK: Produce a set of notes on the mechanisms of cloud development and cloud types. Hand up during first lecture in Term 2 for checking - Those who fail to do so will stay back on Fri afternoon to complete it END

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