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Lecture 1 Topic 1 The atmosphere: radiation and moisture Earth Systems 2: The Hydrosphere ENS00203 Dr Kathryn Taffs School of Environment, Science and Engineering Southern Cross University Lecture outline • • • • • • • • Some basic definitions The atmosphere Solar radiation Electromagnetic spectrum Atmospheric influences atmospheric interaction reflection absorption scattering net radiation balance Atmospheric moisture • • • Temperature • Temperature is: • the amount of energy (or heat) held by any substance • It is measured in: • Celius, Farenheit or Kelvin Air pressure • The pressure exerted by the air above it. • Force exerted against a surface by the collision of gas molecules. Measuring air pressure Mercury barometer Aneroid barometer Change of pressure with elevation Higher ……………………. Lower The Atmosphere • Most weather occurs in the tropopause ~10,000 metres above the Earth’s surface. Energy drives our world • • • The energy for life on Earth, and to drive the atmospheric circulation, comes from the Sun. Hence we need to understand the structure and composition of the atmosphere to understand how solar energy effects life on the surface of Earth. Solar radiation • Average temperature of the Sun’s surface is 6000 K • The Sun emits energy known as electromagnetic radiation. Electromagnetic radiation • Electromagnetic radiation travels in a wave pattern • Each wave has a different length between the troughs and crests respectively Electromagnetic spectrum • The electromagnetic spectrum can be broken into segments according to the wavelength Atmospheric influences • There are a number of factors that affect the solar constant (how much energy reaches the top of the atmosphere) • solar output • solar distance • angle of incidence • Length of day Solar output • Energy from the sun varies over time • this is related to sun spot activity • Sun spots are dark areas of the sun Sun spot activity on 20th August (from http:// www.lmsal.com/YPOP/ ProjectionRoom/ latest_SXT_full.html) associated with increased energy output, particularly in the ultraviolet wavelengths • this is a minor influence on the amount of radiation received by the atmosphere Solar distance • Amount of radiation varies according to distance between the Sun and the Earth which varies seasonally as the Earth’s orbit is elliptical Angle of incidence tilt of the Earth has a • The more dramatic effect on the amount of radiation received at the top of the atmosphere higher the latitude the • The less radiation received because of the greater surface area over which it is spread the thicker the • (and atmosphere it must travel through) • The greater the angle of insolation, the greater the surface area over which it is spread • This significantly affects the amount of radiation received Length of day • The tilt of the Earth’s axis also affects the length of day. • This obviously affects the duration that solar radiation is received Atmospheric interaction with radiation • Once radiation reaches the top of the atmosphere it may be: • scattered • reflected, or • absorbed From: www.teoptics.kulgun.net/Blue-Sky/xt Scattering • Air molecules and fine dust in the path of insolation deflects it. • When the particles are < the wavelength it is known as Rayleigh scattering. • When the particles are > the wavelength it is known as Mie scattering. Absorption small part of insolation is • Aabsorbed by atmospheric • gases amount absorbed depends of the wavelength and the gas type • For example: and ozone absorb • oxygen small wavelengths vapour and carbon • water dioxide absorb longer wavelenths Reflection insolation interacts with the surface • Finally, of the Earth reaching the surface is either • Radiation absorbed or reflected. • The proportion reflected is the albedo varies according to the nature of • Albedo the material, its colour, its roughness, whether it is wet or dry and the angle of incidence of the insolation Global energy budget • Scattering, reflection and absorbance all occur to differing degrees Global energy budget • and the long wave radiation ... Spatial variations of insolation • The amount of insolation received varies spatially across the globe. It is primarily determined by: • latitude, and • cloud cover Effect of latitude • January Effect of latitude • July Cloud Cover • Clouds have a high albedo reflecting about half of radiation received • Clouds are predominantly concentrated in the equatorial belt. • Hence maximum insolation received at the earth’s surface actually occurs in the subtropics at 20°N and 20°S Global distribution of insolation Australian solar radiation From: http://www.bom.gov.au/climate/averages/climatology/solar_radiation/IDCJCM0019_solar_exposure.shtml Balancing global heat • There is a surplus of heat in the equatorial belt (more radiation received than lost by cooling) • At the poles more radiation is lost by cooling than that received • To balance the global heat balance 2 processes occur: • • latitudinal heat transfer vertical heat transfer • These two processes and the basic cause of all of our weather • leading onto topic 2 on global winds Atmospheric moisture • Convection is the vertical movement of air. • It transfers heat from the warm ground surface upwards. • Clouds form where the air temperature equals the dew point temperature Properties of atmospheric moisture: definitions GAS Motion + collisions => partial pressure partial pressure the pressure applied by the collision of gas particles in the atmosphere vapour pressure the pressure applied by the collision of water vapour molecules in the atmosphere • • Properties of atmospheric moisture: definitions saturated • mass of air holding the maximum amount of water vapour at a given temperature unsaturated • when a mass of air is containing less than the saturated amount. Properties of atmospheric moisture: definitions • • • • absolute humidity mass of water vapour in a unit volume of air. relative humidity percentage ratio between the actual vapour pressure and the saturation vapour pressure mixing ratio mass of water vapour per unit mass of dry air dew point temperature temperature at which air must be cooled for saturation to occur Measuring humidity • Humidity is measured using wet and dry bulb thermometers. • The difference in temperature between the 2 values enables us to calculate relative humidity. • In the prac we used Tables, there are also conversion programs available, eg. www.bom.gov.au/lam/ humiditycalc.shtml Daily Relative Humidity The amount of water vapour the air can hold increases with temperature. Condensation For condensation to occur: • air must be saturated (cooled to the dew point) • there must be a surface to condense on (condensation nuclei) • a trigger for the process such as a front or orographic uplift. Lapse rates • • • The rate at which air temperature changes with height is known as the environmental lapse rate (ELR). • 0.65 °C per 100 m When that air is unsaturated this is known as the dry adiabatic lapse rate (DALR) • 1 °C per 100m When that air is saturated it is known as the wet adiabatic lapse rate (WALR) • 0.64 °C per 100m Aerological diagrams Wind speed Height above and ground surface in HPa, and ft and m direction Temperature Temperature Dew point temperature Previous balloon release date and time Balloon release date and time Orographic uplift • As air parcel rises, it cools. • When dew point is reached, condensation occurs • results in cloud development Calculation of cloud level 2500m-1600m=900m, 9x0.64ºC=5.24ºC, thus temperature at C=15ºC-‐5.24ºC=9.6ºC 600 m 25ºC-15ºC=10ºC, @1ºC/100m = 1000m, hence clouds form at 1000+600m=1600m We can calculate the cloud base level and air temperature of a moving air mass using the adiabatic lapse rates Inversions • Occasionally, at some altitudes the temperature abruptly begins to increase with height. • This occurs if a warm layer of air overlies a colder layer • After a short vertical distance the temperature in the warm layer will begin to cool again Condensation: 1. Clouds Latin derivations: • cumulus = “heap” to describe a puffy cloud • cirrus = “curl of hair” to describe a wispy cloud • stratus = “layer” to describe a sheet like cloud • nimbus = “violent rain” to describe a rain cloud Cloud formation • There are 4 main ways that moist air can be lifted to form clouds Orographic uplifting Convective lifting Interaction of air masses Mechanical turbulance Cloud categories • • • • • There are 10 principal cloud types: High clouds; cirrus, cirrostratus, cirrocumulus Middle clouds; altostratus, altocumulus Low clouds; nimbostratus, stratocumulus, stratus Clouds with vertical development; cumulus, cumulonimbus High level clouds Cirrus Cirrocumulus Cirrostratus No precipitation from high level clouds Mid level clouds Altocumulus May produce light showers and if appears on a summer morning may indicate a thunderstorm in the evening Altostratus Often results in precipitation and sometimes snow Low level clouds Cumulus Showers of rain or snow Stratocumulus Often results in drizzle Stratus Often results in drizzle Multi-layer clouds Nimbostratus Heavy rain or snow Cumulonimbus Thunderstorms, lightning, squalls, heavy rain Condensation 2. Fog Fog Cloud formation at sea level Smog Mixture of fog and industrial pollution Dew Condensation on cold surfaces such as glass blades Frost Formation of ice crystals instead of dew Fog There are 2 main types of fog: Advection fog: air flows from warmer to colder area and reaches dew point Radiation fog: Moist air in contact with cooling land surface to reach dew point Practical 1 • Water vapour in the atmosphere • relative humidity, dew point • environmental lapse rates • wet and dry adiabatic rates • Cloud classification • Climate Assignment Climate Assignment • Bring lap top or mobile device • MyGrades for BoM station • Instructions in practical book pracs 1-4 • Podcasts and help files in MySCU, Assignment Resources • Reading 1.1 Readings •Lutgens, F.K. and Tarbuck, E.J. 2007. Heating Earth’s Surface and Atmosphere. In, The Atmosphere. Pearson Prentice Hall, New Jersey. Pp. 32-63. •Reading 1.2 Tarbuck, E.J and Lutgens, F.K. 2009. Moisture, •Clouds and Precipitation. In, Earth Science. 12th Ed. Pearson, Prentice Hall, New Jersey. Pp. 477-511. •Quiz 1 Topic 1 Quiz available through Assessment •Resources