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 associated with increased energy Sun spot activity on 20th August (from http:// www.lmsal.com/ypop/ ProjectionRoom/ latest_sxt_full.html) 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 The tilt of the Earth has a more dramatic effect on the amount of radiation received at the top of the atmosphere The higher the latitude the less radiation received because of the greater surface area over which it is spread (and the thicker the 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 A small part of insolation is absorbed by atmospheric gases amount absorbed depends of the wavelength and the gas type For example: oxygen and ozone absorb small wavelengths water vapour and carbon dioxide absorb longer wavelenths
Reflection Finally, insolation interacts with the surface of the Earth Radiation reaching the surface is either absorbed or reflected. The proportion reflected is the albedo Albedo varies according to the nature of 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 mixing ratio moisture: definitions absolute humidity mass of water vapour in a unit volume of air. relative humidity mass of water vapour per unit mass of dry air dew point temperature percentage ratio between the actual vapour pressure and the saturation vapour pressure 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
Height above ground surface in HPa, and ft and m Wind speed and direction Aerological diagrams 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 Interaction of air masses Convective lifting 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
Readings Reading 1.1 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