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Agricultural Science Climatology Semester 2, 2007 Richard Thompson http://www.physics.usyd.edu.au/Ag/agschome.htm Course Coordinator: Mike Wheatland Chapter 4 Water in our Atmosphere Water in the atmosphere (1) • Water regulates temperature of plants & animals because of the large latent heat of vaporisation. • How water vapour (in air) moves depends on its density, which is related to its temperature. • A mass of air that is less dense than its surroundings will rise in the atmosphere. • A mass that is more dense will sink. • This is the effect known as buoyancy. Water in the atmosphere (2) • Archimedes Principle – object feels a buoyancy force equal to the weight of the fluid (liquid or gas) it displaces. • A body floats (on water) if its weight equals the weight of the water displaced. This is how a duck floats. This stone can ‘t displace enough water to equal its weight so it sinks if the string breaks. What role does pressure play? • Pressure (p) at a given level in a liquid or gas is the same, but it decreases as you rise in the atmosphere (less weight of air above you). • Result is a net force on a body because of the different pressure. This is the buoyancy force. • To calculate changes in pressure: p = po - ρgh (po is at sea level; h is altitude) This assumes gas is not compressible! Only good for small heights, small changes The Barometric Formula (1) P = P0 exp(-h/H) where H = kT/(mg) If we take: k=1.38x10-23 kg m2 s-2 K-1 ; g=9.8 m s-2 ; m=29x1.67x10-27 kg; T=300K; then H = 87,000 (8.7 km) This is ‘scale height’ at which pressure falls to 1/e (37%) of its surface value Compare: Mt Everest is 8.85km high This method comes from the Ideal Gal Law and assumes constant Temperature – also an imperfect model! The Barometric Formula (2) Pressure vs Height 1200 Pressure (mb) 1000 800 600 400 200 0 0 2 4 6 Height (km) 8 10 12 Atmospheric Pressure at Sea Level • Standard atmospheric pressure is defined as 101,325 Pascals (1 atm) which is actually a typical sea level pressure for mid latitude regions on Earth • Pressure is force per unit area. A Pascal is a Force of 1 newton/square metre. A mass of 1 kg has a weight of 9.8 newtons • Synoptic (weather) charts show pressure in units of hectopascals (hPa). Another unit is millibar and 1 hPa = 1 millibar • 1 atm = 1013.25 hPa Weather Map for July 13, 2006 Some thermodynamics • Ideal gas equation generally holds: PV = nRT • If 2 masses of gas have same temperature, volume and pressure, they must contain the same number of molecules – water molecules weigh less than oxygen or nitrogen, so moist air is less dense than dry air. • Remember: it is also true that warm air is less dense than cold air. PV diagrams show how the different processes change the properties of gas ADIABATIC – no heat transfer ISOTHERMAL – constant temperature ISOBARIC – constant pressure ISOCHORIC – constant volume Work = PΔV Dynamics in the atmosphere (1) • Warm air is less dense – it rises • Cool air is more dense – it falls • Most changes occur adiabatically – no heat is transferred during process • If air rises, it will then be in a region of lower pressure - the gas expands, cools down • If air sinks, it will then be in a region of higher pressure - gas is compressed, heats up Dynamics in the atmosphere (2) • Moist air is less dense than dry air • Both kinds cool as they rise, but moist air cools more slowly (some condensation occurs which releases heat) • Rate of change in temperature with height is called adiabatic lapse rate: dry (DALR) is -10 °C/km and moist (MALR) is -6 °C/km Adiabatic Lapse Rate (1) • This is the rate of change of temperature of atmosphere as a function of height assuming no heat exchange with surrounds (definition of adiabatic) • Determines stability of atmosphere (e.g. thermals) and formation of clouds • Meteorologists measure lapse rate using radisondes and compare with predicted rates • It is a negative value – air cools as it goes higher Adiabatic Lapse Rate (2) • Dry Adiabatic Lapse Rate (DALR) is rate of temperature change with height for a rising parcel of dry air. For atmosphere, this is -9.8 degrees C/km. • Moist Adiabatic Lapse Rate (MALR) is similar but for moist air (saturated) and is -4.9 degrees C/km – the difference due to the latent heat released when water condenses • Environment Lapse Rate (ELR) is the actual rate for a stationary atmosphere. Typically -6.5 degrees C/km but this varies from day-to-day and location. Adiabatic Lapse Rate (3) • Look at absolute values of rates • If ELR is larger than DALR and MALR then air is always unstable - increasing formation of cumulus clouds and thunderstorms. Common in afternoon over land when air is heated. • If ELR is less than DALR and MALR then air is always stable and clouds formation is unlikely. Common in morning when air has cooled overnight. • If ELR is between the two values for DALR & MALR, then dry air stable, moist air unstable. Example of intermediate situation U nstable air – parcel rises 2000 m 12 °C 14 °C E nergy release during C ond ensation (latent h eat) M oist air lapse rate = - 6 °C /km 10 °C Stable air – parcel falls D ry air lapse rate = - 10 °C /km 1000 m 20 °C E nvironm ental lapse rate = - 8 °C /km Less dense air rises, m ore dense air sinks Examples of different Lapse Rates: cooling in °C/km of increasing elevation ELR (actual) -4 MALR (moist air) -6 DALR (dry air) -10 Result -8 -6 -10 Dry air stable Moist air unstable -12 -6 -10 Always unstable – Rising air doesn’t cool fast enough Always stable – no clouds form Pressure systems and rain • Falling air – compression, temperature rises, clouds less likely, no rain. High pressure system (deserts). • Rising air – expansion, temperature falls, condensation and clouds more likely, more rain. Low pressure system. Why do clouds form? • In atmosphere - continual evaporation and condensation between gas (water vapour) & liquid (water drops) • Evaporation is easier from small droplets • Condensation needs small particles in air to begin process • Dew point – air is saturated; evaporation & condensation rates are equal • Clouds: water drops condensed on grains Cumulus Clouds Thunderclouds Precipitation and Clouds • Adiabatic cooling occurs when moist air rises • Air becomes saturated, allowing condensation and precipitation as rain, hail or snow • Different types of cloud associated with rain, some contain ice • High clouds (> 5 km) – e.g. cirrus, ice crystals • Middle clouds (2-7 km) – e.g. altostratus, mostly water • Low clouds (< 2 km) – e.g. stratus (rain), nimbostratus (snow) • Vertically developed clouds (via convection) – e.g. cumulus, cumulonimbus (thunderstorms) What other options besides rain? • Snow – lower temperatures produce ice crystals in upper cloud layers • Hail – ice crystals rise and fall in cloud several times resulting in layered growth Golf-ball hail in the USA Humidity (1) • Absolute Humidity is the ratio of the mass of water vapour to the volume of the air (i.e. kg/m3) • Specific Humidity is mass of water vapour to the mass of dry air • Relative Humidity is the ratio of the vapour pressure to that of saturated air. • For a given amount of water vapour in the atmosphere, the saturated air pressure is less at lower temperatures so the Relative Humidity is higher at dawn! • Relative humidity is often mentioned in weather reports as an indicator of rain • Relative Humidity is a measure of comfort – as it is harder to lose heat by perspiration when there is high humidity Humidity (2) Fogs - dew – frosts (1) • Condensation occurs when temperature falls below dew point • If air does not become saturated until temperature below freezing – frosts occur • If air dry & cold – plant cells rupture when temperature gets to dew point; called a killing frost • Avoid killing frosts by spraying water – adds moisture – ice forms on plants and insulates internal cell fluids against freezing Fogs - dew – frosts (2) • Water vapour in the air is invisible • Fog & mist are very small water droplets • Water gets into atmosphere by evaporation (oceans, lakes, rivers) and transpiration (animals, plants) • Water goes back to Earth by precipitation (rain, hail, snow)