Volcanological Meteorology Some background on more specific meteorological topics which we will discuss later in the class With materials from American Meteorological Society and various web sources Lecture #8 Ashfall Grad Course Fall 2009

ATMOSPHERIC SCIENCE Academic Press, Second Edition By John Wallace, University of Washington, Seattle, U.S.A. Peter Hobbs, University of Washington, Seattle, U.S.A. Contents 1.Introduction and Overview 2.The Earth System 3. Atmospheric Thermodynamics 4. Radiative Transfer 5. Atmospheric Chemistry 6. Cloud Microphysics 7. Atmospheric Dynamics 8. Weather Systems 9. The Atmospheric Boundary Layer 10. Climate Dynamics

3 Air Temperature and Altitudinal Relationships in the Atmosphere © American Meteorological Society

4 Troposphere – Lowest layer – Where almost all weather occurs – Temperature decreases with altitude Generally, but with frequent exceptions (e.g., inversion, isothermal layer) Average temperature drop is 6.5 ° C/1000 m (3.5 ° F/1000 ft) – ~ 6 km (3.7 mi) thick at the poles, ~20 km (12 mi) thick at the equator – Upper boundary/transition zone to next layer is called the tropopause

5 Stratosphere – Goes from troposphere up to ~ 50 km (30 mi) – In lower stratosphere, temperature is constant This is an isothermal condition – Above 20 km (12 mi), temperature increases with increasing altitude Stratosphere is warmed by the energy released by ozone absorbing UV radiation – Stable conditions are ideal for jet aircraft travel, but can cause trapping of pollutants (e.g. from volcanic eruptions) in lower stratosphere – Upper boundary/transition zone to next layer is called the stratopause

6 Average Air Pressure Variation with Altitude Expressed in mb

7 A mercury thermometer employs air pressure to support a column of mercury in a tube Air pressure at sea level will support the mercury to a height of 760 mm (29.92 in.) Height of the mercury column changes with air pressure Adjustments required for: – The expansion and contraction of mercury with temperature – Gravity variations with latitude and altitude Air Pressure Measurement

8 An aneroid barometer is less precise, but more portable than a mercury barometer It has a chamber with a partial vacuum Changes in air pressure collapse or expand the chamber This moves a pointer on a scale calibrated equivalent to mm or in. of mercury New ones are piezoelectric – depend on the effect of air pressure on a crystalline substance Home-use aneroid barometers often have a fair, changeable, and stormy scale – These should not be taken literally

9 Air Pressure Measurement Forecasting uses air pressure and tendency values – changes over time Barometers may keep a record of air pressure – These are called barographs

10 Air Pressure Units Units of length – Millimeters or inches Inches typical for TV Units of pressure – Pascal – worldwide standard Metric scale Sea-level pressure = – 101,325 pascals (Pa) – hectopascals (hPa) – kilopascals (kPa) – Bars – U.S. A bar is inches of mercury A millibar (mb) is the standard used on weather maps, meaning 1/1000 of a bar – Usual worldwide range is 970 – 1040 mb – Lowest ever recorded mb (Typhoon Tip in 1979) – Highest ever recorded – mb (Agata, Siberia)

11 Variations in Air Pressure w/Altitude Overlying air compresses the atmosphere – the greatest pressure is at the lowest elevations Gas molecules are closely spaced at the surface Spacing increases with altitude At 18 km (11 mi), air density is only 10% of that at sea level Because air is compressible, the drop in pressure with altitude is greater in the lower troposphere – Then it becomes more gradual aloft Vertical profiles of average air pressure and temperature are based on the standard atmosphere – state of atmosphere averaged for all latitudes and seasons Even though density and pressure drop with altitude, it is not possible to pinpoint a specific altitude at which the atmosphere ends – ½ the atmosphere’s mass is below 5500 m (18,000 ft) – 99% of the mass is below 32 km (20 mi) – Denver, CO average air pressure is 83% of Boston, MA

12 Horizontal Variations in Air Pressure Horizontal variations are much more important to weather forecasters than vertical differences – In fact, local pressures at elevations are adjusted to equivalent sea-level values – This shows variations of pressure in the horizontal plane – This is mapped by connecting points of equal equivalent sea- level pressure, producing isobars

13 Horizontal Variations in Air Pressure Horizontal changes in pressure can be accompanied by significant changes in weather In middle latitudes, a continuous procession of different air masses brings changes in pressure and weather – Temperature has a much more pronounced affect on air pressure than humidity In general, the weather becomes stormy when air pressure falls but clears or remains fair when air pressure rises

14 Horizontal Variations in Air Pressure Influence of temperature and humidity – Rising air temperature = rise in the average kinetic energy of the individual molecules In a closed container, heated air exerts more pressure on the sides – Density in a closed container does not change – No air has been added or removed The atmosphere is not like a closed container – Heating the atmosphere causes the molecules to space themselves farther apart – This is due to increased kinetic energy – Molecules placed farther apart have a lower mass per unit volume, or density – The heated air is less dense, and lighter.

15 Horizontal Variations in Air Pressure Influence of temperature and humidity, continued – Air pressure drops more rapidly with altitude in a column of cold air Cold air is denser, has less kinetic energy, so the molecules are closer together – 500 mb surfaces represent where half of the atmosphere is above and half below by mass This surface is at a lower altitude in cold air vs. in warm air – Increasing humidity decreases air density The greater the concentration of water vapor, the less dense is the air due to Avogadro’s Law We often refer to muggy air as heavy air, but the opposite is true – Muggy air only weighs heavily on our personal comfort factor

16 Air pressure varies continuously

17 Horizontal Variations in Air Pressure Influence of temperature and humidity, continued – Cold, dry air masses are the densest These generally produce higher surface pressures – Warm, dry air masses generally exert higher pressure than warm, humid air masses – These pressure differences create horizontal pressure gradients Pressure gradients cause cold or warm air advection – Air mass modifications can also produce changes in surface pressures – Conclusion: local conditions and air mass advection can influence air pressure

18 Horizontal Variations in Air Pressure Influence of diverging and converging winds – Diverging = winds blowing away from a column of air – Converging = winds blowing towards a column of air – Diverging/converging caused by : Horizontal winds blowing toward or away from some location (this chapter) Wind speed changes in a downstream direction (Chapter 8)

19 Horizontal Variations in Air Pressure

20 Highs and Lows Isobars are drawn on a map as previously discussed – U.S. convention – these are drawn at 4-mb intervals (e.g., 996 mb, 1000 mb, 1004 mb) A High is an area where pressure is relatively high compared to the surrounding air A Low is an area where pressure is relatively low compared to the surrounding air Highs are usually fair weather systems Lows are usually stormy weather systems – Rising air is necessary for precipitation formation – Lows are rising columns of air. Highs are sinking columns of air.

21 Orbit of a Polar-Orbiting Satellite

Geostationary Satellites for Meteorology (and Volcanology!)

23  Visible –Black and white photograph of the planet –Only available during daylight hours –Highly reflective surfaces appear bright white and less reflective surfaces are darker Weather Satellite Imagery

24 Infrared – Available anytime, not just during daylight – Provides temperature comparison of features within image – Whiter = colder Higher cloud tops appear whiter, because they are colder Weather Satellite Imagery

25 Water vapor imagery – Enables tracking of plumes of moisture – Shades of white = increasing moisture – Upper-level clouds appear milky to bright white Weather Satellite Imagery

26 Remote Sensing Measurement of environmental conditions by processing signals that are either emitted by an object or reflected back to a signal source – Radar – Satellites

27 Weather Radar  Complements satellite surveillance  Doppler radar detects movement –Excellent tool to forecast tornadoes

28 Combination of Images  Composite of IR and Radar

What do clouds tell us? Clouds just don’t happen - there’s always a reason A particular cloud’s shape and location depend on (and can therefore tell us about): - the movement of the air - amount of water vapor in air - stability (flat clouds = stable air while puffy clouds = unstable air)

Types of Clouds Sorted by height – High (> 7km) Cirrus (ci), Cirrostratus (cs), Cirrocumulus (cc) – Middle (2-7km) Altostratus (as), Altocumulus (ac) – Low (< 2km) stratus (st), cumulus (cu), strato- cumulus (sc)

Types of Clouds Sorted by structure – Cirrus – Composed of Ice, ‘wispy’ – Cumulus – Detached elements ‘puffy’, ‘cottonlike’ – Stratus – Uniform Layer, ‘sheetlike’ – Nimbo – Precipitation producing Nimbostratus Cumulonimbus

Cirrus Clouds

Cumulus Clouds

Stratus Clouds

Cumulonimbus

What is this?

What this is Cirrus Stratus Snow Cirrus Snow Snow is reflective, but not too cold. Stratus in Mississippi is the same, and the wispy but cold values in Ontario are cirrus Stratus

What is this?

What this is Cirrus Cumulus Cumulo Nimbus Cirrus Cumulus Cumulo Nimbus The low (warm) and scattered clouds in Georgia are cumulus. The scattered by higher clouds in central Florida are cumulonuimbus. The wispy clouds southwest of Florida are cirrus clouds

40 Upper Air Observations Radiosondes – Invented in late 1920s – Transmits altitude readings (soundings) of: Temperature ■ Dewpoint ■ Air pressure – Data is received immediately No need to recover equipment – Rawinsonde A radiosonde that is tracked by direction-finding antennas Provides data on wind direction and speed Dropwindsonde is not launched with a balloon » It is dropped from an aircraft on a parachute – These devices are launched simultaneously worldwide Launched at 0000Z and 1200Z Only 20% of the devices are recovered

41 A radiosonde Launching a radiosonde Graphic measurements of air temperature, pressure, & dew point up to about 30,000 m (100,000 ft)

Upper Air Data

43 Heat Imbalance: Atmosphere vs. Earth’s Surface

44 Heat Imbalance: Atmosphere vs. Earth’s Surface

45 Some of the absorbed solar radiation is used to vaporize water at Earth’s surface This energy is released to the atmosphere when clouds form Large amounts of heat are needed for phase changes of water compared to other substances Latent Heating Latent heat of fusion Latent heat of vaporization

46 Describes how the energy received at the Earth’s surface is partitioned between sensible heating and latent heating Bowen ratio = [(sensible heating)/(latent heating)] At the global scale, this is [(7 units)/(23 units)] = 0.3 Bowen Ratio

47 Sensible Heating Heat transfer via conduction and convection can be sensed by temperature change (sensible heating) and measured by a thermometer Sensible heating in the form of convectional uplifts can combine with latent heating through condensation to channel heat from Earth’s surface into the troposphere – This produces cumulus clouds – If it continues vertically in the atmosphere, cumulonimbus clouds may form

48 Water absorbs or releases heat upon phase changes – This is called latent heat Latent heating – This is the movement of heat from one location to another due to phase changes of water Example – evaporation of water, movement of vapor by winds, condensation elsewhere Phase Changes of Water