Meterology – Part 1

1. Meterology – Part 1 Introduction to the Atmosphere

2. Weather • Weather is the condition of the atmosphere at a particular location and moment • The condition of the atmosphere is expressed according to the following: • Temperature • Humidity (relative humidity, dew point) • Pressure • Wind Speed and Direction • Cloud Cover • Precipitation (type and amount)

3. Climate • Climate is the condition of the atmosphere over many years • The same atmospheric variables are used for climate as weather • Climate data use the following: • Averages (“normal high” = 30 year average high temperatures for a given date and location) • Extremes (“record high” = highest temperature ever recorded for a given date and location)

4. More Definitions • Meteorology – The study of weather variables, the processes that cause weather, and the interaction of the atmosphere with the Earth’s surface, ocean, and life • Climatology – The study of climate, including past climate conditions and possible climate changes in the future

5. The Earth’s Major Surface Features • Our atmosphere receives energy from the Sun • The surface of the Earth exchanges energy and water with the atmosphere • The distribution of land and water plays a major role in determining climatic conditions and weather patterns • 70% of the Earth’s surface is water (Arctic, Atlantic, Pacific, and Indian Ocean) • More than 2/3 of Earth’s land is located in the Northern Hemisphere (Seven continents are: Africa, Asia, Australia, Europe, North America, and South America) • The atmosphere is a thin layer of gases (only 2% as thick as the entire Earth itself) that protects from harmful solar radiation, the air we breathe, and weather

6. The Atmosphere • The atmosphere contains various substances in the solid, liquid, and gaseous phase • Gas molecules are in constant motion and spread out (diffuse) over time • Gravity binds our atmosphere to the Earth • By itself, the atmosphere would be layered with the heaviest gases on the bottom (near the surface), lighter gases on top • Weather processes stir up the atmosphere and keeps it well-mixed

7. Evolution of the Atmosphere • When it formed 4.5 billion years ago, our atmosphere was mostly hydrogen, helium, methane, and ammonia (left-overs) • Volcanic eruptions released water vapor, carbon dioxide, and nitrogen • As the Earth cooled, the water vapor in the atmosphere began to condense and produce clouds and rain • A lot of the carbon dioxide dissolved in the newly formed oceans • 3 billion years ago, blue-green algae evolved in the oceans, and they produced oxygen as a byproduct of photosynthesis. This oxygen leaked into the atmosphere • As oxygen became more plentiful, the formation of ozone was possible in the upper atmosphere, which allowed life to leave the oceans and move onto land • The present day atmosphere formed after billions of years of these processes working

8. Composition of the Atmosphere • Nitrogen = 78% • Oxygen = 21% • Argon = 1% • The above values are rounded. There are also trace gases (in concentrations of parts per million and parts per billion) • Carbon Dioxide = 370 ppm (0.037%) • Methane (0.00017%) • Ozone (0.000004%) • CFCs (0.00000002 %)

9. Water Vapor • The previous slide was the concentration of the “dry” atmosphere • Water vapor is a highly variable gas in the atmosphere – both spatially and temporally • Antarctica can have values very close to 0% • The atmosphere in the very humid tropics can be up to 4% water vapor

10. Trace Gases and Aerosols • Aerosol – a tiny solid or liquid particle in the atmosphere • CFCs (Chlorofluorocarbons), methane, and carbon dioxide are important because they alter the energy balance of the Earth’s atmosphere • In addition, CFCs destroy stratospheric ozone • Source – mechanism that supplies a gas to the atmosphere • Sink – mechanism that removes a gas from the atmosphere • Cycle – The routes by which a gas enters and leaves the atmosphere

11. The Carbon Dioxide Cycle • Nearly half of the carbon dioxide that enters the atmosphere moves between the ocean and plants • Volcanic outgassing is a source of carbon dioxide • Photosynthesis (sink) is the process where a plant takes in sunlight, water, and carbon dioxide to produce plant tissue and oxygen • Photosynthesis is only a temporary sink. When a plant dies, carbon dioxide is released during decay when exposed to the air • Dead plant matter that is buried stores carbon dioxide in the form of fossil fuels (coal, oil)

12. The Carbon Dioxide Cycle

13. Hydrologic Cycle • Water connects the atmosphere with the surface of the Earth • Water is the only substance that exists naturally in the atmosphere in all three phases (solid, liquid, and gas) • Changes of phase is an important part of energy exchange • Sources of Water Vapor • Evaporation from oceans, lakes, and glaciers • Transpiration – release of water vapor by plants • Sinks of Water Vapor • Condensation – phase change from gas to liquid (cloud formation) • Precipitation

14. Methane • The concentration of methane in the atmosphere has doubled since the beginning of the Industrial Revolution • Sources of methane are decay of organic substances, burning of forests, coal mining, and cattle) • Methane is an important gas in “climate change”

15. Chlorofluorocarbons • CFCs are a man-made compound that were used as propellants and refrigerants • CFCs destroy stratospheric ozone • Concentration of CFCs have leveled off since laws were enacted to prohibit their use • CFCs are very stable compounds and will stay in the atmosphere for about 100 years

16. Aerosols • Tiny liquid and solid particles in the air • Examples are smoke, salt, ash, smog, and dust • Measured in microns (one-millionth of a meter) • The amount and type of aerosol can influence the climate of a region by modifying the energy balance • Aerosols can be blown far away from their source (dust found in Greenland ice cores suggest windy conditions at the time) • Aerosols are important in the formation of clouds • Sources are wind erosion, fires, volcanoes, and human activity • Anthropogenic aerosols – particles generated by human activity

17. Atmospheric Pressure and Density • Pressure is the force exerted on a given area • Air pressure results when air molecules move and collide with objects • Air pressure is exerted in all directions • Density is the concentration of molecules, or mass per unit volume • The pressure, density, and temperature of a gas are all related to each other

18. “Top” 1” 1” Meteorology 101 Air Pressure On average, air weighs about 14.7 lb/in2 14.7 lb/in2 =29.92 “inches of mercury” Air Pressure varies over the globe

19. Ideal Gas Law • The relationship between pressure, temperature, and volume is given by the ideal gas law: P = ρ x R x T where P = pressure ρ = (Greek letter rho) density R = a constant T = temperature

20. Ideal Gas Law • You will need to know the Ideal Gas Law, but you will not need to compute anything in this class • Instead, knowing the Ideal Gas Law, you should be able to say what happens to one variable if a change in one of the others occurs (with the third variable a constant) • For instance, what happens to pressure if the density increases (temperature a constant)?

21. Pressure and Altitude • Pressure is measured in terms of inches of mercury, or in millibars • Average sea-level pressure is 29.92 inches of mercury or 1013.25 millibars • Atmospheric pressure always decreases with increasing altitude • Pressure decreases because atmospheric density decreases (fewer molecules banging into objects) • The air pressure measured in Denver will always be less than the air pressure in San Francisco • To subtract the influence of station elevation, air pressure is always mathematically corrected to report what it would be at sea level (sea level pressure)

22. Dividing Up the Atmosphere • The atmosphere can be divided up according to pressure (500 mb layer is about halfway up in the atmosphere) • The atmosphere can also be divided up according to temperature (which does not follow a simple relationship with height) • Averaging out temperature values in the atmosphere, we are able to identify 4 layers

23. Stratosphere 6-8 Altitude (mi) Troposphere Planetary Boundary Layer Temperature Meteorology 101 Layering of the Atmosphere

24. Atmospheric Layers • The layers are identified based on how temperature changes with height • Troposphere – temperature decreases with height • Stratosphere – temperature increases with height • Mesosphere – temperature decreases with height • Thermosphere – temperature increases with height • Separating these layers are “lids”: • Tropopause – separates troposphere and stratosphere • Stratopause – separates stratosphere and mesosphere • Mesopause – separates mesosphere and thermosphere

25. Troposphere • From the surface up to about 6-10 miles (varies with latitude and season – higher in the summer and in the tropics) • Temperature decreases with height because the troposphere is heated by the surface and not directly by sunlight • Almost all of what we call “weather” occurs in the troposphere • 80% of the atmosphere’s mass

26. Stratosphere • Up to about 31 miles • Temperature increases with height because the ozone layer absorbs ultraviolet light and warms up as a result • Lack of mixing and turbulence • Layered portion of atmosphere (“strata”) • Very little mixing occurs between the stratosphere and troposphere (except with thunderstorms and other strong storms) • 99.9% of the atmospheric mass below stratopause

27. Upper Atmosphere • Mesosphere goes up to about 53 miles • Thermosphere goes up to about 75 miles, but there is no clear separation between the thermosphere and interplanetary space • The highest temperatures in the atmosphere are found in the thermosphere due to high energy radiation being absorbed by gases (but it wouldn’t feel hot due to low density) • Ionosphere (charged gas atoms) that can reflect radio waves is found here, as are the aurora

28. Basic Concepts • Front – a boundary between two regions of air that have different meteorological properties, such as temperature and humidity • Cold front – a region where cold air is replacing warmer air • Warm front – a region where warm air is replacing colder air • Stationary front – a front that is not moving • Occluded front – a front where warm air is forced aloft (to be studied further in chapters 9 and 10)

29. Other Concepts • Isotherms – lines on a weather map that connect points of constant temperature • Isobars – lines on a weather map that connect points of constant pressure • Isotachs – lines on a weather map that connect points of equal wind speed • Isopleth – “equal line”; general term describing contours along which any particular variable is constant

30. Pressure Tendency • The trend in pressure (whether it is rising or falling) is an important thing to know in weather forecasting, as you’ll see later • If you see “-3” for pressure tendency, it means that the pressure has decreased 0.3 millibars in the past 3 hours • If you see “12” for pressure tendency, it means that the pressure has increased 1.2 millibars in the past 3 hours • The symbol next to the number shows how the pressure has changed (increased sharply, increased then fell, etc.)

31. Watches, Warnings, & Advisories • The National Weather Service issues watches, warnings, and advisories to the public • Watch – a particular weather hazard may develop in a particular area later on • Warning – a particular weather hazard is occurring (or is imminent) in a particular area • Advisory – a less urgent statement issued to bring to the public’s attention a situation that may impact them • “Lead time,” or the amount of time between the issuance of the above products and the occurrence of the weather hazard, is very important to forecasters (the more, the better as long as you don’t sacrifice accuracy)

32. Meteorology 101 Changing Pressure - Winds L H Take more out than put in – decrease pressure Put more in than take out – increase pressure

33. Cold Warm H L Coldest column = highest pressure ** Warmest column = lowest pressure ** Meteorology 101 Changing Pressure - Temperature

34. H L H L H Meteorology 101 Vertical motions also occur Air “converges” at lows, and rises. Air “diverges” at highs, and sinks.

35. H L Meteorology 101 Reality is more complicated Actual winds around highs and lows

36. L Rising Air near ows Sinking air near ighs H Meteorology 101 • Rising air cools; water vapor in the air condenses to form clouds/precipitation • Lows tend to bring cloudy, wet weather • Sinking air warms and dries out. • Highs tend to bring fair, dry weather.

37. Meteorology 101 Low or lowering pressure = “Lousy” weather

38. H Cold H H Warm L L Meteorology 101 General Circulation

39. Front = Battleground of Air Masses H Cold H H Warm Meteorology 101 • Temperature differences concentrated • Zone of lower pressure where lows (storms) often form

40. Colder Warmer Colder Warmer Colder Warmer Meteorology 101 Cold Front Cold air advances Warm Front Warm air advances* Stationary Front

41. Warm Cold Meteorology 101 What happens when air masses meet at fronts? Cold air lifts the warmer air. Clouds and precipitation form.

42. Ridge Trough Meteorology 101 Upper-Level Features Westerlies - High-Altitude winds blow generally west-to-east 3-6 miles above mid-latitudes. Jet Stream – River of fastest-moving air within the westerlies.

43. Meteorology 101 Reality is messier … Still, highs and Lows move with the westerlies and the jet stream.

44. RIDGE LOW PRESSURE FAVORED HIGH PRESSURE FAVORED TROUGH Meteorology 101 SINKING AIR RISING AIR Highs and Lows form and dissipate in synch with ridges and troughs in the westerlies.

45. H COLD WARM H Meteorology 101 LOW This is your life! Stationary Front separates air masses

46. Meteorology 101 H LOW This is your life! COLD L WARM H Area of low pressure develops along front

47. Meteorology 101 H LOW This is your life! L H Circulation around low sends cold air and warm air advancing

48. Meteorology 101 H LOW This is your life! L H Circulation around low sends cold air and warm air advancing

49. Cloud Shield Precipitation Shield Meteorology 101 LOW This is your life! L Warm Sector Typical cloud and precipitation shield of a low-pressure system and fronts

50. Meteorology 101 Intense lows often take on a “comma-cloud” shape when viewed from space.