Chapter 5

1. Chapter 5 Observing the Atmosphere

2. Figure CO: Chapter 5, Observing the Atmosphere--snowflake © Steve Collender/ShutterStock, Inc.

3. Two Basic Approaches to Observing the Atmosphere • Direct, in situ, or in place methods measure properties of the air that are in contact with the instrument • Indirect or remote sensing methods obtain information without physical contact with the atmosphere being measured

4. Direct Measurement of Surface Conditions • Accomplished by ASOS, the Automated Surface Observing System • Observations often visualized using the station model and the meteogram • Includes temperature, humidity, pressure, wind, humidity and precipitation • The United States’ primary surface weather observing network • Used by the NWS, FAA, DOD

5. Figure B01: Meteogram Source: McIDAS-V

6. Temperature • Mercury and alcohol liquid thermometers, metallic thermometers have been used • ASOS uses an electronic resistance thermometer • Thermometer is shielded from direct sunlight • Thermometer is ventilated • Measures the electrical resistance of a metal wire, usually platinum

7. Figure 02: Stevenson Screen Courtesy of OAR/ERL/National Severe Storms Laboratory (NSSL)/NOAA

8. Humidity—Dew Point • ASOS uses a dew point hygrometer • Based on the principle that a mirror fogs when the temperature equals the dew points • Uses a beam of light focused on a mirror • The mirror is chilled • Light is blocked from the detector when drops or frost form on the mirror • Mirror’s temperature measured with a wire • False reading if something else covers the mirror

9. Figure 03A: Laser dew point.

10. Figure 03B: Laser dew point.

11. Other Humidity Instruments • The wet-bulb thermometer is a liquid barometer with a wet wick around its bulb • The wet-bulb thermometer measures wet-bulb temperature • A psychrometer has a wet-bulb and an ordinary thermometer • Aspirated psychrometer ventilated with a fan • Sling psychrometer whirled by hand • GPS satellites have been used to measure humidity

12. Measuring Pressure • The laboratory standard for measuring pressure is the mercury thermometer, a long glass tube • The aneroid barometer is a partially evacuated flexible metal box that changes size with changing pressure • Used by ASOS • Smaller, more durable than a mercury barometer • Not poisonous • ASOS barometers are electronic with high accuracy and quick response—ASOS’s best instrument

13. Figure 04: Barometer © john rensten/Alamy Images

14. Wind Speed and Direction • Anemometers measure wind speed • Cup anemometer rotates in response to pressure differences • Wind vanes measure wind direction • Typically a pointer in front and fins in the back • The vane rotates until forces are balanced • Wind vane points into the wind • Anemometers and wind vanes mounted at 10 feet above the ground

15. Figure 05: Wind vane and cup anemometer Courtesy of Lewis Kozlosky, NWS/NOAA

16. Wind—Other Devices • Propellers measure wind speed because blades rotate at a speed proportional to the wind • A windsock moves with wind direction and fills proportionally with wind speed • Sonic anemometers use ultrasonic sound waves • No moving parts • Measures wind speed by time difference between transmission and reception of a sonic pulse

17. Measuring Precipitation • ASOS uses the rain gauge • A funnel-like collector above a bucket is heated to melt snow and/or ice • Water is funneled into a tipping bucket • Tipping bucket measures in 0.01 inch increments • Has errors due to splash, wind blowing across the gauge • ASOS doesn’t measure snow • A snowboard is white painted wood • Snow also measured by water equivalent

18. Direct Upper-Air Observations • Radiosondes are radio-equipped meteorological instrument packages carried aloft by a helium-filled balloon • Measure vertical profiles from the surface to 30 km • Temperature and relative humidity measured electronically • Pressure measured with an aneroid barometer • Tracking the position of the balloon gives wind speed and direction, and gives the observation the name rawinsonde • Soundings taken world-wide twice a day

19. Figure 07: Rawinsonde Courtesy of NWS/NOAA

20. Indirect Weather Observations • Active sensors emit energy and measure the energy that returns • Example: radar • Passive sensors measure radiation emitted by the atmosphere, surface, or the sun • Example: visible satellite data • Indirect methods mostly involve light interacting with molecules or objects • Must review laws of optics

21. Reflection and Refraction • Law of reflection: the angle at which light strikes a surface is the same as the angle of reflection • Refraction is bending of light as it passes through a transparent substance like water • The index of refraction is the ratio of the speed of light in a vacuum to the speed of light in a substance • Refraction explains why stars twinkle, and causes objects partly immersed in water to look bent or broken in two

22. Figure 08: Reflection/refraction

23. Figure 09: Critical angle

24. Figure B03: Twinkle of a star

25. Scattering • Scattering is change of direction of light rays when they encounter small particles • Solved mathematically by Mie in 1908 • Rayleigh scattering • Particles small compared to the wavelength of incident radiation • Geometric scattering • Larger particles, like cloud droplets • Explains blue sky, red sunsets, white haze

26. Figure 10: Red sky Courtesy of Pam Knox

27. Figure 11: Crepuscular rays © Wong Chee Yen/Dreamstime.com

28. Figure 12A: Thick clouds appear darker on the bottom © tonobalaguerf/ShutterStock, Inc.

29. Figure 12B: Thick clouds appear darker on the bottom

30. Measuring Visibility • Visibility is the horizontal distance a person with normal vision can see and identify specified objects • Reduced when particles in between scatter or absorb light • ASOS uses an active remote sensing method • A flash of light over a very short distance • Scattered light flash measured by a receiver and converted into visibility • Only one direction, and not identical to human eye

31. Measuring Cloud Ceiling • ASOS uses a ceilometer • An active remote sensing instrument • Uses a laser beam which sends pulses of radiation • Designed with aircraft take-offs and landings in mind • Thin and high clouds are invisible to ASOS • Clouds at the horizon are invisible too

32. Figure 13: Ceilometer

33. Meteorological Satellite Observations • Two basic orbits • Geostationary Earth orbit (GEO) • Orbits at the speed of Earth’s rotation • Stays above the same point on the Earth • Height of orbit is 36,000 km • Must be located over the Equator • Low Earth orbit (LEO) • Often pass over polar regions • Altitude of 850 km • Each orbit slightly to west of previous orbit

34. Figure 14: Satellite orbits

35. Tradeoffs for GEO and LEO • GEO • Continuous view of tropics and mid-latitudes • Poor view of polar regions • Tracks storm systems continuously • GOES, METEOSAT • LEO • Good polar coverage • Excellent detailed snapshots of weather events • Good for studying global weather—2 views per day

36. Interpreting Satellite Imagery • Radiometers: passive remote sensing instruments • One type measures visible light reflected from Earth to space • Brightest images are clouds • Second type measures radiation emitted by the surface or clouds (IR) • Measures heat • Data 24 hours per day • Thick cold clouds appear bright white

37. Figure 15: Satellite Visible image Courtesy of SSEC and CIMSS, University of Wisconsin-Madison

38. Figure 16: Satellite IR image Courtesy of SSEC and CIMSS, University of Wisconsin-Madison

39. Figure 17: A matrix for how different types of clouds appear in visible versus IR images

40. Water Vapor Imagery • Use radiometers that measure radiation between 6.5 and 6.9 microns • Give information about upper and middle troposphere • Give information in both clear and cloudy regions • Black for low concentrations, milky white for high concentrations, and bright white for thunderstorms

41. Figure 18: Satellite water vapor image Courtesy of SSEC and CIMSS, University of Wisconsin-Madison

42. Radar Observations • Radar is an active remote-sensing instrument • Sends out pulses of energy • Measures energy scattered back to the transmitting point • Received signal is called the radar echo • Radar echo indicates the location and intensity of precipitation • Range is a maximum of about 240 km • Scans through 360 degrees and at several different elevation angles

43. Figure 20: Weather radars send out a narrow-beam radio wave that is scattered off precipitation

44. Radar Displays • A nearly horizontal scan is a Plan Position Indicator or PPI • A vertical slice is a Range Height Indicator or RHI • The intensity of scattered radiation is called reflectivity and is displayed using a logarithmic scale in units of decibels or dBZ • 20 dBZ or greater is rain • Above 55 dBZ is usually hail • New data are available about every 5 minutes

45. Figure 21: Radar image Courtesy of NOAA

46. Doppler Radar Data • Motions of particles towards/away from the radar are detected • Displays use cool colors (green, blue) for approaching • Displays use warm colors (red, orange) for receding • Particular signatures indicate rotation (supercell updrafts, tornadoes), divergence (downbursts) • It takes 2 Doppler radars to put together a complete wind field

47. Figure 22A: The Doppler Effect

48. Figure 22B: The Doppler Effect

49. Figure 23: A radar display in Doppler mode of a thunderstorm Courtesy of NOAA

50. Other Radar Capabilities • Dual polarization radars • Can change orientation of electromagnetic fields • Can give information about shape and orientation of particles • Help distinguish type of precipitation (rain, hail, snow) • Wind profilers • An application of Doppler technology • Can determine wind speed without precipitation • Collect the three-dimensional wind field