Chapter 6
The tempest comes out from its chamber, the cold from the driving winds.
Meteorologists measure atmospheric conditions, such as temperature, air pressure, wind speed and relative humidity. Thermometer- measures temperature Barometer- measures air pressure Anemometer- measures wind speed Weather vane- measures wind direction
Hygrometer- measures humidity Meteorologists need a “snapshot” of the atmosphere at one particular moment to develop an accurate forecast. In the US, the National Weather Service, the Federal Aviation Administration, and the Department of Defense establish a surfaceweather observation network known as ASOS.
Automated Surface Observing System Gathers data 24 hours a day, every day It began operating in 1990 This data is essential weather data for aviation, weather forecasting and weather-related research.
Digital forecast- since the atmosphere behaves as a fluid, data collected can be expressed as mathematical equations to then forecast the weather. Digital forecasting is the main method used by present day meteorologists. Analog forecast- compares current weather patterns to similar weather patterns from the past. They look for a pattern analogous to the current pattern.
The most accurate and detailed forecasts are short term. A one to three day forecast is usually accurate for expected temperatures and how much precipitation will occur. All long term forecasts are less reliable. Forecasts for months are usually based on weather cycles or patterns seasonally.
While surface-weather data are important, the weather is largely the result of changes that take place high in the troposphere at altitudes up to 30,000 m. Radiosonde- sensors and battery operated radio transmitter send air temperature, pressure and humidity to a ground station. A radiosonde is attached to a helium or hydrogen balloon.
Radiosonde
Rawinsonde- also measures wind direction and speed. Tracking these balloons has been improved by using GPS systems and the latest computer technology.
Radar- uses radio waves that reflect from particles larger than a specific size. Thus the distance to precipitation can be detected. Doppler effect radar- can be used to determine the speed at which precipitation moves toward or away from a radar station. Doppler effect- change in pitch or frequency due to the motion of a wave. (sound of horn higher as it comes closer and lower as it moves away).
Infrared imagery- this allows observation at night. Used to map cloud cover or surface temperatures. Visible light imagery- like a digital photograph, can track clouds. Water vapor imagery- water vapor, though invisible, absorbs and emits infrared radiation at certain wavelengths. Thus sensors can measure the amount of water vapor present in the atmosphere.
A station model is a record of weather data for a particular site at a particular time. Plotting station models creates a weather map. Numerous symbols have been adopted for use in the preparation of weather maps. Wind direction and speed High, middle and low clouds Cloud cover Current Temperature, Dew Point Barometric pressure
2. A synoptic weather chart is designed to present a view of the whole weather situation at one time. The information is collected and prepared four times daily.
On the left labels are: Temperature Current weather Dew point
On the right they are: Sea level pressure
In the center cloud cover
The long tail represents wind barb
4. 5. omit
6. The temperature on a synoptic weather chart is measured in B. Fahrenheit
8. dew point temperature-The air temperature at which saturation occurs is called the dew point 9. The dew point temperature can be higher than the current temperature
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10. Dew points indicate- the amount of moisture 11. Define dew point- the temperature to which the air would have to cool (at constant pressure and water vapor content) in order to reach saturation.
dew forms on surfaces
such as lawns and automobile windshields as a result of air near the Earth’s surface becoming saturated with water vapor. during the night moist air near the ground cools to the dew point.
Frost occurs when saturation occurs at temperatures below the freezing point. Hoarfrost is usually white and feathery, while a denser form of frost, called rime, occurs when super-cooled water droplets form fog or clouds freeze on a surface.
Both dew and frost
require a clear night with little or no wind, and an atmosphere that contains sufficient moisture to allow condensation to occur. Cool night, morning delight.
12. A state of saturation exists when- the air is holding the maximum amount of water vapor possible at the existing temperature and pressure. 13. Which of the following conditions would represent high humidity? A and c
14. When high relative humidity values exist, describe the weather conditions: A) clouds B) precipitation
As air temperature increases, more water molecules evaporate, some of them eventually condensing into liquid. When an equilibrium occurs between the evaporation rate and the condensation rate, the air has reached the saturation point and can hold no more vapor.
Vapor levels depend upon temperature and vary considerably across the planet, from barely discernible amounts in polar latitudes to nearly 4%of the air by volume in tropical regions. Tropical Rain Forest in Hawaii pictured.
15. Define atmospheric pressure- force per unit area exerted against a surface by the weight of air above that surface. 16. Atmospheric pressure is measured using an instrument called a barometer. 17. As a column of air above a surface increases, the pressure increases.
18. If the dew point temperature is 65 or greater, the weather will be uncomfortably humid. 19. & 20. Discuss cloud cover symbols.
1atm= 101.325 kPa = 1013.25 mb = 760 torr For our weather maps the surface air pressure will be in millibars.
If the reported value is greater than 500;
Add a 9 to the left of the given number And divide by 10
Example 827 Is greater than 500 Add a 9 and divide by 10 9827 becomes 982.7 mb
If the reported value is less than 500 Add a 10 to the initial value and Divide by 10
Example: given 027 This is less than 500 Add 10 and divide by 10 10027 becomes 1002.7 mb
The wind barb indicates the wind direction and wind speed. The wind barb points in the direction from which the wind is blowing. Here wind direction is NE at 15 knots.
1 knot= 1.15 mph
Show conversions from 15 knots to mph Show conversions from 45 mph to knots 25. A pennant represents: 50 knots
Dots, * and then commas. Fog is 3 lines. Write in sandstorm
Rain is liquid precipitation
that falls to the ground from clouds. The size of the drop is 0.5 mm or larger. Much smaller drops may fall as drizzle.
Drizzle usually appears as a fine mist that falls or drifts so slowly through the air that it can appear almost suspended in the atmosphere. Small rainfall totals are not even recorded on the ground.
Let’s go through these maps together: 16Z Wed Nov. 15, 2006 20Z Wed Nov. 15, 2006 23Z Wed Nov. 29, 2006 08Z Thr Nov. 30, 2006 23Z Thr Nov. 30, 2006 02Z Fri Dec. 01, 2006 09Z Fri Dec 01, 2006 HW: Complete WS Surface Observations
The shape of the cold front is called a wedge. The strength of the upward motions is a vigorous rising motion. The cold dense air lifts the warm air. As the warm air rises it condenses producing clouds and precipitation. The location and intensity of precipitation is ahead of the cold front mass. Brings showers and thunderstorms.
The shape of the warm front is gentle sloping. The strength of its upward motions are not as vigorous. The warm air lifts and overrides the colder air. The warm air will then cool and create widespread clouds and light precipitation. Thin clouds will form 1000km in advance of the warm front. As the front gets closer and closer the clouds thicken and light precipitation falls.
http:/ / www.stevemcentee.com/ animation3.html Select each type of front and visualize
Go to Atmospheric Sciences & Illinois http:/ / www.atmos.illinois.edu/ index.html Look at current weather and archive data.
Airborne ice crystals that act like tiny prisms are
responsible for the intriguing optical effects known as haloes and sundogs. When sunlight or moonlight passes through the crystals, they cause the light to be refracted or bent slightly. Because many of the ice crystals are in random orientation, circular haloes result.
At certain points on the
halo, particularly bright patches form, known as sundogs. Haloes and sundogs exhibit considerable variations in coloring.
Ice crystals are not the only agents that refract sunlight, sunlight is bent as it passes through varying elements in the atmosphere, producing mirages and on occasion, the dust-related phenomenon known as the green flash that occurs just as the Sun disappears below the horizon.