Lecture 5 (10/07) METR 1111 Isolining and Upper Air Maps

What are isolines? Isolines (also called isopleths)- lines that connect equal values of a variable In meteorology, we frequently use: - isobars (pressure) - isotherms (temperature) - isotachs (wind speed)

Isolines Isolines make it easy to see different values on a map Allow us to interpolate values at a point Can pick out different features like fronts, jetstreams, drylines, pressure fields, etc. How can I draw an isoline?

Drawing Isolines When drawing isolines/isopleths, keep in mind: Lines cannot cross. They must close themselves off or start and end at the edge of the map You must have greater values on one side and smaller values on the other

Upper Air “Upper air” can refer to anything not on the surface Rawinsonde (weather balloons) launched twice a day from stations across country & world at 0 and 12 Z Measures temperature, pressure, humidity, and wind speed and direction

Upper Air Maps UAM have many stations’ readings at a certain constant pressure level Ex: On an 850mb map, all stations are at a pressure of 850mb. Most common upper level maps: 850mb, 700mb, 500mb, 250mb, 200mb.

Temperature Temperature is in upper left corner Measured in Celsius

Dew Point/Dew Point Depression Dew point or dew point depression is in lower left corner Measured in Celsius Dew point depression = difference between the temperature and dew point Can look for map label or use intuition If you see a dew point greater than a temperature, dew point depression is plotted (here dew pt = -2° C)

Geopotential Height Upper right corner = geopotential height Geopotential height -- the height above sea level that the instrument is at when it reads the specified pressure level. Often truncate last number or two but measured in meters Need to decode geopotential height.

To try clear up confusion in class: The following pattern appears between isobars and lines of constant geopotential height: If we draw lines of constant height on a constant pressure surface it will look like lines of constant pressure (isobars) drawn at a given height (as long as data at about the same elevation above the earth is used) Thus, an area of low heights means you have an area of low pressure An area of high heights means you have an area of high pressure

What does it all mean? Here’s an example if you’re interested: Compare this pattern of constant height lines on a constant pressure surface: With this pattern of constant pressure lines on a constant height surface: The height contours from the first should approximately line up with the isobars from the second (except that they use fewer lines on one). To see this best, look in regions where the lines are closest together (greatest pressure gradient) and compare between the two maps A viewing tip: Click on the first url to display it on your web browser and then cut and paste the second url into the box and hit return—then hit the forward and back button to switch between the two

Decoding Geopotential Heights Not all map-makers use the same truncation rules These are the most common: 850 mbadd 1000m533=1,533m 500 mbmultiply by 10591=5,910m 250 mbmultiply by 10098=10,980m & add 10,000m

Wind Barb Wind symbols are the same as with surface plots Remember a triangle means 50 knots Ex: 1 triangle, 3 long flags and 1short flag add up to =85 kts This wind is 90° (Easterly) at 50 kts

Announcements: Pick up graded hmwks & quizzes after you turn in your quiz Read Ch 6 (Floods & Droughts) Complete homework 5 for next time (available on web)