Viewing and Depicting the Atmosphere

MODIS 03 FEB 2008 1830 UTC http://rapidfire.sci.gsfc.nasa.gov/realtime/

Viewing or Depicting the Atmosphere

Viewing the atmosphere is like viewing a layer cake. You can view a cake from above (Happy Birthday), slice through it to see the layers or stick a straw in it to see the layers without slicing it. Thus the next slides show

1. Map, Plan View, or Bird’s Eye View (2-D from above)

2. *Cross Section or Panoramic View (2-D from the side)*

3. *Sounding (1-D vertical profile)

4. A temperature Map, X-Section, and Sounding.

*The panoramic view is the way we see and photograph the landscape but it is almost never used in TV weather reports. Soundings are also rarely used in TV weather reports. But both are very good at illustrating weather processes and phenomena.

The Sounding or Core

This is a core drilled through a tree to see the annual growth rings. Each tree ring is a compound layer that consists of rapid spring growth (light colored, less dense wood) and slower summer growth (dark colored, dense wood) Similar cores are drilled through glaciers and the great ice sheets. They also have compound annual rings consisting of light winter snow and denser summer snow.

Atmospheric soundings consist of air and are invisible. They must therefore be color coded or graphed for quantities such as temperature and Humidity as functions of height or pressure.

Great Weather Web Site www.rap.ucar.edu/weather

Another great weather website http://weather.unisys.com

In addition to current weather data and forecasts, this site is an archive of images of past weather, sea surface temperatures and hurricanes.

TV Weather Reports

Television weather reports now include magnificent graphics. This presentation illustrates their main ingredients for the morning of 27 August 2011 when Hurricane Irene was battering North Carolina and Virginia. Included are

Satellite images

Radar images

Surface weather maps

Upper air constant pressure charts

Soundings

Visible Satellite Images show reflected light. Cloud tops, snow, and ice appear white, sand gray, and land and water dark.

Infrared Satellite Images are really color-coded temperature maps that record energy radiated by surfaces. High or thick clouds, snow, and ice are typically color coded blue or violet, low clouds, land surfaces and cold waters are color coded yellow or orange, and heated land during day is coded dark red.

Radar Images record microwaves reflected by larger particles in the atmosphere such as raindrops, snowflakes and hailstones. They are coded to indicate intensity. Thunderstorms appear red, winter rain and snow yellow and green, and drizzle blue.

Images with Hurricane Irene (2011) appear in the next 3 slides.

Images of the Atmosphere

Visible Satellite Image of Hurricane Irene at 1545 UTC (1145 EDT) 27 August 2011. Under the clouds, the sky is dark and dismal but viewed from above in Space, clouds appear bright white because they reflect sunlight. Most ground and water appear dark from Space because they reflect poorly.

Infrared (IR) Satellite Image is really a temperature map of the ground or tops of clouds. This image shows that the center of Irene consists of cold, high (and thick) clouds while temperature is much warmer in the clear region around the storm. Look back to the previous slide to see that the clouds in Eastern Kentucky and Tennessee (see the yellow arrow) are barely visible in the IR because their tops are much lower and much warmer.

The radar chart shows where it is raining and is color coded to indicate the intensity of the rain. Rain is most intense in the spiral bands and part of the eye wall around the central eye, where there is almost no rain.

Surface Weather MapsSurface weather maps depict1. Features such asa. Fronts (blue and red lines)b. Low and High pressure areas (L’s and H’s).c. Regions and types of Precipitation (various colors).

2. Variables such asa. Temperature (color filled or dashed lines)b. Pressure (solid lines called isobars)c. Wind (arrows or flags)d. Humidity (using Dew Point Temperature, Td*)

* Dew Point Temperature Td, is a difficult concept. It is the temperature at which air becomes saturated (and dew forms on grass). The smaller the difference between Td and temperature, T, the higher the humidity. On weather maps, Td is never higher than T. When T = Td, Relative Humidity, RH = 100% and you are either in fog or clouds.

http://www.weather.com/maps/maptype/currentweatherusnational/index_large.html

A ‘Friendly’ Surface Weather Map

A ‘Friendly’ Surface Temperature Map

* Weather symbols include Snow (), Rain (), Fog (), Freezing Rain (), Sleet (), Showers (), Thunderstorm ( ). *** Either 900 or 1000 must be added to get the actual Sea Level Pressure, (p = 995.6) which is almost always close to 1000 hPa.

Reading Professional Weather Maps: The Station Model

Most professional weather maps are not so friendly as the last two maps but contain data from weather stations across the country. In order to read a weather map we must first know how to read the weather information at each station, which is presented in a standard coded form called the weather station model. The station model on this slide is highly simplified. The next slide shows a raw, unanalyzed surface weather map for the Southeast United States when Gaston came ashore.

T (F) = Upper left number.Td (F) = Lower left number.Cloud Cover = filled fraction of the circle.Weather type given by symbols to left of circle. Symbol chart is needed.*P (0.1hPa) Upper right number***Wind direction along the shaft from feathers to circle. Direction is named for where it comes from. Wind Speed (knots, kt) given by feathers on the shaft. Full feather = 10 kt, Flag = 50 kt, Double circle = Calm

To read this map you must isolate each weather station and then decode the data using the station model.

Depicting Weather Aloft - Constant Pressure ChartsWind, temperature, and humidity high above ground level are depicted on charts at constant pressure levels - 850, 700, 500, 300, 250, and 200 hPa (hectoPascals or millibars). The Jet Stream, a band of strong winds about 10 km above Sea Level, is often shown as a wavy arrow on simplified maps but is indicated by wavy contour lines in the charts of the next two slides at p = 700 and 200 hPa. The contour lines represent the height at which pressure is 700 hPa are almost parallel to the wind flow. Humidity is color contoured at 700 hPa and wind speed at 200 hPa.

Note that the hurricane appears as a closely contoured bulls-eye at 700 hPa but has disappeared by 200 hPa while the jet stream stretching from west to east across the US-Canada border has much faster winds at 200 hPa than 700 hPa. The jet stream winds are also faster (up to 100 knots here) than the winds in Hurricane Irene!!!

Constant Pressure Chart Station Model KeyT (C) = Upper left number.T-Td (C) = Lower left number.Height (10’s of meters, m) Upper right numberWind direction along the shaft from feathers to circle. Wind Speed (kt) given by feathers on the shaft. Full feather = 10 kt. Flag = 50 kt. Double circle = calm

459-17

12

700 hPaRelative Humidity, RH

200 hPaWind Speed (1 knot = 1.15 mph)

Soundings

Vertical profiles of wind, temperature, and humidity (dew point temperature) are called soundings. They are seldom shown on TV Weather Reports perhaps because they are difficult to read.

The next slide contains a sounding in Tropical Storm Gaston depicted on a skew-T diagram. It shows that temperature generally decreases with height. Lowest temperatures and fastest winds (except in hurricanes) often occur at the tropopause, around 150 hPa (near 17 km) in the tropics and 300 hPa (6 – 8 km) near the Poles.

The Skew-T diagram is the standard weather sounding diagram but is difficult to read.

The horizontal light blue lines are isobars or lines of constant p. The light blue lines that tilt up to the right are isotherms or lines of constant T.

The sounding consists of vertical profiles of T (red line) and dew point Temperature, Td (green line) and wind (feathered arrows).

For example at p = 400 mb, T = -8C, Td = -11C and the wind is SSE at 30 knots

Temperature (ºC) -50 -40 -30 -20 T = -8

Td = -11

Sounding near Irene’s Eye. Air is saturated or very humid. Winds are fast near the ground and weaken with height.