National Weather Service United States Department of Commerce

Learning Lesson: Pie in the Sky


Meteorologists often refer to ridges and troughs in the upper atmosphere when explaining the reason for the weather one experiences. Yet all one sees are lines on a weather map. However, these weather systems are high and low places in the atmosphere as this lesson will demonstrate.

TOTAL TIME 5 minutes
SUPPLIES 8 to 10 clear flat salad tray tops (one set for yourself and/or one set per student). Permanent marker pens.
PRINTED/AV MATERIAL Base map; 500 mb chart
You can prepare this before hand, do it as a demonstration yourself, or have the students perform the demonstration. An Internet search will provide resources to obtain salad trays or ask a local deli to provide some trays. If both top and bottom of the trays are clear then they can be separated to provide two samples.
SAFETY FOCUS Turn Around, Don't Drown


  1. Print base map and position one salad tray on top of map.
  2. Using a black permanent marker, place a dot on the tray over each of the two dots on the map then trace the North America outline onto the salad tray. This is your base background image.
  3. While ensuring the North America outline and two dots line up with the underlying map, add a salad try on top. Place a dot on the tray over each of the dots on the bottom salad tray. These dots will serve as alignment points for the contours to be added later.
  4. Add a second salad tray and place a dot on the tray over each of the dots. Repeat with remaining salad trays. Eventually all salad trays will be stacked with both dots aligned on each tray.
  5. Remove all trays from the map and set the tray with the North America outline aside.
  6. Print 500 mb chart. Aligning with two dots position one salad tray on top of map.
  7. Beginning with the 582 contour (5,820 meter height of 500 mb pressure), trace that contour only onto the salad tray. Label that contour. Set salad tray aside. Note: the different colors on the 500 mb chart are to assist differentiating the different contours. All contours should be of one color other than black.
  8. Take another salad tray, align the dots and trace and label the 576 contour only.
  9. Continue until you have a different contour line on each salad tray. Remember some contours (example 564, 558) are located at more than one position on the map and will need to be added to the appropriate salad tray.
  10. Beginning with the 534 contour tray stack salad trays on top of one another (from lowest to highest contour) for a 3D view of the 500 mb chart.


When the salad are stacked properly it will be readily apparent that ridges and troughs are real high and low places in the atmosphere. So, when the meteorologists mentions an upper level ridge then the 500 mb heights is physically higher in the atmosphere.

Conversely, an upper level low pressure area where the height of the 500 mb level is lower in the atmosphere.

The spacing between contours is governed by the temperature of the atmosphere at that level. The closer the spacing the greater the difference in temperature at that location. It also means the wind speed at that location is faster than where the spacing between contours is greater.

To the right side of the trough axis the air is physically ascending as the height of the 500 mb level increases. These are the regions where precipitation is likely to occur due to the rising motion of the air.

Conversely, the left side of the trough axis the air is descending leading to fair weather.

Trough and ridge axis as well as the locations of where the precipitation is likely to occur.

Live weatherwise

Flash floods are the deadliest natural disaster in the world. They are often caused by stationary or slow-moving thunderstorms that produce heavy rain over a small area.

Hilly and mountainous areas are especially vulnerable to flash floods, where steep terrain and narrow canyons can funnel heavy rain into small creeks and dry ravines, turning them into raging walls of water. Even on the prairie, normally-dry draws and low spots can fill with rushing water during very heavy rain.

Preparations at home and work:
  • Know the name of the county where you live and nearby rivers and streams. Keep a map so you know where storms that may cause flash flooding are.
  • Determine if you are in a flood-prone area. If you are, know where to go if the water starts to rise. Have an escape route if you have to leave quickly.
  • Make a safety kit containing: A flashlight and extra batteries, battery-powered weather radio receiver and commercial radio, extra food and water, first-aid supplies, canned food and a can opener, water (three gallons per person), extra clothing, and bedding. Don't forget special items for family members such as diapers, baby formula, prescription or essential medications, extra eyeglasses or hearing aids, and pet supplies.
  • Know how and when to shut off utilities: Electricity, gas, and water.
  • Get local warning information from cable TV or the NOAA Weather Radio.