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For teachers · Worksheet 4 of 5

What radar is and how it is extrapolated

Radar “sees” rain hundreds of kilometres away thanks to the echo from the drops. And if we know which way it is moving, we can anticipate what will happen over the next hour.

Aim

To understand what a weather radar measures, what it means to extrapolate its image (very short-range forecasting, or nowcasting) and why it only works for a short time.

A radar on the ground sends a pulse towards a rain cloud; part of the energy bounces off the drops and returns to the radar as an echo. radarpulseecho

Activity

  1. 10 min
    What the radar sees. In the Enthusiast view, switch on “Precipitation radar” (under “Observation”). Press ▶: first come the observed images from the last two hours and then the extrapolated ones, from +10 to +90 minutes. Pay attention to the time on each image.
    Open the dashboard
  2. 10 min
    Measuring the speed. Pick a core of rain and compare where it was in two images 30 minutes apart. Use the distance between two towns you know to work out how many km/h it is moving.
  3. 10 min
    When will it arrive? With that speed, work out when it would reach the school. In the simple view, “Next hour” gives the system's answer. Check it by looking out of the window and record it with “What's the weather like at school?” in the class challenge.
    Open the simple view

Questions

  1. What exactly does a weather radar measure? How does it know how far away the rain is?
  2. Why can there be areas with no data, or patches that are not rain?
  3. What are we assuming when we extrapolate the radar image?
  4. Why does the extrapolation get worse after an hour, especially with thunderstorms?
  5. To know whether it will rain in the next 30 minutes, what advantage does radar have over a numerical model?

Answers for teachers
  1. It sends out microwave pulses and measures the energy returned by raindrops, snowflakes or hail (the reflectivity): the stronger the echo, the heavier the precipitation. The distance comes from the time the pulse takes to travel there and back, at the speed of light.
  2. Mountains block the beam; in addition, the beam gains height with distance because of the Earth's curvature and can pass over the rain. There are also false echoes: the ground, rough seas, birds or aircraft.
  3. That the rain moves without changing. The dashboard estimates the movement by comparing two images 30 minutes apart, piece by piece, and shifts the latest one forward.
  4. Because it does not take into account that rain forms, grows and dies away: a shower or a thunderstorm can form or disappear within half an hour. That is why it is useful up to about 60 minutes, and at 90 minutes it should be treated with more caution.
  5. It starts from what is happening right now, in great detail and updated every 10 minutes. A numerical model takes hours to calculate and can place a shower several kilometres from where it actually falls.