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weather picture is then assembled inside the computer, ground
clutter is removed, and the information is delivered to the flight
crew’s displays. The end result is an optimum weather picture from
the nose of the aircraft out to 320 NM (♦page 4-22).
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MultiScan™ Radar Table of Contents
TABLE OF CONTENTS
Title Page
Reflectivity ........................................................................................ 5-1
The Water Molecule and Reflectivity ............................................ 5-1
Bright Band ................................................................................... 5-4
Thunderstorm Reflectivity ............................................................. 5-5
Thunderstorms ................................................................................. 5-8
Introduction ................................................................................... 5-8
Single-Cell Thunderstorm Development ....................................... 5-8
Multi-Cell Thunderstorms ............................................................ 5-11
Steady-State Thunderstorms ...................................................... 5-13
Thunderstorm Characteristics ..................................................... 5-14
Squall Lines .................................................................................... 5-20
Microbursts And Windshear ........................................................... 5-22
Microbursts ................................................................................. 5-22
Windshear ................................................................................... 5-26
Hazardous Weather ....................................................................... 5-31
Introduction ................................................................................. 5-31
Steep Gradient ............................................................................ 5-33
Scalloped Edges, Pendant, Finger, Hook, U-Shape ................... 5-34
Non-Reflective Weather ................................................................. 5-37
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MultiScan™ Radar Reflectivity
AVIATION WEATHER
REFLECTIVITY
THE WATER MOLECULE AND REFLECTIVITY
A water molecule is composed of two hydrogen atoms with positive
polarity and one oxygen atom with negative polarity. Thus, the side of
the molecule where the hydrogen atoms are attached is positive while
the opposite side of the molecule is negative. This is referred to as a
bipolar (having two poles) condition (see figure 5-1).
Figure 5-1 Basic Water Molecule
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AVIATION WEATHER COLLINS
Reflectivity MultiScan™ Radar
Ice crystals are formed when the positive hydrogen atoms and the
negative oxygen atom of individual water molecules lock or freeze the
molecules into an ice crystal lattice. The molecules contained in the ice
crystal are unable to change orientation to respond to and reflect radar
energy and are thus extremely difficult for radar to detect.
When water molecules are in a liquid state they are attracted to each
other but are still free to move and rotate to some degree. Thus, when
radar energy hits these molecules, the poles of the molecule are able to
align and reflect significant amounts of radar energy.
An airborne radar’s pulse energy is optimized to detect water. The
radiated radar energy reflects best when the bipolar (positive and
negative) water molecules are able to align. When water freezes, the
water molecules are locked into an ice crystal lattice and are unable to
align to reflect radar energy efficiently. The same concepts are involved
in microwave cooking where water boils very quickly but frozen foods
take a great deal of time to thaw.
The end result is that ice crystals reflect very little radar energy while
water (rain) is an excellent radar reflector. Figure 5–2 shows different
kinds of precipitation and their ability to reflect radar energy.
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MultiScan™ Radar Reflectivity
Figure 5-2 Reflectivity Characteristics of Precipitation
Airborne weather radar has been optimized to detect rain. Rain drops
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Collins Weather Radar operator’s guide(27)
