getting to grips with surveillance(92)

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meteorological phenomena linked to flight operations.
Note: for A380 aircraft, please refer to 7.1.4 – Weather Radar Function. However, the weather radar basic principles on-board the A380 remain identical to the ones described in this chapter.
6.1.1. RADAR THEORY
6.1.1.1. REFLECTIVITY OF WATER MOLECULES
The weather radar properly detects rains or wet turbulence. Indeed, water molecules in liquid state reflects radar pulses more than ice crystals do. Based on this principle, the reflectivity of precipitations depends on their nature. The radar pulse weakly reflects on dry snow and dry hail. In addition, wet hail presents a higher reflectivity than rain thanks to the size of hailstone combined with the presence of liquid water molecules on their surface. Most of the
1 Autotilt is a registered trademark of Honeywell.

time, the radar pulse is not able to penetrate wet hail. The weather behind wet hail is often hidden.
Similarly to wet hail,
stratiform rain in the vicinity (at or 3000 ft below) of the freezing level returns a large portion of the radar pulse. Indeed, in this area, ice crystals start to melt and are covered of water.
6.1.1.2. REFLECTIVITY OF THUNDERSTORMS
Based on the principle described above, weather radars are optimized to detect rains. Consequently, thunderstorms may be divided into four layers according to the reflectivity of each layer.
.  
The turbulence dome defines an
area of very severe turbulence. It
can reach several thousand feet
above the visible top, when the
thunderstorm is growing

.  
The upper part above the
altitude of –40°C (if applicable)
contains ice crystals only. It reflects
a very small portion of the radar
pulse. This part may be invisible on
the weather radar image whereas it
is clearly visible through the
windshield

.  
The intermediate part from the
freezing level up to the altitude
of –40°C contains ice crystals and
super-cooled water. The super-cooled water reflects a portion of
the radar pulse. Ice crystals absorb
the remainder of the radar pulse

.  
The lower part up to the
freezing level is the most
reflective part of the thunderstorm
due to the heavy rain.

 

The radar or wet top is the highest portion of the thunderstorm the weather radar can detect. It separates the intermediate part from the top part of a thunderstorm.
The visible top is the top of the thunderstorm upper part.

 

6.1.1.3. FREQUENCY BAND
As a general physics rule, the propagation of a wave is closely linked to its length or frequency.
wave celerity
As a reminder: wavelength =
wave frequency
Higher the frequency, shorter the wavelength, weaker the propagation.
　
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