In this page I wont to talk about weather sensors, in particular I'll talk about Aeronautical Weather Sensors.
In airport we can find many sensor such as:

• Visibility Sensors
• Barometers
• Ultrasonic Anemometers
• Thermo -  Hygrometer
• Rain gauge
• Celiometer
• Lightning detector (about this argument, please go to see the dedicate page)

VISIBILITY SENSOR

 

Visibility Sensor measures transparency of the atmosphere and calculates its extension coefficient and meteorological optical range (MOR) values. Using both direct attenuation and forward scatter technologies, it can measure airborne particle sizes once available only from a transmissometer, while having the reliability and cost-effectiveness of a forward scatter visibility sensor.
Its communication can be made using a serial communication RS 232 or RS 485.
Visibility Sensor also give you the present weather using the WMO Code.

​

WMO code 4677: Present weather reported from a manned station.          

00 - Cloud development not observed or not observable                       
01 - Cloud generally dissolving or becoming less developed                  
02 - State of sky on the whole unchanged                                    
03 - Clouds generally forming or developing                                 
04 - Visibility reduced by smoke, e.g. veldt or forest fires, industrial smoke or
     volcanic ashes                                     
05 - Haze                                                                   
06 - Widespread dust in suspension in the air, not raised by wind at or near the
     station at the time of observation                         
07 - Dust or sand raised by wind at or near the station at the time of
     observation, but not well-developed dust whirl(s) or sand whirl(s),
     and no duststorm or sandstorm seen; or, in the case of ships, blowing
     spray at the station                                    
08 - Well-developed dust or sand whirl(s) seen at or near the station during the
     preceding hour or at the time of observation, but no dust storm or sandstorm                                                
09 - Duststorm or sandstorm within sight at the time of observation, or at the
     station during the preceding hour                            
10 - Mist                                                                   
11 - Patches of shallow fog or ice fog at the station, whether on land or sea not
     deeper than about 2 metres on land or 10 metres at sea      
12 - More or less continuous shallow fog or ice fog at the station, whether on
     land or sea, not deeper than about 2m/land or 10m/sea       
13 - Lightning visible, or thunder heard                                    
14 - Precipitation within sight, not reaching the ground or the surface of the
     sea                                                 
15 - Precipitation within sight, reaching the ground or the surface of the sea,
     but distant, i.e. > 5 km from the station                     
16 - Precipitation within sight, reaching the ground or the surface of the sea,
     near to, but not at the station                               
17 - Thunderstorm, but no precipitation at the time of observation          
18 - Squalls at or within sight of the station during the preceding hour or at
     the time of observation                                     
19 - Funnel clouds at or within sight of the station during the preceding hour or
     at the time of observation                           
20 - Drizzle (not freezing) or snow grains, not falling as showers, during the
     preceding hour but not at the time of observation           
21 - Rain (not freezing), not falling as showers, during the preceding hour but
     not at the time of observation                                
22 - Snow, not falling as showers, during the preceding hour but not at the
     time of observation                                             
23 - Rain and snow or ice pellets, not falling as showers; during the preceding
     hour but not at the time of observation                      
24 - Freezing drizzle or freezing rain; during the preceding hour but not at the
     time of observation                                         
25 - Shower(s) of rain during the preceding hour but not at the time of
     observation                                         
26 - Shower(s) of snow, or of rain and snow during the preceding hour but not at
     the time of observation                                     
27 - Shower(s) of hail, or of rain and hail during the preceding hour but not at
     the time of observation                                     
28 - Fog or ice fog during the preceding hour but not at the time of observation                                                            
29 - Thunderstorm (with or without precipitation) during the preceding hour but
     not at the time of observation                                
30 - Slight or moderate duststorm or sandstorm - has decreased during the
     preceding hour                                                     
31 - Slight or moderate duststorm or sandstorm - no appreciable change during
     the preceding hour                                              
32 - Slight or moderate duststorm or sandstorm - has begun or has increased
     during the preceding hour                                    
33 - Severe duststorm or sandstorm - has decreased during the preceding hour                                                         
34 - Severe duststorm or sandstorm - no appreciable change during the preceding
     hour                                                         
35 - Severe duststorm or sandstorm - has begun or has increased during the
     preceding hour                                                     
36 - Slight/moderate drifting snow - generally low (below eye level)        
37 - Heavy drifting snow - generally low (below eye level)                  
38 - Slight/moderate blowing snow - generally high (above eye level)        
39 - Heavy blowing snow - generally high (above eye level)                  
40 - Fog or ice fog at a a distance at the time of observation, but not at
     station during the preceding hour, the fog or ice fog extending to a
     level above that of  the observer                       
41 - Fog or ice fog in patches                                              
42 - Fog/ice fog, sky visible, has become thinner during the preceding hour                                                                   
43 - Fog/ice fog, sky invisible, has become thinner during the preceding hour                                                         
44 - Fog or ice fog, sky visible, no appreciable change during the past hour                                                              
45 - Fog or ice fog, sky invisible, no appreciable change during the preceding
     hour                                                         
46 - Fog or ice fog, sky visible, has begun or has become thicker during
     preceding hour                                                  
47 - Fog or ice fog, sky invisible, has begun or has become thicker during the
     preceding hour                                              
48 - Fog, depositing rime, sky visible                                      
49 - Fog, depositing rime, sky invisible                                    
50 - Drizzle, not freezing, intermittent, slight at time of ob.             
51 - Drizzle, not freezing, continuous, slight at time of ob.               
52 - Drizzle, not freezing, intermittent, moderate at time of ob.           
53 - Drizzle, not freezing, continuous, moderate at time of ob.             
54 - Drizzle, not freezing, intermittent, heavy at time of ob.              
55 - Drizzle, not freezing, continuous, heavy at time of ob.                
56 - Drizzle, freezing, slight                                              
57 - Drizzle, freezing, moderate or heavy (dense)                           
58 - Rain and drizzle, slight                                               
59 - Rain and drizzle, moderate or heavy                                    
60 - Rain, not freezing, intermittent, slight at time of ob.                
61 - Rain, not freezing, continuous, slight at time of ob.                  
62 - Rain, not freezing, intermittent, moderate at time of ob.              
63 - Rain, not freezing, continuous, moderate at time of ob.                
64 - Rain, not freezing, intermittent, heavy at time of ob.                 
65 - Rain, not freezing, continuous, heavy at time of ob.                   
66 - Rain, freezing, slight                                                 
67 - Rain, freezing, moderate or heavy                                      
68 - Rain or drizzle and snow, slight                                       
69 - Rain or drizzle and snow, moderate or heavy                            
70 - Intermittent fall of snowflakes, slight at time of ob.                 
71 - Continuous fall of snowflakes, slight at time of ob.                   
72 - Intermittent fall of snowflakes, moderate at time of ob.               
73 - Continuous fall of snowflakes, moderate at time of ob.                 
74 - Intermittent fall of snowflakes, heavy at time of ob.                  
75 - Continuous fall of snowflakes, heavy at time of ob.                    
76 - Diamond dust (with or without fog)                                     
77 - Snow grains (with or without fog)                                      
78 - Isolated star-like snow crystals (with or without fog)                 
79 - Ice pellets                                                            
80 - Rain shower(s), slight                                                 
81 - Rain shower(s), moderate or heavy                                      
82 - Rain shower(s), violent                                                
83 - Shower(s) of rain and snow, slight                                     
84 - Shower(s) of rain and snow, moderate or heavy                          
85 - Snow shower(s), slight                                                 
86 - Snow shower(s), moderate or heavy                                      
87 - Shower(s) of snow pellets or small hail, with or without rain or rain and
     snow mixed - slight                                           
88 - Shower(s) of snow pellets or small hail, with or without rain or rain and
     snow mixed - moderate or heavy                                
89 - Shower(s) of hail, with or without rain or rain and snow mixed, not
     associated with thunder - slight                                   
90 - Shower(s) of hail, with or without rain or rain and snow mixed, not
     associated with thunder - moderate or heavy                        
91 - Slight rain at time of observation - Thunderstorm during the preceding hour
     but not at time of observation                          
92 - Moderate or heavy rain at time of observation - Thunderstorm during the
     preceding hour but not at time of observation               
93 - Slight snow, or rain and snow mixed or hail at time of observation -
     Thunderstorm during the preceding hour but not at time of observation                                                    
94 - Moderate or heavy snow, or rain and snow mixed or hail at time of
     observation - Thunderstorm during the preceding hour but not at
     time of observation                                                 
95 - Thunderstorm, slight or moderate, without hail, but with rain and/or snow at
     time of observation                                     
96 - Thunderstorm, slight or moderate, with hail at time of ob.             
97 - Thunderstorm, heavy, without hail, but with rain and/or snow at time of
     observation                                                    
98 - Thunderstorm combined with dust/sandstorm at time of observation       
99 - Thunderstorm, heavy with hail at time of observation





Barometer​

Barometer measures the atmospheric pressure. When the pressure changes, the silicon diaphragm bend and changes the height of the vacuum gap in the sensor. As the vacuum changes the capacitance of the sensor changes, which is measured and converted into a pressure reading. The pressure sensor combines three powerful techniques for its superior performance: the use of single crystal silicon as sensor material, the capacitive measurement principle and the air sampling inlet. Silicon offers good elasticity, low hysteresis, excellent repeatability, small temperature dependence and superior long-term stability.
Its communication is a simple RS232 serial communication.









Anemometer

 

An anemometer is a device for measuring wind speed, and is a common weather station instrument. Sonic anemometers, first developed in the 1970s, use ultrasonic sound waves to measure wind velocity. They measure wind speed based on the time of flight of sonic pulses between pairs of transducers. Measurements from pairs of transducers can be combined to yield a measurement of velocity in 1-, 2-, or 3-dimensional flow. The spatial resolution is given by the path length between transducers, which is typically 10 to 20 cm. Sonic anemometers can take measurements with very fine temporal resolution, 20 Hz or better, which makes them well suited for turbulence measurements. The lack of moving parts makes them appropriate for long term use in exposed automated weather stations and weather buoys where the accuracy and reliability of traditional cup-and-vane anemometers is adversely affected by salty air or large amounts of dust.









Thermo -  Hygrometer

 

Automated airport weather stations use a temperature/dew point sensor (hygrothermometer) designed for continuous operation which normally remains on at all times, except during maintenance.
The measurement of temperature is simple compared to the dew point. Operating under the principle that electrical resistance varies with temperature, a platinum wire resistive temperature device measures the ambient air temperature.
In contrast, the dew point measurement is considerably more complex. The dew point sensor contains a chilled mirror that is cooled to the point where a fine film of condensation forms on the mirror's surface. The temperature of the mirror at this condition is equal to the dew point temperature. The hygrometer measures the dew point by directing a light beam from a small infrared diode to the surface of the mirror at an angle of 45 degrees. Two photo transistors are mounted so they measure a high degree of reflected light when the mirror is clear (direct) and scattered light when the mirror is clouded with visible condensation (indirect). With the formation of condensation on the mirror, the degree of cloudiness of the mirror surface increases with the direct transistor receiving less light and the indirect transistor more light. The output from these photo transistors controls the mirror cooling module which is an electronic heat pump that operates much like a thermo-couple in reverse, producing a heating or cooling effect. When the sensor is first activated, the mirror is clear. As the mirror surface temperature is cooled to the dew point temperature, condensations forms on the mirror. The electronics continuously tries to stabilize the signal levels to the power amplifier to maintain the mirror temperature at the dew point. If the dew point of the air changes or if the circuit is disturbed by noise, the loop makes the necessary corrections to re-stabilize at the dew point and maintaining continuous operation.





Rain gauge​

Rain gauge is an meteorological instrument for determing the depth of precipitation (usually in mm) that occurs over a unit area (usually one metre squared) and thus measuring rainfall amount. One millimetre of measured precipitation is the equivalent of one litre of rainfall per metre squared.
Rain gauge is the autographic gauge which can be either of the tilting-siphon type or the tipping-bucket type. The recording chart on an autographic rain-gauge is mounted on a drum which is driven by clockwork and typically rotates round a vertical axis once per day. For a tilting-siphon rain gauge, the rainwater in a collector displaces a float so that a marking pen attached to the float makes a continuous trace on the paper. The two buckets in a tipping bucket rain gauge rest on a pivot so that when one bucket has received 0.2 (or 0.5 mm) of rain it tips by gravity, empties the rainwater and allows the other bucket to start collection. During the tip, an electrical switch is closed and triggers a nearby autographic recorder to register each 'tilt', thus giving a fairly continuous record of precipitation and, in a more sophisticated form, even rainfall intensity. Rain gauges must be sited in as representative a location as possible, but the choice of location is difficult, since many precipitation events are highly localized.​









Celiometer

A ceilometer is a device that uses a laser or other light source to determine the height of a cloud base. Ceilometers can also be used to measure the aerosol concentration within the atmosphere.
Ceilometer consist of a vertically pointing laser and a receiver in the same location. A laser pulse with a duration on the order of nanoseconds is sent through the atmosphere. As the beam travels through the atmosphere, tiny fractions of the light are scattered by aerosols. Generally, the size of the particles in question are similar in size to the wavelength of the laser. A small component of this scattered light is directed back to the lidar receiver. The timing of the received signal can be transformed into a spatial range, z, by using the speed of light. I.e.,



                        distance = cdt/2





​

where c is the light speed in the air.

In this way, each pulse of laser light results in a vertical profile of aerosol concentration within the atmosphere. Generally, many individual profiles will be averaged together in order to reduce the signal to noise ratio and average profiles are reported on a time scale of seconds. The presence of clouds or water droplets leads to a very strong return signal compared to background levels, which allows for cloud heights to be easily identified.
For cloud base determination purpose, due to the ceilometers ability to pick up any particle in the air (dust, precipitation, smoke, etc...), it will give occasional false readings.