ATC has numerous tools available to pilots which can keep pilots safe from adverse weather
ATC Radar Weather Display:
ATC radars are able to display areas of precipitation by sending out a beam of radio energy that is reflected back to the radar antenna when it strikes an object or moisture, which may be in the form of rain drops, hail, or snow
The larger the object is, or the more dense its reflective surface, the stronger the return will be presented
Radar weather processors indicate the intensity of reflective returns in terms of decibels (dBZ)
ATC systems cannot detect the presence or absence of clouds
The ATC systems can often determine the intensity of a precipitation area, but the specific character of that area (snow, rain, hail, VIRGA, etc.) cannot be determined
For this reason, ATC refers to all weather areas displayed on ATC radar scopes as "precipitation"
All ATC facilities using radar weather processors with the ability to determine precipitation intensity, will describe the intensity to pilots as:
"LIGHT" (< 26 dBZ)
"MODERATE" (26 to 40 dBZ)
"HEAVY" (> 40 to 50 dBZ)
"EXTREME" (> 50 dBZ)
ATC facilities that, due to equipment limitations, cannot display the intensity levels of precipitation, will describe the location of the precipitation area by geographic position, or position relative to the aircraft
Since the intensity level is not available, the controller will state "INTENSITY UNKNOWN"
ARTCC facilities normally use a Weather and Radar Processor (WARP) to display a mosaic of data obtained from multiple NEXRAD sites
There is a time delay between actual conditions and those displayed to the controller
The precipitation data on the ARTCC controller's display could be up to 6 minutes old
Enroute ATC radar's Weather and Radar Processor (WARP) does not display light precipitation intensity
When the WARP is not available, a second system, the narrow-band Air Route Surveillance Radar (ARSR) can display two distinct levels of precipitation intensity that will be described to pilots as "MODERATE" (30 to 40 dBZ) and "HEAVY TO EXTREME" ( > 40 dBZ )
The WARP processor is only used in ARTCC facilities
ATC radar is not able to detect turbulence
Generally, turbulence can be expected to occur as the rate of rainfall or intensity of precipitation increases
Turbulence associated with greater rates of rainfall/precipitation will normally be more severe than any associated with lesser rates of rainfall/precipitation
Turbulence should be expected to occur near convective activity, even in clear air
Thunderstorms are a form of convective activity that imply severe or greater turbulence. Operation within 20 miles of thunderstorms should be approached with great caution, as the severity of turbulence can be markedly greater than the precipitation intensity might indicate
Weather Avoidance Assistance:
To the extent possible, controllers will issue pertinent information on weather or chaff areas and assist pilots in avoiding such areas when requested
Pilots should respond to a weather advisory by either acknowledging the advisory or by acknowledging the advisory and requesting an alternative course of action as follows:
Request to deviate off course by stating the number of miles and the direction of the requested deviation
In this case, when the requested deviation is approved, navigation is at the pilot's prerogative, but must maintain the altitude assigned by ATC and to remain within the specified mileage of the original course
An approval for lateral deviation authorizes the pilot to maneuver left or right within the lateral limits specified in the clearance
It is often necessary for ATC to restrict the amount of lateral deviation ("twenty degrees right," "up to fifteen degrees left," "up to ten degrees left or right of course")
The term "when able, proceed direct," in an ATC weather deviation clearance, refers to the pilot's ability to remain clear of the weather when returning to course/route
Request a new route to avoid the affected area
Request a change of altitude
Request radar vectors around the affected areas
For obvious reasons of safety, an IFR pilot must not deviate from the course or altitude or flight level without a proper ATC clearance
When weather conditions encountered are so severe that an immediate deviation is determined to be necessary and time will not permit approval by ATC, the pilot's emergency authority may be exercised
When the pilot requests clearance for a route deviation or for an ATC radar vector, the controller must evaluate the air traffic picture in the affected area, and coordinate with other controllers (if ATC jurisdictional boundaries may be crossed) before replying to the request
It should be remembered that the controller's primary function is to provide safe separation between aircraft
Any additional service, such as weather avoidance assistance, can only be provided to the extent that it does not derogate the primary function
It's also worth noting that the separation workload is generally greater than normal when weather disrupts the usual flow of traffic
ATC radar limitations and frequency congestion may also be a factor in limiting the controller's capability to provide additional service
It is very important, therefore, that the request for deviation or radar vector be forwarded to ATC as far in advance as possible
Delay in submitting it may delay or even preclude ATC approval or require that additional restrictions be placed on the clearance
Insofar as possible, the following information should be furnished to ATC when requesting clearance to detour around weather activity:
Proposed point where detour will commence
Proposed route and extent of detour (direction and distance)
Point where original route will be resumed
Flight conditions (IFR or VFR)
Any further deviation that may become necessary as the flight progresses
Advise if the aircraft is equipped with functioning airborne radar
To a large degree, the assistance that might be rendered by ATC will depend upon the weather information available to controllers
Due to the extremely transitory nature of severe weather situations, the controller's weather information may be of only limited value if based on weather observed on radar only
Frequent updates by pilots giving specific information as to the area affected, altitudes, intensity, and nature of the severe weather can be of considerable value
Such reports are relayed by radio or phone to other pilots and controllers, and also receive widespread teletypewriter dissemination
Obtaining IFR clearance or an ATC radar vector to circumnavigate severe weather can often be accommodated more readily in the en-route areas away from terminals, because there is usually less congestion and, therefore, offer greater freedom of action
In terminal areas, the problem is more acute because of traffic density, ATC coordination requirements, complex departure and arrival routes, adjacent airports, etc.
As a consequence, controllers are less likely to be able to accommodate all requests for weather detours in a terminal area or be in a position to volunteer such routing to the pilot
Nevertheless, pilots should not hesitate to advise controllers of any observed severe weather and should specifically advise controllers if they desire circumnavigation of observed weather
Procedures for Weather Deviations and Other Contingencies in Oceanic Controlled Airspace:
Product Parameters for Low/Medium/High Altitude Tier Radios
When the pilot initiates communications with ATC, rapid response may be obtained by stating "WEATHER DEVIATION REQUIRED" to indicate priority is desired on the frequency and for ATC response
The pilot still retains the option of initiating the communications using the urgency call "PAN-PAN" 3 times to alert all listening parties of a special handling condition which will receive ATC priority for issuance of a clearance or assistance
ATC will:
Approve the deviation
Provide vertical separation and then approve the deviation; or
If ATC is unable to establish vertical separation, ATC must advise the pilot that standard separation cannot be applied; provide essential traffic information for all affected aircraft, to the extent practicable; and if possible, suggest a course of action. ATC may suggest that the pilot climb or descend to a contingency altitude (1,000 feet above or below that assigned if operating above FL 290; 500 feet above or below that assigned if operating at or below FL 290)
Phraseology:
"Standard separation not available, deviate at pilot's discretion; suggest [Climb/Descent] to [Altitude]; Traffic [Location]; Report deviation complete"
The pilot will follow the ATC advisory altitude when approximately 10 NM from track, as well as execute the procedures detailed in paragraph 7-1-12c5
If contact cannot be established or revised ATC clearance or advisory is not available and deviation from track is required, the pilot shall take the following actions:
If possible, deviate away from an organized track or route system
Broadcast aircraft position and intentions on the frequency in use, as well as on frequency 121.5 MHz at suitable intervals stating: flight identification (operator call sign), flight level, track code or ATS route designator, and extent of deviation expected
Watch for conflicting traffic both visually and by reference to TCAS (if equipped)
Turn on aircraft exterior lights
Deviations of less than 10 NM or operations within COMPOSITE (NOPAC and CEPAC) Airspace, should REMAIN at ASSIGNED altitude. Otherwise, when the aircraft is approximately 10 NM from track, initiate an altitude change based on [Figure 2]
When returning to track, be at assigned flight level when the aircraft is within approximately 10 NM of centerline
If contact was not established prior to deviating, continue to attempt to contact ATC to obtain a clearance. If contact was established, continue to keep ATC advised of intentions and obtain essential traffic information
