Nothing on the Scope Is Weather: Inside a Controller's Weather Diversion Decision (2026)

This episode follows Maya Chen, a fictional air traffic controller, through the decisions behind a storm day — first the deviation requests in front of her, then the flow of traffic she has to rebuild around them.

The pilot decides where an airplane flies around weather. The controller decides everything else. The person on the radar approves the deviation or hands back a version that keeps the traffic apart, protects the separation minimum while the route bends, and passes the aircraft to the next sector with an honest account of what it is doing. When the weather stops moving and sits on the route, the decision leaves the scope and becomes a rate — how many arrivals an airport can still accept in an hour — and a fixed ladder of tools for bringing demand down to meet it.

The decision in one page

The pilot owns the weather; the controller owns the separation. The regulation says the pilot in command is "directly responsible for, and is the final authority as to, the operation of that aircraft." The manual puts the boundary the other way round for controllers: the primary function is safe separation, and weather avoidance assistance is an additional service provided only so far as it does not undermine that. 1 2
The scope shows precipitation, not danger. Controllers describe radar-derived weather with four words, from the lowest descriptor to the highest — light, moderate, heavy, extreme. ATC radar cannot see clouds or turbulence, and the order says not to use the word "turbulence" when describing radar-derived weather at all. In a center the mosaic on the display can be up to six minutes old. Controllers must also broadcast hazardous weather that may affect operations within 150 nautical miles of their sector. 2 3
A deviation is a clearance with limits. When a pilot requests it, controllers provide radar navigational guidance or approve deviations around weather, and the order tells them to plan ahead and be ready to suggest alternative routes or altitudes. An approval for lateral deviation authorizes the pilot to maneuver left or right within the lateral limits named in the clearance; it does not release the assigned altitude. Where traffic or airspace prevents the request, the controller must take an alternate course of action that still satisfies the pilot's need to avoid the weather. 3
Three miles is about forty-three seconds. Radar separation is 3 miles inside 40 miles of the antenna in terminal airspace, rising to 5 beyond that; 5 miles in a center, or 3 miles in defined areas up to and including FL230 that meet the order's conditions; and 1,000 feet vertically. At 250 knots, an aircraft covers 3 miles in roughly 43 seconds. 4
The handoff has to say what the aircraft is doing. With an aircraft deviating for weather, the transferring controller must clearly coordinate the nature of the route guidance being provided, and the order stresses the difference between controller-assigned headings, suggested headings, and pilot-initiated deviations. A suggested heading can be left; an assigned heading cannot. 3
Capacity is a number, and the tools are ranked. The airport arrival rate is a dynamic parameter giving the number of arriving aircraft an airport or airspace can accept from the center per hour, used to calculate the interval between successive arrivals. Policy requires the least restrictive method available, and the command center's severe weather plan lists its options in order: expanded miles-in-trail, a severe weather avoidance plan advisory, reroutes, ground delay programs, airspace flow programs, then ground stops. 5 6 7
The spacing restrictions have thresholds. A miles-in-trail restriction of 10 miles or less is coordinated between facilities locally and cannot exceed four hours; 25 miles or more requires a flow evaluation area shared with the command center; 40 miles or more requires a conference with the command center and the facility's general manager being told the reason. A delay is reportable at 15 minutes or more. 6
The delay is assigned on the ground. A ground delay program holds aircraft at their departure airports and assigns arrival slots; the release time is worked out from the estimated time en route and the slot, and delay assignment is computed in 15-minute increments. The stated purpose is to limit airborne holding — a standard holding pattern is right turns, at no more than 200 knots below 6,000 feet, 230 knots up to 14,000, and 265 knots above. 5 8 9
The ground stop is the last rung. A ground stop overrides every other traffic management initiative, and aircraft are not released from one without the approval of whoever issued it. A local ground stop applies where the affected facilities sit wholly inside one facility's area and conditions are not expected to last more than 30 minutes. The order lists the cases it is for: severely reduced capacity, precluding extended airborne holding, and routings unavailable because of severe weather. 10

What it looks like when it goes wrong is a single afternoon. On July 17, 2019, in the New York area, 108 aircraft diverted to alternate airports, 57 were assigned to holding patterns, and the day produced 565 departure cancellations, 535 arrival cancellations and 784 delays. On August 7, 2019, a further 171 airborne flights were held and 217 aircraft diverted. Across the six years from June 2017 to May 2023, the FAA's delay-by-cause reports put weather at 74.26 percent of system-impacting delays over 15 minutes, against 14.92 percent for volume alone. Planners need convective weather impact forecasts four to eight hours out to influence long-haul flights, and two to six hours out for shorter ones. 11

This is an explanation of how the framework works, not instruction for flying or for working traffic. The provisions quoted come from the current editions of the controller and pilot publications, and they change; the rules in effect on the day are the ones that apply.

References

The decision in one page

The pilot owns the weather; the controller owns the separation. The regulation says the pilot in command is "directly responsible for, and is the final authority as to, the operation of that aircraft." The manual puts the boundary the other way round for controllers: the primary function is safe separation, and weather avoidance assistance is an additional service provided only so far as it does not undermine that. 1 2

The scope shows precipitation, not danger. Controllers describe radar-derived weather with four words, from the lowest descriptor to the highest — light, moderate, heavy, extreme. ATC radar cannot see clouds or turbulence, and the order says not to use the word "turbulence" when describing radar-derived weather at all. In a center the mosaic on the display can be up to six minutes old. Controllers must also broadcast hazardous weather that may affect operations within 150 nautical miles of their sector. 2 3

A deviation is a clearance with limits. When a pilot requests it, controllers provide radar navigational guidance or approve deviations around weather, and the order tells them to plan ahead and be ready to suggest alternative routes or altitudes. An approval for lateral deviation authorizes the pilot to maneuver left or right within the lateral limits named in the clearance; it does not release the assigned altitude. Where traffic or airspace prevents the request, the controller must take an alternate course of action that still satisfies the pilot's need to avoid the weather. 3

Three miles is about forty-three seconds. Radar separation is 3 miles inside 40 miles of the antenna in terminal airspace, rising to 5 beyond that; 5 miles in a center, or 3 miles in defined areas up to and including FL230 that meet the order's conditions; and 1,000 feet vertically. At 250 knots, an aircraft covers 3 miles in roughly 43 seconds. 4

The handoff has to say what the aircraft is doing. With an aircraft deviating for weather, the transferring controller must clearly coordinate the nature of the route guidance being provided, and the order stresses the difference between controller-assigned headings, suggested headings, and pilot-initiated deviations. A suggested heading can be left; an assigned heading cannot. 3

Capacity is a number, and the tools are ranked. The airport arrival rate is a dynamic parameter giving the number of arriving aircraft an airport or airspace can accept from the center per hour, used to calculate the interval between successive arrivals. Policy requires the least restrictive method available, and the command center's severe weather plan lists its options in order: expanded miles-in-trail, a severe weather avoidance plan advisory, reroutes, ground delay programs, airspace flow programs, then ground stops. 5 6 7

The spacing restrictions have thresholds. A miles-in-trail restriction of 10 miles or less is coordinated between facilities locally and cannot exceed four hours; 25 miles or more requires a flow evaluation area shared with the command center; 40 miles or more requires a conference with the command center and the facility's general manager being told the reason. A delay is reportable at 15 minutes or more. 6

The delay is assigned on the ground. A ground delay program holds aircraft at their departure airports and assigns arrival slots; the release time is worked out from the estimated time en route and the slot, and delay assignment is computed in 15-minute increments. The stated purpose is to limit airborne holding — a standard holding pattern is right turns, at no more than 200 knots below 6,000 feet, 230 knots up to 14,000, and 265 knots above. 5 8 9

The ground stop is the last rung. A ground stop overrides every other traffic management initiative, and aircraft are not released from one without the approval of whoever issued it. A local ground stop applies where the affected facilities sit wholly inside one facility's area and conditions are not expected to last more than 30 minutes. The order lists the cases it is for: severely reduced capacity, precluding extended airborne holding, and routings unavailable because of severe weather. 10

Nothing on the Scope Is Weather: Inside a Controller's Weather Diversion Decision

The decision in one page

References

Related content

Nothing on the Scope Is Weather: Inside a Controller's Weather Diversion Decision

The decision in one page

References

Related content