Why Low Clouds and Poor Visibility Cause Delays

Airplanes fly through clouds all the time. Heck, thanks to the super intelligent autopilot systems installed on most airplanes they can even land themselves. But despite all of that technology one of the things that regularly causes the most delays is low clouds and poor visibility. But why?

Spacing, The FAA, and the Big Sky Theory of Aviation

Once upon a time there was no Air Traffic Control System within the United States. When commercial aviation was still in its infancy there just weren’t enough flights in the skies to make anyone concerned. Airlines would coordinate their schedules to make sure that generally no two airliners would be in the same place at the same time, local tower controllers would regulate airplanes taking off and landing to make sure that no one ran into each other in that very small area, and for the rest of the flight pilots relied on the “big sky” theory to keep them safe.

The idea here was that the sky was so very vast and there were so very few airplanes that running into each other was statistically improbable. And if they ever did meet, the pilots could “see and avoid” the other aircraft to keep everyone safe.

For decades the system worked, until in 1956 two fully loaded airliners — United flight 718 and TWA flight 2 — collided over the Grand Canyon.

Two years later in 1958 the U.S. Government established the Federal Aviation Administration (FAA) with the objective to design a system to make sure that such an accident never happens again. They worked together with other international organizations to design the rules and regulations that govern commercial aviation to this day.

One of the fundamental rules of this air traffic control system is the idea of keeping “spacing” or separation between aircraft. Accidents and mistakes can happen, but this spacing provides a buffer zone that allows for the mistake to be identified and fixed before another catastrophe happens.

The amount of “spacing” required between aircraft depends on the weather conditions. When visibility is good and there’s not a cloud in the sky pilots don’t need as much space between aircraft, often they are even allowed to maintain “visual separation” between themselves, meaning they can fly in fairly close formation as long as everyone can see each other. When there are clouds and poor visibility that visual separation isn’t possible and the required separation is increased.

How does this impact you, dear passenger? Well, bad weather means increased spacing requirements. Increased spacing requirements means fewer airplanes can fly through the same space in a given period of time. Which, in turn, means that fewer airplanes can take off and land at a given airport.

Especially at busy airports, the schedules are set up with the assumption of good weather. So if there’s poor weather and fewer airplanes can land then there are nearly guaranteed to be delays.

Spacing can slow things down, but it can also bring things to a screeching halt.

Approach Minimums and Why They Matter

Clouds happen, and airplanes still need to land even when the skies aren’t clear. To bring airplanes safely in to land even when they can’t see where they are going airlines and airports developed “approach procedures” that detailed a process for safely landing an aircraft at a specific runway using only the instruments within the aircraft, also called “instrument approach procedures.” The first instrument approach used by a commercial flight was in Pittsburgh, PA in 1938.

The concept behind an instrument approach procedure is to safely guide the aircraft down low enough that the pilots are able to see underneath the clouds, identify the runway, and land visually.

Some approach procedures are more accurate and reliable than others. As a result, in order to maintain the highest level of safety for everyone involved, less accurate approach methods aren’t allowed to get as close to the ground as more accurate methods. And depending on the training of the aircraft crew they might have additional requirements that try to reduce the risk of these instrument approach procedures by not going as low.

But in order to even attempt these approach procedures, the weather needs to be good enough that a pilot might be able to successfully see the runway and land from the lowest altitude they are allowed to descend. These weather requirements are called “approach minimums” and involve visibility, cloud cover, and other factors. If the weather at an airport is below these approach minimums then pilots will wait, either in a “holding pattern” in the air or decide to delay their departure from their original airport, until the weather improves.

Instrument approaches have gotten better and better over the years, to the point where an aircraft can now land itself automatically. So why can weather still slow down and stop aircraft?

Getting from the Runway to the Gate

Getting down is half the battle. Once you’re on the ground you still need to navigate from the runway to the gate. And all of the support vehicles need to be able to safely move around and pick up baggage, refuel the aircraft, and more.

Navigation systems are great in the air, but there hasn’t been a system designed yet that can safely guide an aircraft from the runway to the gate without seeing outside the cockpit. One day this technology will exist, but we just aren’t there yet.

In the meantime visibility will still have a major impact on the ability for aircraft to land at airports, even in an age where autoland systems are common.

Flight Delayed by “Flow Control” – What Is It?

Things were going so well.

You made it through airport security without getting groped. You arrived at your gate¬† with enough time to get one last bathroom break before boarding. You weren’t crushed to death in the mad dash to board when your group was called. And, most importantly, your airplane pushed back from the gate exactly on time with you safely tucked away in your seat.

Then things went horribly wrong.

You probably didn’t notice that your airplane took a left turn on that taxiway instead of a right, bringing it to a holding pad instead of the runway. But when the pilot came on the speaker and told you that “due to flow control” into your destination airport the aircraft can’t take off for a while you became keenly aware that you would now be cooling your heels for a while on the ground.

That’s the worst case scenario, stuck on an airplane for an unknown period of time because air traffic control won’t let you leave. But “flow control” can also strike when you are at the gate before you board.

But, why? Why did air traffic control pull this terrible trick on you?

Let me explain.


The All-Knowing ATC System

Every regularly scheduled commercial flight in the United States is required to file a flight plan for the flight they intend to operate. Wikipedia has some great information on what a flight plan is and the various parts (and no, it’s not the terrible 2005 movie starring Jodie Foster), but the reason why it is important for this topic is the timing. Every flight plan includes not only the expected departure time for the flight, but also how long the flight is expected to take and when they will actually arrive at the destination airport.

Flight plans give the FAA a very accurate picture of how many aircraft intend to reach a specific airport in the future.

Airports can only handle so many airplanes in a given time frame. Not only is there a limit to the number of aircraft that the controllers themselves can safely manage, there are legal requirements as well for how much space needs to be maintained between aircraft. This is called “spacing” and is designed to provide enough time for pilots and controllers to identify potentially hazardous situations and fix them before an accident can occur.

The spacing required between aircraft isn’t a fixed number, either. Spacing requirements are increased when the weather gets bad, which gives pilots and controllers a wider margin of error. Spacing can also vary depending on the kind of aircraft — large and fast aircraft require more spacing than small and slow aircraft to maintain the same margin of safety.

Or, alternatively, things could just be so miserable that the airport is completely closed. For example, if there’s a huge snowstorm or thunderstorm over the airport they may decide to stop releasing flights in that direction.

Based on the weather conditions at the airport and the number and type of aircraft arriving, the air traffic controllers can determine how many aircraft they can safely handle in a given period of time.

Armed with that information, as soon as your flight files their flight pan with the local air traffic control office the ATC system can determine if there’s enough space for your airplane to land when it arrives. If there isn’t enough space (due to poor weather or heavy traffic or any other of a variety of reasons) they can determine roughly how long you would need to wait before it was your turn to arrive.

Can’t You Do That Before We Leave?

Flight plans are typically filed around two hours prior to departure, but pilots are often in the dark about ATC’s plans until after they have already pushed back from the gate.

Just because you have filed a flight plan doesn’t mean that’s what ATC wants the flight to do. Depending on a whole number of circumstances, from weather to other air traffic in the area, ATC will often adjust the flight plan and re-route planes to make them easier to handle.

Usually a pilot only gets this information moments before they push back from the gate and start the engines. As part of the workflow pilots will make sure they have the latest weather report for the airport and any notices that have come out (called a Notice to Airmen or NOTAM) then call the tower to ask for their clearance.

There’s a difference here to note: a “flight plan” is what the pilot intends to fly, a “clearance” is what ATC will allow them to fly.

This is the point at which the pilot will get their first idea of what ATC has in mind for their flight. Sometimes the pilot gets their clearance while still at the gate, other times the pilot is told that their clearance is “on request” meaning that ATC isn’t done analyzing their flight plan and they will get it in a few minutes.

In either case the pilot typically has no idea before all the passengers are on board the plane and they are ready to go. There’s an effort called NextGen where ATC will be able to send flight plans digitally to each aircraft well in advance, but not every airplane is equipped with that system yet.

But Why Am I Delayed On The Ground?

So your flight is being asked to patiently wait its turn. Why not fly slower? Or just get there and then wait your turn? There’s two very good reasons.

First: things may get worse.

Delays are estimates. Sometimes the weather can deteriorate even further and the delays will need to be much longer to accommodate everyone. The airline might even realize that the delay is so long that it makes more sense to cancel the flight. Or maybe they want to bring everyone back to the gate so they can wait comfortably in the lounge instead of in their cramped seat.

In these cases it makes sense to be on the ground. Airplanes can return to the gate if needed, take on more gas to account for future delays, and have a lot more options than if they were in the air.

Second reason: fuel.

Airplanes only carry as much fuel as they need to complete a flight, plus a little more than 45 minutes of reserve fuel. Carrying any more would be expensive and wasteful. So as soon as that airplane pushes back from the terminal and the engines start turning that airplane only has a certain amount of time it can be airborne before it needs to refuel.

Flying slower would eat up more fuel. Airplanes fly as efficiently as possible to burn as little fuel as they possibly can. Fuel costs money and airlines, as I’m sure you can tell, don’t like spending money they don’t have to. So any change from the most efficient route, whether changing speed or anything else, will increase the fuel use for that flight. Increased fuel use means more money spent and a profitable flight can quickly turn into a money pit.

Not to mention the whole running out of gas issue. There aren’t any gas stations in the clouds. At least, not for civilians.

This Sucks

I know. Been there, both as a passenger and a pilot. Trust me, the pilots don’t like it either. They only get paid for the time they are flying, so sitting on the ground twiddling their thumbs doesn’t help anyone in this situation. Just know that there’s a very good reason why you are sitting on the ground instead of in the air, and eventually things will get better.

Why Snow and Ice are Dangerous for Aircraft

No other weather phenomenon causes as much chaos for airports as a snowstorm. Not only does it make things more dangerous for aircraft moving around on the ground, it also poses a huge risk to aircraft in the air and attempting to land as well.

There are three main reasons why snowstorms are so dangerous: reduced visibility, slippery surfaces, and icing.



Flying through clouds has rarely been an issue for pilots and their aircraft. Ever since the early 1930’s pilots have been trained to be able to safely pilot their aircraft from one place to another without needing to ever look out the window.¬†This is extremely useful in situations where the pilot can’t see outside their cockpit, such as when they are flying through clouds or snow. In these situations pilots need to rely on their aircraft’s instruments to get them safely to their destination, hence why it is called “instrument time” or “flying on instruments.”

When pilots can’t see out the window they need to follow a very specific set of rules, called the “Instrument Flight Rules” or “IFR.” These regulations were designed to keep everyone in the air safe by keeping airplanes further apart from each other than normal (to ensure they don’t accidentally run into each other in a cloud) and requiring pilots to use very specific instructions called an “instrument approach” to land at their destination airport. These approaches are specifically designed for each airport and the surrounding city or landscape so that airplanes trying to land won’t accidentally run into any tall buildings or mountains in the area.

Even with the best navigational instruments and the best trained pilots the Instrument Flight Rules still require the pilots to be able to see the runway for takeoff and landing. As aircraft instruments have improved over the years to be more accurate airplanes have been able to get closer and closer to the runway without needing to see the ground, but only a human pilot is reliable enough to safely land an aircraft on a runway. At the moment a human pilot typically needs to be able to see the runway from 200 feet above the ground and 1/2 mile from the end of the runway in order to legally continue the approach and landing, which is why when the cloud cover at an airport drops below 200 feet flights will typically need to cancel or divert to other airports.

Since 1968, some aircraft with very special equipment, specially trained pilots, and designated airports, have been able to use an “autoland” system which allows the flight computer to actually land the aircraft without human intervention. During these approaches and landings the systems are so finely tuned that flight attendants will require passengers to power off all of their electronic devices so that no stray signals will interfere with the instruments. While the computer might be able to get the airplane on the ground, no system has yet been designed to get the aircraft from the runway to its parking spot at the terminal without a human pilot being able to see outside. That’s why even with these systems aircraft still need the clouds to be at least 50 feet off the ground and at least 600 feet of horizontal visibility to land. Otherwise airplanes would land and get stuck on the runway.

Snow storms are notorious for creating situations with poor visibility. Heavy snow clouds can often be very close to the ground, sometimes below the 200 feet required for normal instrument approaches. The large snowflakes can obstruct the pilot’s vision keeping them from seeing the runway until it is too late, meaning the aircraft would be unable to land. And even when the snowstorm has stopped snow blowing across the runway can reduce the visibility to dangerous levels.

Slowing Down and Stopping

Once aircraft are able to see the runway and land, the next question is: should they?

On a normal sunny day with a dry runway, a normal Boeing 737 needs about 4,000 feet of runway to safely come to a stop. That usually isn’t an issue as even at DC’s Reagan National Airport (consistently ranked as one of the most difficult airports to land an airplane) the shortest runway offers a cool 5,204 feet.

If your runway isn’t perfectly dry then the airplane can’t stop as quickly, and as a result it needs more room to stop. On a compacted snow runway the same 737 needs an additional 800 feet to come to a stop, putting it dangerously close to the end of that 5,200 foot strip. If there’s a little bit of ice, or water, or slush, then the pilot needs another 2,400 feet to bring the plane to a stop — well past the end of that runway.

As you’d expect, larger aircraft need longer runways to stop. A Boeing 747 would need 7,400 feet of runway to come to a stop on a good day, significantly more when it has been snowing and the runway is wet.

Ice and snow don’t just increase the distance it takes for an airplane to slow down — it also makes the airplane much more difficult to control. Patches of ice on the runway can lead to uneven braking during touchdown, sometimes causing the airplane to spin. Even when the plane is safely on the ground, ice patches on the taxiway can still cause the aircraft to slip off the pavement.

Ice: An Airplane’s Worst Enemy

When there’s a snow storm blowing through getting rid of the snow and ice on the runway is the easy part. Getting it off the airplanes is the harder and much more important consideration.

Airplanes can only take off and fly when the lift generated by their wings is greater than the weight of the aircraft, and when the thrust of their engines is greater than the drag produced by the aircraft body. Which is why it is so critical in aircraft design and maintenance to keep the weight as low as possible, keep the body aerodynamic to reduce drag, and maintain the shape of the wings to create as much lift as possible. Ice and snow ruin all three of these important characteristics.

First and foremost, snow and ice is heavy. The Boeing 737 has about 1,340 square feet of surface area. If the entire wing were covered in just a half inch of ice — barely enough to be visible — that would add an additional 418 pounds (55.875 cubic feet of water) to the weight of the aircraft. That’s not even counting the ice accumulating on the rest of the body of the aircraft.

In a world where airlines are using lighter paper in their in-flight magazines to save gas, that extra weight could mean the airplane is now too heavy to make it to its destination.

That is, if the aircraft can even get off the ground. Snow and ice not only add weight but they also add drag by changing the shape of the aircraft. Once covered in ice and snow that sleek and shiny jet liner now looks more like a Chia Pet, covered in a fluffy white shag carpeting. All those extra nooks and crannies slow the aircraft down because they increase drag. If there isn’t enough thrust in the aircraft’s engines to overcome that extra drag then the aircraft might not be airborne for very long, if ever.

Ice isn’t just a problem for aircraft taking off and landing, though. Ice can accumulate on an aircraft any time they are in a cloud. Usually aircraft have systems designed to remove the ice and keep the airplanes flying, but if those systems aren’t working properly or if the pilots aren’t paying attention the ice can build up and cause issues with the third force we discussed: lift.

Wings create lift because of their unique shape, which uses the air flowing over the wing to causes pressure under the wing than over the wing and suck the airplane upwards (apologies to my flight instructor for the way over simplified explanation). If the shape of the wing is altered, such as when a good amount of ice builds up on its surface, the wing can’t produce the same level of lift. If the ice builds up past a critical point the wing cannot hold the airplane in the air any longer.

Like I Said, The Worst

I think you get the picture. Snow storms and ice storms are pretty much the mortal enemy of the airplane. They make everything more difficult, and as a result airlines just don’t want to operate anywhere near them. And as a passenger, I can’t blame them. I’ll happily drive or take a later flight if I know things will improve.

It can take ages to recover from a snow storm. Once the snow on the ground is removed it can still be days or weeks of below freezing weather, which means ice can still be an issue on the ground and in the air. And the clouds can linger causing visibility issues for pilots trying to land.

My best advice: be patient. And keep an eye on the forecast.