There are some good comments from the piloting side, but there are other factors involved as well. From that ATC perspective though:
Airports and the surrounding airspace can only handle a limited amount of aircraft before things start to get dangerous. When weather conditions are less than optimal, the number of aircraft the system can handle decreases. For example, an airport may be able to safety handle 60 arrivals an hour in good weather with no risk to safety. During a snow storm, poor visibility may reduce that number to 40 aircraft (or less) per hour. The reduction in capacity means those aircraft may have to wait at their home airports, connections get missed, and generally fewer airplanes can go flying. The last thing anyone wants to to have more airplanes in the air than can safely be allowed to land. So the delays are shifted to departing aircraft, which often translates into flight cancellations.
TL:DR The amount of airplanes an airport can safely handle decreases on a sliding scale as weather conditions deteriorate. This means some flights get cancelled and delayed while others do not.
Very simply put, the weather may not be in your area. Planes are either
3. on Scheduled or unscheduled maintenance.
They don’t take breaks, and they don’t have parking lots where extra planes just hang out. If a plane isn’t flying, it is losing money.
So the plane for your flight has to come from somewhere. If that flight is delayed due to weather, your flight will be too. They can’t just bring a plane around from the back. So other flights out of your airport have birds coming from other places with better weather. Their flights won’t be affected.
TL:DR; weather delays don’t always refer to weather in your area.
It might not be bad weather where *you* are; it could be bad weather at your destination or somewhere along your flight path. Or bad weather at a plane’s origin preventing it from getting to where you are. Planes going to other destinations wouldn’t face the same bad weather, so they don’t get cancelled.
Going to add into what has already been mentioned…
SFO has two parallel runways. It’s scheduled to use both. However, there are times where they cannot safely use both and close one runway. Either visibility or crosswinds. (Annoyingly, one of the common situations is fog reducing visibility… Fog. In San Francisco.) The airport is still operating but any lower priority flight is delayed until it clears. I worked for an airline that had a very small presence in SFO – we had a lot of delays.
I’m sure there are other airports that semi regularly have to close one of their runways due to weather for various reasons.
I am by no means an expert but just going by logic, you can imagine that most planes are designed so that a stiff wind or weather wouldn’t knock them out of the sky. Flights aren’t being delayed/canceled because they’re busy cleaning up all the wreckage.
The delay comes from other reasons. all of the smaller personal/charter flights that actually would be unsafe have to be reorganized or diverted. If a plane has to divert or circle the field a couple times that causes delays. With airports it’s all about knock-on effects. Once one plane is late, the delays will pile up eventually leading to cancellations. Also, remember that the flying conditions at 30k feet are different from ground conditions. So a plane may easily fly above the weather but be delayed from actually landing due to ground conditions. Keep in mind, the last thing an airport wants to do is start delays or cancellations because of just those knock-on effects. Imagine the shitshow it would be if 100% of flights out of an airport were cancelled. Literally millions of dollars are lost every hour the airport is shut down.
One Major Reason is Timing: When conditions are not perfect, they space the planes out farther for safety. Longer time between takeoffs, landings, longer following distances. So you can’t fit as many planes into the air. That means some will have to be cancelled to help avoid a domino effect of delays.