Hundreds of thousands of US airline customers were stranded this week as severe weather grounded planes and led to canceled flights. But storms were only one factor behind the travel nightmare. Staffing shortages, at both US airlines and the Federal Aviation Administration's air traffic control operations, took a bad situation and made it worse.

The situation at US airports was only slightly better Wednesday. As of 8:30 am ET, FlightAware tracking service reported there were 687 flights canceled and another 1,200 delays. Storms in the Boston area caused a groundstop there, keeping planes destined for Logan Airport at the gate or parked on the tarmac at airports around the country.

Although that's better than the 2,200 canceled flights in each of the last two days, or the more than 16,000 delayed flights between Monday and Tuesday, it's hardly a smooth-running operation.

Staffing shortage

The US air travel system is unable to recover quickly from widespread weather problems, because it doesn't have the bodies to deal with the disruptions.

Despite $54 billion of taxpayer funds funneled into airlines to keep them alive during the pandemic, most airlines greatly reduced staff during the first year of the pandemic when air travel, and fares, plunged. They were not allowed to involuntarily layoff staff but they did offer buyouts and early retirement packages. Many also permanently grounded older, less efficient aircraft. Rehiring staff has taken longer than planned.

The result has been that domestic US airline capacity, as measured by the number of available seats adjusted for miles flown, is still down 10% in the current quarter compared to the second quarter of 2019, ahead of the pandemic, according to data from Cirium, an aviation analytics firm.

And when problems occur, finding seats for passengers whose flights have been canceled becomes a problem, particularly at busy travel periods.

This Friday, at the start of the four-day July Fourth holiday weekend, is projected by the Transportation Security Administration to be the busiest air travel day since the start of the pandemic.

Delta CEO Ed Bastian said Tuesday that things are better than they were last summer. But he conceded in remarks to investors that his airline "still [is] not running as optimally as it needs to run. We have improvements we can make. Substantial improvements, over where we were last summer."

FAA problems

Meanwhile United CEO blamed the staffing shortage at the FAA, which he pointed said limited the number of flights into and out of major population centers like the three airports in and around New York City. Because of its hub at Newark Airport, United has been it hardest with canceled and delayed flights during this current meltdown.

"The FAA frankly failed us this weekend," said United Airlines CEO Scott Kirby in an internal company memo shared with CNN. The airline canceled 461 flights this past Saturday and Sunday and delayed another 1,972 flights, according to data from flight tracking site FlightAware.

Kirby says that on Saturday, the FAA reduced arrival rates at its major hub at Newark Liberty International Airport by 40% and departure rates by 75%, which was "almost certainly a reflection of understaffing/lower experience at the FAA."

"It led to massive delays, cancellations, diversions, as well as crews and aircraft out of position," Kirby said. "And that put everyone behind the eight ball when weather actually did hit on Sunday and was further compounded by FAA staffing shortages Sunday evening."

In the memo, Kirby was careful to say the current FAA leadership did not create the current staffing problem, but that they need to deal with them. "To be fair, it's not the fault of the current FAA leadership," he wrote, but added "they are responsible for solving the problem they inherited."

The FAA didn't have much of a response to Kirby's criticism, other than to say, "We will always collaborate with anyone seriously willing to join us to solve a problem," according to a spokesperson.

Unions speak out

But one of the major unions at United said the problem was not just the FAA but also United management.

"The levels of frustration are high and it feels as if there is no solution in sight -- especially for those who have been on duty for extended periods of time," the Association of Flight Attendants said in a memo shared with CNN.

The union message sent Monday to members cites hours-long hold times with company crew schedulers, hinting at problems beyond air traffic control staffing.

"Because hold times are currently in excess of 3 hours as crew scheduling works to update crews in chronological order, we are working to find solutions to relieve some of the backlog," the memo said.

Asked about the flight attendants' complaints, United responded "making sure our flight attendants can reach us quickly is a top priority. We have deployed all available resources to catch up on call volume, including increasing staffing in crew scheduling and mandatory overtime on the scheduling team. We also have ways flight attendants can check in electronically for trips and schedule changes."

Growing frustration

But all of that is little consolation for passenger once again left at the mercy of a non-functioning air travel system.

Paul Thacker told CNN he flew from Madrid to United's hub in Newark on Monday, only to find his connecting flight to Toronto had been cancelled. He spent the Monday night sleeping on the floor at the airport after being unable to find a hotel.

"My suitcase is checked to go to DC, so I have no clothes, toiletries," Thacker told CNN Tuesday. ""I finally gave up and booked a Greyhound to Toronto. I have no clue where I'm sleeping tonight, and I have no clothes. I need a hotel and a store so I can change clothes."

Michelle Maciel told CNN she was on hold for 7 hours trying to rebook a flight from Denver to Portland Oregon after her flight the night before was canceled in the middle of the night after numerous delays.

"That was infuriating," she said about being disconnected from the call. "I have no idea where my luggage is. There's no one to ask unless I wait in another line for several hours. I can't believe I'm 53 years old , I paid for a ticket and I slept on the ground. How is this OK with any of us?"

-- CNN's Pete Muntean and Gregory Wallace contributed to this report