The on-time performance of North American airlines declined more than 10 percentage points between May and June.

The latest monthly report from aviation data analytics firm Cirium shows that June on-time performance among North American airlines studied was just 68.1 percent while in May it was 78.66 percent.

Among North American airlines, Alaska Airlines scored the best, as the only carrier whose on-time performance was above 80 percent in June at 80.3 percent. Even in Alaska’s case however, the carrier’s performance slipped between May and June declining from 84.7 percent one month earlier.

Delta came in second among North American carriers for the month with an on-time arrival rate for June of 79.73 percent, which represents a decline from 88.86 percent in May. American Airlines nabbed the third place slot with on-time arrivals 74.13 percent of the time —again a decline, however, down from May's 83.91 percent.

Rounding out the top five were United and Southwest with on-time arrival rates of 72.61 percent and 68.06 percent respectively for June. That's a decline for both airlines. In the case of United, the May on-time rate was 79.65 percent, while for Southwest 77.75 percent of planes arrived on-time one month earlier.

Overall, the June on-time performance declines among North American carriers ranged from 7.1 percent for United Airlines to as steep as 17.7 percent for Air Canada.

Separately, Delta was the only North American carrier to make the overall global on-time list of airlines. The carrier ranked eight among its global competitors with its 79.73 percent on-time performance. That’s a steep drop from Delta's second place global ranking in May. The top-ranked on-time airline globally for June is Avianca with a 86.61 percent rate. Additional airlines earning top marks globally include LATAM in the second place slot and Qatar Airways in third.

There’s a variety of reasons why the performance of North American airlines may have declined between May and June. Among them, poor weather, a serious shortage of air traffic controllers and impacted visibility amid the Canadian wildfire smoke.

Additional highlights of the new report include:

• North American Airlines cancelled some 17,414 flights in June. That figure is nearly triple the 6,042 cancelled in May.
• The report tracked 694,310 flights among North American carriers in June, which represented 99.75 percent of flights.
• Alaska Airlines had the top completion factor at 99.79 percent.

An on-time flight performance is one in which the plane is gate side within 15 minutes of its scheduled arrival time.