Translation: a team of student researchers found that on days when the skies are cloudy, there are lower peak daytime temperatures and warmer nighttime temperatures. In other words, they just reinvented the wheel. First error: they assumed that the three days of relatively clear skies were abnormal, which is simply not true. Because they assumed that clear weather is abnormal, they associated the clear days with the abnormal lack of commercial flights. But in all probability, the clear days were simply the result of a normal weather pattern, and the temperature results they obtained were what would be expected for similar weather patterns on any given day.
They could just has easily have claimed that the three days of relatively clear weather was due to intensive TV watching, since for three days after 911 most people stayed home glued to their TVs. The 'researchers' could then claim that if people stop watching TV so much, they will lower daytime temperatures and raise nighttime readings.