Here's why snow was reported amid 90-degree temperatures at Philadelphia airport

As record highs sizzle across the United States, snow in July seems impossible. Yet, as temperatures hit a high of 94 degrees on Sunday, a record daily snowfall was reported at the Philadelphia International Airport.

As record highs are reported across the United States, snow in July seems impossible, especially in Philadelphia. Yet, as temperatures hit a high of 94 degrees Sunday, a record daily snowfall was reported at the Philadelphia International Airport.

The National Weather Service (NWS) office in Mount Holly, New Jersey, explained in a social media post that thunderstorms passed over Philadelphia on Sunday and produced small hail. "Since hail is frozen precipitation, this counts as a trace of snow in our climate reports," the post said.

"In a rare case when hail covers the ground and “accumulates” over a weather observing site, the National Weather Service records this in their daily climatology files as 'snowfall,' "AccuWeather Meteorologist Brandon Buckingham confirmed. "Within the climatology files, the National Weather Service lumps together any form of frozen precipitation that accumulates. In reality, this was not snow that fell, as it would be impossible to do so with temperatures in the 90s Fahrenheit at the time," Buckingham further explained.

While it's rare to get even a trace of snow due to hail from thunderstorms in the summer, the NWS says it's happened more often than you might think. From June 1911 to August 2011, the NWS recorded 13 instances of getting a trace of snow at Philadelphia International Airport in the summer months.

Just one day after the snow report, Philadelphia has a chance to hit the 100-degree mark, as Tuesday will be the hottest day of the year so far. The last time Philadelphia hit 100 degrees was on July 18, 2012.

Hail develops when rising air in a thunderstorm, known as the updraft, lifts water droplets high into the atmosphere where temperatures are below freezing.

Before hailstones become too heavy to fall to the ground, the updraft pushes them up repeatedly, freezing more ice around them. The stronger the storm, the more powerful the updraft, resulting in larger hailstones.

Hail is classified by diameter, from pea-sized, one-quarter of an inch across, to softball-sized, 4 inches across.

Statistics show an increase in the number of large hailstones in the United States over the past two years, which has caused meteorologists and researchers to wonder: Is hail getting larger, and if so, is climate change to blame?

Hail measurement is an inexact science

Large and extreme hailstones have always fallen on the Plains but were not well documented until the mid-2000s. One important thing to remember about spotter network data is that more spotters lead to an increase in reports over time. More storm chasers trying to document the largest hailstones could account for some of the recent upswing, but last June remains an outlier.

Because documenting hail size is difficult, only nine states maintain official records on hailstones. Other inconsistencies can also call the data into question. Hail can melt quickly before a measurement can be taken, so spotter reports can be estimated.