snow storm old jerusalem photo snow storm old jerusalem photo A rare snowstorm in Jerusalem covers palm trees. (Photo: Copper Kettle / Flickr)

How often does it snow in the Mediterranean?

This subtropical, beach-heavy region was under more than a foot of snow a few winters ago. Is global warming to blame?

In a subtropical region known for its steaming-hot beaches and long, dry summers, it's hard to imagine waking up to a snowstorm.

But that's exactly what happened in December 2013, when some countries in the eastern basin of the Mediterranean Sea were hit with up to 20 inches of snow. The storm hit Jerusalem head-on, shutting the city down for an entire week. It also blanketed a few inches over Egypt, Jordan and Turkey. Across the region, this rare weather event closed schools, snarled traffic and made international headlines. Residents were left stunned, wondering how cold air could circulate through this type of climate for such an extended period of time and produce so much snow.

snow storm jerusalem photoSnow falls in Jerusalem. (Photo: zeevveez/Flickr)

Meteorologists were surprised, too, but noted it wasn't the first time – the countries around the Mediterranean had been hit with a few paralyzing snowstorms in the past. Barcelona, Spain, was hit with a huge storm in 1962 that covered the city in 20 inches of snow, the biggest snowfall there since 1887, and not seen again till another storm hit in 2010. Italians in Palermo, on the island of Sicily, were pummeled with a huge snowstorm in 2009.

So how does it happen?

These types of unusual weather events typically start with jet streams, rivers of wind above the atmosphere that form along the boundaries of cold and warm air masses. They flow east and west as well as north and south. The greater the temperature contrast, the more powerful the jet stream.

In the northern hemisphere, the jet stream normally circulates air counter-clockwise, powered by opposing low pressure over Iceland and high pressure over the Canary Islands. As a result, air moves swiftly back and forth over the Atlantic, said Jeff Weber, atmospheric scientist at The University Corporation for Atmospheric Research.

But in the 2013 storm, the northern hemisphere jet stream became blocked and eventually buckled, shooting cold air farther south and warm air north. This pushed atypically cold air through the Mediterranean basin, and then it all converged on Israel.

“Israel was impacted by a deep trough (in the jet stream) allowing much colder air to dig south, and since there was moisture available, it came as snow,” said Weber.

While this explains how the heavy snow was produced, it doesn't explain why. That's because the extent to which global climate change has affected this seemingly irrational weather pattern remains unclear. Some climatologists believe that global warming contributes to these unusual weather events by causing more frequent jet stream buckling.

"We do not know exactly how and why these pressure fields moderate and change over time; this is a current research topic," Weber said. "We are looking at the loss of sea ice in the Arctic as a possible cause, as well as a 'freshening' (loss of saline) of the North Atlantic due to increased melt off the Greenland Ice Sheet ... we know these two factors impact the location and magnitude of the high and low pressure fields."

As for that big winter storm, Weber explained that warmer climates carry more moisture, which can translate to snow when exposed to near-freezing temperatures. Without the typical contrasting pressures, jet streams will get stuck and push weather events into places in which they don’t generally occur – like the subtropical Mediterranean region.

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