There are several different 'jet streams' that affect our weather patterns

I've received numerous requests from Press subscribers to explain the upper-level jet stream flows across North America.

Simply put, a jet stream is a high-speed wind about 6 to10 miles above the Earth's surface. Our planet's rotation and the heating of its atmosphere create several jet streams, which typically form in the middle latitudes between areas of temperature and barometric pressure contrasts.

Most major jet streams flow from west to east with the rotation of the planet. Some jet streams move from north to south and vice-versa.

As I witnessed two weeks ago on my trip to Arizona, a jet stream flowing in the same direction that a plane is flying will often mean an early airport arrival. Flying into a jet stream usually means that the plane will be late. Cross-continent flight times can vary as much as an hour depending upon the strength and direction of a jet stream.

There are several different types of jet streams, including the polar and sub-tropical branches.

The Polar Jet Streams, especially during the winter months, can kick up speeds in excess of 200 miles per hour. These jet streams can make storms more powerful by causing violent clashes of widely-opposing air masses. This motion makes the air move up and down between the upper atmosphere and lower atmosphere often resulting in damaging hailstorms, gusty straightline winds, torrential rains and even, as we saw last week in Oklahoma, deadly tornadoes.

The upper-level winds are strongest where the barometric air pressure differences are the greatest, at the boundary line between two widely-opposing air masses. Nature 'abhors a vacuum' and is constantly trying to 'fill-up' deep low pressure systems. This is why powerful jet streams certainly add to the intensity of most storms, again especially during the winter and early to mid spring months.

In addition to polar jet streams, other westerly jet streams develop much farther to the south along lower latitudes between 25 and 30 degrees north. These are the warm, moist sub-tropical jet streams.

Usually more powerful during 'El Nino' sea-surface temperature events, sub-tropical jet streams often bring heavy rains and lowland flooding to California and the Southwest. The current record drought conditions in the southwestern corner of the U.S. are due to extremely weak 'La Nada' and 'La Nina' affected subtropical jet stream flows aloft. Things don't look good for the 2013-14 winter season either in California and the Desert Southwest as they will probably be in a cool, dry 'La Nina' sea-surface temperature cycle in the Pacific waters, the 'opposite' type of conditions from the warm and moist 'Pineapple Express' from Hawaii. But, as I always say, "only time will tell..."

NORTH IDAHO WEATHER REVIEW AND LONG-RANGE OUTLOOKS

High pressure has pushed into the Inland Northwest bringing much warmer temperatures into the summerlike 80s and clear skies to the region. No rains are in the immediate forecast, but I'm still expecting some scattered showers and thunderstorms to develop on the back side of the current warm spell sometime between June 14 and June 20.

Longer-term, it still looks like a long, hot summer for our part of the country. I still see less rainfall than usual between late June and mid September in North Idaho and the rest of the Inland Empire, great conditions for outdoor activites.

We may see as many as 25 'Sholeh Days' above 90 degrees this summer. By the way, Sholeh Patrick, the loving wife of our managing editor, Mike Patrick, hates hot weather, temperatures at or above 90 degrees. That's why I call such torrid readings 'Sholeh Days.' I've done this for nearly a decade. (Thanks to Carla Noonan for asking me to explain this to our newer readers.)

One last thing, my long-range weather outlook for the annual late August North Idaho Fair and Rodeo is still calling for warmer and drier than normal conditions, but not horribly hot. I'll have more details later in the season.

Cliff Harris is a climatologist who writes a weekly column for The Press. His opinions are his own. Email sfharris@roadrunner.com