High above our heads, far beyond the clouds we see from the ground, a powerful current of air circles the planet. Known as the jet stream, it plays a major role in shaping weather patterns and influencing our climate. Though several thousand feet above us, the jet stream’s effects are felt daily, from temperature swings to the direction of approaching weather systems.

While known for its ferocious upper-level winds, jet streams originate from differences in surface temperatures. The Sun, which heats us, doesn’t do so uniformly. Around the equator, the Sun’s rays typically come straight down to the surface, especially around each equinox. On the other hand, Earth’s poles will never receive sunlight so directly, and even have periods during the year where the Sun doesn’t reach them (each pole’s respective Polar Nights, or where there are zero hours of daylight for an extended period of time).

As a result, areas near the equator are hot year-round, while areas near the poles deal with permanent chill.

In physics, it is a long-held principle that heat energy can transfer between objects. For example, an oven can warm a turkey up to an edible temperature by transferring the heat of the oven into the cooler turkey. In our atmosphere, this process works similarly, as energy transfers between warm objects and cold objects. As a result, energy travels between the warm equatorial part of the globe and the cold, polar areas.

Temperature differences can also cause strong pressure gradients. This is because cold air is at a lower pressure than warm air. A new force, called PGF (The Pressure Gradient Force), is the main cause for wind, naturally flowing from high pressure to low pressure. The PGF and energy transfer both move South to North.

One more ingredient needs to be set up to understand the jet stream. Earth rotates, which means that air movement will be disturbed. For this reason, air will naturally bend to the right of where it is heading. In this instance, air moving from the South will then naturally bend to the East.

There are several jet streams around the globe, but the most significant ones are the polar jet stream and the subtropical jet stream. The polar jet, found in both hemispheres, is the stronger and more influential of the two. It forms where cold, polar air meets warmer air, creating a sharp temperature gradient that fuels high wind speeds aloft. Our area can regularly sit on both sides of the Polar Jet. The subtropical jet, farther south, is typically weaker but still important for large-scale weather systems. This one typically stays away from our area until the summer months.

The jet stream can also be the foundation for weather patterns. Large-scale “dips” in the jet stream are regarded as troughs, which are usually known to bring active weather in a region. “Bulges” poleward in a jet stream are known as ridges, which give areas quiet, unremarkable weather.

The position of a jet stream can typically give away the prevailing weather conditions in an area. Locations that are underneath a jet stream are lucky enough to be in a very active area of weather, as low-pressure systems are typically steered along the jet stream. Polar air masses moving through are usually a result of an unseasonable southerly dip in the Polar Jet, bringing arctic air to areas where that event is usually uncommon. Variations in the jet stream are watched closely in the wintertime, as movements in the volatile river of air by teleconnections, such as the El Niño Southern Oscillation, can shift winter-time conditions across the Valley.