{"id":1584,"date":"2022-07-10T11:19:12","date_gmt":"2024-07-10T11:15:12","guid":{"rendered":"http:\/\/ialert.com\/blog\/?p=1584"},"modified":"2026-04-28T01:49:21","modified_gmt":"2026-04-28T01:49:21","slug":"glossary-of-weather-terms-beginning-with-j","status":"publish","type":"post","link":"https:\/\/ialert.com\/blog\/weather-articles\/weather-glossary\/glossary-of-weather-terms-beginning-with-j\/","title":{"rendered":"Glossary of Weather Terms – Beginning with “J”"},"content":{"rendered":"

Curious about what the jet stream is<\/strong> and how it controls your daily weather? Wondering about the difference between the polar jet stream and subtropical jet stream<\/strong>, what a jet streak<\/strong> does to storm development, or what causes the January thaw<\/strong>?<\/p>\n

Jump to weather terms beginning with the letter:<\/h3>\n

A<\/a> | B<\/a> | C<\/a> | D<\/a> | E<\/a> | F<\/a> | G<\/a> | H<\/a> | I<\/a> | J<\/a> | K<\/a> | L<\/a> | M<\/a> | N<\/a> | O<\/a> | P<\/a> | Q<\/a> | R<\/a> | S<\/a> | T<\/a> | U<\/a> | V<\/a> | W<\/a> | X<\/a> | Y<\/a> | Z<\/a><\/p>\n

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Weather Terms Beginning with “J”<\/strong><\/h3>\n

Jet Stream<\/strong><\/h4>\n

The jet stream is a fast-flowing, narrow band of winds near the top of the troposphere, typically between 25,000 and 35,000 feet altitude, that encircles the globe in a generally west-to-east direction. Wind speeds within the jet stream average 100 to 150 mph but can exceed 250 mph during winter, when the temperature contrast between polar and tropical air masses is greatest. The jet stream acts as the atmosphere’s primary steering current: low-pressure systems (storms), cold fronts, and precipitation bands form along and are directed eastward by the jet stream. When the jet stream dips sharply southward into the US, it draws cold Arctic air along with it; when it lifts northward, warm moist air surges into regions that may have been cold for weeks. The jet stream’s position and intensity are among the most important variables in weather forecasting at every timescale from daily to seasonal. Airlines routinely route trans-continental and trans-oceanic flights to exploit jet stream tailwinds, saving hours of flight time and significant fuel costs on eastbound routes.<\/p>\n

Polar Jet Stream<\/strong><\/h4>\n

The polar jet stream is the stronger and more weather-significant of the two Northern Hemisphere jet streams. It forms along the sharp temperature boundary between cold polar air and warmer mid-latitude air at altitudes of 25,000 to 35,000 feet, typically positioned between 40 and 60 degrees North latitude, though its position shifts dramatically with the seasons and with shorter-term weather pattern changes. During summer, the polar jet retreats poleward toward the Arctic as the temperature contrast between air masses weakens, and the US experiences less frequent and less intense storm activity. In winter, the polar jet dips far southward, delivering deep cold outbreaks, powerful nor’easters, blizzards across the Great Plains, and ice storms across the South. When the polar jet stream develops large north-south meanders (amplified Rossby waves), it can lock weather patterns in place for weeks, producing extended drought in some regions and persistent heavy rainfall or flooding in others simultaneously. Disruptions of the polar jet stream are responsible for the “polar vortex” events that bring -20 to -40\u00b0F temperatures to the northern Plains and Midwest.<\/p>\n\n\n\n\n\n\n\n\n\n\n
Polar Jet Stream vs Subtropical Jet Stream<\/caption>\n
Feature<\/th>\nPolar Jet Stream<\/th>\nSubtropical Jet Stream<\/th>\n<\/tr>\n<\/thead>\n
Location<\/td>\n50-60\u00b0N latitude (varies seasonally)<\/td>\n20-30\u00b0N latitude<\/td>\n<\/tr>\n
Altitude<\/td>\n25,000-35,000 feet<\/td>\n35,000-45,000 feet<\/td>\n<\/tr>\n
Average Speed<\/td>\n100-150 mph<\/td>\n75-115 mph<\/td>\n<\/tr>\n
Strength<\/td>\nStrongest in winter<\/td>\nStrongest in winter\/spring<\/td>\n<\/tr>\n
Weather Impact<\/td>\nDrives US winter storms, cold outbreaks<\/td>\nEnhances winter precipitation in South; affects hurricane tracks<\/td>\n<\/tr>\n
Seasonal Shift<\/td>\nNorth in summer, south in winter<\/td>\nLess variable than polar jet<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

Subtropical Jet Stream<\/strong><\/h4>\n

The subtropical jet stream is a weaker, more consistent upper-level wind band located at approximately 20 to 30 degrees North latitude, well south of the polar jet and at a higher altitude of 35,000 to 45,000 feet. It is most meteorologically significant during winter and early spring, when it can strengthen considerably and enhance the transport of warm, moist Pacific and Gulf of Mexico air into the southern US, contributing to winter storm and heavy precipitation events from California to the Gulf Coast states. During El Nino years, the subtropical jet stream typically intensifies and extends further eastward across the southern US, producing an active storm track with above-normal winter precipitation from Southern California through the Gulf states. The subtropical jet also plays a role in hurricane season by providing upper-level wind shear that can inhibit tropical storm development in the Gulf of Mexico. During summer, the subtropical jet weakens and retreats, having minimal influence on US weather patterns.<\/p>\n

January Thaw<\/strong><\/h4>\n

The January Thaw is a period of above-normal temperatures that frequently, though not always, interrupts the cold weather of January in the northeastern US, typically appearing sometime between January 20 and February 5. It is not a meteorological certainty or a scheduled phenomenon, but it is a recognized climatological tendency well-documented in the historical temperature record. A typical January Thaw episode lasts 3 to 7 days and is caused by the temporary northward displacement of the polar jet stream, which allows a persistent high-pressure system to route warmer air from the south into the region. Temperatures during a January Thaw can run 15 to 20\u00b0F above the seasonal normal, feeling remarkably mild in contrast to surrounding weeks, before the jet stream relaxes southward again and cold weather returns. The January Thaw has no official NWS product associated with it and is not reliably forecast more than a week in advance, but climatologists have documented it appearing more often than chance would suggest in the historical New England temperature record dating back centuries.<\/p>\n

Jet Streak<\/strong><\/h4>\n

A jet streak is a concentrated region of maximum wind speed embedded within the broader jet stream flow, essentially a local acceleration zone where winds are significantly faster than the surrounding jet stream. Jet streaks are not simply fast winds; they create critical patterns of divergence and convergence in the upper atmosphere that directly drive surface weather development. Air entering the jet streak (the entrance region) and leaving it (the exit region) behaves differently on each side of the streak axis, creating four quadrants with distinct vertical motion characteristics. In the left-exit and right-entrance quadrants of a jet streak, upper-level divergence promotes rising air and falling surface pressure, conditions that favor explosive cyclone development, severe weather outbreaks, and rapid storm intensification. Meteorologists look for jet streak positioning on upper-level wind charts as one of the primary indicators of where and when significant storm development will occur, often hours to a day before surface changes are apparent.<\/p>\n

Jet Stream Amplification<\/strong><\/h4>\n

Jet stream amplification describes the condition in which the jet stream develops large, persistent north-south meanders, called Rossby waves, rather than flowing in a relatively straight west-to-east pattern around the globe. When the jet stream is amplified or “buckled,” weather systems move slowly or stall entirely instead of progressing normally eastward. This creates blocking patterns that can lock weather conditions in place for weeks: regions under the troughs of the buckled jet receive prolonged cold outbreaks, heavy snow, or persistent rainfall; regions under the ridges bake under heat waves or experience drought. The polar vortex disruptions of January 2014 and January 2019, which brought -20 to -40\u00b0F temperatures to the Midwest and northern Plains, were caused by a dramatically amplified polar jet stream that allowed a lobe of Arctic air to push far south of its normal boundaries. Scientists continue to study whether the rapid warming of the Arctic is increasing the frequency and persistence of jet stream amplification events.<\/p>\n

Joint Weather Operations<\/strong><\/h4>\n

Joint weather operations refers to the coordinated effort among the National Weather Service, US military meteorological units, the Federal Aviation Administration, and other federal agencies to share observational data, issue compatible forecasts, and coordinate severe weather messaging across civilian and military operations. Effective joint weather operations ensure that a fighter pilot, a commercial airline, a ship captain, and a homeowner in the same region receive consistent, compatible weather information and threat assessments. During major disasters, presidentially declared emergencies, or large national events, joint weather operations expand to include FEMA coordination and support for state and local emergency management. The NWS operates Aviation Weather Center and Ocean Prediction Center products as part of this broader framework, and military weather squadrons provide specialized products for tactical operations not covered by civilian services.<\/p>\n

June Gloom<\/strong><\/h4>\n

June Gloom is a regional weather pattern affecting the Southern California coast, and similar west-facing mid-latitude coastlines worldwide, characterized by persistent morning and sometimes all-day low clouds and marine fog during May and June, despite the season being well past the winter storms that bring most California precipitation. It forms when a strong North Pacific High pressure system directs a flow of cool, moist marine air onshore from the Pacific Ocean. This cool air is trapped beneath a temperature inversion created by the warm air above, preventing the marine layer from lifting and forming a stubborn stratus cloud deck that can extend inland for 20 to 30 miles. The marine layer typically burns off by midday as solar heating warms the ground surface enough to erode the inversion from below, but on days when the inversion is particularly strong or the high pressure pattern is unusually strong, the low clouds can persist all day. June Gloom significantly reduces solar energy production, surprises tourists expecting constant California sunshine, and affects coastal aviation with recurring IFR (instrument flight rules) conditions during morning hours. Similar patterns occur along the coasts of Chile, Portugal, Namibia, and California’s own Oregon and Washington coast extension.<\/p>\n

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Frequently Asked Questions, “J” Weather Terms<\/strong><\/h3>\n

What is the jet stream?<\/h4>\n

The jet stream is a narrow band of fast-moving winds near the top of the troposphere, typically between 25,000 and 35,000 feet altitude. Wind speeds average 100 to 150 mph but can exceed 250 mph during winter. The jet stream acts as the atmosphere’s main steering current, it directs the path of storms, cold fronts, and weather systems across the country. When the jet stream dips south, cold Arctic air follows. When it buckles north, mild air floods into northern regions. Airlines routinely fly within jet stream tailwinds to save fuel and reduce eastbound flight times by hours.<\/p>\n

How does the jet stream affect daily weather?<\/h4>\n

The jet stream controls weather by steering low-pressure systems (storms) and cold fronts across the country. When the jet stream dips far south, it draws Arctic air deep into the US, producing blizzards, ice storms, and dangerous wind chills. When the jet stream lifts north, warm, moist air can surge northward, triggering severe thunderstorm outbreaks in spring. An amplified (very wavy) jet stream can lock weather patterns in place for weeks, causing prolonged drought in one region while another experiences weeks of flooding rains. The jet stream’s position is one of the most important variables in both short-term and seasonal weather forecasting.<\/p>\n

What is the difference between the polar and subtropical jet streams?<\/h4>\n

The polar jet stream is the stronger of the two, typically located at 50 to 60 degrees North latitude and 25,000 to 35,000 feet altitude. It forms along the sharp temperature boundary between Arctic and mid-latitude air and is responsible for driving US winter storms and cold outbreaks. The subtropical jet stream is weaker and located closer to 20 to 30 degrees North latitude at a higher altitude. It most significantly affects the southern US in winter by enhancing Gulf and Pacific moisture transport and storm activity. Both jet streams can interact and merge to intensify major winter storm systems, particularly during El Nino years.<\/p>\n

What is a jet streak?<\/h4>\n

A jet streak is a concentrated core of maximum wind speeds embedded within the broader jet stream, where winds accelerate significantly above the surrounding jet stream flow. Jet streaks are important because they create regions of upper-level divergence (rising air) and convergence (sinking air) that directly determine where storms develop and intensify at the surface. Meteorologists look specifically at the four quadrants around a jet streak, the left-exit and right-entrance regions are where upper-level divergence is greatest and where explosive storm development, severe weather outbreaks, and rapidly deepening cyclones are most likely to occur.<\/p>\n

What causes a polar vortex event?<\/h4>\n

The polar vortex is a large area of cold, low-pressure air that normally sits over the Arctic year-round. When the polar jet stream is strong and roughly circular, it contains the cold Arctic air within the polar region. But when the polar jet weakens or amplifies dramatically, developing large north-south meanders, the polar vortex stretches and fragments, allowing lobes of Arctic air to push far south into the continental US and even into Mexico. This produces the extreme cold outbreaks sometimes called polar vortex events, which can bring temperatures of -20 to -40\u00b0F to the northern Plains and Midwest, as occurred in January 2019 and January 2014.<\/p>\n

What is June Gloom?<\/h4>\n

June Gloom is a weather pattern affecting Southern California (and similar west-facing coastlines worldwide) in May and June, producing persistent morning and sometimes all-day marine fog and low clouds despite the otherwise sunny season. It forms when a strong Pacific High pressure system directs cool, moist ocean air onshore, which is trapped beneath a temperature inversion layer. The low clouds typically burn off by midday but on strong marine layer days can persist all day. June Gloom is not hazardous but significantly reduces sunshine, affects solar energy production, and regularly surprises visitors who expect constant California sunshine during what should be the sunniest time of year.<\/p>\n

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