Tag Archives: seasons

Vernal Equinox

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Today is the Vernal Equinox, the first day of spring in the northern hemisphere. The new season officially began at 5:14 UTC, which is 1:14 A.M. Eastern Standard Time.

Astronomical seasons are the result of the tilt of the Earth’s axis, a 23.5-degree angle. Today, as spring begins, the Earth’s axis is tilted neither toward nor away from the sun. As a result, we receive approximately equal hours of day and night.

The vernal equinox usually marks the end of winter’s chill and the gradual return of warmth.  Following our fourth warmest winter on record, however, spring conditions are already in full bloom across many parts of United States.

Image Credit: scijink.nasa.gov

Meteorological Seasons

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Seasonal weather records are officially organized by meteorological season. These are slightly different than the seasons you see marked on calendars.

Our planet takes 365.25 days to complete a revolution around the sun.  As a result, the exact dates for the beginning of the astronomical seasons – equinoxes and solstices – vary from year to year.  To maintain consistent records, meteorologists use groupings of three whole-months to subdivide the year.  These quarters are based on temperature rather than the position of the Earth in its orbit.

In the northern hemisphere, for example, the meteorological winter begins on December 1st and includes the three coldest months of the year, December, January, and February.  The winter solstice, on the other hand, does not arrive until December 21st or 22nd, depending on the year. This year’s meteorological winter recently made headlines as the fourth warmest on record.

In general, the four meteorological seasons arrive about three weeks ahead of the astronomical ones.

Graphics Credit: MF at The Weather Gamut

Deadly March Tornado Outbreak

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Roaring through the Ohio and Tennessee Valleys this past weekend, tornadoes claimed the lives of forty people across five states.  This was one of the deadliest March tornado outbreaks in the U.S. since 1950.

At this point, the National Weather Service has confirmed forty-five twisters in eleven states.  This number, however, is likely to rise as damage investigations continue.  The strongest storm in this widespread outbreak was rated an EF-4, the second highest ranking on Enhanced Fujita Scale. With winds of 175 mph, it devastated the town of Henryville, IN. Another hard hit area was West Liberty, KY. An EF-3 storm, with winds of 140 mph, is reported to have charged through the town’s main street, destroying everything in its path.

Tornadoes, nature’s most violent storms, are spawned by severe thunderstorm activity.  They can occur anywhere in any season, given the proper conditions. That said, they are more typical in the spring, when the atmosphere is transitioning from cold winter weather to summer warmth.

With the spring equinox just around the corner, this tornado season is only getting started.   Unfortunately, more severe weather is likely in the coming months.

Image Credit: NWS/Storm Prediction Center

Winter and the Jet Stream

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The local forecast is calling for a big dip in the jet stream this weekend.  This will drive cold air south over New York City and send temperatures plummeting from their above average readings. You may wonder, what is this meteorological phenomenon that is bringing winter back to the city so abruptly?

Jet streams are bands of strong winds in the upper atmosphere that mark the location of the strongest temperature contrast at the surface. They are born out of the complex interactions among a variety of atmospheric conditions, including the position of warm and cold air masses, and the location of areas of high and low pressure.

In the U.S., the polar jet stream, one of four major global jets, marks the divide between cold arctic air and mild mid-latitude air. Its position is a function of temperature contrasts and therefore shifts throughout the year, depending on the season. In the warm summer months the jet is typically located around 50° to 60°N latitude.  In the winter, when temperature contrasts are increased, it usually shifts to the south.  It has been known to plunge as far south as 30°N latitude.

This winter, however, the jet stream has been staying well to the north with only a few arctic outbreaks so far.  When they do come though, the sudden drop in temperature can be very jarring.

Average seasonal positions of the Polar Jet Stream in the northern hemisphere.

Image Credit: ncsu.edu

Perihelion

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Today, the Earth will reach its Perihelion at 8 P.M. Eastern Standard Time.  This is the point in the planet’s orbit where it comes closest to the Sun.

This annual event is due to the elliptical shape of the Earth’s orbit and the off-centered position of the Sun inside that path.  The exact date of the Perihelion differs from year to year, but it’s usually in early January…winter in the northern hemisphere.  We will be furthest from the Sun at the Aphelion in July.

The planet’s distance from the Sun does not cause the seasons, but it does influence their length.  As a function of gravity, the closer the planet is to the Sun, the faster it moves. Today, the Earth is about 146 million kilometers away from the Sun.  That is approximately 5 million kilometers closer than in early July.  This change in distance allows the planet to speed up by about one-kilometer/second.  As a result, winter in the northern hemisphere is about five days shorter than summer.  The opposite is true in the southern hemisphere.

The word, perihelion, is Greek for “near sun”.

The perihelion position of Earth's orbit.

Image Credit: Academy Artworks

Winter Solstice

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Today is the December Solstice, the first day of winter in the northern hemisphere. The new season officially began at 5:30 UTC, that is 12:30 A.M. Eastern Standard Time.

The tilt of the Earth’s axis, a 23.5 degree angle, is what gives us the seasons.  Today, as winter begins, the northern half of the Earth is tilted away from the sun.  This position brings the northern hemisphere the least amount of the sun’s energy all year.

Today is also the shortest day of the year. Since the summer solstice in June, the arc of the sun’s daily passage across the sky has been dropping toward the southern horizon.  It stopped today at the Tropic of Capricorn, its southern most position. (The word “solstice” is Latin for “sun stand still”).   It will now begin its apparent journey northward and our daylight hours will gradually increase.

Marking the transition to longer days, the winter solstice has been a cause for celebration across many cultures throughout history.

Winter Solstice in Northern Hemisphere

Image Credit: scijink.nasa.gov

La Nina Returns

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La Nina has returned for a second year in a row.  According to NOAA’s winter climate outlook, this oceanic-atmospheric phenomenon will strongly shape our upcoming winter season.

La Nina is a climatic episode associated with the larger El Nino-Southern Oscillation (ENSO) climate pattern in the Pacific Ocean.   During a La Nina event, sea surface temperatures in the eastern Pacific are cooler than normal.  This ocean temperature anomaly influences weather around the globe.

In the US,  La Nina will impact both the temperature and precipitation in many parts of the country. The southern states are likely to see conditions that are both warmer and dryer than normal.  This is not good news for the drought-stricken state of Texas.  The northern tier is expected to experience  below average temperatures with the northwest getting above average precipitation.  The northeast and mid-Atlantic states have a 50/50 chance of seeing irregular conditions from La Nina.  In this region, the Arctic Oscillation (AO), a different oceanic-atmospheric pattern, has a stronger influence on winter weather.

Less predictable than La Nina, the AO continually transitions between positive and negative phases. A negative phase will bring cold arctic air and snowy conditions to the eastern US.  The cold snaps and heavy snow we saw last winter in the northeast were influenced by a very strong negative phase of the AO.  These strong phases can last anywhere from a few days to a few weeks and can be difficult to anticipate in long term forecasts.

The current La Nina event is forecast to last through February.

Photo Credit: NOAA

Northern Lights Dip South

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On Monday night, the Aurora Borealis, also know as the Northern Lights, made a rare appearance in the lower latitudes of the US. These geomagnetic storms are normally limited to the polar regions, where the earth’s magnetic field is strongest.

An aurora is an optical phenomenon that is the result of charged solar particles becoming trapped in the Earth’s magnetic field.  These particles ionize or excite the gases that make up our atmosphere.  As these gases return to their ground state, they emit light energy that we see as neon-like colors dancing in the sky. The different colors can depend on the amount of energy absorbed, but in general, oxygen tends to produce a green light and nitrogen gives off a red glow.

Auroras at the poles are connected to the solar wind, a continuous flow of charged particles outward from the sun. Monday’s unusual display, however, was caused by an intense storm on the surface of the sun and coronal mass ejection.  This tremendous eruption of solar wind temporarily allowed the auroral zone in our atmosphere to expand southward.

Auroral activity tends to be strongest around the seasonal equinoxes.  In the southern hemisphere, auroras are called Southern Lights or Aurora Australis.

Northern Lights                                                                                                                                       

Photo credit: dakotalapse.com

The Science of Fall Foliage

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The science behind the color of autumn leaves is a mixture of bio-chemistry and meteorology.  Atmospheric conditions trigger a biological process in deciduous trees that cause the leaves to change color.  This process allows a tree to protect itself from freezing and survive the winter.

Sunlight, groundwater, and temperature all play a part in the annual phenomenon of fall foliage.  The main variable that starts the ball rolling, however, is sunlight. As the days grow shorter in autumn, less sunlight is available to power photosynthesis, the chemical process that feeds the tree by converting carbon dioxide and water into glucose and oxygen.

The reduced amount of sunlight signals the tree to stop producing food and prepare stores for the winter. The tree turns off its food producers by slowly corking the connection between leaf-stems and its branches.  This blocks the movement of sugars from the leaves to the tree as well as the flow of water from the roots to the leaves.  As a result,  the leaves stop producing chlorophyll, the agent of photosynthesis and the reason for the green color of summer foliage.  As the green fades, other chemicals that have been present in the leaves all along begin to show.  These include xanthophyll and carotene that produce yellow and orange leaves, respectively.

The change of leaves happens every autumn, but the intensity and duration vary from year to year.  This is a result of weather conditions like temperature and rainfall.  With below average temperatures, sugars trapped in the leaves react with sunlight and the cooler air to produce anthocyanin.  This gives us more red and purplish leaves.  Warmer than normal temperatures tend to produce longer displays but with less intense colors.  Drought will cause leaves to turn brownish and break off early.

It should also be noted that the bio-chemical make-up of different species of trees react to seasonal and atmospheric conditions differently. Therefore, the more diverse the forest, the wider the range of colors in Autumn.

Recipe for Autumn Color

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The general “weather recipe” for a colorful autumn includes:

  •  a wet spring
  •  no summer drought
  •  an autumn with warm sunny days and cool nights

This year in the northeast, we have collected all the ingredients needed for a beautiful fall foliage season.  Barring any major wind storms that blow the leaves off the trees or an early frost,  leaf peeping will be in full swing soon.

In New York City the leaves have just started to change, but the color is still a bit patchy.   We typically hit peak color sometime around the last week of October.

First Sign of Autumn in New York City, 2011

Photo Credit: MF @ WeatherGamut