Tag Archives: seasons

Groundhog Day 2015

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Today is Groundhog Day, the midpoint of the winter season.

On this day, according to legend, the weather conditions for the second half of winter can be predicted by the behavior of a prognosticating groundhog.  If the groundhog sees its shadow after emerging from its burrow, there will be six more weeks of winter.  If it does not see its shadow, then spring will arrive early.

The practice of using animal behavior to predict future weather conditions goes back to ancient times.  The particular custom that we are familiar with in the United States grew out of the old world tradition of Candlemas brought by German settlers to Pennsylvania in the 1880s. Today, many communities across the U.S. and Canada continue this age-old ritual with their own special groundhogs.

The most famous of these furry forecasters is Punxsutawney Phil from Pennsylvania – he was portrayed in the 1993 film, “Groundhog Day”. This year, despite an overcast sky, Phil saw his shadow and is calling for six more weeks of wintry conditions.

In New York City, our local weather-groundhog is Charles G. Hogg – more popularly known as “Staten Island Chuck”.  Coming out of his burrow this morning, he did not see his shadow and is predicting an early spring for the Big Apple.

Long-range forecasts are a tricky business, so a difference of opinion is not that uncommon. Either way, the spring equinox is 46 days away.

The Dead of Winter: Coldest Part of the Season

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The “Dead of Winter” is an old saying that refers to the coldest part of the winter season. This annual chilly period, statistically, begins today.

While actual daily weather varies, historical average temperatures in most of North America reach their lowest point of the year between January 10th and February 10th.  This cold period does not begin on the winter solstice, the day we receive the least amount of solar energy, because of a phenomenon known as seasonal temperature lag.

Air temperature depends on both the amount of heat received from the sun and the amount of heat lost or absorbed by the oceans and continents. From the start of winter through mid-February, both the oceans and land are losing more heat than they gain.

These few cold weeks are the climatological opposite of the “Dog Days of Summer.”

Earth’s Perihelion 2015

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The Earth reached its Perihelion today at 6:36 UTC, which is 1:36 AM 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. The Earth will be furthest from the Sun in July.

While the planet’s distance from the Sun is not responsible for the seasons, 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 (3 million miles) closer than in early July. This position 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 word, perihelion, is Greek for “near sun”.

Image Credit: mydarksky.org

Image Credit: mydarksky.org

Winter Solstice 2014

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Today is the December Solstice, the first day of winter in the northern hemisphere. The new season officially begins at 23:03 UTC, which is 6:03pm EST here in New York City.

The astronomical seasons are produced by the tilt of the Earth’s axis – a 23.5° angle – and the movement of the planet around the sun. During the winter months, the northern half of the Earth is tilted away from the sun. This position brings the hemisphere the least amount of solar energy and its coolest temperatures 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 and daylight hours have been decreasing. Today, it reached its southern most position at the Tropic of Capricorn (23.5° south latitude) marking the shortest day of the year. This observable stop is where today’s event takes its name.  Solstice is derived from the Latin words “sol” for sun and “sisto” for stop.

Now, the sun will move northward again in our sky and daylight hours will slowly start to increase. Marking this transition from darkness to light, the winter solstice has long been a cause for celebration across many cultures throughout human history.

Seasons

Earth has seasons because it is tilted on an axis relative to its orbit around the sun.  Image Credit: NASA

winter_solstice-1

The Sun is directly overhead at the Tropic of Capricorn on the Winter Solstice.                     Image Credit: NASA

The Five Seasons of the Sonoran Desert

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Seasons are a way of dividing up the year based on changes in weather and daylight hours. Winter, spring, summer, and fall – the traditional four – are familiar to most people. However, while traveling in southern Arizona recently, I had the opportunity to learn about the unusual fifth season of the Sonoran Desert.

The Sonoran desert, covering a large part of the southwestern US and Northern Mexico, basically divides its summer into two parts. “Fore-summer”, occurring in May and June, is very hot and very dry. “Summer monsoon season” follows it from July to mid-September and brings the region soaking rains.  It is considered the major growing season.

Surprisingly lush by desert standards, the Sonoran Desert is one of the wettest deserts in North America.  This is due to the fact that winter there is considered a second rainy season. While the precipitation that falls between December and January is generally not as intense as during the monsoon months, it tends to be more widespread. Overall, the region averages between 3 and 12 inches of rain a year. Spring and Fall are generally warm and dry.

Autumnal Equinox 2014

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Today is the Autumnal Equinox, the first day of fall in the northern hemisphere. The new season officially begins at 02:29 UTC, which is 10:29 pm Eastern Daylight Time.

The astronomical seasons are a product of the tilt of the Earth’s axis – a 23.5° angle – and the movement of the planet around the sun. Today, as fall begins, the Earth’s axis is tilted neither toward nor away from the sun.  This position distributes the sun’s energy equally between the northern and southern hemispheres.

Since the summer solstice in June, the arc of the sun’s daily passage across our sky has been moving southward and daylight hours have been decreasing. Today, it crossed the equator and we have approximately equal hours of day and night. The word “equinox” is derived from Latin and means “equal night”.

As a transitional season, autumn is a time when the heat of summer fades away and the chill of winter gradually returns. The largest drop in average temperature, however, usually lags the equinox by a few weeks.

Seasons

Earth’s solstices and equinoxes. Image Credit: NASA

Equinox

The Earth’s axis is tilted neither toward nor away from the sun on the Equinox.                   Image Credit: NASA

North American Monsoon

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The summer phase of the North American Monsoon is in full swing. But what, you may wonder, is a monsoon?

While most people associate a monsoon with rain, that is only half the story. It is actually a wind system. More specifically, according to NOAA, a monsoon is “a thermally driven wind arising from differential heating between a land mass and the adjacent ocean that reverses its direction seasonally.” In fact, the word monsoon is derived from the Arabic “mausim”, meaning seasons or wind shift.

In general, a monsoon is like a large-scale sea breeze.  During the summer months, the sun heats both the land and sea, but the surface temperature of the land rises more quickly. As a result, an area of low pressure develops over the land and an area of relatively higher pressure sits over the ocean. This causes moisture-laden sea air to flow inland. As it rises and cools, it releases precipitation. In winter, the dry season, this situation reverses.

Monsoon wind systems exist in many different parts of the world. In the U.S., we have the North American Monsoon that impacts states across the southwest. Summer temperatures in this region – mostly desert – can be extremely hot. Readings in the triple digits are not uncommon. This intense heat generates a thermal low near the surface and draws in moist air from the nearby Gulf of California. In addition, an area of high pressure aloft, known as the subtropical ridge, typically moves north over the south-central U.S. in summer. Its clockwise circulation shifts the winds from a southwesterly to a southeasterly direction and ushers in moisture from the Gulf of Mexico. This combination of heat and moisture rich air produces thunderstorms and heavy rainfall across the region between July and September. In fact, summer monsoon rains are reported to supply nearly 50% of the area’s annual precipitation.

Replenishing reservoirs and nourishing agriculture, these seasonal rains are a vital source of water in the typically arid southwest. Conversely, they can also cause a number of hazards such as flash flooding, damaging winds and hail, as well as frequent lightning.

The summer monsoon officially begins, according to the National Weather Service, when there have been three consecutive days with a dew point above 54°F.

North American Monsoon: Summer Weather Pattern. The thermal low sets up over the desert southwest, while the subtropical high moves into the southern plains. The winds draws moisture (green arrows) from the Gulf of California and the Gulf of Mexico. Source: NOAA/NWS

North American Monsoon: Summer Weather Pattern. The thermal low sets up over the southwest, while the subtropical high moves into the southern plains. Their circulating winds draw moisture (green arrows) from the Gulf of California and the Gulf of Mexico.   Credit: NOAA/NWS

Earth’s Aphelion 2014

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The Earth will reach its aphelion today at 8 P.M. Eastern Daylight Time.  This is the point in the planet’s elliptical orbit where it is farthest from the Sun.

Today, the Earth is about 152 million kilometers away from its nearest star.  That is approximately 5 million kilometers further than during the perihelion in early January. The exact date of the aphelion differs from year to year, but it’s usually in early July.

While the planet’s distance from the Sun does not cause the seasons, it does influence their length.  As a function of gravity, the closer the planet is to the Sun, the faster it moves. So, at the aphelion, the Earth will move more slowly along its orbital path than at any other time of the year. As a result, summer is elongated by a few days in the northern hemisphere.

The word, aphelion, is Greek for “away from sun”.

Image Credit: mydarksky.org

Image Credit: mydarksky.org

Summer Solstice 2014

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Today is the June Solstice, the first day of summer in the northern hemisphere. The new season officially began at 10:51 UTC, which is 6:51 A.M. Eastern Daylight Time.

The astronomical seasons are produced by the tilt of the Earth’s axis – a 23.5° angle – and the movement of the planet around the sun. During the summer months, the northern half of the Earth is angled toward the sun. This position allows the northern hemisphere to receive the sun’s energy at a more direct angle and produces our warmest temperatures of the year.

Since the winter solstice in December, the arc of the sun’s daily passage across the sky has been moving northward and daylight hours have been increasing. Today, it reached its northern most position at the Tropic of Cancer and marks the “longest day” of the year. This observable stop is where today’s event takes its name.  Solstice is a word derived from Latin meaning, “sun stands still”.

Now, the sun will start to move southward again in our sky and daylight hours will slowly start to decrease.

Seasons

The position of the Earth during different seasons. Image Credit: NASA

The Sun is directly overhead on Summer Solstice at the latitude known as the Tropic of Cancer. Image Credit: NASA

The Sun is directly overhead at the Tropic of Cancer on the Summer Solstice. Image Credit: NASA

Vernal Equinox 2014

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Today is the Vernal Equinox, the first day of spring in the northern hemisphere. The new season officially began at 16:57 UTC, which is 12:57pm Eastern Daylight Time.

The astronomical seasons are produced by the tilt of the Earth’s axis – a 23.5° angle – and the movement of the planet around the sun. Today, as spring begins, the Earth’s axis is tilted neither toward nor away from the sun.  This position distributes the sun’s energy equally between the northern and southern hemispheres.

Since the winter solstice in December, the sun has been making an apparent northward journey in our sky toward the Tropic of Cancer.  Today, it crossed the equator. This means we have approximately equal hours of day and night. The word “equinox” is derived from Latin and means “equal night”.

As a transitional season, spring marks the end of winter’s biting chill and the gradual return of warmth.  It is often associated with the ideas of renewal and rebirth.  As such, it has long been a cause for celebration across many cultures throughout human history.

Image Credit: NASA

Earth’s solstices and equinoxes. Image Credit: NASA

Image Credit: NASA

The Earth’s axis is tilted neither toward nor away from the sun on the Vernal Equinox. Image Credit: NASA