Tag Archives: seasons

The Science Behind the Spring Equinox

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Today is the Vernal Equinox, the first day of spring in the northern hemisphere. The new season officially begins at 11:49 PM Eastern Daylight Time.

Our 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. During the spring months, 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 arc of the sun’s apparent daily passage across the sky has been getting higher and daylight hours have been increasing. Today, the sun appears directly overhead at 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, spring is a time when the chill of winter fades away and the warmth of summer gradually returns. The most noticeable increases in average daily temperature, however, usually lag the equinox by a few weeks.

The vernal equinox normally takes place on March 20 or 21, but this year it is arriving a day early. The main reasons for this involve complicated calendar issues related to leap year and daylight savings time. Happy Spring!

Earth’s solstices and equinoxes. Image Credit: NASA

Winter 2019-2020: Seventh Warmest on Record in NYC

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The spring equinox is still a few weeks away, but meteorological winter (December, January, and February) has officially ended and it tied the winter of 1990-91 as the seventh warmest on record in New York City.

The season, with daily highs ranging from 25°F to 69°F, felt like a temperature roller coaster. But in the end, the warmth came out on top. The city’s average temperature for the season, according to the NWS, was 39.2°F. That is an incredible 4.1°F above normal.

In all, sixty-three out of ninety-one days posted above average readings and every month was warmer than its long-term norm. In fact, February 2020 was the city’s fifth warmest February on record.  

In terms of snowfall, the city received a paltry 4.8 inches in Central Park, which is a staggering 16.5 inches below average. That makes the winter of 2019-2020 the fourth least snowy winter on record for the Big Apple.

This winter’s pattern of prolonged periods of warmth separated by a few short-lived cold snaps was largely driven by the North Atlantic Oscillation’s positive phase occurring more often and lasting longer than its negative phase.

The city’s warmest winter on record was the 2001-2002 season with an average temperature of 41.5°F.  Central Park weather records date back to 1869.

Credit: The Weather Gamut

Groundhog Day 2020: Furry Forecasters Predict an Early Spring

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

On this day, according to folklore, 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 try to predict future weather conditions goes back to ancient times. The particular custom that we are familiar with here in the United States grew out of old-world superstitions connected to the date of Candlemas, a Christian feast day, that German settlers brought 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 gained national celebrity status after starring in the 1993 film, “Groundhog Day”. Here in New York City, our local weather-groundhog is Charles G. Hogg. A resident of the Staten Island Zoo, he is more popularly known as “Staten Island Chuck”. This year, both groundhogs are calling for an early spring.

That said, long-range forecasts can be a tricky business. So, we will have to wait and see what actually happens. Either way, the spring equinox is 46 days away.

Credit: National Today

Weather Lingo: The Dead of Winter

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The “Dead of Winter” is an old saying that refers to the coldest part of the winter season. For most of the northern hemisphere, that usually means the month of January.

While actual daily weather varies, historical average temperatures typically reach their lowest point of the year between January 10 and February 10.  In the US, the western half of the country, with the exception of high elevation areas, tends to see its coldest day before the eastern half.  This is because the mountainous areas of the west and much of the eastern US tend to be snow covered in winter, reflecting much of the sun’s heat. Here in New York City, mid to late January is usually the coldest part of the winter season.

This traditional 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 energy 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 frosty weeks are the opposite of the “Dog Days of Summer.”

Credit: NOAA

Perihelion 2020: The Earth is Closest to the Sun Today

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The Earth reached its Perihelion today at 7:48 UTC, which is 2:48 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 147.1 million kilometers (91.4 million miles) away from the Sun. That is approximately 5 million kilometers (3 million miles) closer than it will be in early July. This position allows the planet to speed up by about one-kilometer per second. As a result, winter in the northern hemisphere is about five days shorter than summer.

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

Earth’s Perihelion and Aphelion. Credit: Time and Date.com

The Science Behind the Winter Solstice

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Today is the December solstice, the first day of winter in the northern hemisphere. The new season officially begins at 11:19 PM EST.

The astronomical seasons, which are different than meteorological 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 means the northern hemisphere receives the sun’s energy at a less direct angle and brings us our coolest temperatures of the year.

Since the summer solstice in June, the arc of the sun’s apparent daily passage across the sky has been dropping southward and daylight hours have been decreasing. Today, it will reach its southernmost 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.

Soon, the sun will appear to move northward again 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.

Earth’s solstices and equinoxes. Image Credit: NASA

Weather and Health: SAD

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Autumn and winter, with their crisp temperatures, are favorite seasons for many. But for others, the decreasing daylight hours can bring on a condition known as Seasonal Affective Disorder (SAD).

According to the Mayo Clinic, SAD is a “subtype of depression that comes and goes with the seasons” and is most common in fall and winter. Its exact cause is not fully understood, but researchers say a reduction of sunlight can disrupt the production of serotonin and melatonin – chemicals in the brain that regulate mood and sleep patterns. SAD symptoms include low spirits, lack of energy, difficulty concentrating, and changes in both sleep and eating patterns.

SAD is typically found in places that are far from the equator where daylight is at a minimum in the winter months. A report by the American Academy of Family Physicians says about 6% of the US population suffers from some degree of SAD, with most cases occurring in Alaska.

The most common treatment for SAD is light therapy. This involves sitting in front of a special lamp that gives off a light that is similar to natural sunshine. It has been shown to trigger the brain chemicals that regulate mood. The more serious cases of SAD could require advanced talk-therapy or even medication.

While everyone can feel a little “blue” once in a while, SAD is characterized by a prolonged feeling of depression. It can be a serious condition and should be diagnosed by a medical professional.

Weather affects mood. Credit: hercampus

Why the Sky Looks Bluer in Autumn

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Autumn is well known as the time of year when leaves change color. Not as well known, however, is the fact that the sky also changes shades with the season.

In general, we see the sky as blue because of Rayleigh scattering. This is a phenomenon where the molecules of nitrogen and oxygen that make up most of Earth’s atmosphere scatter the incoming light radiation from the sun. More to the point, they are most effective at scattering light with short wavelengths, such as those on the blue end of the visual spectrum. This allows blue light to reach our eyes from all directions and dictates the color we understand the sky to be.

The arc height of the sun’s apparent daily passage across our sky, which varies with the seasons, determines how much of the atmosphere the incoming light must pass through. This, in turn, affects how much scattering takes place. Simply put, the more Rayleigh scattering, the bluer the sky appears.

That said, humidity levels also play a role. Water vapor and water droplets are significantly larger than nitrogen and oxygen molecules and therefore scatter light differently. Instead of sending light in all directions, they project it forward. This is known as Mie scattering and tends to create a milky white or hazy appearance in the sky.

During the summer months, when the sun is higher in the sky, light does not have to travel as far through the atmosphere to reach our eyes. Consequently, there is less Rayleigh scattering. The warm temperatures of summer also mean the air can hold more moisture, increasing the effect of Mie scattering. As a result, the summer sky tends to be relatively muted or pale blue.

In autumn, the sun sits lower on the horizon, increasing the amount of Rayleigh scattering. The season’s cooler temperatures also decrease the amount of moisture the air can hold, diminishing the degree of Mie scattering. Taken together, these two factors produce deep blue skies.

When this azure hue is contrasted with the reds and yellows of the season’s famous foliage, all of the colors look even more vibrant.

Photo credit: Azure-Lorica Foundation

Why Leaves Change Color in the Autumn

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Autumn is a season well known for its colorful foliage. The often-celebrated aesthetic displays, however, are actually part of a process that trees use to survive the winter.

As daylight hours decrease in the fall, there is less sunlight available to power photosynthesis – the chemical process that provides nutrients to trees by converting carbon dioxide and water into glucose. This, in combination with falling temperatures, signals the tree to stop producing food and prepare for a period of dormancy, which is similar to hibernation.

To do this, a 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, which produce yellow and orange leaves, respectively. Red to purplish colors are the result of anthocyanin, a chemical produced as a result of any remaining sugars trapped in a leaf.

While leaves change color every year, the timing and duration of the displays are largely dependent on temperature and rainfall. Dry, sunny days and cool nights are the ideal recipe for beautiful fall foliage. Warmer and wetter conditions, on the other hand, tend to dull and delay the color change. Extreme circumstances, such as frost or drought, can be a source of stress for trees and cause the leaves to fall off faster.

It should also be noted that different species of trees react to atmospheric conditions differently. Therefore, the more diverse a forest, the wider the range of colors in autumn.

Tree in Autumn. Credit: Melissa Fleming

The Science Behind the Autumnal Equinox

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Today is the Autumnal Equinox, the first day of fall in the northern hemisphere. The new season officially began at 7:50 UTC, which is 3:50 AM Eastern Daylight Time.

The astronomical seasons, as opposed to the meteorological seasons, are a product of Earth’s axial tilt – a 23.5° angle – and the movement of the planet around the sun. During the autumn months, 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 apparent daily passage across the sky has been sinking and daylight hours have been decreasing. Today, the sun appears directly overhead at the equator and we have approximately equal hours of day and night. The word “equinox” is derived from Latin and means “equal night”.

Transitioning from summer to winter, autumn is also a season of falling temperatures. According to NOAA, the average high temperature in most US cities drops about 10°F between September and October.

Earth’s solstices and equinoxes. Image Credit: NASA