The Science Behind the Spring Equinox

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

Saint Patrick’s Day and Rainbows

According to Irish folklore, a pot of gold can be found at the end of a rainbow. In reality, however, it is impossible to locate the terminus of this optical phenomenon.

Refraction and reflection inside a raindrop. Credit: Met Office

For a rainbow to form, rain has to be falling in one part of the sky while the sun is out in another. The water droplets in the air act like prisms that refract and reflect the sunlight, revealing the colors of the visible spectrum. Red is refracted the least and is always on the top of a single bow while blue is on the bottom. Since we only see one color from each drop, it takes a countless number to produce a rainbow.

A double rainbow is seen when the light reflects twice inside the raindrops. Since each reflection weakens the intensity of the light, the second bow appears dimmer. The order of the colors is also reversed, with blue on top and red on the bottom.

That said, these colorful arcs are not physical entities that can be approached. No matter how close they appear to be, they are always tantalizingly out of reach. Nevertheless, most people consider seeing one to be a treasure with no gold required.

With a little luck, you can spot a rainbow if you face a moisture source – clearing rain clouds or mist from a waterfall – while the sun is at your back.

Happy Saint Patrick’s Day!

Rainbow and faint second rainbow form after a rainstorm in Bermuda. Credit: Melissa Fleming

Visualizing Wind Speed

From a light breeze to a strong gale, wind speed can be described in numerous ways. All of which are categorized on the Beaufort Wind Force Scale.

Developed in 1805 by Sir Francis Beaufort, an officer in the UK’s Royal Navy, the scale is an empirical measure of wind speed. It relates wind speed to observed conditions at sea and overland instead of using precise measurements. Simply put, it allows a person to estimate wind speed with visual clues.

Initially, it was only used at sea and was based on the effect the wind had on the sails of a frigate – the most common type of ship in the British Navy at the time. By the mid-1800s, the scale was adapted to also reflect a certain number of anemometer rotations – a device that measures wind speed.

In the early 20thcentury, most ships transitioned to steam power and the scale descriptions were changed to reflect the state of the sea instead of the sails. Around the same time, the scale was extended to land observations. For example, the amount of leaf, branch, or whole tree movement is a visual indicator of the force of the wind.

Today, the scale has 13 categories (0 -12), with 0 representing calm winds and 12 being hurricane force. It is in use in several countries around the globe.

In the US, when winds reach force 6 or higher, the NWS begins issuing advisories and warnings for different environments. For marine areas, force 6-7 winds would prompt a small craft advisory, force 8-9 would warrant a gale wind warning, and a wind reaching force 10-11 would call for a storm warning. Force 12 would constitute a hurricane-force wind warning. On land, winds expected to reach force 6 or higher would cause a high wind warning to be issued.

If the winds are connected to a tropical cyclone, they would be measured on the Saffir-Simpson scale. The same type of special circumstances would also hold for a tornado, which would be measured on the Enhanced Fujita Scale.

Credit: Isle of Wight Weather Ctr

Groundhog Day 2020: Furry Forecasters Predict an Early Spring

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

Fun Weather Facts: Snow

Snow is in the forecast for New York City today. Here are some fun facts to ponder as the flakes fall:

  • All snowflakes, regardless of shape, have six sides.
  • Snow crystals are translucent, not white. The white color we see is caused by sunlight that is reflected off the crystals.
  • Most snowflakes fall at a speed of two to five feet per second. That is roughly the same speed as a person walking casually.

Enjoy the snow!

Credit: Wilson “Snowflake” Bentley

Weather Lingo: The Dead of Winter

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

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

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

From Snow to Freezing Rain: Why Winter Precipitation Can Take Several Forms

The winter season can produce various types of precipitation – rain, freezing rain, sleet, or snow. The form we see at the surface depends on the temperature profile of the lower atmosphere.

All precipitation starts out as snow up in the clouds.  But, as it falls toward the Earth, it can pass through one or more layers of air with different temperatures.  When the snow passes through a thick layer of warm air – above 32°F – it melts into rain.  If the warm air layer extends all the way to the ground, rain will fall at the surface.  However, if there is a thin layer of cold air – below 32°F – near the ground, the rain becomes supercooled and freezes upon impact with anything that has a temperature at or below 32°F.  This is known as freezing rain.  It is one of the most dangerous types of winter precipitation, as it forms a glaze of ice on almost everything it encounters, including roads, tree branches, and power lines.

Sleet is a frozen type precipitation that takes the form of ice-pellets. Passing through a thick layer of sub-freezing air near the surface, liquid raindrops are given enough time to re-freeze before reaching the ground. Sleet often bounces when it hits a surface, but does not stick to anything.  It can, however, accumulate.

Snow is another type of frozen precipitation.  It takes the shape of six-sided ice crystals, often called flakes.  Snow will fall at the surface when the air temperature is below freezing all the way from the cloud-level down to the ground.  In order for the snow to stick and accumulate, surface temperatures must also be at or below freezing.

When two or more of these precipitation types fall during a single storm, it is called a wintry mix.

Precipitation type depends on the temperature profile of the atmosphere. Credit: NOAA

Why the Sky Looks Bluer in Autumn

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