A Look at 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 21:58 UTC, which is 5:58 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.

Earth’s solstices and equinoxes. Image Credit: NASA

Weather Lingo: The Beaufort Wind Force Scale

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 over land 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.

The Beaufort Wind Force Scale. Credit: Isle of Wight Weather Ctr

Why February is Usually the Snowiest Month in the Northeast

A major snowstorm can happen during any month of the winter season, but in the northeastern United States, they tend to happen most often in February. In fact, February is the snowiest month of the year, on average, for most places across the region.

The reason for this has a lot to do with seasonal weather patterns.  That is, certain weather patterns are more likely to develop at different times of the year in different places across the country. In February, that pattern is highly conducive to producing major snowstorms in the northeast.

In general, that set up involves a large ridge in the jetstream over the west coast of the US with a deep, negatively tilted trough, in the east. The trough allows cold air from the north to spill down over the region. This means that any precipitation that falls will likely come down as snow. Another key factor is the warm water of the Gulf Stream, which flows just off the east coast.  Storms that pass over it tend to rapidly intensify. Then, following the jet stream northward, storms often encounter an area of high pressure over eastern Canada that slows their forward movement. As a result, more snow can fall over the same location boosting accumulation totals.

This is reflected in the statistics of the North East Snowfall Impact Scale (NESIS), which shows that the most category 3 or higher snowstorms occur in February. Ranked on a scale of 1 to 5, a category 3 is described as a “major” snowstorm, category 4 is considered “crippling”, and category 5 is an “extreme” event. The classifications are based on the size of the area covered, number of people affected, and snowfall totals.

In New York City, a winter season will produce 25.8 inches of snow, on average. Of that total, 9.2 inches comes in February.

Data Source: NWS

The Folklore Behind Groundhog Day

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 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 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 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.

But 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: CBC

 

Extremely Cold Weather Can Be A Danger to Your Health

An arctic blast is expected to sweep across the northeastern United States this week. With temperatures expected to fall into the single digits, it is important to remember that, like extreme heat, extreme cold can be very dangerous.

Extreme cold causes the body to lose heat faster than it can be generated.  Prolonged exposure, according to the CDC, can cause serious health problems, including hypothermia and frostbite.

Hypothermia is a condition of unusually low body temperature – generally below 95°F.  It impairs brain functions, limiting a victim’s ability to think and move.  Symptoms include severe shivering, drowsiness, confusion, slurred speech, and fumbling.  If left untreated, it can be fatal.

Frostbite is a localized injury to the skin and underlying tissues caused by freezing.  It can cause permanent damage and extreme cases often require amputation.  Areas of the body most often affected include the nose, ears, cheeks, fingers, and toes. Signs of frostbite include, numbness, skin discoloration (white or greyish-yellow), and unusually firm or waxy feeling skin.

While the symptoms of both hypothermia and frostbite can range in severity, victims generally require immediate re-warming and professional medical attention.

To stay safe in cold weather, the American Red Cross recommends:

From Snow to Freezing Rain, 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 super-cooled 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

Perihelion 2019: The Earth is Closest to the Sun Today

The Earth reached its Perihelion today at 5:20 UTC, which is 12:20 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 Historical Chances for a White Christmas

The Holiday Season is here and many people are dreaming of a White Christmas. The likelihood of seeing those dreams come true, however, are largely dependent on where you live.

According to NOAA, a White Christmas is defined as having at least one inch of snow on the ground on December 25th. In the US, the climatological probability of having snow for Christmas is greatest across the northern tier of the country. Moving south, average temperatures increase and the odds for snow steadily decrease.

Here in New York City, the historical chance of having a White Christmas is about 12%. This low probability is largely due to the city’s proximity to the Atlantic Ocean and its moderating influence on the temperature.

This year, with temperatures forecast to be in the 40s on the big day, the city’s already minimal chance for snow has largely melted away.

Snow or no snow, The Weather Gamut wishes you a very Happy Holiday!

Source: NOAA

What is the Winter Solstice?

Today is the December solstice, the first day of winter in the northern hemisphere. The new season officially begins at 22:23 UTC, which is 5:23 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 Lingo: Lake Effect Snow

Winter snowstorms have a variety of names, such as Nor’easters and Alberta Clippers. It all depends on where and how they develop. In the Great Lakes region of the US, the vast bodies of fresh water influence the weather and create something known as lake effect snow.

Lake-effect snowstorms, according to NOAA, develop when cold air blows across the warmer waters of a large unfrozen lake. The bottom layer of the air mass is warmed by the water and allows it to evaporate moisture, which forms clouds. When the air mass reaches the leeward side of the lake its temperature drops again, because the land is cooler than the water. This releases the water vapor as precipitation and enormous amounts of snow can accumulate. The effect is enhanced if the air is lifted upward by local topography.

With the clouds typically forming in bands, the snowfall is highly localized. Some places can see the snow come down at a rate of more than 5 inches per hour, while nearby, others will only get a dusting. The shape of the lake and the prevailing wind direction help to determine the size and orientation of these bands.

Fetch, the distance wind travels over a body of water, also plays a key role. A fetch of more than 60 miles is needed to produce lake effect snow. In general, the larger the fetch, the greater the amount of precipitation, as more moisture can be picked up by the moving air.

The impressive depths of the Great Lakes allow them to remain unfrozen longer into the winter season than more shallow bodies of water. This combined with their massive surface area, make them excellent producers of  lake effect snow. With northwesterly winds prevailing in the region, communities along the southeastern shores of the lakes are often referred to as being in the “Snowbelt.”

Credit: NOAA