Tag Archives: winter

Winter Precipitation

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Snow is probably the most well known type of winter precipitation.  There are, however, several other forms in which precipitation can fall during the cold winter months.  It all 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 of at or below freezing.  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 comes in contact with, including roads, tree branches, and power lines.

Sleet is different than freezing rain – it is a frozen precipitation.  It falls at ground level in 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 any objects.  It can, however, accumulate.

Snow is another type of frozen precipitation.  It takes the shape of multi-sided ice crystals, often called flakes.  Snow will fall at the surface when air temperatures are 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 all 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.

Precipitation type depends on the temperature profile of the atmosphere.

Image Credit: NOAA

Wind Chill

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Temperature is one of the basic elements of weather.  Our perception of it, however, is often influenced by other environmental conditions. Wind, for example, can make a cold day feel even colder.  This phenomenon is called the wind chill factor.

Wind chill is a measure of the apparent or “real feel” temperature.  It calculates the heat loss from exposed human skin through the combined effects of air temperature and wind speed. Essentially, the wind is carrying heat away from the body and allowing the skin to be exposed to cold air.  As the winds increase, heat is carried away at a faster rate and the colder the body feels.  For example, a temperature of 20°F and a wind speed of 5-mph will produce a wind chill index of 13°F.  At that same temperature, but with a wind speed of 10-mph, the wind chill index would be 9°F.

Extended exposure to low wind chill values can lead to frostbite, a serious winter health hazard.

windchillChart Credit: NOAA

Arctic Blast for NYC

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After a mild start to the season, winter has finally found New York City.

A deep dip in the jet stream has ushered in bitterly cold arctic air and brought the city its coldest temperatures of the season to date. While we are in the dead of winter and cold temperatures are expected, today’s high was only 20°F.  That is 18°F below average.  In fact, today was the coldest day NYC has experienced in two years.

These frigid conditions, according to the local forecast, are likely to stay in place for a while.  Temperatures are not expected to get above the freezing mark until early next week. Bundle up!

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Graph Credit: The Weather Gamut

The Dead of Winter

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Today marks the beginning of the “Dead of Winter”.

This old saying refers to the coldest, and often the harshest part, of the winter season. Traditionally, this period runs from January 10th through February 10th in the northern hemisphere.

The “Dead of Winter” is the climatological opposite of the “Dog Days of Summer“.

White Christmas

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The holidays are 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 Unites States – aside from mountainous regions – the greatest chance of seeing snow for Christmas is in cities across the northern tier of the country.  Average temperatures in that region are usually below freezing  by December.  Moving farther south, the probability of having a white Christmas steadily decreases.

Locally, here in New York City, the odds of having snow on Christmas Day are less than twenty-five percent. This low probability is largely due to the city’s proximity to the Atlantic Ocean and its relatively mild air. This year, despite some recent chilly temperatures, NYC is not expecting a white Christmas.

Snow or no snow, the Weather Gamut wishes you a very happy holiday!

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Probability of a white Christmas based on data from 1981-2010.

Image Credit: NOAA

Winter Solstice 2012

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Today is the December Solstice, the first day of winter in the northern hemisphere. The new season officially began this morning at 6:12 AM Eastern Standard Time.

Seasons are the result of the tilt of the Earth’s axis, a 23.5° angle. 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 solar energy all year.

Today is also the shortest day of the year.  Since the summer solstice in June, the sun – in its apparent seasonal journey across the sky – has been making its way south.  Today, it reached its southern most position at the Tropic of Capricorn.  Now, the sun will begin to move northward again and our daylight hours will slowly start to increase.

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

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Winter Solstice

Image Credit:scijink.nasa.gov

Nor’easter

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Hurricane season officially comes to a close later this week. The season for nor’easters, however, has only just begun and typically runs through April.

A nor’easter is an intense type of winter storm that traditionally affects the east coast of the United States from the mid-Atlantic to New England. Given the name, you would think a nor’easter comes from the northeast.  The opposite, however, is true.

These storms develop when a strong area of low pressure to the south moves up the east coast and meets cold air pushing down from Canada. They are infamous for producing powerful winds, heavy rain, snow, and coastal storm surges.  The steady northeasterly wind that blows in from the ocean ahead of these storms is what gives this weather pattern its name.

A Look Back at Winter in NYC

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The phrase, winter warmth, may seem like an oxymoron, but it was the theme of this past season across much of the U.S.  Nationally, it was the fourth warmest winter on record. Locally, in New York City, the season ranked as our second warmest ever with an average temperature of 40.5°F in Central Park.

A highlight of this meteorological winter was the city’s warmest February on record, which registered an average temperature 5.9°F above normal.  These mild temperatures, in turn, meant very little snowfall. Including the pre-season snowstorm at the end of October, the city only saw 7.4 inches of snow, which is 13.5 inches below normal.

Record warm spring temperatures are now following our unseasonably mild winter.

Graph Credit: MF at The Weather Gamut

Remnants of Winter

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Winter seems to be going out like a lamb in the northeastern United States.  Hiking rather then skiing, therefore, seemed appropriate on the last weekend of this record warm season.  I was surprised, however, to still find ice along some of the trails in Lake Minnewaska State Park Preserve. Below is a photo of the remnants of the season.

Photo Credit: MF at The Weather Gamut

Record Warm Winter in the U.S.

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It’s official! This was one of the warmest winters ever in the United States. According to NOAA, this meteorological winter (December-February) was the fourth warmest on record in the lower forty-eight. Registering an average temperature of 36.8°F, the country was 4°F above its long-term average.

Scientists say that a number of factors played a part in producing this unseasonable warmth.  To begin with, the Jet Stream, the boundary between warm southern air and cold arctic air, stayed well to the north this winter. As a result, mild conditions dominated the season and most of the country received below average snowfall. In fact, it was our third smallest winter snow cover in forty-six years of satellite record keeping.  Without snow to reflect the sun’s rays, the exposed ground absorbed solar energy, helping to perpetuate the warm conditions.

Two other significant factors in this winter’s story are La Nina and the North Atlantic Oscillation (NAO).  These powerful climate systems influence the shape and path of the Jet Stream.  La Nina, part of the larger El Nino Southern Oscillation (ENSO), brings cooler than normal water to the eastern Pacific Ocean. As a result, high pressure builds over the cool water and pushes the Jet Stream northward. NAO, the southern branch of the larger Arctic Oscillation, runs in positive and negative phases, depending on the pressure differences between Iceland and the Azores. A positive phase, like the one that dominated this winter, promotes a fairly straight path for the Jet Stream.  It also brings warmer than normal conditions to the eastern two-thirds of the U.S.

Given these constraints, the Jet Stream only managed to dip south a few times this season.  When it did, we were abruptly reminded that it was, in fact, still winter.  Those few cold blasts, however, never lasted very long. On the whole, it felt like a year without a winter.