From Snow to Ice, Winter Precipitation Can Take Several Forms

The winter season can produce various types of precipitation – rain, freezing rain, sleet, or snow. It 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

Auroras: What Causes the Northern and Southern Lights

Auroras occur throughout the year, but the long nights of winter at high latitudes provide an optimal environment in which to see this amazing natural phenomenon.

These colorful patterns of light that dance across the night sky are the result of charged particles from the sun interacting with the Earth’s atmosphere. Originating in a massive explosion on the sun known as a coronal mass ejection, the protons and electrons travel nearly 93 million miles before some of them reach the Earth. When they encounter the planet’s magnetic field, they are pulled toward the poles, where the magnetic force is strongest. There, they interact with atmospheric gases and produce the variety of colors we see. The main factor in which colors are displayed, however, is altitude. Different gases, such as nitrogen and oxygen, vary in concentration at different levels of the atmosphere.

Oxygen molecules can generate green auroras up to 150 miles above sea level and red auroras further up. Nitrogen molecules produce blue lights up to 60 miles above the ground and violet colored lights at higher levels. Auroras, in general, extend from 50 miles to as high as 400 miles above the Earth’s surface.

The word aurora is Latin for dawn. Therefore, the “aurora borealis”, the northern lights, means “dawn of the north”. At the South Pole, the lights are known as the “aurora australis”, which means “dawn of the south”.

Aurora Borealis over Alaska. Credit SmithsonianMag

A January Thaw in NYC

The calendar says January, but it felt more like spring in New York City on Tuesday. The temperature soared to 60°F in Central Park, a staggering 22°F above average for this time of year and 2°F shy of the record high for the date.

This unseasonable warmth is part of the “January Thaw” currently taking place in a large part of the eastern United States. After an extended cold blast, this is a period when winter’s grip relaxes a bit and temperatures rise at least 10°F above normal for a few days. It does not necessarily occur every year, but when it does, it is usually in mid to late January, hence the name.

With temperatures in the 50s since Saturday, many New Yorkers have been out in the parks enjoying the break from the cold. It is, however, still January. So, keep your winter gear handy.

Melting ice on The Lake in Central Park, NYC. Credit: Melissa Fleming

Fun Facts about Snow

If you enjoy winter and a good snow day, here are some fun facts about snow to ponder when 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!

Snowflakes come in a variety of shapes, but all have six sides or points. Credit: Wilson “Snowflake” Bentley

Perihelion 2018: The Earth is Closest to the Sun Today

The Earth reached its Perihelion today at 5:34 UTC, which is 12:34 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 is closest to the Sun during the northern hemisphere’s winter. Credit: TimeandDate.com

Weather Lingo: Alberta Clipper

The winter season can produce a number of different types of storms. One of these is an Alberta Clipper.

These systems originate in western Canada, on the lee side of the Rocky Mountains. As  Pacific air spills downslope, an area of low pressure develops. From there, it gets caught up in the jet stream and moves to the southeast across the US. Traveling over land, these systems lack a significant source of moisture and generally do not produce much snow- usually around 1 to 3 inches. However, they are known for their strong winds and bitterly cold temperatures.

This type of quick-hitting storm takes its name not only from its place of origin near Alberta, Canada but also from the clipper ships of the 19th century – the fastest ships of the time.

Credit: NOAA

How the Santa Ana Winds Help Wildfires Spread

The Santa Ana winds are notorious for exacerbating wildfires in southern California.

These strong winds blow warm, dry air across the region at different times of the year, but mainly occur in the late autumn. They form when a large pressure difference builds up between the Great Basin – a desert that covers most of Nevada and parts of Utah – and the coastal region around LA. This pressure gradient funnels air downhill and through the passes of the San Gabriel and San Bernardino mountains toward the Pacific. According to the NWS, the Santa Ana winds can easily exceed 40 mph.

Originating in the high desert, the air starts off cool and dry. But as it travels downslope, the air compresses and warms. In fact, it warms about 5°F for every 1000 feet it descends. This dries out the region’s vegetation, leaving it susceptible to any type of spark. The fast-moving winds then fan the flames of any wildfires that ignite.

The Santa Ana winds are named for Santa Ana Canyon in Orange County, CA.

Credit: NWS

The Supermoon and Other Moon Names

The first and only supermoon of 2017 will rise on Sunday.

Supermoons are the result of the moon’s elliptical orbit around the Earth. They occur when the moon reaches perigee – its closest point to our planet (less than 223,694 miles). As it is so close, a supermoon looks 7% larger and 16% brighter than an average full moon. When seen near the horizon – where buildings or mountains provide a foreground – an illusion is created that makes the super moon look even bigger. They happen about every thirteen months or so.

When the moon is furthest from Earth – at apogee – it is called a micro-moon.

Full moons occur every 29.5 days when the moon is on the side of the Earth directly opposite the Sun. It reflects the sun’s rays and appears as a beautiful silver disk in the sky.

Ancient civilizations used full moons as a guide to schedule important activities, such as hunting and farming. They gave them each a name based on the dominant weather pattern or typical animal and plant activity during a particular month. In North America, according to National Geographic, native tribes used the moon names listed below. Many are still in use today.

  • January: Wolf Moon or Ice Moon
  • February: Snow Moon
  • March: Worm Moon or Sap Moon
  • April: Sprouting Grass Moon
  • May: Flower Moon
  • June: Strawberry Moon
  • July: Buck Moon
  • August: Sturgeon Moon or Grain Moon
  • September: Harvest Moon
  • October: Hunter’s Moon
  • November: Beaver Moon
  • December: Cold Moon or Long Night Moon

A blue moon is when a second full moon occurs in a single month. Given the uneven nature of our calendar system, these happen roughly every 2.5 years.

The apparent size of the moon as seen from Earth. Credit: KQED

Climate Change Indicator: Arctic Sea Ice

There are a number of key indicators, beyond our rising global temperature, that show Earth’s climate is changing. One of these is Arctic sea ice.

Measured via satellite since the late 1970’s, the extent and thickness of sea ice tend to vary from year to year but both have been in an overall decline for decades. According to NASA, the melt season in the Arctic has increased by 37 days since 1979.

Sea ice extent, the area of ocean with at least 15% sea ice, has a strong seasonal cycle. It typically peaks in March as winter ends and then declines during the summer, reaching a minimum in September. In March 2017, it hit a record low maximum for the third year in a row. The record minimum occurred in September 2012.

According to the National Snow and Ice Data Center, the average age of Arctic sea ice is also changing. Thick multi-year ice – the ice that lasts through at least one melt season – has decreased 11% per decade since the satellite era began. That means there is more first-year ice, which tends to be thin and brittle. This is troublesome because it is more vulnerable to warming temperatures and wave action.

Sea ice is frozen ocean water. It forms, grows, and melts in the ocean. In contrast to land ice (glaciers), it does not contribute to sea level rise. However, as it melts it creates a global warming feedback loop. Ice is lighter in color and reflects more sunlight than dark ocean water. So, as more ocean water is exposed, more of the sun’s energy is absorbed. This drives temperatures up even further and causes more ice to melt.

The Arctic is now warming twice as fast as the rest of the planet – a phenomenon known as “Arctic amplification.”  At this rate, scientists expect the region to be ice-free in summer by the 2030s.

Credit: NSIDC

Weather Lingo: ACE

There are a number of different ways to gauge a hurricane season. One of these is the Accumulated Cyclone Energy index, which is widely referred to as ACE.

It expresses the combined intensity and duration of individual cyclones and provides a measure of activity for an entire hurricane season. For a single storm, it is calculated by summing the squares of the maximum sustained wind speeds measured every six hours while they are at least tropical storm strength. This number is then divided by 10,000 to make it more user-friendly. Overall, the stronger and longer-lived a storm is, the higher its ACE value.

The ACE for a season is the sum of the ACE values from individual storms that occurred that year. NOAA considers a season with an ACE of 111 or higher to be above average, while an ACE of 66 or lower is regarded as below average.

The Atlantic hurricane season runs from June 1 through November 30.

Source: CSU

(Note: Year to date the Atlantic Basin has had 13 storms with a combined ACE of 202 and there are still two months left in the 2017 hurricane season.)