Technical Weather Terms Go Viral

This winter produced some record cold temperatures and significant snowfall across the eastern two-thirds of U.S.  It also brought a few technical weather terms into our mainstream vocabulary.

In January, bitterly cold arctic air moved south over a large section of this country and the term “polar vortex” became ubiquitous.  Although it sounds ominous, the phrase literally describes what it is – a pattern of winds spinning around the North Pole.

More recently, as a powerful nor’easter moved up the eastern seaboard, “bomb” became a weather buzzword.  Also known as “explosive cyclogenesis”, it is a meteorological expression that describes the rapid intensification of a low-pressure system.  More specifically, it means the surface pressure of a system is expected to drop by at least 24 millibars in twenty-four hours.  In general, the lower the pressure, the stronger the storm.

The weather phenomena described by these phrases can occur every winter.  Why did they become so popular this year?  Was it because they happened more frequently this season?  Is it because they work well as catchwords or tags on Twitter?  Perhaps, it was a bit of both.

The Dead of Winter

The deep freeze that gripped much of the U.S. this week has begun to thaw.  The “Dead of Winter”, however, is just beginning.

While actual daily weather varies, this old saying refers to what is statistically the coldest part of the winter season.  Between January 10th and February 10th, average temperatures reach their lowest point of the year in the northern hemisphere.

These few weeks are the climatological opposite of the “Dog Days of Summer“.

The Polar Vortex

The Polar Vortex has been making headlines across most of the United States recently.  But, what exactly is it?

According to NOAA, the northern hemisphere polar vortex is a high altitude low-pressure system anchored over the Arctic.  More specifically, it is “the pattern of winds around the North Pole.”  It is always present, but tends to be stronger in the winter.

The configuration of the polar vortex – smooth or wavy – determines how much cold air escapes the region. Driven by the temperature difference between north and south, these winds typically circle the pole from west to east in a smooth pattern that bottles up the Arctic’s cold air.   When these winds weaken, the pattern becomes wavy and cold air pushes southward.

The connection between warming conditions in the Arctic (decreasing the temperature difference between north and south) and extreme weather events in the mid-latitudes is an active area of research. Some scientists suggest that increasing Arctic temperatures may be responsible for disrupting the pattern of the polar vortex.

The polar vortex is shown in purple.  Image Credit: NOAA

Polar Vortex configuration: Smooth vs Wavy.  Image Credit: NOAA

Today is Earth’s Perihelion

The Earth reached its Perihelion today at 12 UTC, which is 7AM 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 about 146 million kilometers away from the Sun.  That is approximately 5 million kilometers closer than in early July.  This position allows the planet to speed up by about one-kilometer/second.  As a result, winter in the northern hemisphere is about five days shorter than summer.

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

Image Credit: Academy Artworks

Image Credit: Academy Artworks

Heat Index

Temperature is one of the basic elements of weather.  Our perception of it, however, is often influenced by other factors.  In summer, this is usually humidity.

The heat index, developed in the late 1970’s, is a measure of the apparent or “real feel” temperature when heat and humidity are combined.  Since the human body relies on the evaporation of its perspiration to cool itself, the moisture content of the air affects comfort levels. Basically, as humidity levels increase, the rate of evaporation decreases and the body can begin to feel overheated.  For example, an air temperature of 92°F combined with a relative humidity level of 60% will produce a heat index value of 105°F.

The National Weather Service issues heat advisories when the heat index is forecast to be at least 95°F for two consecutive days or 100°F for any length of time.  Extended exposure to high heat index values can lead to serious health hazards.

Heat-IndexImage Credit: NOAA

How Road Salt Works

A major snowstorm is expected here in the northeast tomorrow.  In preparation, salt trucks have been out in force across the region.

Rock Salt – large pieces of NaCl – removes snow and ice from roads and walkways by lowering the freezing point of water.  Essentially, it melts frozen precipitation into a briny liquid and makes it harder to re-freeze, even if the air temperature is below freezing. In extremely cold conditions, however, when temperatures fall below 15°F, rock salt becomes less effective at de-icing surfaces.

While salting roads can help reduce traffic accidents, it does have negative environmental impacts. So, as with diet, it is best to use salt sparingly.

Alberta Clippers

The winter season can produce a number of different types of storms. Recently, Alberta Clippers have been dominating the forecast in the eastern United States.

An Alberta Clipper is a fast moving winter storm system that originates in the Canadian Rockies, specifically in the province of Alberta.  Once formed, these storms tend to get caught up in the jet stream and travel southeast across the US.  Alberta Clippers, unlike nor’easters, usually do not produce large amounts of snow.  Moving quickly over land, they generally lack significant moisture.  Clippers are, however, known for their strong winds and cold temperatures.

This type of storm system takes its name not only from its place of origin, but also from the clipper ships of the 19th century – the early days of meteorology.  Back then, clippers were the fastest sailing ships of the time.


Image Credit: NOAA

Winter Precipitation

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

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

The Dead of Winter

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“.