Tag Archives: definitions

Nor’easter

Posted on by

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.

The Dog Days of Summer

Posted on by

The “Dog Days” of summer have arrived. This popular saying refers to the hottest and most oppressive days of the season. Varying by latitude around the globe, they traditionally run from July 3rd to August 11th in the United States.

Rooted in astronomy, this non-technical weather phrase is linked to Sirius – the Dog Star.  During most of July and August, Sirius rises and sets with our Sun.  As one of the brightest stars in the night sky, ancient Greeks and Romans believed its conjunction with the Sun added extra heat to the summer months, causing temperatures to soar.  Today, we know that light from this distant star does not affect our weather, but the name has endured.

Aphelion

Posted on by

The Earth will reach its aphelion today at 11 P.M. Eastern Daylight Time.  This is the point in the planet’s elliptical orbit where it is farthest from the Sun.

Today, the Earth is about 152 million kilometers away from its nearest star.  That is approximately 5 million kilometers further than during the perihelion in early January. The exact date of the aphelion differs from year to year, but it’s usually in early July.

The planet’s distance from the Sun does not cause the seasons, but it does influence their length.  As a function of gravity, the closer the planet is to the Sun, the faster it moves. So, at the aphelion, the Earth will move more slowly along its orbital path than at any other time of the year. As a result, summer is elongated by a few days in the northern hemisphere.

The word, aphelion, is Greek for “away from sun”.

Image Credit: astronomy.edu.au

Koppen System of Climate Classification

Posted on by

Climate, the long term statistical average of weather conditions, helps describe the character of a region.

Technically, every place on Earth has its own climate. To deal with such tremendous variety, scientists set up categories.  They organize locations with similar characteristics into groups. The ancient Greeks were among the first to attempt climate classification.  They divided the world into three zones: torrid, temperate, and frigid.  Today, the most widely used scheme is the Koppen Climate Classification System.

Wladimir Koppen (1846-1940) was a German climatologist working at the turn of the 20th century.  He developed a climate system based on monthly and annual averages of temperature and precipitation. Koppen also believed that a region’s vegetation was its best expression of climate. So, the natural distribution of native plants heavily influenced his zonal boundaries.

The Koppen system recognizes five principle climate groups, each with various sub-categories.  They are:

  • Tropical: Tropical Rainforest, Tropical Monsoon, Tropical Savanna
  • Dry: Semi-arid, Arid
  • Temperate: Mediterranean, Humid Sub-tropical, Marine West Coast
  • Continental: Warm Summer, Cool Winter, Sub-arctic
  • Polar: Tundra, Ice Sheets

Over the years, people have expanded this system to better suit their needs.  While the number of sub-categories differ, most are based on Koppen’s original idea.

Pineapple Express

Posted on by

A Pineapple Express is a non-technical term used to describe a weather pattern that originates in the tropical Pacific and impacts the west coast of the United States. More specifically, it is a type of atmospheric river.

According to NOAA’s Earth System Research Laboratory, atmospheric rivers are “relatively narrow regions in the atmosphere that are responsible for most of the horizontal transport of water vapor outside of the tropics.” Coming in a variety of shapes and sizes around the globe, they form when strong winds associated with storms draw moisture into a thin area ahead of a cold front. The strongest atmospheric rivers can carry as much as 15 times the amount of water that flows through the mouth of the Mississippi River.

Directed by the jet stream, a Pineapple Express transports moisture from the Hawaiian tropics to the west coast of the US. They are famous for bringing warm, moist air and heavy rain to California, Oregon, and Washington. Just a few of these events, according to NOAA, can supply the region with 30% to 50% of its annual precipitation. While beneficial on one hand, they can also be dangerous. If these systems stall over an area, they can cause major flooding and landslides.

The term Pineapple Express is named for the tropical Pacific’s popular fruit.

The atmospheric river known as the pineapple express. Image credit: cimss/wunderground

The atmospheric river known as the pineapple express.  Image credit: cimss/wunderground.

Rev. 2014

Meteorological Seasons

Posted on by

Seasonal weather records are officially organized by meteorological season. These are slightly different than the seasons you see marked on calendars.

Our planet takes 365.25 days to complete a revolution around the sun.  As a result, the exact dates for the beginning of the astronomical seasons – equinoxes and solstices – vary from year to year.  To maintain consistent records, meteorologists use groupings of three whole-months to subdivide the year.  These quarters are based on temperature rather than the position of the Earth in its orbit.

In the northern hemisphere, for example, the meteorological winter begins on December 1st and includes the three coldest months of the year, December, January, and February.  The winter solstice, on the other hand, does not arrive until December 21st or 22nd, depending on the year. This year’s meteorological winter recently made headlines as the fourth warmest on record.

In general, the four meteorological seasons arrive about three weeks ahead of the astronomical ones.

Graphics Credit: MF at The Weather Gamut

Perihelion

Posted on by

Today, the Earth will reach its Perihelion at 8 P.M. 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.  We will be furthest from the Sun at the Aphelion in July.

The planet’s distance from the Sun does not cause the seasons, but 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 change in distance 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 opposite is true in the southern hemisphere.

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

The perihelion position of Earth's orbit.

Image Credit: Academy Artworks

Flag Trees

Posted on by

Objects that indicate the direction of the wind are known as weather vanes.  Not all, however, are man made.

Flag trees, for example, are living weather vanes.  They show the direction of the prevailing wind in a particular location.  On mountain ridges, where trees are constantly exposed to strong winds, the windward facing branches of a tree are often stunted or killed. On the leeward side, the trunk shelters the branches.  This allows them to grow to normal size.  As a result, the tree is sculpted to the shape of the wind and resembles a flag waving in a breeze.

These deformed trees are common features in the windswept sub-alpine landscape, but are not limited to that terrain.  Flag trees can also be seen in the tropics, where the strong, sustained trade winds contort trees in a similar fashion near the equator.

Flag Trees

Photo Credit: jmu.edu

Global Warming v. Climate Change

Posted on by

We often hear the terms “Global Warming” and “Climate Change” used interchangeably to describe our current environmental conditions.  They are, however, slightly different in scope.

Global warming refers to the rising average temperatures of the atmosphere and oceans across the entire planet.  Scientists report that the Earth’s average surface temperature increased 1.4°F in the last century.  The largest increases have been noted over the past 30 years.

Climate Change refers to the diverse impacts that these increasing temperatures are causing around the globe.  As the additional heat trapped in our atmosphere is circulated around the planet, established weather patterns are being altered.  Different regions, therefore, are being affected in different ways.  Some are getting wetter and others dryer. Many areas are also seeing an increase in the frequency and intensity of extreme weather events.  In addition, coastal regions are being impacted by rising sea levels.

While the Earth’s climate has changed numerous times throughout its long history, the change happening now is occurring very quickly and the vast majority (97%) of experts agree that human activities are the main cause. Regardless of what we call it, Global Climate Change is a subject of great concern that affects us all.

Weather v. Climate

Posted on by

The daily high temperatures this month seem like they are riding a roller-coaster.   While these variations are par for the course in a transitional month like October, they often get people talking about the weather.  In listening to some of these conversations, however, I frequently hear the terms “weather” and “climate” confused.  To set the record straight, weather and climate are related, but different.  The main difference between the two is time.

Weather is the short-term state of the atmosphere at a particular time and place.  It references current local conditions like temperature, wind, and precipitation.  Driven mainly by the relative differences in temperature and air pressure from one location to the next, weather is constantly changing.

Climate is the long-term, ”normal” weather conditions of a given region.  It refers to the prevailing weather that we expect for particular seasons based on past experience and tends to be stable for the course of centuries.  Over extended periods of geologic time, however, climate does change. For example, our planet has seen the extremes of both ice ages with massive glaciers and warmer periods with higher sea levels.

While the Earth’s climate has changed a number of times throughout its long history, the changes happening today are occurring more quickly than they have in the past.  Experts say rising global temperatures – the result of increasing greenhouse gases in the atmosphere – are causing weather patterns to shift and, in turn, regional climates to change.