Autumn Considered the Second Season for Tornadoes

Spring is the season most commonly associated with twisters in the United States. Autumn, however, can be just as dangerous and is known as the “second season” for tornadoes.

According to NOAA, approximately 1200 tornadoes touch down in the US every year. While most occur in “Tornado Alley”, in the central part of the country, activity there tends to peak in May. The second season is most active in the mid-south, an area often referred to as “Dixie Alley”.

During the transitional months of autumn, the jet stream frequently dips south bringing cooler air into the region. At the same time, warm, moist air is flowing in from the Gulf of Mexico. When these two different air masses meet, the local weather can get very active. In fact, some of the largest tornado outbreaks of any month have occurred in October and November.

Credit: USTornadoes

Credit: USTornadoes.com

Credit: USTornadoes

Credit: USTornadoes.com

The Dew Point Makes a Difference

“It’s not the heat, it’s the humidity.” This common refrain heard throughout much of the eastern U.S. in summer refers to how the amount of water vapor in the air affects human comfort. Since the main source of body cooling is evaporation of perspiration, the higher the moisture content of the air, the less evaporation takes place and the warmer we feel. One measure of atmospheric moisture is the dew point temperature.

The dew point, as defined by the NWS, is the temperature to which air must be cooled in order to reach saturation. In other words, the difference between the air temperature and the dew point temperature tells you how much moisture is in the air. The greater the distance between the two, the dryer the air is. Conversely, the closer they are together, the higher the moisture content of the air.

While everyone has a different tolerance for humidity, in summer – when air temperatures tend to be high – a dew point temperature of 50°F is generally considered comfortable. Dew points in the 60s are thought of as muggy and once they reach the 70s or higher, the air can feel down right oppressive. On the opposite end of the spectrum, dew points in the 40s or lower are considered dry. Dry air has its own set of comfort issues, including skin irritations.

Why Air Temperature Decreases with Height

While visiting Colorado recently, I had the opportunity to explore Rocky Mountain National Park, and it was largely a vertical experience. Within its borders are 72 named peaks that reach above 12,000 feet in elevation. Traveling from the Beaver Meadow Visitor Center – elevation 7,840 feet – to the Alpine Visitor Center – elevation 11,796 feet – the drop in temperature was anything but subtle.

The reason for air being cooler at higher elevations is twofold. First, the sun’s rays heat the Earth’s surface, which in turn, radiates that warmth into the atmosphere. As you climb in altitude, there is less surface area of land available to heat the air. Second, as air rises, it expands and cools. This is because air density and pressure aloft are lower than at the surface.

The exact rate at which the temperature decreases with height – the environmental lapse rate – varies with location and daily conditions. On average, however, for every 1000 feet gained in elevation, the temperature drops by about 3.6°F.

Image Credit:British Geographer

Image Credit: The British Geographer

Heat Lightning

When lightning flashes in the sky without a clap of thunder, it is called “heat lightning.” This, however, is a bit of a misnomer as it is not a special form of lightning and it is not generated by heat. In fact, it is just normal lightning that is too far away for the sound of thunder to be heard.

Lightning travels near the speed of light – 186,000 miles per second. Even at a distance of 10 miles, you will see it almost instantly. Thunder, on the other hand, travels at the speed of sound – about one mile in five seconds near the ground. It is often refracted through the atmosphere and reflected by the Earth’s surface, dissipating the sound. For this reason, thunder is rarely heard at a distance of more than 10 miles from where the lightning actually occurred.

The origins of the term, “heat lightning”, are probably linked to the fact that it is most often seen on hot, humid summer nights.

North American Monsoon

The summer phase of the North American Monsoon is in full swing. But what, you may wonder, is a monsoon?

While most people associate a monsoon with rain, that is only half the story. It is actually a wind system. More specifically, according to NOAA, a monsoon is “a thermally driven wind arising from differential heating between a land mass and the adjacent ocean that reverses its direction seasonally.” In fact, the word monsoon is derived from the Arabic “mausim”, meaning seasons or wind shift.

In general, a monsoon is like a large-scale sea breeze.  During the summer months, the sun heats both the land and sea, but the surface temperature of the land rises more quickly. As a result, an area of low pressure develops over the land and an area of relatively higher pressure sits over the ocean. This causes moisture-laden sea air to flow inland. As it rises and cools, it releases precipitation. In winter, the dry season, this situation reverses.

Monsoon wind systems exist in many different parts of the world. In the U.S., we have the North American Monsoon that impacts states across the southwest. Summer temperatures in this region – mostly desert – can be extremely hot. Readings in the triple digits are not uncommon. This intense heat generates a thermal low near the surface and draws in moist air from the nearby Gulf of California. In addition, an area of high pressure aloft, known as the subtropical ridge, typically moves north over the south-central U.S. in summer. Its clockwise circulation shifts the winds from a southwesterly to a southeasterly direction and ushers in moisture from the Gulf of Mexico. This combination of heat and moisture rich air produces thunderstorms and heavy rainfall across the region between July and September. In fact, summer monsoon rains are reported to supply nearly 50% of the area’s annual precipitation.

Replenishing reservoirs and nourishing agriculture, these seasonal rains are a vital source of water in the typically arid southwest. Conversely, they can also cause a number of hazards such as flash flooding, damaging winds and hail, as well as frequent lightning.

The summer monsoon officially begins, according to the National Weather Service, when there have been three consecutive days with a dew point above 54°F.

North American Monsoon: Summer Weather Pattern. The thermal low sets up  over the desert southwest, while the subtropical high  moves into the southern plains. The winds draws moisture (green arrows) from the Gulf of California and the Gulf of Mexico.  Source: NOAA/NWS

North American Monsoon: Summer Weather Pattern. The thermal low sets up over the southwest, while the subtropical high moves into the southern plains. Their circulating winds draw moisture (green arrows) from the Gulf of California and the Gulf of Mexico.   Credit: NOAA/NWS

Earth’s Aphelion 2014

The Earth will reach its aphelion today at 8 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.

While the planet’s distance from the Sun does not cause the seasons, 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: mydarksky.org

Image Credit: mydarksky.org

Summer Solstice 2014

Today is the June Solstice, the first day of summer in the northern hemisphere. The new season officially began at 10:51 UTC, which is 6:51 A.M. Eastern Daylight Time.

The astronomical 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 summer months, the northern half of the Earth is angled toward the sun. This position allows the northern hemisphere to receive the sun’s energy at a more direct angle and produces our warmest temperatures of the year.

Since the winter solstice in December, the arc of the sun’s daily passage across the sky has been moving northward and daylight hours have been increasing. Today, it reached its northern most position at the Tropic of Cancer and marks the “longest day” of the year. This observable stop is where today’s event takes its name.  Solstice is a word derived from Latin meaning, “sun stands still”.

Now, the sun will start to move southward again in our sky and daylight hours will slowly start to decrease.

Seasons

The position of the Earth during different seasons. Image Credit: NASA

The Sun is directly overhead on Summer Solstice at the latitude known as the Tropic of Cancer.  Image Credit: NASA

The Sun is directly overhead at the Tropic of Cancer on the Summer Solstice. Image Credit: NASA

Green Skies

Thunderstorms are fairly common in the late spring and summer in the United States. Every once in a while, though, they can be severe. When they are, the sky often turns green. You may wonder, what causes this odd coloration?

According to scientists, the phenomenon of green skies is not completely understood. The leading theory, however, involves the dense moisture content of cumulonimbus clouds and the time of day. Most thunderstorms develop in the late afternoon, a time when the sun’s rays have to travel a long way through the atmosphere before reaching the ground. This causes the light we see around sunset to be reddish-yellow. Thunderstorm clouds contain large amounts of rain and hail. This water and ice scatters blue light. So, when these towering clouds form in the late afternoon, the two colors mix to give the sky a green or blue-green appearance.

While severe thunderstorms can produce tornadoes, a green sky does not necessarily mean a twister is coming. Nonetheless, the color is associated with dangerous weather. If you see a thunderstorm heading your direction and the sky appears green, you should seek shelter immediately.

Green Sky.  Image Credit: Sky7WX

Green Sky.   Image Credit: Sky7WX

Smoky Haze Fills the Air in NYC

The smell of smoke filled the air in New York City Monday morning.  Its source was a 1,600-acre brush fire in Wharton State Forest, NJ – about 90 miles away.

Burning since late Sunday, the smoke was trapped near the ground by a local temperature inversion. This is a weather phenomenon where the temperature in the atmosphere increases with height instead of decreasing.  Essentially, the inversion layer acted like a lid and caused the smoke to spread out horizontally rather than vertically.  A low level wind from the southeast then carried the smoke toward the city.

The smoky haze prompted the EPA to issue an air quality alert for the NYC area. With a spike in the pollutant known as “fine particulate matter”, this was the first time this year that the city’s air quality dropped below “moderate” on the agency’s AQI scale.

Smoky haze fills the air in NYC.  Image Credit: WPIX.

Smoky haze fills the air in NYC.  Image Credit: PIX11.

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.