Weather Lingo: Blue Norther

Autumn is a transitional season when the heat of summer fades away and the chill of winter gradually returns. But, sometimes winter can be aggressive and show up overnight.

When this type of rapid temperature change happens, it is often called a Blue Norther. This is a fast-moving cold front marked by a quick and dramatic drop in temperature. A fall of 20 to 30 degrees in just a few minutes is not uncommon. They also usher in a dark blue sky and strong northerly winds. Hence, the name.

Blue Northers are most common in the central US, where there are few natural barriers to slow or block arctic air masses from moving south. They can occur throughout the year, but are most common between November and March.

One of the most famous examples of this weather phenomenon was the “Great Blue Norther” of November 11, 1911. As the front passed through the southern plains, temperatures dropped from highs in the 70s and 80s to the teens in just ten hours. In Oklahoma City, for example, the temperature reached a record high of 83°F in the afternoon and then plummeted to a record low of 17°F by midnight. Both records, according to the NWS, are still in place.

Twenty-four temperature changes from The Great Blue Norther of 1911. Credit: FOX

How Wildfires Get Named

Wildfires, like major storms, are named for ease of communication and historical reference. But unlike hurricane names which are chosen from a pre-determined list each season, wildfires are labeled on a rolling basis.

According to CalFire, a wildfire is named as soon as it is reported. The moniker is usually selected by the dispatcher who takes the call or the initial first responders on the scene. Driven largely by geography, the names reflect a landmark such as a canyon, creek, or road, near where the fire started. For the sake of simplicity, they often tend to be one word titles. Although efficient, this can lead to some ominous or misleading names such as the “Witch Fire” of 2007 in San Diego or the “Easy Fire” that is currently burning in Simi Valley, CA.

If a fire occurs repeatedly in the same place, it will get a name and number such as “Bear Fire 2” or “Canyon Fire 3”.

While this may seem like a free-wheeling way to do things, the National Interagency Fire Center offers guidelines for the best practices in naming a fire. They advise against using the names of people, companies, or private property. They also discourage the use of “deadman” in any fire name.

Regardless of how arbitrarily selected or innocuous a name may sound, fires will ultimately be remembered for the destruction they cause.

Kincade Fire 2019. Credit: KTVU

How the Santa Ana and Diablo Winds Help Wildfires Spread

The Diablo and Santa Ana winds are notorious for exacerbating wildfires in northern and southern California, respectively.

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 large desert that covers most of Nevada  – and the California coast. This pressure gradient funnels air downhill and through mountain canyons and passes toward the Pacific. Squeezing through these narrow spaces, the wind is forced to speed up. According to the NWS, they 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.

These infamous zephyrs are named for the places from which they tend to blow. The Santa Ana Winds are named for Santa Ana Canyon in Orange County. The Diablo Winds take their moniker from Mount Diablo, which sits northeast of San Francisco.

Credit: Insider Inc

What Causes the Smell of Rain?

Rain is often associated with particular smells. But, rain itself is odorless. So, where do these aromas come from?

The distinctive scent that lingers in the air after a rainstorm is known as petrichor. It is the product of two reactions that occur when rainwater hits the ground. Its main driver is a soil-dwelling bacteria called actinomycetes. These microorganisms thrive in moist conditions, but as the soil dries out, they produce spores. These are then released into the air by the moisture and force with which the rain hits the ground. This happens at the same time the rainwater is mixing with oils that were secreted by plants onto nearby rocks and soil during times of dryness. Together these reactions produce the musky petrichor smell, which is particularly strong after a long dry spell. The term was coined in 1964 by two Australian scientists, Isabel Joy Bear and RG Thomson. They derived it from the Greek words, “petra” meaning stone and “ichor”, the term used to describe the blood of the gods in ancient mythology.

A different, and often more pungent, rain smell is associated with thunderstorms. After the powerful electric charge of a lightning bolt splits the oxygen and nitrogen molecules in the atmosphere, they often recombine as nitric oxide. This, in turn, interacts with other atmospheric chemicals to form ozone (O3). When people say they can “smell the rain coming”, this is the scent they detect as it often arrives in the wind ahead of an approaching storm.

Credit: freepik

Weather Lingo: Haboob

During the summer months, a change in wind direction known as a monsoon brings a major shift in the weather for the southwestern US. The season is well known for producing intense dust storms known as haboobs.

Haboobs form when thunderstorms collapse and create a strong downward flow of wind. When this downdraft of air hits the ground ahead of the storm, it blows the loose sand and soil from the desert floor high up in the air, creating a giant wall of dust.  Rising quickly, haboobs often reach heights between 5000 and 8000 feet and can span out nearly 100 miles in length. Traveling at speeds ranging from 30 to 60 mph, they can cover large distances rather quickly.

Often called “black blizzards”, these storms turn day into night.  Engulfing entire communities in dust, they cause respiratory problems and create serious travel hazards both in the air and on the ground. Luckily, they usually only last a few hours.

Many dry regions of the world experience haboobs, but they were first described in Sudan, along the southern edge of the Sahara Desert. As such, the word comes from Arabic and means, “blowing or blasting furiously.”

A haboob moves across Phoenix, AZ in August 2018. Credit: ChopperGuy/Twitter

What is the North American Monsoon?

The summer phase of the North American Monsoon is underway. But what, you may wonder, are monsoons and how do they impact the United States?

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 landmass and the adjacent ocean that reverses its direction seasonally.” In fact, the word monsoon is derived from the Arabic word “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, this situation reverses and a dry season takes hold.

Monsoon wind systems exist in many different parts of the world, with the most famous one setting up over India and Bangladesh. In the US, we have the North American Monsoon that impacts states across the southwest. Summer temperatures in the region, which is 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 northward over the southern Plains in summer. Spinning clockwise, this shifts the winds in the area 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. Monsoon rains reportedly supply 50-70% 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, dust storms, hail, and frequent lightning.

The wet phase of North American Monsoon typically runs from mid-June to the end of September.

The North American Monsoon pulls most air (green arrows) inland over the typically arid southwest region of the US. Source: NOAA/NWS

Summer Safety: What is a Rip Current and Why is it Dangerous?

Summer vacation season is in full swing across the US. As millions of people head to beaches to have fun and beat the heat, it is important to remember that the ocean is a dynamic environment that can pose a number of hazards for swimmers. Chief among these are rip currents.

Rip currents are fast, localized channels of water moving away from the shoreline. According to NOAA, they are a result of “complex interactions between waves, currents, water levels, and nearshore bathymetry.” They can form in several different ways on any beach with breaking waves. That said, they are typically found at breaks in sandbars and along permanent structures that extend out into the water such as jetties or piers.

Moving at speeds up to 8 feet per second – which is faster than an Olympic swimmer – rip currents can easily drag unsuspecting swimmers hundreds of yards out to sea.  While they will not pull anyone underwater, they can cause fatigue and panic. According to the U.S. Lifesaving Association, rip currents are responsible for 80% of all surf zone rescues. Nationally, they cause more than one hundred deaths every year.

To spot a rip current, look for a gap in the breaking waves.  This is where the water is forcing its way back out to sea. The area also usually appears murky and darker than the surrounding water.  On guarded beaches, red flags often indicate hazardous conditions for swimmers.

If caught in a rip current, the Red Cross recommends not trying to swim against it.  Instead, they say to swim parallel to the shoreline until you are out of the current. Once free, you can start swimming back toward the beach.

For more information on beach safety, visit: http://www.redcross.org/prepare/disaster/water-safety/beach-safety

Credit: NOAA

A Tale of Two Highs: The Science Behind the Excessive Heat in the Northeast

Summer is the season for warm weather and even the occasional heatwave. But, the excessive heat that is gripping the eastern United States this weekend is exceptional. Its source is essentially a tale of highs – two areas of high pressure, that is.

The first is a Bermuda High. This is a large, semi-permanent, area of high pressure situated off the east coast. Spinning clockwise, it is strongest in the summer months and often steers hot, humid air from the Gulf of Mexico toward the northeast. It is usually the main cause of heatwaves in the region.

This current heatwave, however, is getting an extra boost from a second area of high pressure that is sitting over the central US. Also spinning clockwise, it is funneling hot air aloft from the southwest toward the northeast. Traveling eastward, this hot air must pass over the Appalachian Mountains, which run parallel to the eastern seaboard from Georgia to Maine. Following the topography downslope on the lee side of the mountains, the air compresses and warms even further. This is producing the exceptionally high air temperatures, such as the upper 90s and triple digits reported in cites across the region.

Combining this excessively hot air with the humidity being pumped into the area by the Bermuda High, the heat index or real feel temperatures are well above 100°F in many places.

This type of weather is more than just uncomfortable, it is dangerous. To avoid health complications, the American Red Cross recommends avoiding strenuous outdoor activity, drinking plenty of fluids, and cooling off in air-conditioned spaces when possible.

Credit: NOAA/NWS

Aphelion 2019: Earth Farthest from Sun Today

The Earth will reach its farthest point from the Sun today – an event known as the aphelion. It will officially take place at 22:10 UTC, which is 6:10 PM Eastern Daylight Time.

This annual event is a result of the elliptical shape of the Earth’s orbit and the slightly off-centered position of the Sun inside that path. The exact date of the Aphelion differs from year to year, but it’s usually in early July – summer in the northern hemisphere.

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, Earth is about 152 million kilometers (94 million miles) away from the Sun. That is approximately 5 million kilometers (3 million miles) further than during the perihelion in early January. That means the planet 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 the sun”.

Earth’s Perihelion and Aphelion. Credit: Time and Date.com

The Science Behind the Summer Solstice

Today is the June Solstice, the first day of summer in the northern hemisphere. The new season officially begins at 15:54 UTC, which is 11:54 AM Eastern Daylight Time.

Our astronomical seasons are a product of 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 tilted 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 getting higher and daylight hours have been increasing. Today, the sun will be directly overhead at the Tropic of Cancer (23.5°N latitude), its northernmost position, marking the “longest day” of the year. This observable stop in the sun’s apparent annual journey is where today’s event takes its name. Solstice is a word derived from Latin and means “the sun stands still”.

While today brings us the greatest number of daylight hours (15 hours and 5 minutes in NYC), it is not the warmest day of the year.  The hottest part of summer typically lags the solstice by a few weeks. This is because the oceans and continents need time to absorb the sun’s energy and warm up – a phenomenon known as seasonal temperature lag.

Earth’s solstices and equinoxes. Image Credit: NASA