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

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

Weather Lingo: A 500-Year Flood

Flooding rain events are becoming more common in many parts of the United States. But, the terms used to describe some of them, such as “100-year flood” or “500-year flood”, can be misleading.

While these phrases can make it sound like the flooding is cyclical, only happening once every 100 or 500 years, it is not. Storms are not on a schedule. Rather, these terms refer to statistical probabilities. For example, the “100-year flood” means there is a 1% chance of a flood of that magnitude happening in any given year in a given location. For the 500-year flood, the probability is 0.2%.

Given the right conditions, it is possible to see multiple 100-year or 500-year flood events in a relatively short period of time. Ellicott City, MD, for example, recently saw two 1000-year flood events only two years apart. For more information, watch the video below from the NWS.

Weather Lingo: The Beaufort Wind Force Scale

From a light breeze to a strong gale, wind speed can be described in numerous ways. All of which are categorized on the Beaufort Wind Force Scale.

Developed in 1805 by Sir Francis Beaufort, an officer in the UK’s Royal Navy, the scale is an empirical measure of wind speed. It relates wind speed to observed conditions at sea and over land instead of using precise measurements. Simply put, it allows a person to estimate wind speed with visual clues.

Initially, it was only used at sea and was based on the effect the wind had on the sails of a frigate – the most common type of ship in the British Navy at the time. By the mid-1800s, the scale was adapted to also reflect a certain number of anemometer rotations – a device that measures wind speed.

In the early 20thcentury, most ships transitioned to steam power and the scale descriptions were changed to reflect the state of the sea instead of the sails. Around the same time, the scale was extended to land observations. For example, the amount of leaf, branch, or whole tree movement is a visual indicator of the force of the wind.

Today, the scale has 13 categories (0 -12), with 0 representing calm winds and 12 being hurricane force. It is in use in several countries around the globe.

In the US, when winds reach force 6 or higher, the NWS begins issuing advisories and warnings for different environments. For marine areas, force 6-7 winds would prompt a small craft advisory, force 8-9 would warrant a gale wind warning, and a wind reaching force 10-11 would call for a storm warning. Force 12 would constitute a hurricane-force wind warning. On land, winds expected to reach force 6 or higher would cause a high wind warning to be issued.

If the winds are connected to a tropical cyclone, they would be measured on the Saffir-Simpson scale. The same type of special circumstances would also hold for a tornado, which would be measured on the Enhanced Fujita Scale.

The Beaufort Wind Force Scale. Credit: Isle of Wight Weather Ctr

Weather Lingo: 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 “feels-like” 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 5mph will produce a wind chill index of 13°F.  At that same temperature, but with a wind speed of 10mph, the wind chill index would be 9°F.

Extended exposure to low wind chill values can lead to frostbite and hypothermia, serious winter health hazards.

What is the Winter Solstice?

Today is the December solstice, the first day of winter in the northern hemisphere. The new season officially begins at 22:23 UTC, which is 5:23 PM EST.

The astronomical seasons, which are different than meteorological 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 winter months, the northern half of the Earth is tilted away from the sun. This position means the northern hemisphere receives the sun’s energy at a less direct angle and brings us our coolest temperatures of the year.

Since the summer solstice in June, the arc of the sun’s apparent daily passage across the sky has been dropping southward and daylight hours have been decreasing. Today, it will reach its southernmost position at the Tropic of Capricorn (23.5° south latitude), marking the shortest day of the year. This observable stop is where today’s event takes its name. Solstice is derived from the Latin words “sol” for sun and “sisto” for stop.

Soon, the sun will appear to move northward again and daylight hours will slowly start to increase. Marking this transition from darkness to light, the winter solstice has long been a cause for celebration across many cultures throughout human history.

Earth’s solstices and equinoxes. Image Credit: NASA

Weather Lingo: Lake Effect Snow

Winter snowstorms have a variety of names, such as Nor’easters and Alberta Clippers. It all depends on where and how they develop. In the Great Lakes region of the US, the vast bodies of fresh water influence the weather and create something known as lake effect snow.

Lake-effect snowstorms, according to NOAA, develop when cold air blows across the warmer waters of a large unfrozen lake. The bottom layer of the air mass is warmed by the water and allows it to evaporate moisture, which forms clouds. When the air mass reaches the leeward side of the lake its temperature drops again, because the land is cooler than the water. This releases the water vapor as precipitation and enormous amounts of snow can accumulate. The effect is enhanced if the air is lifted upward by local topography.

With the clouds typically forming in bands, the snowfall is highly localized. Some places can see the snow come down at a rate of more than 5 inches per hour, while nearby, others will only get a dusting. The shape of the lake and the prevailing wind direction help to determine the size and orientation of these bands.

Fetch, the distance wind travels over a body of water, also plays a key role. A fetch of more than 60 miles is needed to produce lake effect snow. In general, the larger the fetch, the greater the amount of precipitation, as more moisture can be picked up by the moving air.

The impressive depths of the Great Lakes allow them to remain unfrozen longer into the winter season than more shallow bodies of water. This combined with their massive surface area, make them excellent producers of  lake effect snow. With northwesterly winds prevailing in the region, communities along the southeastern shores of the lakes are often referred to as being in the “Snowbelt.”

Credit: NOAA

Weather Lingo: Indian Summer

Autumn is a season known for colorful leaves and falling temperatures. Every once in a while, however, summer warmth makes a resurgence. When this happens, it is often dubbed “Indian Summer”.

This weather phenomenon, according to the NWS glossary, is defined as “an unseasonably warm period near the middle of autumn, usually following a substantial period of cool weather.”  In the northeastern US, it is generally associated with an area of high pressure to the south that ushers warm air northward.

In popular use since the 18th century, the exact origins of the term are a bit foggy. One of the more reasonable explanations behind this unique phrase suggests a connection to when Native Americans began their hunting season, but no one knows for sure.

In other parts of the world, this summer-like weather goes by a variety of different names. In Europe, a number of countries associate the unusual warmth with the nearest saint’s day. It is known as “St. Luke’s Little Summer” if it develops in October or a “St. Martin’s Summer” if it occurs in November. In temperate parts of South America, it is simply known as “Veranico” (little summer).

The timing and intensity of these autumn warm spells vary from year to year. Nevertheless, when they do occur, they usually only last a few days. So, as we inevitably move toward winter, enjoy them while you can.

Fall foliage. Credit: Melissa Fleming

Hurricanes, Typhoons, and Cyclones: What’s the Difference?

As Hurricane Lane makes its way toward Hawaii, many people have been asking me why the storm is not being called a typhoon given that it is taking place in the Pacific. The answer is all about location.

Hurricanes, typhoons, and cyclones are all the same type of storm – tropical cyclones. They are just called different things in different parts of the world. It’s like the way people in certain parts of the US say “soda” when referring to a cold fizzy drink, while people in other parts of the country use the word “pop”.

The term hurricane is used for tropical cyclones in the northern hemisphere from the Greenwich Meridian (0°) westward to the International Date Line (the 180° line of longitude). That includes the Atlantic basin as well as the eastern and central Pacific. The eastern Pacific is defined as everything north of the equator from the west coast of the North American continent to 140°W. The central Pacific, where Hawaii is located, extends from 140°W to 180°W.

Typhoon is the word used for storms west of that line, any area known as the western Pacific. If a hurricane crosses the International Date Line and maintains its strength, it will be renamed as a typhoon. In 2014, for example, Hurricane Genevieve became Typhoon Genevieve when it crossed into the western Pacific.

Across the southern hemisphere, all tropical cyclones are simply called cyclones.

These powerful storms, regardless of what we call them, can pose a threat to life and property. All warnings should be taken seriously.

Credit: American Red Cross

Weather Lingo: Humidity

“It’s not the heat, it’s the humidity.” This old adage heard throughout much of the summer in the eastern US, refers to how the amount of water vapor in the air affects human comfort. Since the body’s main source of cooling is evaporation of perspiration, the more moisture there is in the air, the less evaporation takes place and the warmer we feel. Two ways to indicate atmospheric moisture content are relative humidity and the dew point temperature.

Relative humidity (RH) measures the actual amount of moisture in the air compared to the total amount of moisture that the air can hold. It is expressed as a percentage and is commonly used in generic weather reports and apps. A high RH can produce fog and a low RH can cause rapid dehydration in both people and plants – important information for some sectors such farmers and crews fighting wildfires. But, since warm air can hold more moisture than cool air, the relative humidity changes as the air temperature changes.

The dew point temperature, on the other hand, is an absolute measurement and is often the preferred metric of meteorologists. It is the temperature to which air must be cooled in order to reach saturation. In other words, when the air temperature and the dew point temperature are same, the air is saturated and the relative humidity is 100%. If the air were to cool further, the water vapor would condense into liquid water, such as dew or precipitation.

The classic example of this phenomenon is a glass of cold liquid sitting on a table outside on a warm, muggy day. The beverage cools the air around it and beads of water form on the outside of the glass. The temperature at which the beads of water form is the dew point.

Simply put, the closer the dew point temperature is to the air temperature, the more humid it feels. In summer, when the air is warm and can hold a lot of moisture, a dew point temperature in the 50s 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 oppressive. On the opposite end of the spectrum, dew points in the 40s or lower are considered dry, and dry air has its own set of comfort issues