Weather 101 - the "Blue Norther"

I've lived in north Texas and I can attest to the unusual character of the "Blue Norther" (a.k.a., a "norther") in the Southern Great Plains. Meteorologically, a norther is just the arrival of very strong cold front. As the front passes winds shift from a southerly direction to a northerly one (hence the name "norther"). Temperatures often plummet with the frontal passage. However, skies are often cloud-free with and shortly after frontal passage. So during daylight hours, skies are often cloud-free and very blue. Although one historical perspective notes that the blue may refer to the chill of the arriving arctic or polar air mass, more common usage keys on the blueness of the sky.

Record high temperatures can often precede northers. On January 3, 2009, Dallas, TX, for example, tied its high for the date (last set in 2006) at 84 degrees Fahrenheit.

Tulsa, OK actually reached a record high of 78 (old record of 75 set in 2004) followed by temperatures dropping 39 degrees within 5 hours after the norther's passage. Some might say that the temperature dropped by 50% or was cut in half (in fact, some TV meteorologists have done this type of fuzzy math). But be careful. The Fahrenheit scale does not have an absolute zero reference point (only degrees Kelvin does). So "half" of 78 degrees Fahrenheit is NOT 39!

A satellite perspective

With clear or mostly clear skies, it is easy to see the cold front in infrared satellite images. Infrared measuring tools (known as radiometers) onboard environmental satellites can detect variations in ground and ocean surface temperatures. Strong temperature gradients, such as are observed with northers, are very easily detected in infrared images (Figure 1), but are often impossible to see in visible images (Figure 2 with frontal position superimposed).

Aftermath

Following the norther's passage, low clouds may roll in from the north or skies can remain cloud-free. It simply depends upon how moist the atmosphere is in its lowest levels. Temperatures will likely remain cold for several days. As warmer air eventually returns, it can override the colder air (warm air is less dense than cold air) and create extensive low cloudiness and even periods of freezing rain, freezing drizzle or snow. Although not a classic norther, several large and intense arctic air masses invaded north Texas in December 1983. This resulted in a record 295 consecutive hours of below freezing temperatures in the Fort Worth area. Frozen home water pipes, burst water mains, icy roads, and travel and power disruptions led to major economic losses and significant personal inconvenience (trust me because I was there!).

Another recent norther

Another norther in mid-December 2008 brought 40 to 50 degree temperature drops in fewer than 6 hours to many central U.S. locales. The temperature at Norman, OK (near Oklahoma City) dropped from 73 degrees at 2pm CST to 32 degrees at 6pm CST (a 41 degree fall). The temperature dropped 18 degrees between 2pm CST and 3pm CST alone.

One of my interns, a student at the University of Oklahoma, captured this event while standing in front of a bank thermometer. As the temperature fell, he donned appropriate attire, documenting the temperature change both in scientific and human terms.

Still other northers

There are northers in other places, each with its special weather accompaniments. According to the American Meteorological Society's Glossary of Meteorology (not available either online or hard copy without purchase), northers in California bring cold, dry conditions in winter and warm to hot and dry conditions in the summer. Northers here are not linked necessarily to the passage of strong cold fronts but rather just the presence of a strong pressure gradient.

The norther in the Atlantic Ocean west of Portugal involves a clockwise wind flow around a large high-pressure system to its west and northwest. There is also a norther that occasionally brings strong northerly winds to the Chilean coast. And the norther in Australia is a hot dry wind that blows from the interior desert regions to the southeast coast.