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Weather Blog

Weather geeks unite!

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What happens when you put almost 4,000 meteorologists, climatologists, social scientists, and data miners in one convention center? People talk about the weather, of course!

Last week was the 99th Annual Meeting of the American Meteorological Society, and for the first time in over a decade, I attended. The size of the meeting was overwhelming to say the least. With dozens of conferences and symposia embedded within the larger meeting, there was no shortage of options to choose from. Some were technical, getting into the minutia of radar, satellites, programming and coding. Others were discussions about how to effectively communicate confidence or lack thereof in a forecast, risk to different subsections of the public, and what we know and don’t know about climate change.

With the partial government shutdown in place, there were fewer attendees this year. Many who work for the EPA, NOAA and NASA were unable to travel. That situation led to several talks being canceled because the speakers and moderators worked for those federal agencies. For example, I was disappointed to learn that one keynote speech was canceled because Administrator of NASA Jim Bridenstine, who was the speaker, could not attend.

While our cohorts and the education and information they would have imparted were certainly missed, there was still an abundance of insight and wisdom to be gained. I spent most of my time in the sessions focused on education, communication, and risk mitigation. I rubbed elbows with broadcast meteorologists, federal employees (who were willing to foot their own travel bills), and employees of private industries – all of whom were trying to improve their understanding of current scientific knowledge and share their professional experiences in the field.

In future posts, I’ll share some of what I gleaned from my trip. For now, suffice it to say that there are thousands of people from around the world who traveled to Phoenix, Arizona, to spend nearly a week talking about the weather, how it affects everyone, and what we can do to keep lives and property safe.

Weather Blog

Oh, the pressure!

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(Originally written for Allen Publishing’s print products published on January 3, 2019)


Meteorologists talk about barometric pressure or atmospheric pressure when explaining the forecast. We use the terms “low pressure” and “high pressure” to describe the causes for different types of weather. We throw around terms like millibars as if everyone understands what we’re talking about, but do they? Not necessarily.

So, what is pressure in weather terms?

If pressure in general is the force exerted on a unit area, then atmospheric pressure is the force exerted by the atmosphere on a specific area. We use the term barometric pressure almost interchangeably with atmospheric pressure because we use barometers to measure atmospheric pressure.

There are different types of barometers and units used to quantify pressure. The two most common units are inches of mercury and millibars (mb). In a mercury barometer, a column of mercury is used to physically show the amount of pressure on the column. The column will expand or shrink dependent upon the pressure exerted on it by the air surrounding it. The measurement is taken in inches of mercury. Measured this way, the global average air pressure at the surface of the earth is 29.92 inches of mercury.

Millibars are used more often by meteorologists. They are the lines typically seen on weather maps of sea level pressure. It’s not really a metric system unit, although it would be easy to assume that’s the case since in most other situations, the two most common units are based on the British and metric systems. In fact, mercury barometers can give their readings in millimeters of mercury, so they are the metric system version as well as the British system version of pressure reading.

Instead, a millibar is a pressure unit of 1,000 dynes per square centimeter, which is an old way of measuring pressure. Millibars are convenient for reporting atmospheric pressure despite the fact that you don’t see it used for much else. Measured this way, the global average air pressure at the surface of the earth is 1013.25 mb.

When the atmosphere exerts more force per area, we call the pressure “high.” When it exerts less, we call the pressure “low.” Often during the winter, high pressure systems in the mid-latitudes will exert pressure in the 1030 mb to 1040+ mb range. Low pressure systems crossing the continent can bring pressure in the range around 990 mb. By contrast, at her peak intensity, Hurricane Florence’s pressure was measured at 939 mb.

Atmospheric pressure plays a huge role in our weather. High pressure brings clear skies because the air is sinking to ground level. Low pressure brings unsettled weather – possibly even stormy – because the air is rising, which creates clouds and rain. The area directly under the center of a high-pressure system often has calm or very light winds. The area between a system of high pressure and one of low pressure can be quite breezy because the air wants to move from high pressure to low pressure. The closer the two systems are to one another, the windier it is.

In the Northern Hemisphere, the winds around a high-pressure system flow in a clockwise direction away from the center, and the winds around a low-pressure system go counter-clockwise toward the center. Remember, the air wants to go toward where the pressure is lower.

The movement of these systems determines the hour-to-hour and day-to-day forecasts for any given location. They bring our weather.

One thing to note: for this discussion, I’m writing about the weather and the pressure at the earth’s surface. Meteorologists also consider the air pressure at different levels above the ground, which is why you sometimes hear us mentioning an upper level or mid-level low bringing a change in the forecast.

map example of pressure

Credit: National Weather Service. The blue H on the map represents the center of high pressure. The red L represents the center of low pressure. Winds rotate clockwise and outward from the high pressure center and counter-clockwise and inward toward the center of low pressure.

Weather Blog

2018’s notable weather

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(Originally written for Allen Publishing’s print products on January 10, 2019)

The weather word for 2018 across much of North Carolina was “wet.” In fact, records for the wettest year on record fell one after the other in the last few weeks of the year.

According to my count on the Southeast Regional Climate Center’s CLIMPER tool, which maps data recorded at official and coop weather stations, 24 locations – excluding duplicate reporting stations such as Greensboro area versus the Greensboro airport – broke records for the wettest year on record.

2018 reord rainfal map

Credit: Southeast Regional Climate Center. Map showing cities and towns in the region with record-breaking or near record-breaking precipitation in 2018.

Raleigh-Durham International Airport recorded 60.29 inches of precipitation, which made 2018 its wettest year in the 74 years reports have been made at that station. Other record-breakers included Wilmington with 102.4 inches, New Bern with 79.18 inches, Greensboro with 64.11 inches, and Asheville with 79.49 inches.

Hurricanes Florence and Michael certainly assisted in reaching those milestones. However, anyone who tried to get yardwork done on the weekends can tell you more often than not, those plans were rained out, or at the very least soggy. In fact, the majority of the weekends in 2018 produced reported rainfall across the state.

Another record that fell at RDU International back in January was the most consecutive hours at or below 32 degrees when we hit 158, surpassing the record of 157 set in 1982. We spent the first 7 days of the year at or below freezing. The snow and ice that fell during a winter storm lingered in some areas for a week as the temperature struggled to get warm enough to make a difference.

RDU temp plot for January 2018

Credit: National Weather Service. RDU temperature plot for January 2018 showing a stretch of extremely cold weather that month.

Across the country, there was another trend in 2018 worth noting. Only this time, it was due to a lack of something happening. For the first time since 1950, there were no tornadoes rated EF-4 or EF-5 reported anywhere in the United States. Ironically, the same day I read that headline was the day I saw an abstract for a research letter entitled “Increasingly powerful tornadoes in the United States.” To be fair, the article reported on a study of the period 1994-2016.

Much of the country experienced a late winter/early spring with unusually cold weather, which limited tornado activity during what is usually the peak season for Tornado Alley. Of course, tornadoes don’t have to be highly rated on the Enhanced Fujita scale to be deadly. Ten people died as a result of tornadoes in 2018. While much lower than average, the number still represents tragic loss.

With last year behind us, we can now look ahead to 2019 and speculate on what the weather will bring. Perhaps an abnormally cold late January due to the weakening of the polar vortex? Possibly a few more inches of wintry precipitation? Or maybe a more active tornado season? Only time will tell for sure.