Feeling The Pressure: The Value of A Barometer

(Published August 2014)

Every cruising boat should have a reliable barometer on board. Invented by Italian physicist and mathematician Evangelisata Rorricelli in 1643, a barometer helps a weather-wise boater stay abreast of developing weather patterns. The approach of a strong low pressure system with high winds, a squall line, or a hurricane is typically signaled by falling barometric pressure.  The steepness of the decline in pressure provides valuable insight into the strength of the approaching system. In contrast, rising barometric pressure usually heralds the arrival of fair weather and light winds. The ability to measure the rise and fall of barometric pressure only scratches the surface, however. What is barometric pressure, and what does it represent?

Barometric Pressure Defined
Weather occurs in the troposphere, the lowest layer of the atmosphere that extends upward from the Earth’s surface to a height of between four and twelve miles. (The height of the troposphere varies by latitude, and is highest near the Equator and shortest at the poles.) Although it seems counterintuitive, air molecules within the troposphere have mass, and their weight can be calculated. A one-inch square column of air extending from the Earth’s surface to the top of the troposphere  weighs approximately 14.7 pounds. Barometric pressure is simply a measure of the force exerted by the weight of air molecules pressing down on the Earth’s surface.

Figure 1: The weight of a 1" x 1" air column from the surface to the top of the troposhere.

For a moment, let’s assume that the height of the troposphere is consistent across the United States. Since barometric pressure is essentially the weight of air above a location, Denver would almost always observe lower barometric pressure than a city located near the coast, because Denver is approximately one mile above sea level. The much shorter air column above Denver contains fewer air molecules and therefore weighs significantly less than a city at sea level.   

In order to simplify analysis and forecasting of weather patterns across the country, weather maps are adjusted to sea level pressure. Sea level pressure is calculated by increasing the observed pressure at a station by a factor based upon the station’s altitude.

The traditional method of expressing barometric pressure is inches of mercury (inHg). However, the majority of meteorologists and modern forecast graphics use millibars (mb), A website for converting between inches of mercury and millibars can be found here.

Figure 2: 12-hour surface forecast valid at 12Z on July 29, 2014.

Weather Forecasting Graphics
The red Ls and blue Hs that appear on the surface forecast in figure 2 identify the center of areas of low and high pressure. Each high or low has a four-digit number nearby indicating its barometric pressure in millibars. For example, the low in eastern Washington State was forecast to have a pressure of 1013 mb, while the high straddling the Dakotas was forecast to be 1023 mb. (A legend and description of the rest of the meteorological shorthand that appears on the chart can be found here.)

Except at the extremes, there is no barometric pressure threshold that marks the distinction between low and high pressure. The low in eastern Washington is shown as 1013 mb, but I could have easily provided an example where 1013 mb was associated with a high.  Highs and lows depend on the season and the overall weather pattern across the country.  Furthermore, lows may have frontal boundaries, such as cold and warms fronts. Highs, on the other hand, never have frontal boundaries.

Using A Barometer
Although traditional decorative barometers report pressure, a modern digital barometer offers many advantages. Digital barometers, such as the Weems & Plath model 4002, display pressure in either millibars or inches of mercury on an easy-to-read screen. Most digital barometers also maintain a record of observations for 48 hours or longer, and display the pressure trend on a graph. The ability to compare the current pressure to observations from the previous 24 or 48 hours is extremely valuable in weather forecasting, and the digital barometer provides a clear advantage over traditional non-recording barometers. Some digital barometers also have an alarm setting which can alert you when the barometric pressure is falling faster than a specified rate.

Let’s review a few examples where an on-board barometer would have alerted you to approaching inclement weather.

Hurricane Arthur

Hurricane Arthur was an early season tropical cyclone that brushed the Carolinas before heading out to sea. NDBC Buoy 41036 was located close to the storm's track and recorded the precipitious drop in barometric pressure as it passed. The combined graph of wind and pressure observations (click here) show that gusts of approximately 70 knots were associated with the storm.

Surface analysis valid at 18Z on July 3, 2014.
Barometric pressure observations from NDBC Buoy 41036.

Lake Erie: Storms on July 27, 2014

The surface analysis showed a cold front approaching Lake Erie from the west, while a warm front was approaching from the southwest. The steady decline in barometric pressure observations from NDBC Buoy 45005 in Lake Erie's western basin clearly indicated that unsettled weather was approaching. Thunderstorms erupted across Lake Erie in the early evening of the 27th prompting many severe thunderstorm and special marines warnings (click here for radar image).

Surface analysis valid at 18Z on July 3, 2014.
Barometric pressure observations from NDBC Buoy 41036.

Lake Michigan: Strong Winds on July 13, 2014

The surface analysis valid at 8:00 pm on July 12, 2014 indicated a cold front approaching Lake Michigan from the northwest. Thunderstorms accompanying the passage of the cold front produced winds gusts of nearly 35 knots (click here for radar image).

Surface analysis valid at 18Z on July 3, 2014.
Barometric pressure observations from NDBC Buoy 41036.

Hurricane Sandy: October 2012

Hurricane and later Superstorm Sandy was a large late season Atlantic storm that brought destruction to a broad area. NDBC Buoy 44009, which marks the entrance to Delaware Bay, captured the dramatic drop in pressure and corresponding high winds as the storm passes. Even NDBC Buoy 45005 in western Lake Erie observed fall in pressure from Sandy (click here for graph).

Figure 2: 12-hour surface forecast valid at 12Z on July 29, 2014.