- A drop of 33 hPa in 24 hours removes about 340 kg of air above every square meter.
- The jet stream evacuates air upward, deepening the depression without any visible suction from the sea.
- The sea rises by roughly 65 to 70 cm beneath the storm’s center, and France only receives its periphery.
Ten tonnes of air per square meter
From 980 to 947 hectopascals in 24 hours: that is the drop forecasters anticipate for storm Boelo off the coast of Scotland. At the heart of the depression, that corresponds to nearly 340 kg of air less above every square meter. The figure is not a metaphor; it comes from a high-school level calculation, detailed below.
One square meter of ground already carries about 10 tonnes of air.
Atmospheric pressure is simply the weight of the air column that sits above the point where it is measured. At sea level, 1013 hPa, the hydrostatic calculation from Purdue University states: 101,300 divided by 9.81, which is about 10,300 kg per square meter. That is the mass of a seawater column about 10 meters high, which no one feels pressing on their shoulders because air also pushes from inside the body.
The conversion is simple. 1 hPa equals 100 newtons per square meter, roughly 10.2 kg after dividing by 9.81. Each hectopascal of loss therefore lightens the column by roughly ten kilograms. Between 980 and 947 hPa, we lose 33 hPa, about 336 kg, and if the center drops to 946 hPa, we reach 347 kg, hence the “nearly 340 kg” in the title.
Austen, the day before, followed the same path in a more modest way: La Chaîne Météo records a pressure drop from 1006 hPa on Thursday at 0 h to 981 hPa on Friday at 0 h. That is 25 hPa, about 255 kg of air less per square meter. For Boelo, 340 kg corresponds to 340 liters of water, i.e., nearly two full bathtubs removed from each square meter in a single day.
The jet stream pulls air upward
This air did not escape sideways. It was evacuated aloft, at about ten kilometers up, where the jet stream—a ribbon of very fast winds encircling the Northern Hemisphere—flows. La Chaîne Météo explains that Boelo develops rapidly under the push of a powerful jet.
The Met Office blog dedicated to Austen describes the mechanism. The storm crosses the jet and enters its “left jet exit region,” the left exit region, the portion of the jet that favors upward air movement. If air escapes upward faster than it arrives from below, the column lightens and the surface pressure collapses. The storm then deepens from above, with no visible suction exerted from the sea.
Beyond 24 hPa in 24 hours, meteorologists speak of a “bomb.”
The official term is explosive cyclogenesis, or “weather bomb” on the British side. Austen barely crossed the threshold with its 25 hPa. Boelo clearly exceeds it, since meteorologist Yann Amice, cited by actu.fr, indicates that its pressure should drop from 980 to 947 hPa in only 24 hours. The Met Office named the depression on Thursday, October 22, the second named storm of the season after Austen.
Tight isobars, swollen sea
A brutally deep low pressure tightens the isobars, those lines that connect points of equal pressure on a map. The air rushes toward the center, and the stronger the pressure gradient over a short distance, the stronger the wind. According to La Chaîne Météo, gusts will frequently reach 90 to 105 km/h along coasts, islands, and Scottish highlands, with peaks of 120 to 140 km/h, and up to 150 km/h at sea.
France will only feel the storm on its periphery, since the center passes well north of Austen. Météo-France foresees gusts of 80 to 90 km/h on the Channel coasts Saturday evening and during the night from Saturday to Sunday. Across much of the northern half, they should remain at 50 to 70 km/h. Vegetation, sometimes weakened by the intense drought of the summer, may still yield more easily.
The sea also responds to the pressure. Hydrographers count roughly one centimeter of water per hectopascal lost, reports Science & Vie, which estimated about 35 cm of sea rise under Austen (978 hPa). For Boelo, the delta from the 1013 hPa reference reaches 67 hPa at 946 hPa, yielding around 65 to 70 cm beneath the storm’s center.
This rise is static, independent of waves. A 9-meter wave does not lift the sea by 9 meters; it rises and falls around an average level. The 65 to 70 cm are a theoretical order of magnitude, valid under the center, and they move with it.
What the barometer will say this weekend
A home barometer is enough to track the phenomenon from afar. On Saturday, as the afternoon and evening proceed, the needle should fall while Boelo plummets toward 946 hPa near the Hebrides, northwest of Scotland. Of course, your device will not display such values, since the center lies hundreds of kilometers away, but the trend will be the same. A barometer typically shows sea-level pressure, so check its setting before interpreting the drop.
On Sunday, the rise begins.
Boelo then fills back in as it moves toward southern Norway. La Chaîne Météo expects it at 959 hPa on Sunday noon over the North Sea, then at 973 hPa on Monday noon. Between these two checkpoints, the pressure gains 14 hPa, that is about 143 kg of air returned to each square meter in 24 hours. The barometer needle will rise accordingly, but in a more modest fashion.
Even at its deepest, Boelo removes only about 6.6% of the standard air column mass: at 946 hPa, more than 9,600 kg still remain above each square meter.
Sources: meteoconsult.fr | lejma.fr
