Read more: “Strange skies: Seven wonders of the atmosphere“
While the seas rise and fall in thrall to the moon, our ocean of air moves to a more complex beat. The heat of the sun combines with lunar gravity to drive a daily cycle affecting pressure and motion in the atmosphere. These air tides have surprising power, reaching down to trigger landslips and up into space to jam radio links and snare satellites.
At low altitude, air tides are barely noticeable against the background of ordinary weather. Yet as the tidal waves of pressure travel upwards, their energy is transmitted to much thinner air, which moves more violently, reaching hurricane speeds. A few hundred kilometres up, a high tide can make the atmosphere significantly denser, which increases the drag on orbiting satellites. Engineers must allow for this to keep track of them.
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Tides may also interfere with radio communication, according to of the University Corporation for Atmospheric Research in Boulder, Colorado. She suspects that air tides could help form bubbles of plasma in the upper atmosphere that , including GPS data.
An entirely new source of tides has been discovered in the last few years, during a lull in solar activity. Much of the time solar storms and flares buffet our atmosphere’s outer layers, creating disturbances in the electrically charged ionosphere, but the sun has been very quiet since around 2006. These calm conditions revealed subtle patterns in the density of charge in the ionosphere, patterns that seem to match particular regions in the tropics (). These regions are marked by intense thunderstorm activity, and Hagan says the daily formation of tropical rain clouds sends a gentle pressure wave into the ionosphere, where it increases charge density. So terrestrial weather influences space weather.
Though insignificant at the surface, air tides can still shift the ground under our feet. In Colorado, a mass of earth and rock called Slumgullion Slide is slowly slipping downhill. William Schulz at the US Geological Survey in Denver has found that the speed of the slip correlates with air tides (). Tides create a pressure imbalance that changes contact forces between soil particles at the base of the landslip, he says.
