You see that bit of purple in the map (whole U.S. version? That bit of purple, or what it represents, is what has given us rain the past two days. You may have heard a forecast say something about an "upper-level disturbance" moving through the area. An upper-level disturbance is analogous to an eddy in a stream. Above the Earth's surface energy moves around the globe in giant waves, known as long, or Rossby, waves. Short waves occasionally travel through the long waves. Along with them the short waves bring a kind of torque known as vorticity. Most TV weatherpeople are forbidden from saying "vorticity" but Gothamist trusts our reader's intelligence. The movement of vorticity is related to air diverging, or spreading out, higher up in the atmosphere. Since the atmosphere can't have a vaccuum, air beneath the divergence gets pulled upward. Short waves, are also often the cause of the turbulence that causes the captain to turn on the fasten seat belt signs.
Air that rises always cools, and when moist air cools to the dew point, condensation occurs and clouds form. Keep the air rising and the clouds continue to grow and sooner or later precipitation starts falling. This upper-level disturbance is headed out to sea, but don't put away those duck shoes just yet as another, much more intense, short wave will be passing over us on Thursday.
