What is stable air as opposed to unstable air? You hear those terms by many weather authorities, but what do they mean? We are entering the time of year when western Washington gets its greatest number of days with unstable air.
The primary weather ingredients in the lower atmosphere are heat and moisture. Both of these elements play key roles in whether an air mass is stable or unstable. An unstable air mass usually is warmer and more moist closer to the surface than the air above it.
Like letting a helium balloon go, if a parcel of air near the surface rises and keeps going, that is unstable air and the air mass is considered convective. A stable air mass would result in the parcel of air either to rise and stop or rise rather slowly with little cloud development thanks to relatively warmer air aloft.
During our spring season, the longer days and higher sun angle result in warmer temperatures in the lowlands of western Washington.
Yet we still get cooler air aloft rolling inland from the Gulf of Alaska. This pattern often results in an unstable air mass. If moisture is present and it often is, clouds, showers and even thunderstorms develop. On the calendar, our western Washington convective season usually begins around March 1.
Spring, the transition season between winter and summer, is the peak season for showers and thunderstorms in western Washington. The other transition season in the fall has a secondary peak. If you encounter rain and then sunshine, and it repeats seemingly again and again, those are usually showers associated with an unstable air mass. And that is why they are called spring showers.