Spring 2018

What, exactly, constitutes "spring'?  Scientifically, Spring began at 10:15 a.m. (MDT) on Tuesday, 20 March this year, which was the time of the vernal equinox.  At the vernal equinox, days are approximately 12 hours long, with the length of the day increasing as the season progresses.  Here, in the United States, acknowledged months of spring are March, April, and May.  However, in the Southern Hemisphere it is now autumn, or fall; in New Zealand, spring is from 1 September through 30 November.  Swedish meteorologists define the beginning of spring as the first occasion on which the average temperature is above 0 degrees Celsius (32 F) for seven consecutive days, thus their dates for spring vary with the latitude and elevation.  Spring and "springtime" refer to the season, and also to ideas of rebirth, rejuvenation, renewal, regrowth, and resurrection.
   During early spring, the axis of the Earth is increasing its tilt relative to the sun, and the length of daylight rapidly increases for the relevant hemisphere.  The hemisphere begins to warm significantly, causing new plant growth to "spring forth," possibly giving the season it's common name. Snow begins to melt, swelling streams with runoff, and frosts become less severe.  In climates that have no snow, and frosts are rare, ground temperature increases more rapidly.
   Many flowering plants bloom at this time of year, in a long succession, sometimes beginning when snow is still on the ground, and continuing into summer.  In  normally snow-less areas, spring may begin as early as February (in the Northern Hemisphere) or in August (in the Southern).  Many temperate areas have a dry spring and a wet autumn, which brings flowers in this season, more consistent with the need for water, as well as warmth. Sub-arctic areas may not experience spring until the month of May or June.
  While spring is a result of the warmth caused by the changing orientation of the Earth's axis relative to the sun, weather is affected by other, less predictable events.  Rainfall follows trends related to longer cycles, like the solar cycle, or events created by ocean currents and temperatures.  Unstable spring weather may occur more often when warm air begins to invade toward the north, while cold is is pushing south.  Flooding is also most common in and near mountainous areas during this time of year, because of melting snow. "Tornado Alley" is most active in the spring, especially since the Rocky Mountains prevent the surging hot and cold air masses from spreading eastward, and instead force them into direct conflict.  Besides tornadoes, supercell thunderstorms can also produce dangerous large hail and very high winds.  And more so than in winter, the jet streams play an important role in unstable and severe weather in the spring.
   Spring in the Southern Hemisphere is different in several significant ways from the Northern Hemisphere for several reasons.  They include: (a) there is no land bridge between Southern Hemisphere countries  and the Antartic zone capable of bringing in cold air without the temperature-mitigating effects of great tracts of water; (b) the vastly greater amount of ocean water in the Southern Hemisphere at all latitudes; (c) the Earth (at this time) has an orbit that brings it in closer to the Southern Hemisphere for its warmer seasons; (d) there is a circumpolar flow of air uninterrupted by large land masses; (e) there are no equivalent jet streams; and (f) the peculiarities of the reversing ocean currents in the Pacific.
  ** In recent decades, season creep has been observed, which means that many phrenological signs of spring are occurring earlier in many regions - by around two days per decade. **