Climate Change 2.0 – Part 2 : Seasons

By Tsahi Frankovits, January 14, 2018

In the previous article we've learned that the frequency of summery days over the years have grown while wintery days portion in recent years dropped to under 20%. But what does it mean in terms of seasons?

When thinking about seasons we refer to several manifestations of the season: We refer to its start and end day (or its length), to the intensity of the weather phenomenon during the season, to extreme weather events etc. Under the framework of the Global Warming discussion it is usually about extreme weather conditions and average temperatures.   But, in order to examine weather patterns during the seasons we first need to know when the seasons start and ends.  This is the object of this post.

When does the winter start and when does it end?

The arbitrary answer is given by the "official" dates of winter -  December 21^st (the shorter day of the year) is the first day of winter and March 20^th (the equilibrium day) is defined as the last day of winter/first day of spring. Summer, by this convention, start on June 21^st (the longest day) and ends on September 21^st (the second equilibrium day).

Unfortunately, climate conditions are not necessarily "obeying" to these rules, so we need a more objective, facts driven, measure. Using my daily weather patterns classifiers, we can define a new measure of start/end day of the seasons. For our purpose, I've defined the turning point of the two major seasons (Summer and Winter) as follows:

If the number of summery days during the last 21 days exceeds 10 (i.e. more than 50%) than we are at Summer, and when it drops to below 10 days, Summer is over. The same applies to Winter. The other two seasons are derived thereafter.

Using this method over the entire period of 50 years, and looking at 5-year periods (to avoid localities), I found that since the early 90' summer is getting longer on both sides (starts earlier and ends later) so that Summer duration is now approximately 155 days vs. 123 days during 1965-1970.  While summer is longer, Winter is getting shorter -  mainly by ending earlier. During 1965 through 1976 Winter usually lasted until day 80-87 of the year (~last 10 days of March), but in recent years winter ends some 18-25 days earlier. See in the next two charts.

These are not good news to a country who sits on the edge of the desert and strives for natural water resources, though Israel was smart enough to identify the growing gap between its water needs and its natural water resources and developed large water desalination facilities.

In my next post of this series I will use the definitions of start/end days of summers and winters over my 50 years sample to discover main weather characteristics changes along the years.