There are, generally speaking, three schools of thought on what the seasons are. These are for the Northern Hemisphere; reverse them for the Southern Hemisphere.
-Meteorological seasons, where winter is the entirety of December - February, spring is March - May, summer is June - August, and fall is September - November
-Astronomical seasons, where winter is about December 21 - March 20, spring is about March 20 - June 21, summer is about June 21 - September 23, and fall is about September 23 - December 21
-Solar seasons, where winter is about November 7 - February 6, spring is about February 6 - May 6, summer is about May 6 - August 6, and fall is about August 6 - November 7, using the solstices as midpoints of summer and winter.
Seasons are generally thought of in non-tropical climates as "summer = hot", "winter = cold", and in between is spring and fall. For most climates, the meteorological system is the best approximation of this (except for many oceanic climates, where seasonal lag is strong enough that the astronomical seasons are often the best approximation). Solar seasons are ridiculous (where in the world is there a Northern Hemisphere climate where November 10 is colder than February 10?)
But these three classifications are just that: approximations. Seasons don't start precisely at a solstice or a month boundary. In most of the world, they're defined by temperature. The precise temperatures that define "summer" and "winter" are going to vary wildly from climate to climate, so it would make sense that if the average temperature is within a certain percentage of the temperature in the warmest month (the height of summer) that it would be called "summer", and vice versa for winter. Weatherspark uses 20 percent, which seems like a reasonable number.
For my climate (Bowling Green, KY), the coldest month (January) averages at 34 F / 1 C (high 43 F / 6 C, low 25 F / -4 C) per Intellicast. The warmest month (July) is 79 F / 26 C (high 89 F / 32 C, low 68 F / 20 C). That gives us a 45 F / 25 C range.
(Intellicast and Weatherspark are excellent sources, because they give an average high and low for every day of the year).
20 percent of that is 9 F / 5 C, which means that "winter" is when average temperatures are below 43 F / 6 C, and "summer" is when average temperatures are above 70 F / 21 C. Going through Intellicast, we find that the average temperature crosses that 43 mark on March 2 and November 30.
The 70 mark is surpassed on May 30 and dropped below on September 15. This means that for Bowling Green, KY:
Winter: November 30 - March 1
Spring: March 2 - May 29
Summer: May 30 - September 14
Fall: September 15 - November 29
which is pretty close to the meteorological reckoning, with the exception of late summer.
San Francisco is well-known for its summers being delayed. With a January average at 52 F / 11 C and a September (!) average of 64 F / 18 C, its 12 F / 7 C spread is barely enough to say it has "seasons", but anything within 2 1/2 degrees of those averages is our threshold (below 54.5 F for winter, above 61.5 F for summer)
Winter: December 2 - February 13
Spring: February 14 - July 24
Summer: July 25 - October 27
Fall: October 28 - December 1
Fairbanks, Alaska is on the other end, with seasons that peak early. January, the coldest month, averages -6 F / -21 C and July, the warmest month, is 62 F / 17 C. That's a 68 F / 38 C spread, which means that anything within 14 F / 9 C of those peaks is winter or summer (i.e. below 8 F / -14 C for winter and above 48 F / 9 C for summer).
Winter: November 6 - March 6
Spring: March 7 - May 14
Summer: May 15 - September 9
Fall: September 10 - November 5
Monday, July 16, 2018
Sunday, June 18, 2017
What are the seasons where I live - Part 2: The Results
See Part 1 first.
The season breakdown in Nashville looks like this.
Winter: November 29 - March 4
Spring: March 5 - May 29
Summer: May 30 - September 16
Fall: September 17 - November 28
"Guaranteed" seasons (100% likelihood)
Winter: none
Spring: March 26 - May 6
Summer: June 27 - August 17
Fall: October 7 - October 27
"Possible" seasons (at least one year in the 58)
Winter: October 28 - March 25
Spring: January 8 - June 26
Summer: May 7 - October 6
Fall: August 18 - January 12
Interestingly, no date was found to have been in winter every year during the 58 year span. January 5-7 and January 13 - February 7 were in winter during 98% of years (57 of 58), but due to a warm January-February 1990, spring started on January 8 that year, whereas a warm December 2006 and early January 2007 meant that winter didn't start until January 13. This gives January 8-12 the unique distinction of being able to fall in three different seasons (fall, winter, and spring).
Only April and July were "guaranteed" to be fully in a particular season: April in spring, and July in summer.
Here's a chart of each season's starting and ending date.
The season breakdown in Nashville looks like this.
Winter: November 29 - March 4
Spring: March 5 - May 29
Summer: May 30 - September 16
Fall: September 17 - November 28
"Guaranteed" seasons (100% likelihood)
Winter: none
Spring: March 26 - May 6
Summer: June 27 - August 17
Fall: October 7 - October 27
"Possible" seasons (at least one year in the 58)
Winter: October 28 - March 25
Spring: January 8 - June 26
Summer: May 7 - October 6
Fall: August 18 - January 12
Interestingly, no date was found to have been in winter every year during the 58 year span. January 5-7 and January 13 - February 7 were in winter during 98% of years (57 of 58), but due to a warm January-February 1990, spring started on January 8 that year, whereas a warm December 2006 and early January 2007 meant that winter didn't start until January 13. This gives January 8-12 the unique distinction of being able to fall in three different seasons (fall, winter, and spring).
Only April and July were "guaranteed" to be fully in a particular season: April in spring, and July in summer.
Here's a chart of each season's starting and ending date.
Here is the likelihood that a particular date is in a particular season:
When are the seasons where I live? Part 1 - The Explanation
I live near Nashville, TN, a climate that is classified as a Cfa by the Koppen classification, the C indicating a coldest month average between 0 and 18 C (32 to 64 F), the F a relatively even precipitation distribution (no dry season) and the A a hot summer, with a warmest month average of 22 C or warmer (72 F+).
Nashville is a cold-end Cfa; if winters were a few degrees colder, it would be a Dfa.
The average (mean of high/low) in January is 3.2 C / 37.4 F in January and 26.3 C / 79.4 F in July. With a 23.1 C / 42.0 F difference, temperature is the main driver of seasons here. Folks, this is a climate that gets four distinct seasons.
What mean temperature defines each season? A good rule of thumb is that temperatures in the top 20% of the average range (or higher) are summer, and vice versa for winter. 20% of 23.1 C / 42.0 F is 4.6 C / 8.4 F, which would suggest that average temperatures of 7.8 C / 45.8 F or colder are "winter" and 21.9 C / 71.0 F or warmer is "summer". Since the Dfa cutoff of 72 was so close, and summer stayed the longest season (it's more stable than winter), I rounded up that number.
I came up with anything colder than 46 F (8 C) would be considered "winter", and warmer than 72 F (22 C) would be considered summer. But in many climates, Nashville included, temperatures jump all over the place. March 1, 1997 for instance had a mean temperature of 74 (23 C). On the opposite end of the spectrum, October 7, 2000's mean temperature was 44 (7 C). Should those dates be considered "summer" and "winter" respectively? No, they're just unseasonable heat and unseasonable cold.
So I use a 21-day running average, with the value being for the 11th day (the day in the center). This reduces the impact of unseasonable days and gives a more long-term look.
For instance, the day I got my car, May 4, 2013, had a mean temperature of 8 C / 47 F (high 10 C / 50 F, low 7 C / 44 F). But the running average for that day was 15.7 C / 60.2 F, because it takes into account the mean temperature from April 24 - May 14, 2013 (the 21 days with May 4 as their center).
If the 21-day running average was below 46, it was "winter", above 72, it was "summer". The in-between areas were called "spring" and "fall".
Usually, it was straightforward; the 21-day running average would fall below or rise above 46 or 72 and would not return until the end of the season. But sometimes it didn't work that way. For instance, January 25-31, 2012 averaged above 46 in their 21-day running averages, but February 1-20, 2012 returned to below 46. Since February 1-20 was 20 days and January 25-31 was 7 days, January 25-31 was considered "winter", as the 20 days outnumbered the 7. Hey, mild spells are part of winter here too.
I took data from January 1, 1959 to December 31, 2016 and found when each season lasted during the 58-year span (a total of 58 winters, 58 springs, 58 summers, and 58 falls). I then found out how likely each day was to fall in a particular season. For instance, today, June 18, fell in summer 98% of years (it was spring in 1961), thus it would be considered a summer day as it fell in summer the majority of years. For ties (29 of 58 years), the date was given to the warmer season; September 16 and November 28 were ties and were given to summer and fall, respectively.
I also have data up to June 17, 2017, but didn't want to use an incomplete year in my data set.
Nashville is a cold-end Cfa; if winters were a few degrees colder, it would be a Dfa.
The average (mean of high/low) in January is 3.2 C / 37.4 F in January and 26.3 C / 79.4 F in July. With a 23.1 C / 42.0 F difference, temperature is the main driver of seasons here. Folks, this is a climate that gets four distinct seasons.
What mean temperature defines each season? A good rule of thumb is that temperatures in the top 20% of the average range (or higher) are summer, and vice versa for winter. 20% of 23.1 C / 42.0 F is 4.6 C / 8.4 F, which would suggest that average temperatures of 7.8 C / 45.8 F or colder are "winter" and 21.9 C / 71.0 F or warmer is "summer". Since the Dfa cutoff of 72 was so close, and summer stayed the longest season (it's more stable than winter), I rounded up that number.
I came up with anything colder than 46 F (8 C) would be considered "winter", and warmer than 72 F (22 C) would be considered summer. But in many climates, Nashville included, temperatures jump all over the place. March 1, 1997 for instance had a mean temperature of 74 (23 C). On the opposite end of the spectrum, October 7, 2000's mean temperature was 44 (7 C). Should those dates be considered "summer" and "winter" respectively? No, they're just unseasonable heat and unseasonable cold.
So I use a 21-day running average, with the value being for the 11th day (the day in the center). This reduces the impact of unseasonable days and gives a more long-term look.
For instance, the day I got my car, May 4, 2013, had a mean temperature of 8 C / 47 F (high 10 C / 50 F, low 7 C / 44 F). But the running average for that day was 15.7 C / 60.2 F, because it takes into account the mean temperature from April 24 - May 14, 2013 (the 21 days with May 4 as their center).
If the 21-day running average was below 46, it was "winter", above 72, it was "summer". The in-between areas were called "spring" and "fall".
Usually, it was straightforward; the 21-day running average would fall below or rise above 46 or 72 and would not return until the end of the season. But sometimes it didn't work that way. For instance, January 25-31, 2012 averaged above 46 in their 21-day running averages, but February 1-20, 2012 returned to below 46. Since February 1-20 was 20 days and January 25-31 was 7 days, January 25-31 was considered "winter", as the 20 days outnumbered the 7. Hey, mild spells are part of winter here too.
I took data from January 1, 1959 to December 31, 2016 and found when each season lasted during the 58-year span (a total of 58 winters, 58 springs, 58 summers, and 58 falls). I then found out how likely each day was to fall in a particular season. For instance, today, June 18, fell in summer 98% of years (it was spring in 1961), thus it would be considered a summer day as it fell in summer the majority of years. For ties (29 of 58 years), the date was given to the warmer season; September 16 and November 28 were ties and were given to summer and fall, respectively.
I also have data up to June 17, 2017, but didn't want to use an incomplete year in my data set.
Sunday, January 15, 2017
Hottest Day of the Year in Bowling Green, KY
The hottest day of the year where I live can vary by more than three months.
Since 1948, it's been as early as June 1 (in 1951) and as late as September 23 (in 1961).
June 1-15: 6 times
June 16-30: 7 times
July 1-15: 14 times
July 16-31: 16 times
Aug. 1-15: 12 times
Aug. 16-31: 9 times
Sept. 1-15: 5 times
Sept. 16-30: 1 time
The average date is July 24, though it's been shifting later. The record period is 69 years, so I divided it up into three 23-year intervals.
1948-1970: July 23
1971-1993: July 22
1994-2016: July 29
The average temperature is a toasty 97.52 F / 36.40 C on the hottest day of the year, but the hottest day of 2012 (June 29) was an amazing 109 F / 43 C! 2004's hottest day, on the other hand, was just 91 F / 33 C.
91-93 F: 10 times
94-96 F: 16 times
97-99 F: 28 times
100-102 F: 7 times
103-105 F: 4 times (all 103)
106-109 F: 4 times
There is no real trend of it getting hotter or colder over time.
1948-1970: 97.65 F
1971-1993: 97.17 F
1994-2016: 97.74 F
Bowling Green, KY data used 1948-1954, 1973-2000, 2002-present.
Nashville, TN data used 1955-1972 and 2001 due to a lack of Bowling Green data for those years.
Since 1948, it's been as early as June 1 (in 1951) and as late as September 23 (in 1961).
June 1-15: 6 times
June 16-30: 7 times
July 1-15: 14 times
July 16-31: 16 times
Aug. 1-15: 12 times
Aug. 16-31: 9 times
Sept. 1-15: 5 times
Sept. 16-30: 1 time
The average date is July 24, though it's been shifting later. The record period is 69 years, so I divided it up into three 23-year intervals.
1948-1970: July 23
1971-1993: July 22
1994-2016: July 29
The average temperature is a toasty 97.52 F / 36.40 C on the hottest day of the year, but the hottest day of 2012 (June 29) was an amazing 109 F / 43 C! 2004's hottest day, on the other hand, was just 91 F / 33 C.
91-93 F: 10 times
94-96 F: 16 times
97-99 F: 28 times
100-102 F: 7 times
103-105 F: 4 times (all 103)
106-109 F: 4 times
There is no real trend of it getting hotter or colder over time.
1948-1970: 97.65 F
1971-1993: 97.17 F
1994-2016: 97.74 F
Bowling Green, KY data used 1948-1954, 1973-2000, 2002-present.
Nashville, TN data used 1955-1972 and 2001 due to a lack of Bowling Green data for those years.
Saturday, January 7, 2017
Season Lengths in Nashville, TN: Part 1, 1959-1963
I've decided to try a little experiment. Where I live, it feels like summer when temperatures average >22 C / 72 F and winter when they average <8 C / 46 F. This also gives us (roughly) a 3.5 month summer and 3 month winter, which makes sense because we do tend to get a bit of seasonal lag in September.
I've used a 21-day running average (the day, 10 days prior, and 10 days after) to determine when each season began and ended. For instance, the temperature for November 5, 2014 was 49.8, taking the means of all days from October 26 - November 15.
If the temperature dropped below 46 and stayed there, or fell below 46 and stayed there longer than any late fall >46 spells, I called it winter. For instance, in 2014, it was below 46 from Nov. 8 - 28 and above 46 from Nov. 29 - Dec. 2 before falling below 46 for the rest of the winter. Therefore, I called anything from November 8 winter. Ditto for summer, but with 72. For spring, replace "fell below" with "rose above" and "late fall >46" with "late winter <46"
On average:
Winter: November 26 - March 1
Spring: March 2 - May 31
Summer: June 1 - September 15
Fall: September 16 - November 25
Update to my original Sept. 20, 2016 post. I've decided to do every season from 1959-present in Nashville. I will also probably make a blog in the near future detailing all of my weather musings and interesting statistics.
This is the first release, Dec. 4, 1958 - Nov. 28, 1963.
Winter 1958-59: ? - March 13 (Peak: December 16, 31.9 F)
Spring 1959: March 14 - May 27
Summer 1959: May 28 - October 2 (Peak: August 23, 80.4 F)
Fall 1959: October 3 - November 14
Winter 1959-60: November 15 - March 24 (Peak: February 28, 27.8 F)
Spring 1960: March 25 - May 24
Summer 1960: May 25 - September 16 (Peak: July 30, 81.1 F)
Fall 1960: September 17 - November 27
Winter 1960-61: November 28 - February 11 (Peak: January 29, 28.9 F)
Spring 1961: February 12 - June 26
Summer 1961: June 27 - September 17 (Peak: July 24, 79.9 F)
Fall 1961: September 18 - December 3
Winter 1961-62: December 4 - March 17 (Peak: January 7, 29.9 F)
Spring 1962: March 18 - May 6
Summer 1962: May 7 - September 12 (Peak: August 14, 80.2 F)
Fall 1962: September 13 - November 27
Winter 1962-63: November 28 - March 4 (Peak: January 22, 25.6 F)
Spring 1963: March 5 - June 1
Summer 1963: June 2 - September 13 (Peak: July 29, 79.5 F)
Fall 1963: September 14 - November 28
I've used a 21-day running average (the day, 10 days prior, and 10 days after) to determine when each season began and ended. For instance, the temperature for November 5, 2014 was 49.8, taking the means of all days from October 26 - November 15.
If the temperature dropped below 46 and stayed there, or fell below 46 and stayed there longer than any late fall >46 spells, I called it winter. For instance, in 2014, it was below 46 from Nov. 8 - 28 and above 46 from Nov. 29 - Dec. 2 before falling below 46 for the rest of the winter. Therefore, I called anything from November 8 winter. Ditto for summer, but with 72. For spring, replace "fell below" with "rose above" and "late fall >46" with "late winter <46"
On average:
Winter: November 26 - March 1
Spring: March 2 - May 31
Summer: June 1 - September 15
Fall: September 16 - November 25
Update to my original Sept. 20, 2016 post. I've decided to do every season from 1959-present in Nashville. I will also probably make a blog in the near future detailing all of my weather musings and interesting statistics.
This is the first release, Dec. 4, 1958 - Nov. 28, 1963.
Winter 1958-59: ? - March 13 (Peak: December 16, 31.9 F)
Spring 1959: March 14 - May 27
Summer 1959: May 28 - October 2 (Peak: August 23, 80.4 F)
Fall 1959: October 3 - November 14
Winter 1959-60: November 15 - March 24 (Peak: February 28, 27.8 F)
Spring 1960: March 25 - May 24
Summer 1960: May 25 - September 16 (Peak: July 30, 81.1 F)
Fall 1960: September 17 - November 27
Winter 1960-61: November 28 - February 11 (Peak: January 29, 28.9 F)
Spring 1961: February 12 - June 26
Summer 1961: June 27 - September 17 (Peak: July 24, 79.9 F)
Fall 1961: September 18 - December 3
Winter 1961-62: December 4 - March 17 (Peak: January 7, 29.9 F)
Spring 1962: March 18 - May 6
Summer 1962: May 7 - September 12 (Peak: August 14, 80.2 F)
Fall 1962: September 13 - November 27
Winter 1962-63: November 28 - March 4 (Peak: January 22, 25.6 F)
Spring 1963: March 5 - June 1
Summer 1963: June 2 - September 13 (Peak: July 29, 79.5 F)
Fall 1963: September 14 - November 28
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