Why is June 1st the start of meteorological summer? It's science!
While most wait until June 21 for the start of summer, meteorologists begin the season 'early'. Why? It's all about the science!
Did you know that there are actually two different sets of seasons, and thus two different summers, every year? Astronomically, the First Day of Summer falls on the Solstice, around the 21st of June. However, meteorologically, the season begins three weeks earlier, on the 1st of June. This is the story behind Meteorological Summer.
Throughout the year, we typically use four different dates on the calendar to mark the passage of the seasons, each keyed to the Sun's journey across our skies.
In the northern hemisphere, the June Solstice corresponds to when the Sun reaches its highest point in the sky. Following that, the September Equinox is when it appears directly above the equator, as it heads from north to south. Then, on the December Solstice, the Sun reaches its lowest point in the sky, and on the March Equinox, it appears above the equator again, this time going from south to north. Meanwhile, in the southern hemisphere, this is all flipped on its head, with the June Solstice marking when the Sun is lowest in the sky, and so on.
These dates, and the seasons that lie between them, have been observed for centuries.
DON'T MISS: Destructive Atlantic hurricanes can still happen during El Niño
The reason for the seasons, and for these specific dates, is the tilt of Earth's rotational axis.
As our planet orbits the Sun, it traces out a flat, nearly perfect circle along what we call the ecliptic. However, the ecliptic doesn't line up with Earth's equator. Instead, the equator is tilted with respect to the ecliptic, by approximately 23.4 degrees. The source of this tilt is likely the colossal impact of Theia, the Mars-sized protoplanet that struck Earth early in its formation, which is the most probable reason for the existence of the Moon.
These satellite views of Earth show the start of the four seasons. From left to right, with respect to the northern hemisphere, we see the summer solstice, fall equinox, winter soltice, and spring equinox. Credit: NASA
So, during each of our year-long journeys around that orbit, Earth's tilt causes our perspective on the Sun to change.
For half of the year, it climbs higher in our sky each day, until it reaches its highest point. For the rest of the year it gets lower in the sky every day, until it reaches its lowest point. Then, the pattern starts all over again.
This 'solargraph' image captures the Sun's path across the sky, day by day, between June 21 and December 21, 2023, from atop Weather Network Headquarters. Credit: Bret Culp
READ MORE: What is a solargraph? How to record the Sun's seasonal journey across our sky, all in one image
Using this astronomical timing to define our seasons works just fine, in general.
However, specifically when it comes to keeping track of our weather and climate, they don't work quite as well. The start and end dates of the astronomical seasons typically fail to capture the weather that most defines a particular season. Also, they definitely do not mesh well with how we keep records of weather conditions throughout the year.
Shifting alignment
When keeping weather and climate records, consistency is essential.
Daily, weekly, monthly, and even yearly records satisfy this requirement quite well. A day is always 24 hours, a week is always 7 days, and except for the occasional leap year, we can count on each month staying the same length, and each year having 365 days. This helps atmospheric scientists to easily make comparisons, find extremes, and track trends in their weather records.
Comparing seasonal trends is important too. However, astronomical seasons are anything but consistent. Due to the influence of the Moon and the other planets, slight changes are introduced into the timing of Earth's orbit and rotation. As a result, the exact day and time of the equinoxes and solstices changes year to year. This throws an added complication into the process.
Without meteorological seasons for comparison, the 'above normal', 'normal', and 'below normal' regions of this Summer Forecast map would be much more difficult to predict. (The Weather Network)
It's not that comparisons can't be made. These days, computers make tallying weather records between any start time and end time a trivial thing. Even making comparisons between inconsistent time spans isn't that big of a problem, even though it isn't ideal.
That's just the modern considerations, though. Back when Societas Meteorologica Palatina established the first meteorological network in 1780, all record-keeping and calculations were done by hand. This practice continued until computers became more mainstream in the latter half of the 20th century.
So, for the first 300 years of weather records, making seasonal comparisons and calculating seasonal trends would have been far more cumbersome if they used astronomical seasons.
FROM SPACE: Look up! The two brightest planets converge in the sky in June
To better align the seasons with how weather records were kept, meteorological seasons were invented.
Each meteorological season is still three months long. However, unlike their astronomical counterparts, they all align precisely with our calendar months.
Meteorological spring begins on the 1st of March, meteorological summer starts on the 1st of June, meteorological fall begins on the 1st of September, and meteorological winter starts on the 1st of December.
Therefore, they start and end on the exact same dates every year, bringing consistency to any comparisons that are made.
Does it make that much of a difference?
Meteorological seasons do more than bring consistency to seasonal comparisons. They also tend to capture the most 'representative' temperatures of each season.
This graph plots average daily temperatures for seven cities across Canada for Winter, Spring, Summer, and Fall. The start and end dates for both meteorological and astronomical seasons are indicated. (Data from Environment and Climate Change Canada)
Canada is a very large country, and the different regions of the country experience different weather throughout the year. Even so, when the average monthly temperatures for different cities from coast to coast are plotted on the same graph, they all experience roughly the same trends with regards to the seasons.
As the above graphs show, compared to the astronomical seasons, meteorological seasons are much better at capturing the most representative temperatures for each season — coldest for winter, hottest for summer, and transitional for spring and fall. That makes them even more useful for tracking trends.