What determines the first day of spring?

There are three different ways we can tell when spring is coming: astronomical, meteorological and ecological.

Astronomical way

Possibly the most common way of determining the first day of spring is by timing it with the equinox.

Equinox is derived from Latin and means ‘equal night’. The name is apt because an equinox refers to that point in the year when day and night are shared equally in both hemispheres. This point is the specific moment the sun crosses the celestial equator, during which daytime is equal length to night-time.

This means that each hemisphere will get 12 hours of daylight and 12 hours of darkness. There are only two points in the year when this happens: the start of spring and the beginning of autumn.

In the Northern Hemisphere, this usually takes place on March 20 or 21 and is called the March equinox. In the Southern Hemisphere, this occurs on September 1, and is aptly called the September equinox.

Why is the equinox called the equinox?

The word equinox has Latin roots. It stems from two words: aequus, which means“equal” and nox, which means “night.” It refers to the fact that on the day of the equinox, most places on Earth will experience it as equally split between day and night.

Meteorological way

Determining the first day of spring using meteorology is a lot simpler.

Why?

Because meteorological seasons are arranged so that they match our Gregorian calendar. Dividing the twelve months into four different seasons makes it easier to compare seasonal and monthly statistics.

Meteorologists then just go by average temperatures each month, with the three warmest months defined as summer, the three coldest months as winter and the intervening gaps as spring and autumn.

Spring, as defined meteorologically, can also start on different dates in different regions.

For example, in countries like the US and the UK, the spring months are March, April, and May.

In the Southern Hemisphere countries of Australia and New Zealand, on the other hand, spring begins in September and ends in December

On the other side of the world, in Brazil, spring months are September, October and November.

In the Himalayan states of India, spring is a short two-month season starting in March and ending in April.

Ecological way

The ecological (or phenological) way of determining the first day of spring relies on certain biological indicators in a given place.

In Australia, for example, spring begins when the wattles start to bloom while in northern American states, people know that it’s spring when the bluebirds arrive.

In other words, the blossoming of certain plant species or the activities of specific animals are all treated as indicators of spring.

So, what is the official first day of spring?

Now that we`ve looked at the different ways we can define when the spring season begins, it’s clear that to identify the official first day of spring, you’ll first need to decide which definition to use!

This is because the first day of spring changes depending on the definition you’re using.

Let’s start with the meteorological definition. As we said, it’s the simplest definition because it’s based on the Gregorian calendar, which divides the year into twelve months and then splits those twelve months into four equal seasons (of three months).

In the southern hemisphere, this means that the seasons are divided as follows:

Summer: December, January