Spring: What It Is and When It Starts, From Equinox to Climate

Why spring begins, and what the equinox marks

In the Northern Hemisphere, spring begins, astronomically speaking, with the March equinox: the moment when, according to NASA, the center of the Sun crosses the plane of Earth’s equator. It is the same mechanism behind every season, the tilt of Earth’s axis relative to the plane of its orbit and the planet’s position along that orbit, not its distance from the Sun. That day the Sun lights up both hemispheres almost equally, one of the few moments of the year when both poles receive sunlight at the same instant.

In 2026 the March equinox falls on March 20, at 14:46 UTC, according to NASA Science. From that moment on, the Northern Hemisphere sees the sun rise a touch earlier and set a touch later, while the air gradually turns milder; at that exact same instant, the Southern Hemisphere shifts into autumn, with later sunrises, earlier sunsets, and the first dry leaves dropping to the ground. The two hemispheres are always in opposite seasons, when it’s spring in the north it’s autumn in the south, and the roles swap right on schedule at the September equinox.

The word “equinox” comes from Latin, “equal night” (equi-, equal, and nox, night), but the balance between hours of daylight and hours of darkness is not perfect. As NASA notes, Earth’s atmosphere refracts sunlight, slightly stretching the perceived length of daylight beyond a precise 12 hours: at the equator, an equinox produces about 12 hours of daylight and 12 of darkness, but not down to the minute.

Practical example: picture two photographs taken on the same day in March, one in Milan and one in Sydney. In the first, trees show their first buds and the days are stretching toward summer; in the second, the leaves are starting to turn and the days are shrinking toward winter. It is the same equinox, read from two opposite hemispheres.

Astronomical spring vs meteorological spring: two calendars

There is more than one way to mark the start of spring. According to NOAA/NCEI, seasons can be defined in two ways: the astronomical ones, based on Earth’s position relative to the Sun and bounded by two equinoxes and two solstices, and the meteorological ones, which meteorologists and climatologists group into three-month blocks based on the annual temperature cycle.

The practical difference is in the length. Astronomical seasons last between 89 and 93 days, because Earth’s orbit around the Sun is slightly elliptical; meteorological seasons, instead, always cover March, April and May in the Northern Hemisphere, a fixed length that makes it simpler to compare climate statistics from one year to the next.

Practical example: a meteorologist comparing the average temperature of ten different springs prefers the meteorological definition precisely because it always starts on the same day; with the astronomical definition, which shifts from year to year, the comparison would be less direct.

One example of how spring climate is measured in practice comes from the United States: according to NOAA/NCEI, meteorological spring 2025 (March-May) in the contiguous United States recorded an average temperature of 54.1°F, about 12.3°C, 1.8°C above the historical average, the second warmest spring in a 131-year record; average precipitation was 8.90 inches, about 226 mm, in the wettest third of the same historical record. These are US figures, not a universal value, but they illustrate well the variability in temperature and precipitation that characterizes seasonal statistics wherever they are gathered; the same underlying warming trend is described, on a global scale, in the article on the greenhouse effect.

The natural world waking up: phenology

Spring’s most visible effects are not only climatic. Phenology, as the USA National Phenology Network defines it, is the study of the timing and cyclical patterns of natural events, in particular those tied to the annual life cycles of plants and animals: the budding of leaves, the arrival of migratory birds, the flowering of plants, the onset of fruit ripening.

With the first flowers, what USA National Phenology Network scientists call the season’s first food web switches on: native bees, flies and beetles feed on nectar and pollen, and in turn become an early food source for birds and other animals emerging from winter. It is the same moment when, leaf by leaf, photosynthesis restarts in plants that had slowed down or paused through the winter.

Phenology is not just observation: it has concrete applications. It helps predict events tied to human health such as allergies and mosquito season, optimize planting, fertilizing and harvest in agriculture, manage invasive species and forest pests, and understand how carbon cycles in ecosystems change over time. The US environmental agency EPA, for instance, uses an index of leaf and bloom dates as a climate change indicator: scientists compare, year over year, when flowers bloom and when leaves emerge to measure, indirectly, how much the climate is warming.

What happens to the body: allergies and mood

Spring makes itself felt on the body too, and not always pleasantly. The main effect is pollen allergies: according to the AAAAI (American Academy of Allergy, Asthma & Immunology), in spring the biggest culprits are trees, in particular birch, cedar, cottonwood and pine; grasses dominate in summer and ragweed in autumn. Pollen irritates the nose, roof of the mouth, throat and eyes, causing sneezing, nasal congestion, a runny nose and watery eyes; in the more troublesome cases it can lead to sinus infections, disrupt sleep and reduce performance at school or work.

The numbers, in the United States, are not marginal. According to a CDC analysis based on the 2024 National Health Interview Survey, 31.7% of US adults have a diagnosed allergic condition (seasonal, eczema or food); the pollen component alone affects 25.2% of adults, with a marked gender gap, 29.5% of women against 20.7% of men. These are US figures, but the underlying biological mechanism, the immune system reacting to pollen, does not change from one continent to another.

Practical example: according to the AAAAI, people with spring allergies can ease their symptoms with a few simple habits, cutting back on outdoor activity on high-pollen days, keeping windows closed at home and in the car, and showering after coming inside to remove pollen from hair and skin. These are tips reported by the source, not medical advice from this article.

The other effect involves mood. Seasonal affective disorder (SAD), described by the NIMH as a form of depression with a recurring seasonal pattern, is best known in its winter version, with symptoms that begin in late autumn and resolve in spring or summer; it is the same mechanism of light and circadian rhythm explored in the article on human emotions. There is also a less common pattern, though, that shows up in spring or summer itself, with symptoms opposite to the winter ones, insomnia instead of oversleeping, poor appetite instead of carbohydrate cravings, restlessness and agitation instead of social withdrawal. The NIMH lists reduced serotonin levels, disrupted melatonin production and, in some cases, a vitamin D deficiency among the biological causes common to both patterns; the disorder affects women more often, and people living at northern latitudes. It is, in a sense, the opposite of what happens during heat waves, where it is an extended excess of heat, not a change of season, that puts the body under stress.