1. Why March and September differ in temperature even though both equinoxes have similar day and night lengths
The spring and autumn equinoxes are set as markers when day and night are nearly equal in length. According to the Japan Meteorological Agency’s public relations magazine, the twenty-four solar terms were first determined by the sun’s movement—specifically day length—and then intermediate points like the start of spring were placed between them. These terms originated in ancient China more than 2,000 years ago [2]. So, is day and night truly equal on the equinox? In Tokyo in 2026, sunrise was at 5:45 and sunset at 17:52, making the day 12 hours and 7 minutes long, and the night 11 hours and 53 minutes [4]. The day is about 14 minutes longer because sunrise and sunset are defined by the top edge of the sun, and atmospheric refraction makes the sun appear higher than it actually is. Still, day and night are "almost" equal.
Next, let’s look at temperature. Using normal values from 1991 to 2020, Tokyo’s monthly average temperatures are 5.4°C in January, 6.1°C in February, 9.4°C in March, 14.3°C in April, and 23.3°C in September [1]. The difference between March and September is 13.9°C. This compares monthly averages, not the specific temperatures on the equinox days themselves. However, it shows a clear trend: even though sunlight is similar, spring is much colder than autumn. Why does the temperature shift when sunlight is almost the same? First, we must check what determines the strength of sunlight.
2. How the Earth's tilt changes the strength of sunlight
The Earth’s axis of rotation is tilted about 23.4 degrees relative to its orbital plane. Because it keeps this tilt while orbiting the sun, the Northern Hemisphere receives stronger sunlight and has longer days when it tilts toward the sun. The summer solstice is the day when the sun is highest at noon and the day is longest [5]. In middle school science classes, the change in the sun’s noon altitude and day length is taught as the reason for the amount of solar energy hitting the ground and changes in temperature. Summer has a high noon altitude, while winter has a low one [6]. The spring and autumn equinoxes fall exactly in the middle. Around these times, the sun’s height and day length are almost the same in spring and autumn. This suggests that the highest temperature should occur at the summer solstice, when sunlight is strongest. But is that true?
3. Why temperatures rise late even as sunlight increases in spring
Sunlight is used to warm the ground and the sea. In spring, sunlight increases day by day, but the added heat is first used to warm the ground and seawater, and only then does the air temperature catch up [7]. This is similar to how a room with a heater warms up: the floor and walls heat up first, and then the air becomes warm. However, on Earth, the sea absorbs heat as well as the ground, so the delay is thought to be much larger. This "lag" also appears in summer. Fukuoka’s normal temperature peaks in early August, more than a month after the summer solstice (late June), when sunlight is strongest [2]. Tokyo’s normal values show the same trend. January is 5.4°C and February is 6.1°C, staying low for a while even after sunlight begins to return. The sharp rise to 9.4°C in March and 14.3°C in April comes later [1]. In spring, even though sunlight strength is the same as in autumn, the temperature has not yet caught up. In autumn, the ground and sea, warmed during summer, are still holding heat. Is this delay the same everywhere? Let’s compare Japan, surrounded by sea, with an inland city.
4. Why spring comes slowly in Japan: Fukuoka and Xi'an compared
Japan’s spring advances "slowly" because of the strong influence of the sea. Weather station materials compare the normal temperatures of Fukuoka and Xi’an (Chang’an), the ancient capital of China [2]. In deep winter, Xi’an is colder than Fukuoka. But in spring, Xi’an’s temperature rises rapidly, catching up to Fukuoka around April, and becoming higher than Fukuoka in May and June [2]. The reason is the difference in how sea and land warm up. The sea warms and cools more slowly than land, so island nations like Japan are strongly affected by the sea, causing seasons to progress slowly. Inland cities like Xi’an, where land warms easily, see seasons progress faster [2]. The fact that Fukuoka’s temperature peaks in early August is influenced by the sea. Both the fact that Japan’s spring is colder than autumn and that summer heat comes more than a month after the summer solstice are linked to the property that "the sea absorbs heat and changes slowly."
5. How migratory high-pressure and low-pressure systems make spring weather change
Spring is the time when temperature is catching up, so warm and cold days alternate. One major reason is the migratory high-pressure system. According to the Japan Meteorological Agency’s glossary, migratory high-pressure systems move eastward, alternating with extratropical cyclones, and are common in spring and autumn [3]. Weather is often clear between high-pressure systems, but clouds spread and rain is likely when a low-pressure system approaches. So, every time these systems swap, the weather and temperature change. The pattern of repeating cold and warm days in early spring is described by the term "three cold days, four warm days." However, the exact number of days between changes is not fixed. This shows that spring warmth is not "gradual every day," but rather rises overall while repeating warm and cold days.
6. How to compare March and September in your town using normal values
On the Japan Meteorological Agency’s website, you can see monthly normal values (averages over the past 30 years) for each city. Try comparing March and September in your town to see if the difference is similar to Tokyo’s "9.4°C and 23.3°C." Choosing a coastal town and an inland town might show different differences. On the National Astronomical Observatory of Japan’s calendar page, you can check sunrise and sunset times. You can verify that day length is almost the same on the spring and autumn equinoxes using numbers from your own town. The fact that temperatures differ despite similar sunlight becomes clear just by placing two numbers side by side. After that, it is interesting to read the "why" part again in sources like the Fukuoka District Meteorological Observatory’s public relations magazine [2].