Seasons are divisions of the year that correspond to periods of similar weather, ecology, or daylight hours. Seasons are the result of the Earth’s 23.5° tilt on its axis, such that through the year, the sun moves in its overhead latitude at midday, from north to south of the equator, to the limits of the latitudes of Cancer and Capricorn. This means that the sun’s energy reaching the north and south regions of the earth varies through the year and repeats each year.
Living in temperate regions we think of the four seasons; spring, summer, autumn, and winter, but in other latitudes the seasons are different. In the Arctic and Antarctic there are essentially two seasons; winter when the sun doesn’t rise and summer when the sun doesn’t set. In tropical regions the temperature may be hot all year and the seasons are defined by rain; dry, rainy, monsoon. A variety of dates and even exact times are used in different countries or regions to mark changes of the calendar seasons.
Here in the temperate southern climate of New Zealand we refer to the four seasons. But when those seasons are, is the subject of some confusion. Since the year has 12 months, each season lasts about three months. However, the dates when the seasons begin and end, vary depending on whether you ask an astronomer or a meteorologist. The astronomer is likely to say that spring starts at the spring equinox when there is almost exactly equal day and night length. This is around the 20th March for the northern hemisphere and 23rd September for the southern hemisphere (the equinoxes and solstices vary a little from year to year). But a meteorologist will say spring starts from the 1st March in the northern hemisphere and the 1st of September in the southern hemisphere.
|Astronomic||Starts ~21st Mar||Starts ~20th Jun||Starts ~23rd Sep||Starts ~21st Dec|
|Meteorologic||1st Mar-31st May||1st Jun-31st Aug||1st Sep-30th Nov||1st Dec-28/29th Feb|
|Astronomic||Starts ~23rd Sep||Starts ~21st Dec||Starts ~21st Mar||Starts ~20th Jun|
|Meteorologic||1st Sep-30th Nov||1st Dec-28/29th Feb||1st Mar-31st May||1st Jun-31st Aug|
Of course, the weather in any location does not suddenly change from winter to spring on either the astronomical or meteorological start of spring. The change is gradual and subject to variations. You can often get very spring-like days in winter and wintery days in spring. Delicate plants planted too early in spring are at risk of frost damage. Gardeners frequently gamble on whether to get seedlings planted early to give them a head start at the risk of them being killed or damaged by a cold snap.
There are also variations on when the warmer spring weather arrives, depending on where you are in New Zealand. In the far north it may never get cold enough for frost damage but in the south or at higher altitudes there may be risk of frost even later in spring. You will find some information on the regions of New Zealand and the hardiness rating of plants here.
Life in the garden has a rhythmic calendar known as the circannual rhythm, which is regulated by the photoperiod or length of day/night. So, for example, bulbs will emerge in spring even if the spring-like weather has not yet arrived. But other conditions also influence the timing of bulb emergence, spring flowering, autumn senescence, etc. E.g., temperatures, moisture, and sunshine hours will slow or hasten the change of season. Predicting when spring will start in the garden is going to happen is an annual guessing game. The first exciting signs of the arrival of spring are the emergence of spring bulbs, tree blossom, pollen on your car and flowering shrubs. Less exciting are the emergence of weeds.
Spring Prediction 2021
NIWA are continuing their prediction of warmer than average mean temperatures for the August and September which would suggest an early start to spring. Soil moisture levels are about normal so that will not inhibit growth, and rainfall is predicted to be about normal for the next couple of months.