It is May, the time of year when growers constantly watch the weather because the risk of frost is high. Many growers and gardeners consider the May full moon as an important factor for determining when the last frost of spring will occur. It is also considered again for the risk of frost in the fall. This is based on the old weather proverb “clear moon, frost soon”. This year, there are two full moons in May, leading to worry of the potential for another frost at the end of the month.
As part of my efforts to determine the effects of climate change on the weather in southern Ontario, I determined the last frost date (the last day the air temperature went to 0.0°C or colder) for each year for the past 90 years at Delhi and the last 135 years at Ottawa. Looking at the historical dates of the full moon over that period, it was easy to determine how many days away from a full moon each last frost occurred, and whether frosts were any more common on nights with a full moon.
Based on this analysis, the last frost averaged 7.7 days from a full moon (before or after) in Delhi and 7.5 days from a full moon in Ottawa. Since there are 29.5 days between full moons, this means that roughly half of the frosts at both locations occurred between the first quarter and the third quarter phases of the moon (half moon to half moon), and the other half occurred in the other half of the cycle. In other words, there was absolutely no effect of the moon on the last frost date. Figure 1 shows how many times the last spring frost occurred in each phase of the moon at both locations. While the concern is always the full moon in May, some of the dates that coincided with the full moon actually occurred in April. Despite the same moon phases at each location, the last frost at Delhi was frequently not on the same night as the last frost in Ottawa.
Figure 1. The number of last spring frost events at Delhi (1934-2024) and Ottawa (1890-2024) in relation to the number of days before or after a full moon. The phase of the moon for each number of days before or after a full moon is illustrated by the moon icons above the graph.
This makes sense for a couple of reasons:
- There is at least one full moon every month of the year. Why would the full moons in May and September/October specifically lead to frost?
- The same full moon occurs on the same night everywhere from the tropics to the Arctic. In the tropics, they never worry about frost. In the southern US, the last frost may occur in March or April. In northern Ontario, there could easily be a frost in June. It wouldn’t make sense for frost risk to follow the moon calendar when there is a gradual transition in risk from south to north.
If we return to the old proverb “clear moon, frost soon,” it actually doesn’t say anything about a full moon. The keyword is “clear”. At this time of year, frost usually occurs under very clear skies after a cold front. If there happens to be a full moon at that time, it is very prominent and can lead to the impression that the moon has something to do with the frost. When the moon is not present, it doesn’t get any attention.
For the next frost event, focus on the weather forecasts, particularly for clear skies, calm winds at night, and a low dew point. The phase of the moon does not affect the risk of frost.
