We have had a couple of frosts in the last few days (6th and 11th of October 2021), though they were not that severe. Here in Ballarat spring frosts are not that unusual, last year we had a frost on the 12th October, in 2017 we had one in the first week of November. So why do we get Frosts so late in the spring when other regions in similar latitudes do not?
The answer depends on several factors.
Clearly the length of days (number of daylight hours) is a significant factor in determining the likelihood of a frost, you do not get frosts in the middle of summer when the days are long.
For a frost to occur there needs to be a cloudless sky with no wind. Warm air rises, but clouds trap hot air in the lower atmosphere, usually just enough hot air is trapped to keep the air temperature high enough to prevent a frost. If the night is clear but there is wind, this wind movement will usually stop a frost forming. There are exceptions to this rule. In the dead of winter freezing can occur even if there is wind and cloud around. But this phenomenon is rare, at least in Ballarat.
Warm air rises and cool air sinks to the lowest point. As a result, frosts tend to be more severe and frequent at the bottom of a valley then on top of nearby hills.
In Eric Newby’s book A Short Walk in the Hindu Kush he describes hiking in 40 deg temperatures during the day but after sunset the temperature dropping to below freezing. That is because he was walking at a high altitude. Put simply, the higher the altitude, the lower the air density, heat rises more quickly when the air density is low. As the general rule of thumb, air temperatures drop about 1°C for every 100 metres in altitude. As Ballarat’s altitude is about 500 metres above sea level the temperature here is on average 5°C lower than at sea level.
The frost we had on the 11th of October was a long way away from the shortest day (21st/22nd June), in fact the 11th of October is 113 days after the shortest day. While the length of the day is a good indicator of the risk of frost it is not the only factor in play. If you count back 113 days from 21st June you arrive at the date of 1st March, which is a good two months prior to there being any likelihood of a frost in Ballarat. This discrepancy between the risk of frost and the length of time before and after the shortest day is due to the fact the soil stores heat over summer and slowly releases it as the weather cools down in Autumn. Similarly, in spring as the weather warms up, the average soil temperature takes a long time to rise. If it is a chilly night and the soil is cold, there is likely to be a frost, but if the soil is warm then it doesn’t’ matter how cold the night is there is little chance of a frost as this warm ground will slowly give off heat at night, thereby warding off a frost.
Water is even a more stable heat sink than soil. The temperatures of oceans and large lakes remain very stable throughout the year. Their temperatures will gradually fall in autumn as the weather cools and rise again in spring as it warms up. But this change will be much slower than that of air or soil temperatures. On a chilly winter’s night, the water temperature will drop more slowly than that of the air and the ground near the shoreline. But even this slower temperature gives off some heat, thus keeping the air over the water warmer than that of nearby land. This air temperature imbalance is what creates fog, it is formed as the warm moist air over water gradually mixes with the cooler air over land. While it may seem cold when you are surrounded by fog the air temperature is in fact much warmer than if it was a clear still night. Fog, like cloud, acts as a blanket trapping heat. When there is a fog there usually will not be a frost. Fog will occur in the absence of nearby water, but its creation involves the same mixing of warm moist air and cooler air.
Yes, this is a somewhat truncated explanation of why frosts occur, but hopefully these points will help gardeners determine when the first and last frosts are likely to occur in their area, and therefore when it is safe for them to plant frost sensitive summer vegetables.