By Vince Condella Published Apr 30, 2003 at 5:05 AM

They say golf is a game of inches. We could say that weather is a game of feet, especially when it comes to frosty nights and the gardening season.

It's that time of year when many people are heading out to the garden to plant their flowers and vegetables. Those with the green thumb are keeping their eye on the radar for the necessary moisture that the soil requires, but they are also watching the overnight low temperature. A frost or freeze after the seeds are in the ground can spell a delay in the growing season. But when it comes to overnight temperatures, numbers can be deceiving.

The coldest nights occur when skies are clear and the wind is light or calm. A cloud cover acts like the lid on a pot of hot water. As long as the lid covers the pot, the pot remains warm for a long time. Once the lid is lifted, heat is allowed to rise away from the water and it cools.

Clouds work the same way. They trap the outgoing heat energy (longwave radiation) that typically rises from the Earth's surface. Once the sun sets, outgoing energy is greater than incoming energy and the temperature begins to cool. If the sky is clear and the winds are light or calm, temperatures drop quickly.

Wind is an important factor on a cold, clear night because a breeze can keep things stirred up. A breeze allows some of the escaping warmth to mix back down towards the ground and temperatures do not cool off as much as they would with a calm night.

Another factor to watch is the dew point. If there is a large difference between the air temperature and the dew point temperature, the air is quite dry. Moisture in the air will help trap warmth as it moves away from the Earth's surface. A lack of moisture in the air causes temperatures to drop rapidly after sunset.

There is an old forecasting rule that states the overnight low temperature will drop as low as the dew point value at 3 p.m. While that rule is a bit simplified and does not apply to all situations, it does recognize that a low dew point preceding a clear, calm night could lead to quite a chill.

Temperatures are typically measured in an instrument shelter that houses the thermometers. The shelter is white in color to reflect sunlight, with louvered slats on the sides to allow air to flow freely through it. The thermometers are allowed to sample the air without overheating due to direct sunlight. A thermometer in a shelter is about five feet above the ground. However, lawns and gardens are at, well, ground level! Those five feet can make quite a bit of difference.

If the night is clear and the wind is calm, warmth radiates away from the ground quickly. The lowest layer of air, e.g. the first few inches, cool the quickest. The air several feet above the ground will also cool, but not as rapidly. During the long hours overnight, ground temperature may drop to 30 degrees while the air temperature measured by the thermometer in the instrument shelter may be 35 degrees.


A 30-degree temperature at garden level is enough to freeze any moisture created by dew formation, and this can cause a frost. Yes, a frost can occur even with the "official" temperature at the 5-foot level at 35 degrees.

One more factor to consider is the heat from an urban area. This is often called the "urban heat island." It is simply the warmth from the daytime heating remaining in the bricks and concrete of the city long after the sun has gone down. Urban areas tend to stay warmer at night. Even locations near Lake Michigan tend to stay warmer on a clear, calm spring night. The Lake Michigan water temperature is usually between 40 and 45 degrees in April and early May. This is often warmer than the air at night. So the lake can act as a source of warmth at night even though it acts as a source of cooling during the day.

In Milwaukee, the average last occurrence of 32 degrees or less in spring is April 25. This would technically be the average last freeze date. Keep in mind that this is only an average date and it applies to Mitchell International Airport where official weather records are kept. Outlying areas away from the city can have average last freeze dates well into May. And because we can get frost even with an air temperature of 35 degrees, frost can appear on our lawns and gardens for at least another month or more.

The latest freeze (32 degrees) on record in Milwaukee is May 27, 1961. The earliest date of the last occurring freeze was March 29, 1878, when weather records were kept in downtown Milwaukee.

When it comes to the garden, the real culprit is the freeze, not the frost. And the duration of the freeze is important. When temperatures dip below 32 degrees at ground level for four hours or more, it can prove deadly to sensitive plants. That's when the green thumbs have to get back to work and start all over again.