As we head through the Memorial Day Weekend, a chance of showers remains in the forecast, although that chance falls off as the weekend progresses. For Saturday, a weak low dropping down from the north will replace the low to our south which has brought all the rain earlier in the week. Cloudy skies and scattered mainly afternoon showers and thunderstorms on Saturday will turn into partly cloudy skies with only isolated afternoon showers on Sunday and Monday, with temperatures warming up into the 70s for the latter half of the holiday.

clear night

Yesterday I mentioned that the air temperature doesn’t have to drop below freezing to freeze a plant. Solid objects can lose heat two different ways: conduction and radiation (convection only applies to liquids and gasses.) Conduction heat transfer occurs when a warm object comes into direct contact with a colder one. When air cools, it draws heat from a plant until they are both the same temperature, at which time heat transfer stops. And if that was all that was going on, then as long as the air didn’t drop below freezing, then your plant wouldn’t freeze. And on a cloudy night, that holds true. But when radiational cooling occurs, all bets are off.

On a clear night, any object can also lose heat directly by bypassing the atmosphere and going directly into outer space by radiation. As a result, the cooling of your plant isn’t limited to the temperature of the air. So even if the air temperature is several degrees above the freezing mark, any solid object (ground, car, picnic table, tomato plant…) can easily cool to below freezing.

There are a couple of things that limit how much cooling below air temperature an object can get by radiation. If the skies are cloudy, the long wave radiation emitted is blocked, and the object will essentially cool to the air’s (ambient) temperature. And even if it’s clear, conduction of the now warmer air will limit how much colder the object can get. But just how cold can something get due to radiation loss? Let’s look back in history.

roman fort desert

Put yourself in the sandals of the Roman legionnaires: marching all day across the hot sands of the Sahara Desert, with nothing but warm camel’s milk to slake their thirst. What’s a centurion to do when he gets a hankering for an iced frappuccino? How about making ice in the desert? Here’s how it might have actually happened:

According to some historians, the Romans dug a broad, deep hole in the desert sand, and placed a broad container (like a bronze shield) in the bottom, filling it with water. At night, the water was left exposed to the clear skies, and cooled significantly due to radiational heat release. At the end of the night, the water was covered and then insulated with a deep bed of straw, which kept it cool through the hot day. At night, it was uncovered and the process continued, with the water cooling a little more each night than it warmed each day until it froze, putting a camel’s milk smoothie on the menu.

While there is some disagreement as to whether this actually happened, it is certainly possible in theory.