Illustration (Image credit: Wix)
High pressure anchored over northern Africa will continue to pump warm, but dry air our way over the next few days. As it slowly progresses northward, dust levels will continue to increase, and temperatures will warm into the mid 20sC/70sF even in the higher elevations. Dust concentrations may even become dangerous to our health on Friday or Shabbat, prior to a change to cooler, if not colder weather to follow.
Motzei Shabbat (Saturday night) will be quite mild, but winds will switch to the west and northwest and our temperatures (and some might say our weather) will start a precipitous downward trajectory that will eventually remind us that it's winter. Moisture arriving with the cold air may also bring showers or even periods of rain from Sunday into at least mid-week.
Of course, this is Israel, and we live in a southern Mediterranean climate, so cold isn't really cold, and wet is hardly ever white. Still, it's good for the plants and trees (including those that bear fruit) to have at least a few more weeks of normal winter weather. They need the cold to prepare themselves for spring, which will hopefully bring new growth and new fruits. In fact, if they don't receive enough chill hours then blossoming can be delayed or not occur at all. It can even happen that the female flowers on a tree will bloom at a different time than the male flowers, which is sort of like going on a date but getting the time or even day wrong. So, we should be happy that winter is scheduled to return.
In fact, my guess would be that until this week -- although our temperatures have not been particularly cold at night -- that our plants have been building up chill hours just fine. The reason is that chilling hours are optimal for plants between 37 and 45 degree Fahrenheit or about 3 to 7 Celsius. Plants can still "chill-out" so to speak if temperatures reach up to 12 Celsius, but above about 18 C they lose chilling hours. Moreover, if temperatures get much below 3 Celsius, plants don't gain any chill hours. So, perhaps, our winter temperatures have been just right. Depending on plant type, fruiting plants and trees need between 200 and 800 hours. If we've had optimum chilling nights since December, then we've already had close to 600 cold hours.
Jerusalem forecast (click here for updated national forecasts):
Image credit: The Jerusalem Herald
By the way, if you don't think that cold hours are important, and that fruiting trees need to bloom on time, then take my own situation as an example. The first night I met my wife, we had agreed to meet at the "Off the Square" restaurant. I arrived on time and waited dutifully (but with expectation) for my date to arrive. The seconds clicked off my watch and a half hour passed.
Then a miracle happened. I called my wife-to-be’s mother and spoke fluent and understandable Hebrew. I don't think this has happened again (that's why she speaks English to me unless she's angry). Her mother hung up and made a quick phone call. My poor wife-to-be was waiting for me too, but she was waiting in the wrong place. Not realizing that the restaurant had moved, she figured that it was time to head home. True, this was more a problem of space, rather than time, but another moment (in time) and we might not have met -- but her phone rang and it was her mother telling her that I was waiting outside. I was accumulating chill hours in the hope that spring would soon come.
That's a bit sappy, but true.
Dr. Lynn is a lecturer at The Hebrew University of Jerusalem, Earth Sciences Department. He is also CEO of Weather It Is, LTD, a company that specializes in reducing weather risk.