February was a wild month for us–weather-wise, at least. The month started with 10 straight days of fog, including 7 days of dense fog — where visibility dropped to 1/4 of a mile. We only had one series of storms hit us in February, but it was a whopper… dropping feet of snow in the Sierra Nevada, lowering snow levels to the foothills, and bringing over 2″ of rain to Fresno, Madera, and Merced in just 3 days in the middle of the month. We then ended the month on a warm note, and records were broken: not record highs, but 4 (including one tie) records for warm overnight LOWS. The 80 we hit on the last day of the month wasn’t a record high, but it was only the 14th time Fresno has ever recorded an 80 temperature in February — ever! And records go back over 140 years in Fresno.

Even though the old expression goes ‘ March – in like a lion, out like a lamb’, don’t look for any wild weather as we start the month of March. The sunshine will continue, but temperatures will cool back down to near average for this time of the year in a few days. We should be looking at highs in the 60’s by the middle of the week.

There IS still a chance of fog for the next couple of days: a little patch of fog could form early Sunday morning near Visalia and Tulare, and Monday morning could bring a little more widespread fog, but still not much. Any fog that does form won’t last long, as we’re really at the end of our fog season now.

And what a fog season it’s been! 40 days of dense fog this season in Fresno, which is the most we’ve had in 23 years. Our days of dense fog have trended down quite a bit in the past 30 years. Since the early 1990’s, we’ve been averaging only 24 days of dense fog per season in Fresno. Compare that to what we used to see in the 1960’s, 70’s, and 80’s: 40 days! So, 50 years ago, fog like we saw this winter was typical, not unusual!

Dense fog days in Fresno have dropped 40% in the past 30 years, going from an average of 40 per year back then to only 24 dense fog days per year now (this year notwithstanding). Further south, dense fog days in Bakersfield have dropped 33%, going from an average of 24 in the 60s, 70s, and 80s, to only 15 now. So, why has there been such a big drop in dense fog days in the past 30 years?

It’s not from just one reason alone, but a combination of reasons. The first is agriculture. Back in the 1960s and 70s, row crops (like tomatoes or cotton) ruled the San Joaquin Valley, and there were few orchards of almonds, pistachios, or walnuts. Times have changed since then, and orchard acreage in the Valley of those nut trees has soared, growing from around 200,000 acres grown in the 1970s to over 1,700,000 acres now.

Forming dense fog requires two main ingredients: long, clear, cool nights, and plenty of moisture. After row crops are harvested in the fall, the fields often sit fallow through the winter. The tilled soil absorbs rain easily, which evaporates on a sunny winter afternoon, loading the atmosphere up with moisture to form fog later that night. However, many times the soil of an orchard is more hard packed by harvesting and other farm equipment, making it harder for rain to soak into the ground. Instead, it runs off into storm drains, and leaves the surface drier, giving the atmosphere less moisture to work with to form fog.

The second major reason for less fog lately is that our temperatures have warmed up in the past 30 years. To form fog at night, you need the temperature to cool down to the dew point. When it does, the humidity reaches 100%, and fog begins to form. If you raise the overnight temperatures, you’ll cool down to the dew point less often in the winter… which will lead to less fog. In the past 30 years, Modesto has seen their average overnight low temperature in the winter months (November through February) warm almost 1.5 degrees! Fresno has seen their average overnight low in the winter months rise 2.4 degrees in the past 30 years… leading to a dramatic reduction in fog.

This season, rain started early, and by Thanksgiving we were off to our wettest start to a rain year ever… which led to our very foggy winter. We’re not going to do that every year, so foggy winters like this one will become increasingly rare going forward. Eventually, we’ll be able to tell our kids, “I remember when the fog was so thick for days on end…” and they may not believe us!