Not sure how people in the Central Valley (California) are feeling these days weather-wise, but for me it is a satisfying sight today – a neat blanket of clouds over the Central Valley as seen in this MODIS satellite image!
The milky white layer over the valley that you see in the attached image is not snow; it is clouds, locally called Tule fog, which often forms in winter under a high-pressure system. It has persisted for several days. The Central Valley is like a bowl, and it traps anything that forms or comes within it.
You might be wondering at what height these clouds/fogs are. We could look at radiosonde data (from weather balloons) to find that out, but unfortunately the Central Valley does not have a radiosonde station (you know it is a poorer region despite being the producer of more than 25% of the food in the US). Still, we can estimate the cloud height from topography: the surrounding mountains rise to around 1 km, so we can say these fog/clouds are below that level.
But the valley does not only trap clouds or fogs. It also traps air pollutants like ozone and particulate matter. When winds are stagnant and an inversion sets in, pollutants from vehicles, industry, and agriculture accumulate. That is why many cities in California (e.g., Bakersfield, Modesto, Fresno) rank among the most polluted in the United States.Activate to view larger image,
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