Above the Clouds: Forecasting Inversions
Everyone has probably come across an image of someone posing on a mountainside above the clouds with a dramatic sunrise in the background. Few experiences are quite as jaw dropping as starting out on what looks like a gloomy day hiking up the trail eventually seeing the sun bleed through the clouds and then reaching the summit to find crystal clear skies and an ocean of clouds below. Is coming across these sort of conditions just luck or is there a way to predict find yourself above the clouds?
Well, the answer is actually a little bit of both. We call this phenomenon a cloud inversion and it is actually more common than you think, especially depending on where you live. I will dive into the details on what cloud inversions are and how they occur so you can better predict the generally unpredictable nature of these beautiful low cloud decks. This guide will hopefully help you be able to forecast, predict, and eventually find yourself enjoying a sunset or sunrise above the clouds. It is a true bucket list experience!
Where do Cloud Inversions Occur?
First thing to note is for the sake of this post, while cloud inversions can happen in many different places I am going to focus exclusively on cloud inversions of the West Coast as they are much more common, predictable, and familiar to me. Having said that, the most common spots to encounter cloud inversions on the West Coast is near and along the Pacific coast. The closer to the ocean the more common these cloud inversions occur. In some cases, if the conditions are prime enough the inversion can be pulled further inland typically terminating at the crest of the highest mountains for example in the Pacific Northwest, from the crest of the Cascades westward. Further south in California, areas west of the crest of the Coast Ranges are most common.
How do Cloud Inversions Occur?
Three main factors go in to allowing cloud inversions to occur; air temperature, ocean temperature, and atmospheric winds. Essentially these three processes interacting together will produce a cloud inversion. The first thing to note is time of year is everything. Whether you are in San Diego or Seattle cloud inversions tend to happen in the warmer months from late spring to early fall, though that's not say they don't happen in winter but it tends to be less often and less predictable. Why summer though?
The answer goes back to the three processes on going in cloud inversion formation. On the West Coast, the Pacific Ocean is quite chilly no matter the time of year. Strong currents from Alaska are pulled south down the West Coast keeping the water cold (yet nutrient rich) year round. This doesn't mean too much most of the year, especially when the air temperature is similar or lower than the ocean temp (ocean temp typically in the 50s). In the summer months however the air temperature becomes much warmer than the steady ocean temperatures. This allows evaporation to occur above the ocean and surrounding coastlines. As night falls and the temperature drops, the water vapor in the air condenses into a shallow deck of stratus clouds, known as a marine layer. This cloud deck generally sits just off the ground (maybe 1,000 feet above sea level) with the cloud tops only reaching 3,000-5,000 feet above sea level. In addition for this to occur the surface winds need to be very light to nearly non existent which is much more common in summer on the West Coast. Winters are typically too stormy and active.
Now that we have a cloud deck over the ocean we need to somehow pull those clouds onto land. How does that happen? The answer lies in the atmospheric winds above. Like I mentioned before during the warmer months the jet stream (atmospheric winds) tends to weaken as there are less extreme temperature differences in the northern Hemisphere between the arctic and tropics. What is a long wet stormy period of low pressure system (stormy weather) after low pressure system from October to April, is replaced with a period of high pressure systems (calm weather) as the jet stream weakens. Unlike low pressure systems that are generally progressive being pushed along by a fast jet stream, the high pressure systems of summer tend to get stuck and hang around for weeks sometimes months at a time.
During these periods the days are warm and sunny and the winds are calm. That is not to say there is no wind at all. When the summer highs set up in the Pacific Northwest the winds in the atmosphere though light can blow in from the ocean, which we call onshore flow. This onshore flow will push the marine layer from the ocean into the surrounding coastlines. Most summer mornings on the West Coast (Seattle, San Francisco, Los Angeles, etc) begin with grey gloomy skies and cool temperatures infamously known as the June Gloom. However, the grey gloom tends to only last a few hours as by lunch time the intense summer sun will burn off the thin shallow layer of clouds resulting in a glorious mild sunny afternoon.
Depending on the strength of the high pressure system the cloud deck can get drawn deeper inland reaching the crest of the Cascades or Coast Ranges sometimes even reaching the Sierra in the interior of California. It is during those stronger events when you can find yourself strolling along an airy ridgeline in the Cascades, high above the clouds below.
Witnessing these events is one of those things you just have to see to believe, it is unlike even flying above the clouds on an airplane. Since you are grounded in one place you can really sit and watch the clouds move and interact with the topography. Sometimes the clouds seem to roll in wave like patterns, other times they seem to drift aimlessly in and out, in and out. In other cases the clouds will roll over a ridgeline and evaporate before reaching the other side, and sometimes they rise and descend as if the Earth is breathing. Just absolutely incredible!
To review, during calm clear weather in summer, warm air temperatures combined with cool ocean temperatures create low clouds that are then pulled inland due to onshore flow. This is how you get cloud inversions. The behavior of cloud inversions is finicky and delicate with no ever being a guarantee, but keep an eye on these factors and your chances of encountering this phenomenon is greatly increased.
Below is a gallery of images I've taken over the years from cloud inversions I have encountered. Enjoy!
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