The magic of the winter sunrise (...so, we have to start waking up early?)

A word on seasonal variation of colorful sunrises/sunsets in California

"Red sky in morning, sailor take warning..."

Some of our most observant Escaypers have realized that a lot of the good winter skies seem to be sunrises.

This winter, our sunsets seem to be filled with disappointment. Lots of 70% skunk, if not 100% skunk. "Very few clouds are expected to light up," the Escaype app says, day after day, all across the forecast region. Sure enough, the clouds are there, looking beautiful, but sunset comes and nothing happens. 

We've had better luck with sunrises. Sure, there are plenty of skunk days, but there have been several 100/0 days. With less fog than the summer months, and the sun rising in the east-southeast, it's a perfect time to shoot sunrises in many of our favorite places along the coast. 

But, of course, a lot of us aren't morning people. Work starts early enough, traffic sucks, and setting an alarm for sunrise only adds insult to injury. 

Those of you who have been with us for a while may recall that in the summer, when there were good clouds in the afternoon, they burned much more frequently at sunset. Some might even say we had better luck with sunsets in the summer than we've had lately.

What's going on? Does Mother Nature hate us? Or is Jeff just trying to make troll you and make you wake up early?

Turns out it's a real phenomenon, and there's a scientific explanation for it. 

First, we have to understand a couple of the general patterns in the atmosphere in our area. In general, our weather features move from west to east. Sometimes it's from the northwest, sometimes southwest, but it almost always has a westerly (from-the-west) component. These winds are known as the prevailing westerlies, and dictate the flow of weather systems between ~30 and ~60 degrees latitude. 

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A little science background for the prevailing westerlies (optional):

In the atmosphere, temperature differences create pressure differences, which create winds. The reason for this circulation is the found in our earth's Hadley Cells, which guide much of the general circulation in the atmosphere. It starts with temperature. The equator receives the most solar warming, so air rises from the surface, leaving low pressure in its wake. There's less air at the surface than there was before, so the pressure falls. The poles are just the opposite: less sunlight, less heating, so the air sinks and creates high pressure. In between, areas around 30 degrees latitude typically have high pressure at the surface, and at 60 degrees we find low pressure. (This is why most of Earth's deserts are found around 30 degrees, and many of the wettest areas are around 60 degrees and the equator. The Bay Area is 37-38 degrees latitude; Los Angeles is 34. Note that LA is drier, and NorCal is wetter, and gets wetter still as you go north towards the Pacific Northwest.)

Air naturally flows from areas of high pressure to areas of low pressure. It will act to even things out. (Think of a a balloon; you know how badly that air wants to leave the high pressure inside and get out). So, at the surface, we'd expect the winds to flow away from the high pressure at the poles, or from north to south; and also to the south towards the low pressure near the equator, and to the north from the 30-degree highs to the 60-degree lows. We call this driving force the pressure gradient. But things get a little more complicated because Earth rotates, so the Coriolis effect must be considered. 

The Coriolis "force" directly opposes the high-to-low pressure gradient, and, using a geostrophic wind approximation, points 90-degrees to the right of the direction of motion. Between 30 and 60 degrees latitude in the northern hemisphere, this leaves us with a northerly Coriolis "force" and a westerly wind. In the troposphere, except near the ground, the flow is typically very close to geostrophic, and the winds are indeed westerly. 

If a little bit of that went over your head, don't worry -- it's a couple hours of lectures to explain fully. Let's go back to the photography part. 

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Another thing we note about most low-pressure weather systems, which typically bring clouds, is that they tilt westward with height, meaning the low pressure at the surface (think: low clouds and rain) is further west than the low pressure in the mid-troposphere (think: mid-level clouds), which is further west still than the low pressure in the upper troposphere (think: high clouds). Also recall the prevailing westerlies: the weather system as a whole moves from west to east. This is why, during most winter weather systems, we will see leading high clouds before the storm, then the high clouds will gradually lower into mid-level clouds, then the low clouds and rain finally arrive. For some storms, the rain can be at least 12 hours behind the leading high clouds. For other storms, the low pressure trough is sloped more steeply, and there may be just a thin band of leading high clouds. Occasionally, there's none at all. 

So, what does this mean for our skies? Remember, the sun sets in the west... 

If we have a sky filled with high or mid-level clouds ahead of an approaching weather system, more often than not, the clouds will be lower to the west and high/thin to the east, because the bulk of the storm is still out to sea. This setup causes a skunk at sunset and a burn at sunrise, and it is why there are more dramatic sunrises than sunsets in winter. 

As the figure shows, if the timing of the system is right, a massive sunrise burn can happen. If the timing is off, and the rain is overhead at sunrise, then we probably won't get anything. But, regardless of the timing of the system, this setup will almost never yield a good sunset, because it will be usually blocked by thick clouds out to sea no matter what's over your head. 

As the storm passes, there may be more mid and high clouds than just the leading bands. But they often pass over while it's raining, so we're socked in and get nothing. By the time the low clouds clear, the juicy stuff is often long gone. Sometimes, storms will weaken just before they reach us in California, so the low clouds and rain band may be thinner than the clouds above, or never come at all, so we can get some mid and high clouds without being socked in. In this case, and usually only in this case, do we get a good winter stormy sunset with a major weather system. They're often localized, as the burn is only visible from places that aren't socked in by low clouds. 

Some winter sunsets can happen with upper-level weather features; for example, an area of low pressure in the upper troposphere that causes high clouds but never extends below 20,000 feet, so we get the high clouds and nothing else. These often occur when major weather outlets forecast "mostly sunny" skies -- it's because there is no threat of rain. But it may give you exactly what you're looking for as a photographer.

With the major systems, though, sunrises are much more likely to work out than sunsets. 

How do you know when it's going to be one of those exception cases, where the rain band breaks up before the storm makes landfall, or there's no low clouds at all out there? Unfortunately, it's nearly impossible to do -- unless you have the help of a model that analyzes what's happening at all the different heights in the troposphere, to see just what kind of setup we're dealing with, and if it's expected to work out. 

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But what about summer? 

In the summer, the weather features in the Northern Hemisphere in general are weaker, and the atmospheric flow is weaker. This is largely because much more heating is occurring at the poles in the summer than the winter, so the polar high pressure weakens, and the overall circulation is not driven as strongly. 

This allows a certain weather pattern to set up, with high pressure nearly stationary over the eastern Four Corners region, which drives a clockwise circulation that pulls moist air from the Gulf of Mexico and Gulf of California up towards our area. It's known as the North American Monsoon, and it causes thunderstorms over the Sierra and much of the Southwest and Mexico, occasionally making it into California. When this happens, the clouds are coming from the southeast, so when they make it as far west as the bay area, they're likely to burn at sunset, because there's probably nothing but clear skies (except for possible fog, which as we explained in a previous article, does not block a burn) to the west, so the light isn't blocked. You won't see this setup in winter, with the stronger temperature gradient from the equator to the north pole. 

Also, because of the weaker circulation in summer, we don't really get many strong Pacific weather systems, so the kinds of epic sunrises that we see in winter become increasingly rare. 

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Remember, these are general guidelines for a "textbook case". Many real-life weather systems follow textbook cases fairly well, but others are curveballs. The models from which we pull raw data for our Escaype models usually do a pretty good job at handling these systems -- a far better job than any of us could do on our own with satellite imagery and trends. But it's a handy trick to making educated guesses, and now you can tell yourself, no, you're not crazy -- more often than not, sunrises really are where the party's at in winter. 

And remember, the sunrises will never be later than they are right now (I mean, 7:20am really isn't that bad). And nothing wakes you up better than an incredible sunrise, and capturing a portfolio shot. 

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