Friday, August 9, 2013
In many parts of the country, air conditioning is necessary during the summer months. Even in places with high and low temperatures similar to us (usually places that are close to the Canadian border, such as International Falls, Minnesota and Portland, Maine) generally have higher relative humidities. And Chicago? Fu’gett about it. And Texas? Fer’get bout it even more.
For that, I am extremely thankful. I get hot pretty easily, especially after the sun sets, and it’s very difficult, nay, impossible for me to sleep when I am overheated. You can always put on more clothes, but you can’t keep on taking them off. I generally take a cold shower right before bed so I can cool myself off. With a little meditation and relaxation, you can stay calm while the unheated tap water seeps down your back, and when it’s time to get out, you feel oh so good.
This is abnormal. I get pretty hot at night, but most people don’t need the whole cold shower ritual. All people, myself included, benefit from what is called a “marine push” or “onshore flow.” In a typical Western Washington marine push event, cool air off the upwelled coastal waters of the Pacific flows into the lowlands through the Chehalis Gap and Strait of Juan de Fuca. This relatively cool and moist air brings cooler temperatures and clouds during the morning
But why do we even get an onshore flow? It’s not just Seattle – much of the West Coast experiences a similar phenomenon of cloudy mornings and sunny afternoons. San Francisco comes to mind.
|Golden Gate Bridge on May 3, 2009 at 7:33 P.M. – Picture by Wikimedia user runner310|
However, each time I have woken up after an onshore flow event, I’ve noticed that the sky is initially hazy. This haze lessens as the sun burns off the stratus clouds in the morning, and it continues to burn off as the temperature rises into the afternoon. I’ve done some research and I think I have arrived at a correct conclusion. Before I spill the beans and give you the answer, though, let’s go over what an onshore flow or marine push event is and how it forms.
|Mass Ascension from the International Albuquerque Ballon Fiesta. Taken October 13, 2006 by Eric Ward and retrieved from Wikimedia Commons.|
Now let’s take a look at Washington and Northern California sea-surface temperatures. Some parts are not shown because clouds were covering the area at the time of the shot, and these types of satellites cannot approximate the water temperature if clouds are covering the surface.
Much colder water is near the coast. Off Washington, the water is around 50 degrees. Off Brookings (which is actually in extreme southwest Oregon), the water is even colder: 44 degrees in spots. In both locations, there is a massive temperature difference between the water immediately off the coast and the temperatures of the land adjacent to the coast. Since cold air is denser than warm air, a localized dome of high pressure remains on the coast while the pressure drops over the inland areas each day as the air warms and becomes less dense. This creates a strong pressure gradient from high pressure over the coast to low pressure over land during the day, and this creates a very strong seabreeze. At night, the temperatures are around the same, so no breeze occurs. On another note, some places actually experience land breezes, in which the wind travels offshore because the water offshore is warmer than the temperatures on land. This generally occurs at night. The Pacific Northwest doesn’t get much of a land breeze because at night there is not that much of a temperature discrepancy between the land and the ocean.
Often times, these breezes leave evidence that they have occurred, even when they are not actually occurring. Take a look at this picture of a windswept tree off the coast of northern California. Since the pressure gradients off northern California and southern Oregon are very strong due to the strong temperature differences off the coast and onshore in the summer, the area is currently being investigated for the potential for offshore wind farms. There are already plenty of wind farms onshore in that area.
|Photo Credit: Ted Goth – April 2010|
Now, our feature presentation… the marine pushes that are unique to Western Washington.
Due to our sea breeze, all this moist air blows into Western Washington through the Chehalis Gap and the Strait of Juan de Fuca.
As the sun sets and temperatures over Western Washington drop, this moist air off the ocean condenses into stratiform clouds over the lowlands, giving us our classic cloudy mornings.
* All these pictures were based off of ones from a KOMO article on the same thing, which you can find here: http://www.komonews.com/weather/faq/4306832.html. Tanner Petersen, CEO of WeatherOn, did the graphics for me. Send him a thank you by spreading the word about WeatherOn.
Portland actually does NOT get these marine pushes because there is no passage for air to flow through. Therefore, highs are generally 5-10 degrees warmer in the summer there than in Seattle. Marine pushes are not only a function of cold water and warm land… there has to be a pathway for the air to flow through.
My original stipulation for writing this blog was my observation that the visibility was worse in the morning and got better as the day went on. I could not explain this… the amount of particulate matter in the atmosphere didn’t change as the day went on, so why would visibility change? Well, I did some research, and figured it out.
When the relative humidity is high, water is more likely to bond to particulate matter in the atmosphere. When water bonds to particles, it makes them bigger, thus resulting in lower visibility. As the day goes on, the relative humidity decreases due to daytime heating (although the dewpoint stays the same, which means there is the same amount of water vapor in the atmosphere), and less water bonds to particulate matter. Therefore, visibility increases!
I could have just written those last two paragraphs, but I decided to give you the whole spiel. There’s no point in me explaining the finer points of a concept if you don’t have the basics down!
Thanks for reading, and enjoy the lightning tonight!