Weather Fair And Foul: Mexico’s West Coast; Remote North Atlantic Islands


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The Faroe Islands

The Faroe Islands are the tiny speck north-northwest of Scotland in the upper right corner of the weather map. It was between developing storms this morning and those brave enough to venture there were rewarded with a glimpse of the three-minute total solar eclipse. Analysis courtesy of NOAA.

Every Friday, Decoded Science takes you to places that are nice — and not so nice.

This week we feature the (nice) weather along the west coast of Mexico, and the (not so nice) weather in the Atlantic Ocean at sixty-two degrees latitude.

Why those two places?

The first is a no-brainer: The weather’s gorgeous.

The second one: Well, there was a reason to be there earlier today.

A Rare Celestial Event

Occasionally, a massive fusion furnace (the sun) lines up with a piece of airless rock (the moon) to provide a spectacular visual display for the inhabitants of the only body in the universe known to harbor sentient life.

A few hours ago, the moon passed between the earth and the sun, and those in the right spot saw the sun entirely disappear for up to three minutes.

A Remarkable Coincidence

It is an amazing fact that the sun, much bigger, and the moon, much closer, appear the same size from the earth. Because the orbit of the moon is not round, it sometimes appears bigger and sometimes smaller than the sun. This eclipse took place only 14 hours after the moon had reached perigee, its closest approach to the earth, and thus the eclipse was total and lasted for the maximum time possible.

When the moon is farther from the earth, the eclipse is ‘annular’ — a ring of the sun is visible around the moon.

The path of totality passed directly over the Faroe Islands, about 200 miles north-northwest of Scotland and about half way between Iceland and Norway. Except when there’s an eclipse, you probably wouldn’t want to visit.

Perpetually Foul Weather

Though the Faroes are located in the path of the Gulf Stream, that river of warm water has weakened to the point where water temperatures are in the 40s in the winter and 50s in the summer. Air temperatures don’t stray much from the water temperatures.

What makes The Faroes a gloomy place is its location in the path of a wet oceanic flow. Rain falls 260 days per year; yet the Faroes’ annual rainfall is only 50 inches, not much more than New York’s, and less than Miami’s.

An occasional powerful storm lashes the Faroes, but mostly there’s a dreary daily drizzle.

Showery weather with a few breaks in the clouds this morning allowed those intrepid enough to venture to this bleak place a fleeting glimpse of the sun in total eclipse at 9:46 local time (5:46 in New York).

On To Nicer Places: The West Coast Of Mexico From Puerto Vallarta To Lazaro Cardenas

The weather on the Mexican west coast from 18 to 20 degrees north latitude has a distinctly schizophrenic personality. Fair to the point of boredom in the winter, it turns frog-strangling wet (the step above ‘raining cats and dogs’) in the summer.

In Puerto Vallarta, the average March rainfall is three-hundredths of an inch, and the average number of days in March on which rain falls, to the nearest integer,  is ZERO. This contrasts with rain on half the days and a total of over six inches in July.

Pacific Ocean weather at 20 degrees latitude

Dry high pressure dominates Pacific Ocean weather at 20 degrees latitude. The Mexican coast is at the far right. Analysis courtesy of NOAA.

In the winter, this section of the Mexican coast near 20 degrees latitude enters the global desert belt which circles the earth at an average position of 30 degrees latitude.

In the summer, tropical weather takes over, with high humidity and frequent thunderstorms. An occasional tropical storm blows in off the ocean, but for the most part the storms stay offshore.

In March, temperatures along the coast from Puerto Vallarta to Lazaro Cardenas are in the pleasant low 80s in the daytime and comfortable-for-sleeping low 60s at night.

By July, the temperatures reach a humid lower 90s by day and stay in the sticky upper 70s at night.

The water temperature in March is in the mid 70s. Storms in the middle of the Pacific Ocean create large waves for excellent surfing, but there are also protected coves suitable for leisurely swimming.

The following was contributed by Geoscientist Jennifer Young:

Caption: Earthquakes of at least magnitude 6 since 1915. Image by USGS.

Caption: Earthquakes of at least magnitude 6 since 1915. Image by USGS.

Tectonic Activity On The West Coast Of Mexico

The section of coast stretching from Puerto Vallarta in the north to Lazaro Cardenas in the south may be a tropical paradise for a weatherman, but you might want to take your hard hat and brush up on your earthquake drill.

In the past century, this short stretch of coast has experienced 144 earthquakes of at least magnitude 5, as recorded on the United States Geological Survey’s real time earthquake map. Thirteen of these have been of M7.0 or larger; other earthquakes may have gone unrecorded.

What’s Causing The Earth To Shake So Much?

Tectonically speaking, this part of Mexico lies at the northern end of the subduction zone which runs the length of Central and South America and is known in this area as the Middle America Trench. (Not far to the north the boundary becomes one dominated by lateral movement, the San Andreas fault zone.)

The subduction zone, where the oceanic Cocos plate meets and descends beneath the South American continent, is represented on the map by the red line which follows the coast. The second red line, further offshore, is the boundary between the Cocos and Pacific plates.

Although earthquakes occur across the map area they are of different types, because the tectonic mechanisms are different. The oceanic ones are largely caused by the plates moving apart, while those along the subduction zone are caused by the plates moving together. Those of the latter type tend to be larger and include all but one of those bigger than M7.0.

The Mexican earthquake of 1985

The Mexican earthquake of 1985 caused massive damage. Photo courtesy of USGS


The Region’s Biggest Quake: M8.0

Though frequent and damaging, these earthquakes have killed perhaps fewer people than might be expected, although no detailed figures are available.

The outstanding exception is the 1985 M8.0 earthquake, whose epicenter was in Colima province, in the middle of this stretch of coast. The official death toll was 9,500 although the USGS notes that “According to some sources, the death toll from this earthquake may be as high as 35,000”.

Many of these deaths were in Mexico City, hundreds of miles from the epicenter. Historically, Mexico City is built upon the site of a former lake; the effect of this particular earthquake was to amplify the shaking of the ground — and the buildings upon it.

A Spectacular Sight; Don’t Get Too Close

Subduction zones have plenty of volcanoes, and right now the Colima volcano is in full flow. Some pretty good fireworks on show, but don’t get too close — or you’ll definitely need that hard hat.

It’s Been A Busy Week

Vernal equinox, start of Spring, total eclipse of the sun, and moon at perigee (technically it’s a Supermoon because it looks so big, but it’s not as spectacular as the full moon Supermoons). Wouldn’t it be nice to just lie on a dry, sun-drenched beach with a pleasant breeze and the temperature about 80?

That’s what you’d experience in Decoded Science’s Fair Weather pick of the week in Mexico. But what’s the weather like where you live? Drop us a comment — fair or foul.

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