Thursday, October 6, 2022

Inches of Mercury = Feet of Water

In the HVAC business, we measure air pressure in inches of water.  In the weather business, we measure it in inches of Mercury, which is 13 times heavier.

When I was a technician in the air handling lab at Carrier, we used Merriam Inclined Manometers to measure air pressure in ducts as well as pressure in Pitot tubes to measure airflow (Pitot tubes are also used to measure airspeed in airplanes). The pressures were small and measured in inches of water.  We'd use an inclined manometer such as shown above, so we could detect these tiny differences in pressure.

Play with a drinking straw and a glass of water sometime, and you'll realize how little pressure a few inches water really is.

Atmospheric pressure is about 14 PSI at sea level, or about 30 inches of Mercury.  We live at the bottom of a gravity well, and are being smothered with a layer of air which bears down on us.  But, like fish at the bottom of the ocean, we don't notice this pressure as it is equalized within our bodies.  Sure, once in a while we notice changes in pressure when going up or down mountains - our ears "pop" to equalize the pressure inside.

The recent hurricane flooded Ft. Myers beach and the videos were pretty horrific - water ten feet deep washed over the island in parts, with waves several feet high.  Houses on the ground or even older houses on stilts, stood no chance.  Only modern houses made of concrete and built to hurricane standards, withstood the storm.

But why was the water to high?   Well part of it was that the storm hit at high tide.  Part was the wind itself blowing the water in its path.  But part of it, I think, is due to barometric pressure.  Again, an inch of Mercury is equal to about 13 inches of water (Mercury is 13.5 times as dense) so if the barometric pressure drops by one inch of Mercury, the atmosphere is basically sucking up the ocean a whole foot.

Normal barometric pressure is about 29.9 inches of Mercury.  A "normal" range is about 29.6 - 30.2 inches Hg (752-767 mm Hg) at sea level.  Rarely (at sea level) do readings exceed 30.4 inches Hg (773 mm Hg) except for occasional arctic highs in January. Rarely (at sea level) do readings fall below 29.5 inches Hg (749 mm Hg).

But during a hurricane, pressure can drop precipitously.  For each category of hurricane, the pressure can drop further:

2 — Moderate: 28.50 to 28.91 inches of Hg
3 — Extensive: 27.91 to 28.47  inches of Hg
4 — Extreme: 27.17 to 27.88  inches of Hg
5 — Catastrophic: less than 27.17  inches of Hg
If "normal" pressure is around 30 inches of Mercury and it drops to 27 inches, that equals a delta in pressure of over three feet of water.  It is not hard to see how that, combined with the wind, rain, and tides, can cause the ocean to rise to catastrophic levels.  Even without a hurricane present, changes in pressure can result in abnormally high or low tides.

We were fortunate on Jekyll Island that the hurricane went off the coast (which they tend to do, as a result of the pattern of the Gulf Stream as it passes Georgia) and also that it went by at low tide.   We've had high tides before where the water seems to be within a foot or so of the roadway.  Throw in wind, rain, and low pressure, and water could easily wash over the island as it did back in the late 1800s.

It is not a matter of "if" but when a hurricane will wash over a barrier island.  Barrier islands, by their nature, are not permanent.  Our little island, for example, is migrating Southward.  People who bought "beach homes" on the South end of the island are now about a half-mile from the beach.  Meanwhile, the North end erodes.  Brides come to "Driftwood Beach" at the North end to get their wedding photos taken (and sometimes get married there!) but it is not a beach of driftwood.  Rather, as the island has eroded, wave upon wave of old oak and pine trees were inundated by the ocean, which killed the trees and eroded their root base, until they fell over and became picturesque "driftwood" even though they hadn't done any drifting.

Our island has also moved Westward, as barrier islands on the East coast tend to do.  The State of Georgia spent a lot of money hauling in "rip-rap" (rocks) to try to quell the erosion (and save a few homes and hotels).  In addition to the staggering cost (not only for the rocks and their placement, but the destruction of the roads as dump truck after dump truck brought rocks for almost a year) there are other issues with rip-rap.  Where it finally ends, it tends to accelerate erosion locally. And some say it tends to cause sand to wash away, so nothing is left but the rip-rap - or a rock beach.

Other methods have been tried and used on barrier islands.   Pumping sand from out deep and spraying up on the beach is one technique that is quite popular, particularly in Florida.   But such techniques provide only short-term results.  Sand eventually finds it own level, and if you scour out a hole in the ocean, a half-mile offshore, the sand you pumped to the beach will eventually find its way back to that hole.  Any child building sand castles on the beach could tell you that.

Some argue that maybe we shouldn't be developing all this land so close to the sea.  And there is a valid argument there.  Back in the day, developers would bulldoze the barrier dunes and build right on the beach, mere feet above high tide.   Houses in places like Kitty Hawk have fallen into the sea as a result.  In more recent years, better building practices and better construction practices have been enacted.  It is a no-no to even walk on some barrier dunes, lest you destroy the fragile plants holding them in place.

Sand is a funny thing.  We visited Saugatack Michigan, home of the "Dunes" - which we found out were entirely artificial.  Seems they clear-cut the forest to build a port town on Lake Michigan, but the sand started blowing once the trees were gone, burying their new town and building a mountain of dunes near the shore.   It has happened more than once on the Great Lakes, too.

So, building a house on sand is an act of faith.  Even paving a road on sand is an exercise in futility.  Building on sand near the ocean - well, expect to get washed away.   Back in the day, people put up "beach shacks" and expected them to be washed away on occasion.  When that happened, you gathered up more scrap lumber (probably from the shack that blew onto your property) and built again.  Today, we are driving concrete pilings into the sand and building concrete houses with cat-5 hurricane windows - and so far, they seem to be surviving these storms.

I wonder, though, looking at these videos of Ft. Myers Beach, whether some of these houses - and hotels and condos - really "survived" the hurricane.  The agitation of the water, scouring at the foundations and pilings, surely must have had some effect on the structural integrity of these buildings.  What's more - the beach has washed away in parts and is now lapping at the door of some of these high-rises. And some of these high rises were not high enough - the first floor, eight feet above the ground, was still flooded and utterly washed out.  How high do we have to build?   10 feet?  15 feet?  20 feet?

I guess we will have to wait and see.  I suspect that given how much money is involved, they will pump sand onto Ft. Myers Beach to build a buffer between the condos and hotels (including the new Jimmy Buffet Margaritaville under construction!) and perhaps put in sea walls or rip-rap.  It is a bandaid stop-gap measure, but the people with money will make sure it happens.

And the new houses going up will be "hurricane-proof" to the extent possible, or the insurance companies won't write on them.   The cost of insurance will become even more staggering and this will create a dichotomy of haves and have-nots.  Estero island (Ft. Myers Beach) will likely move more upscale, as only the very wealthy will be able to afford the staggering property taxes and hurricane insurance.

The have-nots?  They will likely have to live with the modern equivalent of the "beach shack" - the RV, Mobile Home, or Park Model - that is paid-for in cash and uninsured.  Like the beach shack of yore, when it blows away, you just shrug and buy another one, for cheap.

All that being said, if you live near water and the barometric pressure drops by an inch or more, seek some high ground, because that water is going to be sucked up by the sky, a foot or more!