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What’s the Difference Between a Stalactite and a Stalagmite?

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Well-meaning geologists ended up confusing plenty of folks when they named stalactites and stalagmites. Both of these similar-sounding structures—typically formed in limestone caves—are capable of stretching over 27 feet in length. But what's the difference between them, and how do such strange ornaments grow in the first place?

Let’s clear up the terminology. Here’s a helpful (and widely-used) phrase you can use to sort out which is which: “Stalactites hold tight to the ceiling and stalagmites might touch the ceiling.” In other words, stalactites form on the roofs of caves and dangle downward like rocky icicles. Stalagmites, in contrast, are based on the floor and stretch upwards, only occasionally coming into contact with the overhanging ceiling.

An alternate memorization method goes as follows: “stalactite” is spelled with a “t,” as in “top.” “Stalagmite” uses the letter “g," as in “ground.”

According to the Oxford English Dictionary, these two terms descended from the Greek word stalaktos, which means “to drip." This is because trickling rainwater is responsible for the conical objects’ formation. When rain seeps through limestone, the water extracts carbon dioxide gas from the rock. What results is a weak carbonic acid that penetrates the stone and deposits a patch of calcite on the caves’ roof. As the water continues dripping, more and more calcite is added to the spot, eventually producing a lengthy stalactite.

But what about stalagmites? There’s a reason these are generally found directly underneath stalactites—all that dripping water has to land somewhere, after all. When a drop finally hits the cave floor, it deposits even more calcite there, this time in an unassuming mound. The liquid keeps dripping off the tip of a stalactite and the lump keeps rising, leaving us with a surging stalagmite. To call this process gradual would be a gross understatement. In limestone caverns, the usual growth rate is under 10 centimeters per millennium. 

It’s also worth noting that both stalagmites and stalactites belong to a larger geological group known as “speleothems." This is an extensive family of differently-shaped mineral formations that also includes globular “cave popcorn” and stunningly-beautiful “flowstones." Additionally, lava is occasionally involved in stalactite creation, leading to some strange-looking results. 

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Big Questions
Where Is the Hottest Place on Earth?
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The summer of 2017 will go down as an endurance test of sorts for the people of Phoenix, Arizona. The National Weather Service issued an extreme heat warning, and planes were grounded as a result of temperatures exceeding 120 degrees. (Heat affects air density, which in turn affects a plane’s lift.)

Despite those dire measures, Phoenix is not the hottest place on Earth. And it’s not even close.

That dubious honor was bestowed on the Lut Desert in Iran in 2005, when land temperatures were recorded at a staggering 159.3 degrees Fahrenheit. The remote area was off the grid—literally—for many years until satellites began to measure temperatures in areas that were either not well trafficked on foot or not measured with the proper instruments. Lut also measured record temperatures in 2004, 2006, 2007, and 2009.

Before satellites registered Lut as a contender, one of the hottest areas on Earth was thought to be El Azizia, Libya, where a 1922 measurement of 136 degrees stood as a record for decades. (Winds blowing from the nearby Sahara Desert contributed to the oppressive heat.)

While the World Meteorological Organization (WMO) acknowledged this reading as the hottest on record for years, they later declared that instrumentation problems and other concerns led to new doubts about the accuracy.

Naturally, declaring the hottest place on Earth might be about more than just a single isolated reading. If it’s consistency we’re after, then the appropriately-named Death Valley in California, where temperatures are consistently 90 degrees or above for roughly half the year and at least 100 degrees for 140 days annually, has to be a contender. A blistering temperature of 134 degrees was recorded there in 1913.

Both Death Valley and Libya were measured using air temperature readings, while Lut was taken from a land reading, making all three pretty valid contenders. These are not urban areas, and paving the hottest place on Earth with sidewalks would be a very, very bad idea. Temperatures as low as 95 degrees can cause blacktop and pavement to reach skin-scorching temperatures of 141 degrees.

There are always additional factors to consider beyond a temperature number, however. In 2015, Bandar Mahshahr in Iran recorded temperatures of 115 degrees but a heat index—what it feels like outside when accounting for significant humidity—of an astounding 163 degrees. That thought might be one of the few things able to cool Phoenix residents off.

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Big Questions
How Does Autopilot Work on an Airplane?
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How does autopilot work on an airplane?

Joe Shelton:

David Micklewhyte’s answer is a good one. There are essentially a few types of features that different autopilots have. Some autopilots only have some of these features, while the more powerful autopilots do it all.

  • Heading Hold: There’s a small indicator that the pilot can set on the desired heading and the airplane will fly that heading. This feature doesn’t take the need for wind correction to desired routing into account; that’s left to the pilot.
  • Heading and Navigation: In addition to holding a heading, this version will take an electronic navigation input (e.g. GPS or VOR) and will follow (fly) that navigation reference. It’s sort of like an automated car in that it follows the navigator’s input and the pilot monitors.
  • Altitude Hold: Again, in addition to the above, a desired altitude can be set and the aircraft will fly at that altitude. Some autopilots have the capability for the pilot to select a desired altitude and a climb or descent rate and the aircraft will automatically climb or descend to that altitude and then hold the altitude.
  • Instrument Approaches: Autopilots with this capability will fly preprogrammed instrument approaches to the point where the pilot either takes control and lands or has the autopilot execute a missed approach.

The autopilot is a powerful computer that takes input from either the pilot or a navigation device and essentially does what it is told to do. GPS navigators, for example, can have a full flight plan entered from departure to destination, and the autopilot will follow the navigator’s guidance.

These are the majority of the controls on the autopilot installed in my airplane:

HDG Knob = Heading knob (Used to set the desired heading)

AP = Autopilot (Pressing this turns the autopilot on)

FD = Flight Director (A form of navigational display that the pilot uses)

HDG = Heading (Tells the autopilot to fly the heading set by the Heading Knob)

NAV = Tells the autopilot to follow the input from the selected navigator

APR = Tells the autopilot to fly the chosen approach

ALT = Tells the autopilot to manage the altitude, controlled by the following:

VS = Vertical Speed (Tells the autopilot to climb or descend at the chosen rate)

Nose UP / Nose DN = Sets the climb/descent rate in feet per minute

FLC = Flight Level Change (An easy manual way to set the autopilot)

ALT Knob = Used to enter the desired altitude

This post originally appeared on Quora. Click here to view.


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