What Do Pilots Use to Detect Turbulence?

iStock.com/Maravic
iStock.com/Maravic

by Joe Shelton

I think the concept of "turbulence" has gotten a bad reputation.

Just like the wind, turbulence isn't an on or off thing; it's a spectrum.

Step outside one day and a light breeze might be hard to feel, while the next day you might have trouble holding onto your hat because the wind is so strong. But most of the time it's somewhere in between those extremes.

Turbulence is exactly the same. Some days, some places, nothing. Some days it can rattle your teeth (or turn your stomach). But most of the time when turbulence exists, it is light or at worst case slightly annoying.

How do pilots detect turbulence?

Weather forecasts provide estimates when there might be turbulence. The thing to remember is the noun forecast: It's not a guarantee that it will be turbulent, nor is the lack of mention of turbulence in a forecast a guarantee that there won't be turbulence.

In addition, turbulence can be widespread as well as very localized.

I typically detect or expect serious or widespread turbulence as well as local turbulence in one of three of ways:

  • Cumulus clouds: If the clouds are tall and vertical and/or getting taller, then there is a good chance there is turbulence around. The taller the clouds and the faster they are growing, the worse the probable turbulence. Especially in the clouds. The worst example of that would be a thunderstorm. The turbulence within a thunderstorm can tear an aircraft apart.
  • Hot days: Also known as convection, warm and especially hot days mean that the hot air is rising and the reciprocal, cold air is descending. That's a recipe for turbulence. Depending upon the temperature and the aircraft's altitude the turbulence can be irritating or it can be very uncomfortable.
  • Wind: Wind can "tumble," especially downwind of mountains—often for many miles downwind—and it can even be turbulent over mountains, as winds are encouraged to rise following the upwind mountain side.

With very few exceptions, for the most part turbulence isn't dangerous. At least to aircraft. Pilots know how to manage turbulence, often simply by slowing the aircraft's airspeed and/or changing altitude.

However, clear-air turbulence (CAT)—severe turbulence that happens in what otherwise seems to be calm, clear air—can cause injuries to passengers who aren't wearing seat belts or, worse, are walking. And CAT is very difficult to detect until you experience it.

(By the way, clear-air turbulence got its name because although turbulence is often accompanied by clouds, this particular form isn't. Hence the name.)

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

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Why Do We Celebrate Christmas on December 25?

If Jesus wasn't born on December 25, does this rule still apply?
If Jesus wasn't born on December 25, does this rule still apply?
Jon Tyson, Unsplash

Each December, Christians throw a collective birthday bash to celebrate the anniversary of Jesus’s arrival on Earth. But without a birth certificate—or any other official record of his actual birthdate—in existence, December 25 seems like an arbitrary day for all our Christmas traditions. So how did early observers choose it?

When Was Jesus Really Born?

Since the Bible doesn’t name a month or even a season for Jesus’s birth, historians have relied on other context clues to estimate when it occurred. Shepherds tend sheep in the Nativity story, which people often cite as evidence that Jesus was more likely born during the spring. Others argue that Israel’s mild winter temperatures allow sheep to graze even in December. According to Slate, it’s also possible that sheep set aside for religious sacrifices may have been given free rein, frigid night or not.

The Adoration of the Shepherds by Sebastiano Conca, 1720.J. Paul Getty Museum // Public Domain

One clue pointing specifically to December 25 comes from the story of Mary’s cousin Elizabeth, who approached old age without having given birth to any children. One day, her husband, a priest named Zacharias, was burning incense in the temple when the angel Gabriel appeared to him with good news: Elizabeth would bear a son. Early Christians guessed that Zacharias was probably in the temple for Yom Kippur, which they believed always took place on September 24 (it actually shifts year to year based on the Jewish lunisolar calendar). Nine months after September 24 is June 24, so they chose that as the birthdate—and feast day—of Elizabeth and Zacharias’s son, John the Baptist. When Gabriel later visited Mary to let her know that she’d bear a son, too, he mentioned that Elizabeth was in her sixth month of pregnancy. That means Jesus would’ve been conceived in late March, and born in late December—the night of December 24, to be exact, or the early hours of December 25.

Another theory suggests that Christians arrived at December 25 based on an ancient Jewish idea that prophets die on their birthday. During the 3rd century CE, theologists like Tertullian and Hippolytus dated Jesus’s crucifixion to March 25, since it happened around Passover. But to Sextus Julius Africanus, it was less about when Jesus was born and more about when he first came to Earth; in other words, he believed Jesus’s death and conception coincided on March 25, and thus his birth occurred on December 25 [PDF].

The Early History of Christmas

Even if Zacharias was in the temple on September 24, Gabriel did visit Mary exactly six months later, and Jesus was born right on his due date, it’s still possible that we celebrate Christmas on December 25 for a different reason altogether.

While 3rd-century Christians were busy worshiping the Son of God, some of their pagan counterparts were busy worshiping the Sun God. In the 270s, Roman emperor Aurelian popularized the cult of Sol Invictus, or “The Unconquered Sun,” whose feast day was celebrated on December 25. According to John Carroll University history professor Joseph F. Kelly, other Romans revered a Persian god, Mithra, whose feast day also may have fallen on December 25. There was also Saturnalia, an annual Roman festival that ran from December 17 to December 23. In short, many ancient Romans were well-accustomed to celebrating something in late December by the time Christianity entered the mainstream.

A painting of Saturnalia festivities by Antoine Callet, 1783.Themadchopper, Wikimedia Commons // Public Domain

That happened during Constantine’s rule over Rome in the early 4th century. In 313, Constantine and his fellow ruler Licinius issued the Edict of Milan, which basically legalized Christianity and condemned the ongoing persecution of anyone who practiced it. Constantine was a devout Christian himself, and he spent the rest of his reign spreading the religion throughout the empire. The first known record of December 25 as Jesus’s official birthday is from 336, the year before Constantine died. Because it’s mentioned in a volume containing other important religious dates, some have assumed that a celebration probably occurred on that day, and 336 is often cited as the first known “Christmas.”

Whether Christians celebrated Christmas on December 25 before 336 may forever be unknown, but we do know that the custom quickly caught on (spending the holiday watching A Christmas Story marathon wouldn't come until much later). By the end of the 4th century, Christian bishops were holding Christmas Mass all over Rome, and pagan festivals soon fell out of fashion. The fact that Christmas essentially replaced those earlier December traditions could be a coincidence, but some believe it was by design: Since Romans were already primed for parties on December 25, the Church could’ve been trying to co-opt a built-in subscriber base.

In summary, the origins of Christmas are just as subject to interpretation as Jesus’s actual birthdate—so feel free to play Christmas music whenever you want.

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