# What causes turbulence?

Air turbulence usually occurs in the higher levels of the atmosphere, which is why you don't notice it unless you're in an airplane. It happens when upward-and downward-moving currents of air mix (convective mixing). This typically is noticed while flying through a cloud or near a jet stream.

# Why do meteorologists refer to the Reynolds number when talking about turbulence?

The Reynolds number is a mathematical result computed by calculating the ratio of inertial to viscous forces. Put in more understandable English, it measures how fluids move through an area of defined diameter. It is named after English physicist and engineer Osborne Reynolds (1842-1912), who was interested in how water flows through rivers and in waves and tides. However, it can also be used in terms of air flowing through the atmosphere, and thus has applications in meteorology, where Reynolds's formula is used to calculate air turbulence.

Air particles clearly reveal a dangerous microburst. Sometimes, airplanes can be caught in these dangerous bursts of air. (NOAA Photo Library, NOAA Central Library; OAR/ERL/National Severe Storms Laboratory)

# What is an "air pocket"?

Air pockets are caused by turbulent air. Many people who have ridden in airplanes are familiar with them, as they cause that bouncing sensation as you hit an updraft, followed abruptly by a downdraft, in quick succession.

# How does air flow around low and high pressure systems?

Air tends to flow toward a low pressure system and away from a high pressure system. In the Northern Hemisphere, the air will spiral in a counterclockwise direction as it moves toward a low pressure center, and it will move in a clockwise direction as it shifts away from a high pressure center.

# What is the Intertropical Convergence Zone?

The Intertropical Convergence Zone (ITCZ) is a band around our planet where the winds from the Southern and Northern Hemispheres rub against each other. The

What are the "Doldrums"?

Sailors used to call the low-pressure area near the equator the "Doldrums" because winds here tend to be very light and variable and, thus, difficult to sail in. The Doldrums are found near where the trade winds originate. The expression "in the doldrums" has come to mean a person who is feeling inactive, listless, or lethargic.

resulting convergence causes air to rise and is an ideal place for tropical storms to form. Generally near the equator, the ITCZ changes position depending on the effects of the Sun and seasonal cycles. Usually, there is one ITCZ, but sometimes a double ITCZ can form. Land beneath the ITCZ receives much more rain (up to 200 days annually) than land areas outside the zone, and shifts in the zone can lead to wet or dry weather.

# What is the moist tongue?

Sometimes meteorologists come up with some pretty fanciful descriptive terms. Moist tongue falls into that category! It refers to humid tropical air moving in the direction of either the North or South Pole. A moist tongue regularly occurs in the spring and summer over the U.S. Central Plains, where it works in conjunction with a jet stream to produce storms.

# What are the horse latitudes?

The horse latitudes are two high pressure belts characterized by low winds about 30 degrees north and south of the equator. Dreaded by early sailors, these areas have undependable winds with periods of calm. In the Northern Hemisphere, particularly near Bermuda, sailing ships carrying horses from Spain to the New World were often becalmed. When water supplies ran low, these animals were the first to be rationed water. Dying from thirst, the animals were tossed overboard to conserve water for the men. Explorers and sailors reported that the seas were "strewn with bodies of horses," which may be why the areas are called the horse latitudes. The term might also be rooted in complaints by sailors who were paid in advance and received no overtime when the ships slowly traversed the area. During this time, they were said to be "working off a dead horse."