Lesson 4

One haunting phenomenon created by winds are known as Aeolian sounds. Aeolian sounds are tones generated when wind flows past an object, creating oscillating vortices that shed rhythmically from either side of the obstacle. These vortices create pressure fluctuations that your ears interpret as sound, which is often the sound of a hum, whistle, or low drone. When this wind encounters an object like a wire, pole, twig, or even a blade of grass, the airflow separates and forms alternating swirling eddies downstream, which is called vortex shredding. Think of when there is construction and the two lanes on the road diverge as there is a barrier preventing the left lane from going straight. This is vortex shredding where swirls of winds, or eddies take different paths creating an oscillating area of low-pressure in which surrounding higher-pressure tries to fill.  

The following wind described below are downslope winds, which are called katabatic winds. Katabatic winds are driven by the force of gravity. On the contrary, anabatic winds are upslope winds and are driven by surface heating. The two winds below are examples of katabatic winds. Another example are mountain-valley circulations where nighttime cooling causes air to sink causing a cool, katabatic wind. During the day, the surface heats up creating upslope winds up the mountain, which forms a warm, anabatic wind. 

Chinook Winds

One well-known winds are called the Chinook Winds. The Chinook winds are warm, dry, westerly winds that descend on the leeward side of the Rocky Mountains. As discussed in lesson 1 with Parcel Theory, when air descends, the parcels warm and compress at the dry adiabatic lapse rate (9.8 C/km). Since this is a warm wind, as the downslope winds; especially in the winter, can cause snow to rapidly melt as temperatures can rise upwards of 20 to 40 degrees in the matter of minutes to hours. This is why it is sometimes referred to as the "Snow Eater". This is also a dry wind because on the windward side of the Rockies, air is forced upward through orographic lift causing parcels to expand and cool and condense into clouds and precipitation. Most of the moisture is lost on the windward side due to it precipitating out, so the leeward side leads to warm, dry air descending the slopes of the Rocky Mountains. If conditions are dry at the surface on the leeward side with low relative humidity, these downslope winds can also cause a wildfire risk. 

Santa Ana Winds

Another type of downslope winds is the Santa Ana Winds. The Santa Ana winds are strong, dry, downslope winds that blow from the Great Basin toward coastal Southern California, often producing gusts of 40-65 mph and extremely low humidity. They come from a high-pressure system over the Great Basin (Nevada/Utah region), which creates a pressure gradient that pushes air from the interior toward the coast. As the air descends the mountains toward sea level, it warms and compresses, which can cause downed trees and powerlines and elevated fire weather concerns. 

A dust storm is a wall of dust and debris lifted into the air by strong winds, often miles long and thousands of feet high. They are most common in arid and semi-arid regions, especially North Africa, the Middle East, Central Asia, and China. They often form from strong winds over dry, loose soil. This can be either from the outflow of a thunderstorm or strong surface winds blowing across a dry surface, lifting sand and dirt into the air. Many dust storms, especially in the U.S. Southwest, are triggered by thunderstorm outflow boundaries, producing a fast-moving wall of dust called a haboob. Dust storms have significant impacts and can reduce visbility to near zero causing dangerous driving conditions and multi-vehicle pileups. Fine dust can cause respiratory issues, eye irritation, and worsen existing conditions. This massive broom that is the atmosphere sweeping across the dusty ground is important to watch out for and can be particularly dangerous. 

Monsoons are one of the most powerful examples of seasonal atmospheric circulations, which are driven by global wind patterns. Many people associate "monsoons" with torrential rainfall, which certainly can happen, but that is only half of the story. A monsoon is a seasonal reversal of wind direction that produces distinct wet and dry seasons around the world. They are indeed not just rain, but a seasonal wind system that shifts direction for months at a time. Before diving into how a monsoon forms, a fundamental concept of heat capacity is important to note. Heat capacity is the amount of energy required to raise the temperature of an object or substance by one degree. Let's say summer just started and your neighborhood pool just opened up. While it is certainly hot outside, the water is certainly cold. In fact, you dip your foot into the water and it's freezing. Now you just want to lay out and sunbathe. A heat wave comes a few days later with temperatures heating into the 90s to near 100 degrees. After a few days of the hot weather, you return back to the pool and notice that the water is warm like bath water. In an essence, this is heat capacity. Water has a higher heat capacity than air as it takes way more energy to heat up the water and the surrounding air. This is why it takes the pool a lot longer to heat up than the surrounding air. 

Now, there are two kinds of monsoons. The summer monsoon, known as the wet season, allows land to heat up faster than the ocean. This creates a thermal area of low-pressure on land and moist, ocean air flows inward. Air rises, cools, and condenses and produces heavy rainfall. The other half of the story is the winter monsoon, known as the dry season. During the winter, land cools much faster than the ocean. High-pressure forms over land causing dry, continental air to flow offshore toward the ocean. As a result, dry conditions form over land. Monsoons are tied to the annual migration of the Intertropical Convergence Zone (ITCZ), which shifts north and south with the seasons depending on where the most sunlight and daytime heating occur. As the ITCZ moves, it drags the monsoon circulation with it. Thus, monsoons occur in many tropical and subtropical regions including South Asia (India, Bangladesh, Pakistan), Southeast Asia, West Africa, Australia, and the Southwestern United States (Arizona and New Mexico). Several locations rely on these monsoonal patterns as part of their annual rainfall, but if too strong, a summer monsoon can bring flooding, landslides. On the contrary, a strong winter monsoon can bring drought conditions and damage agriculture, water supply, and ecosystems.