In this lecture, we'll discuss the forces that generate winds. Wind is simply how mass is transferred in the atmosphere. Wind transfers a surplus of mass that we can measure at high pressure to a deficit of mass at low pressure.
Measuring and then analyzing the distribution of pressure is a fundamental forecasting technique. That's why you see all the H's and L's on a weather map. Remember, pressure is simply force per area. and force is just mass times acceleration. So pressure readings are really giving us the distribution of mass in the atmosphere.
The difference in pressure between two locations is called a gradient and the force that's enacted is called the pressure gradient force. Wind is a vector which means it has two properties. We measure wind speed with an anemometer and wind direction with a wind vane.
Wind is always named for the direction it's coming from. So a wind coming from the east would be an easterly wind. We know it's heading west, but we're more interested in where it's coming from. That's because we want to know what kinds of weather conditions might be headed our way.
A north wind can bring cold conditions called cold air advection. A south wind in Arizona often brings moisture. The vertical pressure gradient is more extreme than any horizontal pressure gradient. It's caused because pressure decreases with height.
The upward directed pressure gradient force is usually balanced by the downward directed force of gravity. The balance between the two forces is called hydrostatic balance. That balance allows horizontal winds to dominate.
Since horizontal forces tend to dominate, we'll focus our understanding on those operations. Winds will initiate motion as a result of a pressure gradient. Once the parcel is moving, it could be impacted by other forces.
Newton's law tells us that the object will accelerate in the direction of the net force. The Coriolis effect is an apparent force caused by the rotation of the Earth. A parcel that's moving in the atmosphere will move in the direction of the pressure gradient force. With the Earth rotating underneath the parcel, the parcel will end up in a different location. That deflection is the Coriolis effect.
The deflection in the northern hemisphere is to the right of the path of motion. In the southern hemisphere, the deflection is to the left of the path of motion. Coriolis only affects large-scale wind systems. The maximum Coriolis parameter is at and it's zero at the equator. As the wind speed of the system increases, the Coriolis effect increases.
The Coriolis is responsible for how winds move around pressure centers. The global winds operate in the same locations and directions throughout the year. They help steer large-scale weather systems.
Remember, winds are named for where they're coming from. So the trade winds are coming from the northeast or southeast and range between 0 and 30 degrees north and south. The westerlies generally come from the southwest or northwest between 30 and 60 degrees north and south.
And the easterlies are between 60 and 90 degrees north and south. Local scale winds are just as important. Local scales encompass wind regimes the size of a city or region.
These winds are often controlled by the creation of a thermal low. A thermal low is where low pressure is created by surface heating. The land sea breeze is a diurnal wind. That means it has a 24-hour periodicity. It's caused by the differential heating of land next to a large body of water.
Land heats up much faster than water, building a thermal low during the day. This creates the onshore wind called the sea breeze. At night, the land cools off much faster than the water, creating relatively high pressure over land.
This forces the offshore rush of wind called the land breeze. Another diurnal wind is generated by incident angles. Slopes are able to intercept radiation more readily than flat valley surfaces.
During the day, slopes warm relative to the valley floor, creating a thermal low. This generates an upslope wind called the valley wind. At night, the slopes cool and the cold air descends downslope, creating the nighttime mountain wind. Local-scale winds are studied to understand dispersal of pollutants, like chemical, biological, or radiological particles.
Emergency managers plan for these scenarios, and we often hold practice sessions where responding agencies can coordinate their efforts as we run a drill. A monsoon is not a thunderstorm. It's a seasonal reversal of winds. Every continent except Antarctica experiences a monsoonal flow.
During the warm season, the landmass can build a thermal low, causing moist winds to move onshore. Winds in Arizona are predominantly westerly during most of the year. In summer, a thermal low can build over the Four Corners region, generating more southerly winds.
Winds moving from the Gulf of Mexico are Gulf of California will bring moisture into Arizona. That moisture can build the convective thunderstorms that highlight our monsoon season. Downslope winds are generically called katabatic winds.
There are a lot of regional katabatic winds like the Fenn in the Swiss Alps, the Leviche in Spain, and the Sirocco in the Mediterranean. What's important to know about a katabatic wind is that as it moves downslope, the air temperature increases, and the air moves away from saturation, which means it's also a dry wind. The Chinook wind moves down the east slope of the Rockies. The wind is also called a snow eater as the warm downslope winds melt the winter snow, which can then generate dangerous flooding. The regional Santa Ana wind is a downslope wind.
The Santa Anas usually run October through February in Southern California. They come from high pressure that builds over the Great Basin Desert. As the winds move in a clockwise manner around the high pressure, they diverge and descend to the lower coastal areas. The winds warm as they descend and dry out as they move away from saturation. Hot dry winds move through the coastal passes and can kick up any fires caused by natural events or humans.
The Santa Anas have caused billions of dollars in damage and forced evacuations of San Diego and Los Angeles. Prevailing winds can be harnessed to generate electricity. Wind turbines turn like pinwheels, generating electricity in turbines.
Strong winds can also cause problems to an ecosystem, like soil denudation or physical damage. Shelter belts can be installed from natural or artificial materials to block or disrupt the wind flow within the area. Shelter belts can modify the temperature profiles, and reduce things like soil erosion and evapotranspiration.