Wind Flow

• If the weather is fair after the barometer rises, it will not last.

• It is wet and there is a sudden rise in the barometric pressure, it will not be nice for very long outside.

• In England (where these rules took place) the barometer will rise the most for north and east winds.

If the barometer is unsettled or unsteady:

• If the barometer is unsteady, you can expect unsettled weather.

• If the barometer says it is rainy and rises to the middle, you can expect a short period of good weather.

• If the barometer says good weather and it falls to say changing weather, you can expect bad weather.

So, in most cases a rising or high barometric pressure means the weather will be without precipitation, while a low barometric pressure indicates rain.

WIND FLOW

Let's try to put together a better picture of air movement around the earth. You know that warm air rises, so it makes sense that when it does this you will see a low-pressure system on the ground. The air has risen so the pressure near the ground decreases. Like any system of gases, a low-pressure system is ripe for the movement of air around it to be sucked into the low-pressure. This horizontal movement of air is called advection.

On the other hand, cool air will fall creating a high pressure zone near the ground. Air in the atmosphere will always flow from an area of high pressure to low-pressure. If you think about it on a molecular basis, it makes sense. High pressure areas mean lots of molecules or molecules with higher kinetic energy; these will naturally travel to low pressure areas with fewer molecules at a lower kinetic energy. Figure 22 shows you how complex it can get when you consider that air masses are moving all over our globe:

Figure 22.

Around the earth, the whole thing becomes a series of circles. In any given area, you will see a circle of low-pressure with warmer air rising upward, cooling, and then sinking downward. There will be horizontal flow at all times from areas of high pressure to lowpressure. Anytime you see wind, it will be because of advection.

Convection is something you already know about. It helps to create weather too by allowing temperatures to rise and fall according to their warmth and other things, like the amount of water vapor they have in them. Warm air that ascends with humidity can no longer hold this humidity as it cools, so clouds form and it rains or snows. Cooler air falls and warms up as it reaches the earth. It can now hold a lot more moisture without raining and the warmth allows the air to accept evaporated water from the ground. You can see now how the whole thing is one big cycle. Around the globe, there are global wind belts where air is continually moving, affecting the climate in the region on a yearly basis. Within these regions are smaller air pressure systems that create local weather.

You can also see that the wind you experience in your neighborhood is basically advection at work. There is continually moving air from areas of high-pressure to lowpressure. In a large high-pressure system, the air will hold back winds and you will see light winds and fair weather. In some low pressure systems, you will see wind because the air is moving into it. Let's look at some specific local wind patterns.

Have you ever heard of sea breezes? In any large body of water, the air is more humid and on land. Humid air holds onto its heat so it doesn't change as much over time. If there is any large temperature difference between the air on land and the air at sea, you will see a difference in the pressure of air between the two places. This is what creates breezes.

A sea breeze happens when the air is warm over land and cool over the ocean. This will happen mainly in the summertime. The cool air from the ocean then flows as a gentle breeze on to the land and about 10 to 20 kilometers per hour. Land breezes are seen in the wintertime when the air cools more over land that it does over the water. In this case, the breeze will come from the land out to the ocean. It can also be seen when air cools differentially in the daytime versus the nighttime, as seen in figure 23.

Land breezes are seen at night and sea breezes are seen in the daytime.

These local breezes are moderating to the area of land near the oceans or large bodies of water. You see this in areas like Southern California, with the breezes moderating the air up to 60 miles away from the shoreline.

Monsoon winds are much bigger than the land and sea breezes, but they are based on the same phenomenon. You see them on hot summer days from sea to the land or in the opposite direction in the wintertime. These are so strong that you will see thunderstorms and monsoon rains. You see them all the time in India; they contribute to the monsoon rains most people in the area depend on for crops and drinking water. They used to help sailors get back home from Africa or go there when goods were trafficked between India and Africa.

Figure 23.

You also can have valley and mountain breezes. Daytime means the air overheats on the slopes than in the valleys, drawing cool air up into the mountains, causing a valley breeze. At nighttime, the reverse happens and you get a mountain breeze that flows

downhill. Katabatic winds are these same breezes on steroids, forming over high plateaus, where the air gets cold in the wintertime. This cold air sinks downward, causing large winds coming off of these high plateaus. In Greenland and Antarctica, these winds are both strong and cold.

Chinook winds are related to mountain ranges. Air is forced upward up and finally over a mountain range. Westerlies from the Pacific push these winds over the Sierra Nevada Mountains. This air is naturally warm and moist. Once over the mountain range, this air cools and, because of this, it cannot hold that moisture anymore. It may rain or snow; it also forms high pressure on the leeward side of the mountain. This is the side opposite where the wind is coming from. The air drops down, forming hot and dry winds on the lands near this leeward side. Chinook winds cause rapid temperature increases, melting snow on the leeward side, and what's called the "rain shadow effect", which is the formation of deserts brought on by hot dry winds. All of the precipitation has been left behind on the mountain tops.

Santa Ana winds are seen during the late fall and winter months in the same area of California. East of the Sierra Nevada, the atmosphere cools. Cooling means that a high pressure system is created. Winds travel downhill and always in a clockwise direction. The temperature rises and there is less humidity in the air mass, creating large wind flow in the southwest from the land to the ocean. These hot and dry winds make the California landscape even drier, which is why they lead to large-scale fires in Southern California.

You often associate winds with cold days and bad weather but, in reality, winds can be hot as well. You see this with the Santa Ana winds. You can get similar winds in any desert; these can be linked to great monsoon storms. Remember that deserts have mostly sand or dirt but little vegetation. The winds pick up these particles. You will get what's called a haboob, which is a severe dust storm at the beginning of these wind fronts. Whirlwinds of dust form in these areas as hot air rises, creating pockets of low pressure that spin. They do not last long but can be damaging. The dust also provides fodder for monsoon storms over any nearby ocean. Figure 24 shows what a haboob looks like: