## Wind

This page describes how to determine wind direction and relative wind speed using twocommon types of weather maps, 500 mb height maps, which we have already studied, and surface weather maps. The main reason to go over this material now is that we are going to try to describe both the surface winds and the 500 mb winds that are typical with middle latitude storm systems. Later in the semester we cover hurricanes and the winds both at the surface and at 500 mb are important in understanding hurricane strengthening and movement.

You are watching: Wind speed may be roughly indicated by

In describing weather, wind is generally taken to mean the horizontal movement of air. By convention, the wind direction is the direction from which the wind is blowing (e.g., a north wind means the air is moving from north toward south) and the windspeed is the speed at which the air is moving relative to the ground. Surface winds blow along the surface of the Earth where we live. But there are also winds blowing along horizontal paths at altitudes above the ground surface, for instance, the 500 mb winds that have been previously discussed. As shown in the skew-T diagrams, the horizontal winds often change considerably at different altitudes. Weather maps show the pattern of air pressure on horizontal surfaces. Using this information we can visually determine the approximate wind pattern.

On this page we will look at how wind information can be inferred from common weather maps, which depict how air pressure changes along horizontal surfaces.

There is more detail on this page than what is covered in course homework and exams. After studying this page, les-grizzlys-catalans.org 336 students are expected to: Understand that the root cause of winds are horizontal variations in air pressure, which are called pressure gradients. A horizontal pressure gradient results in a horizontal pressure gradient force, which forces air from higher pressure toward lower pressure. Although the pressure gradient force is the driving force for wind, the large scale wind direction is NOT in the direction of the pressure gradient force. This is because as the air starts to move, there are other forces that act on the air. You are not expected to understand these other forces even though they are briefly discussed in the text below. Realize that strong pressure gradients (and strong winds) are indicated by close packing of the contour lines on weather maps. Be able to determine the approximate wind direction and relative wind speed of (a) winds at the 500 mb altitude using 500 mb height maps, which we have already done, and (b) winds at the ground surface using surface weather maps with isobars of sea level air pressure.Understand the difference between sea level pressure and station pressure and why sea level pressureinstead of station pressure is shown on surface maps. Realize that 500 mb winds are parallel to the height contours, while surface winds blow slightly across the isobars toward lower pressure. Know what is meant by horizontal divergence and convergence or air. Know the relationships between divergence and convergence and forced upward and downward air movements. Know that surface winds converge toward areas of low pressure, which forces air to move vertically upward above the surface low. Clouds and precipitation form where air moves vertically upward. Thus, surface low pressure areas are often associated with precipitation. On surface weather maps, strong surface storms are usually closed lows surrounded by tightly packed isobars. Know that surface winds diverge away from areas of high pressure, which forces air to move vertically downward on top of the surface high. Fair weather with no precipitation is most likely wherw air moves vertically downward. Thus, surface high pressure areas are often associated with nice weather. Recall that there is a region of upper level divergence (near the top of the troposphere) just downwind of 500 mb troughs. You should now better understand why upper level divergence forces air to move vertically upward. We did not explain the cause of the upper level divergence itself. Recall that there is a region of upper level convergence (near the top of the troposphere) just downwind of 500 mb ridges. You should now better understand why this forces air to move vertically downward. We did not explain the cause of the upper level convergence itself.

All winds result from horizontal differences in air pressure. Basically, airis forced or pushed from regions of higher air pressure toward lower air pressure. Consider a sealed box of air. The pressure of the air inside the box pushes outwardon the box. One way to increase the air pressure inside the box is to pump moreair into the box. This increases the force of air pushing out. If you pop a hole in the box, air will be forced outward, from higher pressure inside the box towardlower pressure outside the box. This is an example of wind on a small scale.This alsoexplains why air rushes in when you open a vacuum sealed container (likea jar of food); it explains why air rushes out when you opena carbonated beverage sealed under high pressure (like soda or beer); andhow it is that we breath (expanding our lungs initally lowers the airpressure inside, causing higher pressure air outside to rush in). Inall cases, air is forced to move from high toward low pressure andthe greater the difference in pressure, the stronger the force on the air, andthe faster the air moves. For small scale air motions, like the soda can, the wind blows directly from higher toward lower pressure. For large scale motions,like those we are going to study with weather maps, the wind direction will notbe directly from higher to lower pressure since other forces must be considered.

Recall that the average air pressure at an altitude of sea level is about 1000 mb. This is just an average as the air pressure changes with both time and location. At any given moment in time, there is a spatial pattern of sea level air pressure. Weather maps showing the pattern of sea level air pressure at specific times are constructed using contours of equal pressure, called isobars. The figure below on the left shows a small portion of a sea level weather map. Two labeled isobars 1004 mb and 1000 mb (contours of sea level air pressure) show the pressure pattern. The air pressure gets smaller as you move from left to right. Consider a box of air located between the 1004 and 1000 mb contours. The air pressure pushing inward on the left side of the box is stronger than the air pressure pushing inward on the right side of the box. Thus, there is a net force that pushes the box of air from higher toward lower pressure (left to right). This force is called the pressure gradient force. The root cause of all winds are horizontal changes in air pressure or pressure gradients.

A simple example of how contours of sea level pressure (isobars) are used to depict the spatial pattern of sea level air pressure. Since the pressure on the left side is greater than the pressure on the right side, there is a net force that pushes air from higher toward lower pressure.
 "> Another example showing a pattern of sea level pressure using isobars. Since the pressure on the right side is greater, the direction of the Pressure Gradient (PG) and Pressure Gradient Force (PGF) points from right to left. The PG is calculated between isobars using the definition PG equal change in pressure divided by the change in distance. Since the PG (and hence the PGF) are stronger on the left side, the wind will be stronger on the left side.

Since air pressure falls rapidly along the vertical direction, there is always a strong upward directed pressure gradient and pressure gradient force. For example, an average 500 mb height is 5.5 km above sea level, while the average sea level pressure is about 1000 mb. The average vertical pressure gradient (change in pressure divided by change in distance) is (500 mb)/(5.5 km), which is much larger than any horizontal pressure gradients that are ever measured in the les-grizzlys-catalans.orgsphere. The strong upward pressure gradient force, however, does not cause the air to move upward due to hydrostatic balance (click for figure), in which the upward pressure gradient force is balanced by the weight of the air above. The strong upward pressure gradient force is just enough to hold up the weight of the air above it. The near balance of forces in the vertical means that air does not easily move up or down in the les-grizzlys-catalans.orgsphere. However, even very small changes in air pressure along a horizontal surface (horizontal pressure gradients) result in winds (horizontal movement of air) since there is no balancing force acting against the pressure gradient. What is important in determining horizontal winds is to identify how the air pressure changes along a horizontal surface.

## Reading the Pressure Pattern on Surface (or Sea Level) Weather Charts

Station pressure is defined as the barometer reading observed at ground level at a given meteorological station. A barometer is an instrument used to measure air pressure. The surface level air pressures measured allaround the world can be plotted on a weather map to show the pattern of air pressurechanges. The problem with using measured station pressure though is that not allweather observation stations are located at the same altitude. Higher altitude locationswill measure lower air pressure than lower altitude stations, since air pressure fallsoff so rapidly as one moves upward in elevation.

Recall that winds are caused by changes in air pressure along a horizontal (constant elevation) surface and that changes in pressure along a vertical direction are largely balanced by the weight of the air above and do not cause much wind. Cities separated by just a few kilometers might have very different station pressures due to differences in station altitude. Thus, to properly monitor horizontal changes in pressure, surface barometer readings must be adjusted to a common altitude to eliminate the pressure differences due to differences in station altitude. Again, the purpose of the altitude adjustment to the measured station pressure is to have all the pressure measurements be at the same altitude because it is the change in pressure along a horizontal surface that causes winds.

Altitude adjustments are made so that a barometer reading taken at one elevation can be compared with a barometer reading taken at another to compute the horizontal change in air pressure. Station pressures are normally adjusted to a altitude level of mean sea level and the adjusted pressure is called Sea level pressure. The size of the adjustment depends primarily on how high the station is above sea level. For example, Tucson is about 2500 feet (760 meters)above sea level. A typical station pressure for Tucson would be in the range 910-930 mb, which is muchlower than a station pressure measured at a location near sea level, like San Diego,which is typically around 1013 mb. In order to eliminate the pressure difference that is dueto elevation differences, i.e., the part that does not contribute to winds along the ground, the Tucsonstation pressure is converted to sea level pressure, which is basically what the air pressurewould be in Tucson if Tucson were at the altitude of sea level. You should realize that forTucson (and any other location located at an altitude above sea level) the sea levelpressure will be greater than the station pressure. This is simply because air pressurewill always decrease as one moves upward in the les-grizzlys-catalans.orgsphere.

Compare the current Tucson station pressure with the altitude-adjusted sea level pressure, which are shown in this link underthe heading "Pressure".You may also wish to look at thesea level pressure graphover the last 24 hours, which was determined by adjusting the measure station air pressure on campus.Stations located at even higher altitudes will measure smaller stationpressures and a larger adjustment is needed to convert to sea level pressure. A list of current station and the adjusted sea level pressures for cities near the Denver,Colorado area is shownin this link. Look at the column headed "mb or hpascals" and comparethe first row (station pressure) with the third row (adjusted sea level pressure) for eachof the observation stations listed. Noticethat the higher elevation stations have a lower station pressure. The other columns in thetable are just different units for measuring air pressure. The basic concept of adjusting the station pressure to account for pressure differences caused by stationsbeing at different altitudes should make sense to you. However, the details of how exactlythis is done can become complicated and you are not expected to know how to calculatesea level pressure from station pressure. You should understand why it is necessary though.

 ">
Sea level pressure (in millibars) is what is plotted on surface weather charts. Isobars are lines connecting points of equal pressure. The analysis of the sea level pressure data allows for the pressure pattern to be visualized. Again, the reason we plot out thepressure pattern is that winds are forced by changes in pressure along horizontal surfaces. These "maps" are called sea level pressure charts or surface weather maps. An example of a surface weather map withisobars is shown on the left.The surface pressure map indicates a strong low (L, 985 mb) over Nebraska and a weak high (H, 1016 mb) near the Great Lakes. Lows mark where a center of lowest pressure isfound and highs mark where a center of highest pressure is found.Note that the change in pressure along a horizontal direction can be determined by examining the pattern of isobars. The more closely packed the isobars, the greater the pressure gradient which is the change in pressure divided by the distance over which that pressure change happens. The stength of the pressure gradient is what determines the strength of the winds. The greater the pressure gradient, the stronger the windspeed. Thus, on surface weather maps, the strongest winds are happening where the isobars are closest together and the weakest winds are happening where the isobars are spaced furthest apart. You can see that the isobars on April 8 were tightly packed over Colorado, so the pressure changed rapidly over the relatively short distance from the center of the low to western Colorado. This strong pressure gradient produced wind gusts over 100 mph, and the storm caused \$13.8 million in damage in Colorado in a two-day period. Meanwhile weak winds wouldbe expected across much of the eastern United States where the isobars are spread apartand the pressure gradient is weak.

The current sea level weather chart for the United States can be found in ~ WW2010 at the University of Illinois (Click on the image labeled "isobars"). On most days, you will not find strongpressure gradients (closely packed isobars) on surface weather maps as seen in the example above.Fortunately, strong pressure gradients and strong winds at the surface only happen occasionally inassociation with strong weather systems. However, the weather is more interesting when strongweather systems are present, and I would like you to be able to identify strong weather systemson surface weather maps. The links below show the surface weather maps on two days with strong storm systems:Sea level chart for August 29, 2005 (Hurricane Katrina)Sea level chart for October 29, 2012 (Superstorm Sandy)The tight packing of isobars is easily seen on the Superstorm Sandy map. At first glance Hurricane Katrina looks much weakerin that the isobars are not as tightly packed. However, notice that the spacing of the isobars near Katrina is every8 mb, instead of every 4 mb like it is on the rest of the map (and most other maps). So to properly compare the pressure gradientson the maps, near Katrina there should be another isobar drawn between each of the isobars that encircle the center of the storm.

So far we have only discussed how to determine the relative strenth of the wind, but not the winddirection. The relationship between the pressure pattern and direction that surface winds are blowing is explained after the next section on upper air charts.

## Reading the Pressure Pattern on Upper Air Weather Charts

Horizonal winds blowing at different altitudes above sea level are also veryimportant in determining what is going on with the weather. Upper air weather charts are drawn to visualize pressure patterns at differentaltitudes. We have also used the 500 mb upper air chart to geta picture of the large-scale weather pattern around the North America.

While surface weather charts depict the pressure pattern at a fixedaltitude (sea level), upper air charts depict a pattern showing howthe altitude of a fixed pressure surface changes. There are maps showingthe height pattern at 850 mb, 700 mb, 500 mb, 300 mb, and so on. We have previouslylooked at 500 mb height maps and discussed 500 mb winds. It is veryimportant to realize that the height patterns shown on upper air maps give you the sameinformation about changes in pressure along a horizontal surface that surface maps do, just at differentaltitudes. Thus, the pattern of height contours indicate how air pressure variesalong horizontal surfaces, for a horizontal surface located where the air pressure is 500 mb. Air is forced or pushed from higher heightstoward lower heights and the more closely spaced the height contours, the strongerthe pressure gradient and the stronger the winds. This is simply a re-statement of what waspreviously described about 500 mb winds. Again this only covers the strength of the wind, basedon the pressure gradient information obtained from examining the upper air height contours on a weather map,but not the wind direction.

## Determining wind direction and relative wind speed from weather charts

Because the driving force for all wind is the horizontal change in pressure, thegreater the horizontal change in pressure (or more precisely the pressure gradient), the greater the windspeed. The pressure gradient is the horizontal changein pressure divided by the horizontal change in distance.On a weatherchart, the magnitude of the pressure gradient can be seen byexamining the spacing between the contour lines of the map (isobars on the surfacemap or height contours on the upper air map). Where the lines are closest together,the horizontal change in pressure is stronger, and the winds are stronger. In otherwords, higher windspeeds are found where the contour lines are closest together.The force exerted on air by changes in air pressure is known as the pressure gradient force. The direction of the pressure gradient force is from higher pressure toward lower pressure. Thus on weather maps, the pressure gradient force points most directly from higher contour values toward lower contour values and is perpendicular to the contours, i.e.,if the pressure gradient were drawn on weather maps, it would be shown as arrows pointing from high toward low contour values on the map (examples are provided below). Since the pressure gradient force is the root cause of all winds, you might think the wind direction would be directly from high to low pressure, but this is not the case due to the Earth"s planetary rotation.

Over short distance scales, air moves in the direction forced by the horizontal pressurechanges, i.e., directly from high toward low pressure. This is the case for the examplesof opening a jar of food or a can of soda mentioned above. However, for large-scaleair motions (like the ones depicted on weather maps), the actual wind direction isturned away from this direction because the Earth is rotating. This phenonemon is calledthe Coriolis effect or Coriolis force. The details of the Coriolis effect aredifficult to understand, so we will not go into them. Basically, it comes about becausewe are observing the wind from a rotating frame of reference. We are attached to the surfaceof the Earth and are rotating with it, while the air above is not attached and thus doesnot have to rotate with it. It is important to understand that the Coriolis effect is onlyimportant for motions that traverse long distances or last long enough for the Earth to move significantly in its rotation. Thus, the Coriolis effect is not significant whenshooting a basketball and does NOT affect the direction that water swirls down a drain.The Coriolis effect is significant for determining the direction of large scale windsfrom weather charts, the direction of ocean currents, or the paths of long-range missles andairplanes.

See this basic description of the Coriolis effect.Play the merry-go-round video, which is a good demonstration for how the Coriolis force works. Here is a link to a YouTube Video of the merry-go-round that does not require Adobe Flash.Notice that for anobserver looking down on the merry-go-round in a frame of reference that is not rotating sees the ballmove in a straight line, which can be easily explained by the laws of motion, i.e., the ball was pushedin one direction and it moves in that direction. However, for someone observing the ball on the merry-go-round in a frameof reference that is spinning clockwise, the ball appears to curve to the left. In order to explain this using thelaws of motion, there needs to be a force that causes the ball to curve. This force is known as the CoriolisForce and will be important when observing motions on a rotating frame of reference. Since the Earth is rotatingand we are on it, we observe motions from a rotating frame of reference. The linked video shows the merry-go-roundrotating clockwise. This is the situation in the southern hemisphere of the Earth. For observers in the northernhemisphere, the rotation is counterclockwise, and the Coriolis force will deflect objects to the right.Here is a link to a National Geographic Video demonstration of the Coriolis Effect on a merry-go-round and a longer video explaning the Coriolis Effect produced by Nova

### Upper Air Weather Charts (e.g., 500 mb map)

Therefore, the Coriolis force turns the wind to the right (of the pressure gradient) in the northern hemisphereand to the left (of the pressure gradient) in the southern hemisphere. We will only worry about the northern hemisphere.At all altitude levels above the ground surface (includes all upper air charts, but not surface charts),the wind direction is 90° to the right of the direction of the pressure gradient force. The pressure gradient force isdirected from high heights (or pressures) toward low heights (or pressures). Thus, on upper air chartsthe wind moves parallel to the height contours, with lower heights to the left of the wind direction.This was the basic rule given to estimate wind speed and direction on 500 mb charts in the previous reading page. Now youshould understand a bit more about why the wind blows as it does at 500 mb. See figures below.

 Sample 500 mb map showing height contours in black. The direction of the pressure gradient force (from high to low heights)is shown in red. The wind direction at 500 mb, shown in blue, is turned 90° to the right of the pressure gradient force due to the Coriolis Effect. Sample 500 mb map depicting a closed low. The direction of the pressure gradient force is inward toward the low. The winddirection is 90° to the right of the pressure gradient force and flows counterclockwise around the center of the closed low.

### Surface (or sea level) Weather Charts

Because air moving along the ground surface is slowed by friction with the ground, there is a third important force,a frictional or drag force due to contact with the ground, which complicates the direction of air flow along the ground.A general rule of thumb is that the wind direction just above the ground surfaceis only turned about 60° to the right of the pressure gradient force, instead of 90° right of the pressuregradient force as it is on uppper air maps. Thus, on surface weather chartsthe wind direction, rather than being parallel to the isobars, points about 30° toward lowerpressure. I suggest using a two step process to determine wind direction using a surface weather map.Step 1 is to find the direction that is parallel to the isobars with lower air pressure to the left and higher air pressure to the right. This is 90° to the right of the direction of thepressure gradient. This is the same as the wind direction on 500 mb maps. Step 2 isto take this direction and turn it 30° toward lower pressure. This is the wind direction. See figures below.

 Sample surface map showing isobars of sea level pressure in black. The direction of the pressure gradient force (from high to low pressure) is shown in red. The direction 90° to the right of the pressure gradient force is shown in green. On surface maps the wind direction,shown in blue, is not parallel to the contours, but is 30° toward lower pressure as shown. Sample surface map depicting a closed high. The direction of the pressure gradient force is outward away from the center of the high. Asdepicted in the caption under the figure to the left, to find the wind direction, first find the direction 90° to the right of the pressure gradient force, then the wind direction is 30° toward lower pressure. The wind pattern near a surface high pressure is clockwisearound the high, but also spiraling outward away from the high.

## Review/Summary of What to Know About Winds

More detail was provided in the previous section than what les-grizzlys-catalans.org 336 students are expected to knowfor homework and exam questions. This section is a review of the important points.

In our les-grizzlys-catalans.orgsphere, the horizontal movement of air is typically much greater than the vertical movement of air. Thus, when we speak of winds, we are generally referring to the horizontal movement of air. This does not mean that vertical motion is not important. It just means vertical motion is not as strong as horizontal motion. Vertical motion is very important in the formation of clouds and precipitation. As shown with skew-T diagrams, horizontal winds change in both speed and direction as you move vertically up and down. For example, there are surface winds that blow along the ground surface, 700 mb winds blow at the altitude where the air pressure is 700 mb, 500 mb winds that blow at the altitude where the air pressure is 500 mb, and so on. We have already used 500 mb height maps to determine the wind direction and relative wind speed of 500 mb winds. There are also 700 mb maps and so forth that we will not study in this class. We will only look at 500 mb height maps and sea level or surface maps.This section reviews how to determine wind information from the contour lines plotted on these maps.

Horizontal winds are caused by horizontal variations in air pressure, which are called horizontal pressure gradients. The horizontal pressure gradient is defined as the change in air pressure divided by the change in the distance over which the pressure change happens. The strength of the pressure gradient is the rate at which air pressure changes along a horizontal surface. The stronger the pressure gradient, the stronger the winds. On weather maps, the strength of the pressure gradient is indicated by the spacing of the contour lines, i.e., the closer the spacing of the contours, the stronger the pressure gradient force and the stronger the winds.

The contours on 500 mb maps represent the height of the 500 mb pressure level in meters above sea level.It is important to realize that the height contour pattern on 500 mb maps depicts the pressure gradient at the altitude where the air pressure is 500 mb just as the isobars on surface maps depict the sea level pressure pattern along the surface.(You can just accept this since we did not take the time to derive or explain why the 500 mb height contour pattern isbasically equivalent to a contour pattern of air pressure at a fixed altitude. If you are interested, you may contact the instructor for more explanation.)The pressure gradient force points from higher toward lower 500 mb height. Thestrength of the pressure gradient (and the strength of the wind at the 500 mb height level) is determined by the spacing of the contour lines, i.e., the closer the spacing of the contours, the stronger the pressure gradient and the stronger the winds.

On surface weather maps, the contour lines are lines of equal sea level pressure, called isobars, rather than the height of an equal pressure surface, such as the 500 mb height on 500 mb maps. The average sea level air pressure is 1013.25 mb (roughly 1000 mb), but it varies with both time and location. A surface weather map shows a snapshot of the sea level air pressure pattern valid at the time indicated with the map label. The contoured pattern of sea level pressure is used to estimate surface winds. It is similar, butnot exactly the same, as how 500 mb height contours are used to estimate 500 mb winds.

 ">
Shown on the left is a surface weather map with isobars. The labeled contours in green represent the pattern of sea level air pressure. Two regions are circled on the map. The yellow circle highlights an area with a large pressure gradient, which is indicated by the tight spacing of the contour lines. Strong surface winds are expected in this area due to the large pressure gradient. The blue circle highlights an area with a much smaller pressure gradient, which is indicated by the large spacing between the contour lines. Weaker surface winds are expected in this area due to the smaller pressure gradient.

We will not routinely look at surface maps in this class. I want you to know what strong surface storms and hurricanes look like on surface weather maps. Strong surface storms typically show up as closed lows with a tight packing of the isobars (contour lines) around the center of the low. The low pressure system centered over the state of Nebraska on the map is an example of a strong surface storm. Examples of hurricanes are shown below.

The surface wind direction relative to the pattern of isobars is generally the same as it is for 500 mb wind direction and height contours. The surface wind direction is mostly parallel to the isobars with lower pressure to the left of the wind direction, but there is one very important difference: the surface wind is not fully parallel to the isobars, the surface wind slightly crosses the isobars toward lower pressure. The average angle between the surface winds and the isobars is about 30°. When I look at a surface map with isobars, I start by visualizing the winds pattern as parallel to the contours, then shift this pattern to point slightly toward lower pressure.

The figure below shows the wind direction for straight line contours of 500 mb height, like what you may see on a 500 mb map, on the left side and sea level pressure (isobars), like what you may see on a surface weather map on the right side. As described in previous reading material, the 500 mb winds blow parallel to the height contours (along the contour lines without crossing them) with lower heights to the left of the wind direction. For surface winds, the wind direction is nearly the same, however, it is not fully parallel to the contours. The surface wind moves slightly toward lower pressure such that the wind direction crosses the isobars at an average angle of about 30°.

 Portion of a 500 mb height map with straight line contours. The black lines are contours of 500 mb height. Red arrows show the direction of the 500 mb wind, which is parallel to the height contours with lower heights to the left of the wind direction. Portion of a surface weather map with straight line contours. The black lines are isobars or contours of sea level pressure. Red arrows show the direction of the surface wind, which crosses the isobars at an angle of 30° toward lower pressure.

The wind trajectories around closed lows in the contour pattern are shown below. The wind trajectory represents the path followed by air moving along with the flow. At the 500 mb height level, the winds are counterclockwise around the center of the low. Again this is parallel to the contours with lower contour values to the left of the wind direction. The surface wind pattern around a closed low is counterclockwise as well, but also crosses the contour lines toward lower pressure. Thus, the surface wind around closed lows moves counterclockwise and spirals in toward the center of the low.

 Portion of a 500 mb map with a closed low. The black lines are contours of 500 mb height. Red arrows show the wind trajectory or air flow pattern. The air flow is parallel to the height contours and counterclockwise around the center of the closed low. Portion of a surface map with a closed low. The black lines are isobars or contours of sea level pressure. Red arrows show the wind trajectory or air flow pattern. The flow is counterclockwise and converging toward the center of the closed low.

The fact that surface winds blow slightly toward low pressure is important in understanding why surface low pressureareas are associated with clouds and precipitation and why surface high pressure areas are associated with fair weather.This is discussed in the next section. les-grizzlys-catalans.org 336 students are responsible for understanding the material below.

### Surface Winds, Forced Rising and Sinking Air, and Weather near Surface Low and High Pressure

An important consequence of the fact that winds just above the ground surface do not blow parallel to the isobars, but slightly toward low pressure (and away from high pressure), is that sea level pressure pattern and winds can force airto move upward or downward. Later in the semester we will show that upward moving air is associated with cloud formation and precipiation,while downwarn moving air is assoicated with fair weather and a lack of clouds. First a couple of definitions. The wind pattern is said to be convergent in regions where there is a horizontal inflow of air toward the region and divergent in regions where there is a horizontaloutflow of air away from a region. The pattern of the arrows in the figures below show examples examples of purely convergent and divergent airflow.
 Instructive images for horizonal convergence of air. Arrows show horizontal wind pattern. Left side shows pure covergencetoward a line, while right side shows pure convergence toward a point. In realistic surface air flow, the wind pattern is justslightly convergent toward lines or points of low pressure. Instructive images for horizonal divergence of air. Arrows show horizontal wind pattern. Left side shows pure divergence away from a line, while right side shows pure divergence away from a point. In realistic surface air flow, the wind pattern is just slightly divergent away from lines or points of high pressure.

Of importance for weather, when the horizontal air flow at the surface is convergent (flowing together), air is forced to rise (move vertically upward). Conversely, when the horizontal air flow at the surface is divergent (flowing away), air is forced to sink (move vertically downward). To help you understand this relationship, think of air as a fluid that flows, just as water or even a tube of toothpaste. Squeezing a tube of toothpaste from the bottom is horizonal convergence. Toothpaste is forced upward by the convergence, just like air will be forcedupward when there is horizontal convergence of air at ground level. In other words, squeezing air togethernear the ground forces some air to rise upward, since it cannot go down into the ground. Conversely, if you couldpull apart a tube of toothpaste from the bottom, then paste from above will move down to fill the space. In otherwords, when air is flowing out of a region at the surface of the earth (divergence), air will sink down fromabove to fill the space.

Since the winds at the surface flow toward lower pressure and away from higher pressure, surface low pressureareas are associated with surface convergence, forced rising air, and a good possibility for clouds and precipitation,while surface high pressure areas are assoicated with surface divergence, forced sinking air, and generallyfair, cloud-free weather. The figure below also shows that rising and sinking air can be forced by convergenceand divergence from the top of the troposphere as well. We previously showed that upper level divergence happens justdownstream of 500 mb troughs and this forces rising air motion along with the possibility of clouds and rain, whileupper level convergence happens just downstream of 500 mb ridges and this forces sinking air motion with fairweather expected.You should understand the relationships betweenthe surface pressure pattern, surface winds, covergence or divergence, rising or sinking air, and the expected weather conditions. Following are two links that review this information, which you may find useful. One is a WORD document about winds and weather maps which includes figures and the other is an image document showing horizontal convergence, divergence, and vertical motion.

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 Relationship between horizontal covergence and divergence and vertical air motion. -- Left side shows that sinking air motion (downward moving air) is forced by horizontal covergence at the top of the troposphere and divergence at the surface or bottom of the troposphere. Convergence in the upper troposphere happens just downwind of 500 mb ridges. Divergence in the lower troposphere takes place near surface high pressure areas. -- Right side shows that rising air motion (upward moving air) is forced by divergence at the top of the troposphere and convergence at the surface or bottom of the troposphere. Divergence in the upper troposphere happens just downwind of 500 mb troughs. Convergence in the lower troposphere takes place near surface low pressure areas. Clouds and precipitation form in regions where air is ascending or moving upward.

Look again at the surface map forApril 8 shown above on this page. You should now realize that the winds near the surface low overNebraska will be counterclockwise and inward (converging), which forces air to rise upward. The windswill also be relatively strong due to the strong pressure gradient force (tight packing of the isobars).In fact, weather systems that produce both strong surface winds and significant precipitation will havesurface low pressure at their center. The tight pressure gradient results in strong winds and the convergence and forced rising motion produces clouds and precipitation. The strongest of these lowpressure systems are the epic storms of history. These include winter storms as well as hurricanes.The surface map for 18Z on Thursday, January 4, 2018, which has an example of a strong winter stormjust off the northeastern coast of the United States, is shown below.

 Surface map from 18Z on Thursday, January 4, 2018. The labeled contours are isobars or lines or equal sea level pressure.Notice the strong surface low pressure (963 mb at center) just off the northeast coast of the United States, whichindicates a strong winter storm. The color shading represents the surface wind speed in nautical miles per hour (knots).The strongest winds are found where the isobars are closely packed together. Note that winds are stronger over theoceans and Great Lakes relative to land areas, even when the isobar spacing is the same. This is because there is lessfrictional drag over the smooth water surface relative to the rougher land surface, which results in slower wind speedsover land for the same pressure gradient.

Other examples of recent strong low pressure storms are Hurricane Katrina in 2005 and also Superstorm Sandy in 2012.The tight packing of isobars is easily seen on the Superstorm Sandy map. At first glance Hurricane Katrina looks much weaker in that the isobars are not as tightly packed. However, notice that the spacing of the isobars near Katrina is every 8 mb, instead of every 4 mb like it is on the rest of the map (and most other maps). So to properly compare the pressure gradients on the maps, near Katrina there should be another isobar drawn between each of the isobars ithat encircle the center of the storm. Both of these surface low pressure storms had strong surface convergence of airand forced rising motion with heavy precipitation.Generally strong winds are possible wherever strong pressure gradients form, not just with surfacelow pressure areas. Strong pressure gradientscan occur in association with strong surface high pressure as well. The difference is that surface high pressureforces air to sink vertically, so there will not be much in the way of clouds and precipitation accompanying thestrong winds.