Winds at the 500 mb Height Level; Southwest Monsoon Season

Wind direction and speed at 500 mb

The wind at 500 mb can be determined from the height pattern. The wind blows parallel to the height contours with lower heights to the left of the wind direction. In other words, if you are moving with the 500 mb level wind at your back, lower 500 mb heights will be toward your left and higher 500 mb heights will be toward your right. More information on winds and weather maps is provided below and in the next page of the reading material. During the winter months across the United States, this means that the 500 mb winds generally blow from west toward east, but follow the wavy pattern of the height contours. This is because the air temperature generally gets colder as one move toward the north and 500 mb heights generally get lower. This general west to east air motion at the 500 mb height level weakens in the summer as the temperature differences between the tropics and the Arctic becomes weaker. Keep in mind that this is the wind several thousand meters above the Earth's surface and not the wind that would be felt on the ground.

The wind speed is faster where the height lines are closer together, and slower where the height contours are spaced further apart. If this helps, you can think of the 500 mb height lines as channels through which the air moves. The wind follows the wavy pattern of the lines, and gets squeezed together where the lines get closer together, resulting in faster winds. This is just how water would flow in a hypothetical channel. A couple of examples are shown below.

In the first example for 500 mb winds the lines with arrows represent the trajectory of the air flow at various places within the 500 mb height pattern. Regions of strong and weak wind speeds based on the spacing of the height contours are marked on the map. In the second example for 500 mb winds arrows are again used to mark wind trajectories. It is also emphasized that the flow around closed highs is clockwise and the flow around closed lows is counterclockwise. A visualization of the relationship between the 500 mb height pattern and the winds at high alitudes can often be seen by looking at a movie of the 500 mb height pattern superimposed over a satellite image that is sensitive to the motions of clouds and water vapor at high altitudes. The weather data available from the UA Department of Atmospheric Sciences produces a movie showing the evolutions of the 500 mb height pattern over the last 24 hours (one frame per hour) superimposed on what is known as a satellite "water vapor image." The water vapor image is able to "see" or "sense" the presence of clouds and relatively high water vapor concentrations at high altitudes. Although, it is most sensitive to clouds and water vapor at altitudes higher than the 500 mb height (lower air pressure), the winds at those altitudes are often similar to those at 500 mb. When you look at the loop, notice that the features in the water vapor image generally move parallel to the 500 mb height pattern with lower heights to the left of the wind direction. It can be a bit tricky because the 500 mb height pattern is changing with time along with the winds. Link to 500 mb movie for the last 24 hours

The 500 mb winds have a large influence on the motion of smaller-scale weather features, such as the movement of surface low pressure areas, the movement of hurricanes, and the movement of individual thunderstorm cells. In this context, smaller-scale means small compared to the size of the larger scale trough and ridge pattern. For these reasons, the winds at the altitude of the 500 mb pressure level are often referred to as the "steering level" winds. In fact where thunderstorms do form, they generally move as a unit in the direction of the winds near the 500 mb pressure level. This is something that we will pay attention to when local (Tucson, AZ) thunderstorms form. Note that we have only covered how to interpret the 500 mb winds based on the height pattern. We have not discussed the forces that cause the wind to blow.

Overview for North American Monsoon

The North American Monsoon (sometimes called the Mexican Monsoon or the Arizona Monsoon or the Southwest United States Monsoon) is experienced as a dramatic increase in rainfall from an extremely dry late spring season to a rainy summer season. The North American Monsoon region includes northwestern Mexico and parts of the US desert southwest. While the most notable change in our weather is the increase in moisture and precipitation during the monsoon season, it should be pointed out the meteorological definition of a Monsoon is simply a seasonal shift in wind direction. Monsoonal shifts in winds around the world are often associated with changes in precipitation, but they need not be.

On average, the climate of Earth is characterized by a belt of high 500 mb heights at around 30° North latitude. This semi-permanent ring of high 500 mb heights is called the subtropical high. Lower 500 mb heights are found both to the north and to the south of the subtropical high region. The wind flow at 500 mb is parallel to the height contour pattern, with lower heights to the left of the wind direction. This means the 500 mb winds are generally west toward east north of the subtropical high and east toward west south of the subtropical high. (See Figure showing average 500 mb height pattern and winds.) For most of the year, Tucson is north of the subtropical high and the 500 mb winds over Tucson are generally west to east. At lower levels in the atmosphere, air south of the subtropical high is humid tropical air, while the air near the subtropical high is much drier.

The North American Monsoon is associated with the northward movement of the position of the subtropical high that occurs from June through September. As the subtropical high moves to the north of Tucson, the 500 mb winds shift direction to become east to west. This is an indication of the monsoonal wind shift that happens over the southwestern US and northern Mexico. (see Figure showing average 500 mb height pattern and winds during the monsoon season.) During the spring months of the year, the subtropical high pressure cell is south of Arizona, and Arizona experiences generally westerly air flow with generally dry air masses. During the summer monsoon season, the center of the 500 mb subtropical high moves north of Arizona. This places Arizona in generally easterly air flow with much more humid, tropical air masses. The specific center of high heights that forms in this part of the world during the summer season commonly shows up as a closed high on a 500 mb height map and is often called the monsoonal high. Rainfall during the monsoon season is not continuous. It varies considerably, depending on a variety of factors. There are usually distinct "burst" periods of heavy rain during the monsoon, and "break" periods with little or no rain. Monsoon precipitation, however, accounts for a substantial portion of annual precipitation in northwest Mexico and the Southwest U.S. Most of these areas receive over half their annual precipitation during the monsoon season.

Southwest Monsoon 500 mb Pattern and Air Flow

As mentioned above, the typical 500 mb flow over the southwestern United States is generally westerly (west to east). This is a consequence of the fact that for most of the year, the average air temperature between the ground and 500 mb gets colder as one moves toward the north, which means lower 500 mb heights are found toward the north and the corresponding 500 mb winds are west to east (parallel to the height contours with lower heights to the left of the wind direction). One indicator of the southwest monsoon is to have higher 500 mb heights form to the north of southern Arizona. This results in a wind shift at 500 mb, with 500 mb winds that have an easterly component, i.e., winds from east toward west. This can be understood by applying the rule for 500 mb wind direction. With higher 500mb heights to the north of Tucson and lower 500 mb heights to the south of Tucson, the 500 mb wind direction will be generally east to west.

The meteorological definition of a Monsoon is simply a seasonal shift in wind direction. Often a monsoon season is also characterized by a shift in precipitation (as it is in the southwestern United States and India), but it need not be. The term monsoon is often misused, even by many TV weather reporters. It is incorrect to call a summer thunderstorm a "monsoon" or to say it is "monsooning" outside during a summer thunderstorm. There is a monsoonal shift in winds that happens every year in the southwestern United States and northwestern Mexico though. This shift in wind patterns is most easily seen by looking at the seasonal shift in wind direction that occurs at the 500 mb pressure level. Below are links to higher resolution monthly average 500 mb height maps that have been annotated to show the monsoonal shift in wind direction that takes place during the months of July and August over the southwestern United States. In the figures below the 500 mb height contours are shown every 10 meters, instead of the more common every 60 meters. This is done to better show the wind direction at 500 mb in Tucson. The bold 'H' indicates where the highest 500 mb heights are located in the vicinity of the southwestern United States. The bold arrow indicates the 500 mb wind direction in Tucson based on the average height pattern shown. Notice that the average wind direction is southwest (from the southwest) during the month of June. This is prior to the monsoon season. During the monsoon months of July and August, the 500 mb high moves to the north and east of Tucson and the average wind direction is from the east and southeast. In September, the high center moves back to the south of Tucson, the wind direction shifts back to southwest, which indicates the end of the monsoon season.
June Average 500 mb heights and Tucson wind direction
July Average 500 mb heights and Tucson wind direction
August Average 500 mb heights and Tucson wind direction
September Average 500 mb heights and Tucson wind direction

Please also read over the following link, North American Monsoon article by the NWS. You should understand graphics 2 and 3, which show the monsoonal wind shift at the 500 mb height levels (Graphic 1 shows wind patterns just above the ground surface, which we are not covering in this course). Graphic 3 shows one favorable position for the "monsoon high" in the sense that there are southeast winds at 500 mb over Tucson. As long as there is an easterly component to the 500 mb wind direction, Tucson will generally be favored for thunderstorms. Basically if the high center is north of Tucson, there will be an easterly component to the 500 mb wind direction. The chance for thunderstorms also depends on whether or not there is sufficient water vapor in the atmosphere as discussed in the later section.

In reality, the local 500 mb height pattern changes from day to day and a monsoonal height and wind pattern comes and goes throughout the months of June through September. It is common for there to be periods of several days to a week with favorable conditions for thunderstorms followed by several days to a week of unfavorable conditions. The officially defined monsoon season runs from June 15 through September 15. When the center of highest heights is south of Tucson, rain generally does not fall in Tucson. Worse yet, when the high center is directly overhead, not only is it generally rain-free, but also quite hot as the core of warmest air is right over us. The best chances for summer rains in Tucson happen when the 500 mb high center is north of our area. An example of a 500 mb height pattern that is unfavorable for summertime thunderstorms in Tucson is shown in the map for 0600 GMT, June 23 (Second example above). On that map, there is a high center located to the southwest of Tucson, and the 500 mb windflow over Tucson is from the northwest, which is not favorable for clouds and rain. An example 500 mb height pattern which is more favorable for thunderstorms in Tucson is shown in this example of a 500 mb pattern favorable for thunderstorms in Tucson. The timestamp at the bottom of the map 00Z, Thursday, August 9, 2012 corresponds to a local time in Tucson of 5 PM Wednesday, August 8, 2012. The approximate center of highest heights is indicated with an "H" along with several arrows showing the wind trajectory at various places on the map. One thing to point out is that the 500 mb height over Tucson is above 5940 meters. This is well above the average height of just over 5880 meters for Tucson during the month of August. Thus, we would expect warmer than average temperatures in the area, which was the case as a record high of 107°F was recorded. With regard to the possibility of summer thunderstorms, notice that the high center is north of Tucson and the 500 mb wind flow is northeasterly (from the northeast). This is a favorable wind direction for monsoon season thunderstorms in Tucson as there is an easterly component to the wind direction. In fact thunderstorms did form in the mountains to the east and northeast of Tucson in the early afternoon and by 5 PM had moved into the Tucson area. Remember that the movement of individual thunderstorms is often in the direction of the 500 mb winds. An interesting weather product provided by the department of Atmospheric Sciences is a movie loop of the 500 mb height pattern over the previous 24 hours. In addition to the 500 mb height pattern, satellite-derived water vapor imagery is superimposed on the images. Without going into details, the water vapor imagery distinguishes areas of high water vapor content and clouds. This allows you to visualize the movement of these features with respect to the 500 mb pattern. Bright white cloud masses are generally steered around by the 500 mb winds.

Why Are East to West 500 mb Winds More Favorable for Monsoon Season Thunderstorms in Tucson?

Increase in Water Vapor

The monsoon season happens every year in the southwestern United States. While a monsoon is defined by the seasonal change in winds, monsoons are often associated with changes in humidity and precipitation. The change in water vapor content during the North American Monsoon is obvious when looking at the changes in precipitable water over a year, for example, Tucson daily average precipitable water for 2011.

Using 500 mb Maps to Make Monsoon Season Forecasts for Tucson

Given the importance of the local 500 mb wind direction with regard to the possibility for monsoon season thunderstorms developing in Tucson, I want to direct you to some forecast maps that clearly show the local 500 mb height pattern and winds. Before looking at the forecast maps, the section below explains how to read the wind symbols that are plotted on some weather maps.

Wind Barb Symbols on Weather Maps

The wind barb symbol is a common way to indicate the wind speed and wind direction at points on weather maps. The wind barb consists of a line segment starting at the location (point on a map) where the wind information is provided and points in the direction from which the wind is coming with shorter line segments and flags attached to the end of the direction line segment to specify wind speed. The wind direction is based on standard compass directions with north oriented straight up. Thus, for a wind from the north the barb sticks straight up, from the east the barb points right, from the south the barb points straight down, and from the west the barb points left. Wind speed is indicated by the line segments and flags attached to the end of the barb. There are long and short line segments. Each long line segment represents 10 knots of wind (there can be up to 4 long line segments). A short line segments represents an additional 5 knots of wind. If the wind speed is less than 5 knots, then an open circle is drawn on top of the location where the wind is indicated. By the way a knot is a nautical mile per hour (1 knot = 1.15 miles per hour). You should look over the links below.

(Figure D Reading wind barb symbols) ; (Legend for wind speed)

500 mb forecast maps with wind barbs

The link provided at the beginning of the next paragraph will open the most recently available forecasts of 500 mb heights and winds from the the United States' Global Forecast System (GFS) model. The information is provided from Pivotal Weather. Once the page loads, you can use the control buttons on the top to see the model forecast for 0 to 384 hours into the future (0 - 16 days). Note the zero hour forecast shows the current 500 mb pattern and is not actually a forecast. The time stamp above the upper left corner of the map tells you the time (GMT) of the forecast (remember that local Tucson time is 7 hours earlier than GMT). The 3 digit number after the capital 'F' tells you how many hours into the future between the model starting time and the forecast. For example, 'F192' means that the forecast is for 192 hours after the model started running and 'F000' means the forecast is for zero hours into the future, which is simply the point from which the model forecast started. You should be able to read the 500 mb height pattern and wind barb symbols. You should be able to verify that the relative wind speeds and wind directions shown on the map are generally consistent with the height pattern shown, i.e., the direction of the wind is parallel to the height contours with lower height values to the left of the wind direction and higher wind speeds are found where the contour lines are closest together. Analyses of sample maps are provided as examples below the link. Unfortunately, the contouring routine used by Pivotal Weather does not alway produce smooth height contours, although in most cases the little wiggles are not important.

Here is a link to the Current GFS forecast of 500 mb heights and winds (Courtesy of Pivotal Weather). The controls above the map allows you to play a movie of the forecast maps or select a single frame. Using the 500 mb height pattern, you can make crude forecasts for Tucson based on the information provided above. Remember that in addition to the 500 mb wind direction, the amount of precipitable water vapor in the atmosphere is also an important factor to consider and no information about the water vapor content of the atmosphere is provided on 500 mb height maps. You can also get a forecast of the precipitatable water vapor, GFS forecast of precipitable water (from Pivotal Weather). The color shading, with key at bottom of map, shows the forecast for precipitable water vapor in units of inches. The contour lines on the map are isobars of sea level pressure, which will be discussed on the next reading page. In the next section, an example forecast analysis for Tucson based on 500 mb height maps is provided.

Example 500 mb Forecast Analysis for Tucson from August 2012

As you may know if you are from Tucson or as you may learn if you follow local weather during the monsoon season, accurately predicting whether or not Tucson will get thunderstorms on a given day is very difficult. Looking at forecasts like these, we can determine if the large scale weather pattern is favorable for monsoon season thunderstorms, but making precise forecasts for exactly where thunderstorms will form and move is much more difficult. The department of Atmospheric Sciences runs a local weather model, which makes forecasts for a portion of the southwest US and northwest Mexico. If you are interested in an alternative weather forecast for Tucson, check out Regional Weather Forecast Models and Products for southeast Arizona from ATMO WRF Model. Keep in mind that we have only covered a few aspects about thunderstorm formation during the monsoon season. There is much more to consider when forecasting the possibility of thunderstorms on a given day. Some of these, such as instability in the atmosphere, which is required for thunderstorms and favorable wind shear profiles, which is how the horizontal winds change with increasing height, will be covered later in the semester.