Interpreting Surface and Upper Air Charts

Surface Charts
The thin lines that appear on a surface weather chart (below) are contours of constant sea level pressure (measured in millibars), referred to as isobars. On most charts issued by NOAA, isobars are drawn and labeled in four millibar (mb) increments. For example, the isobar that crosses the Florida panhandle from the Gulf of Mexico is the 1012 mb isobar (annotated version). Any weather station located along that isobar as it continues to the northeast would have a sea level pressure of 1012 mb. Since they are drawn in four millibar increments, the isobar immediately adjacent to the 1012 mb isobar would be 1008 mb or 1016 mb depending upon whether the pressure was increasing or decreasing in that direction.

The purpose of isobars is to identify areas of low and high pressure and delineate frontal boundaries. Centers of high or low pressure are further identified by the placement of an H or L respectively. A review of the surface chart for April 4, 2011 (below) shows that isobars are rarely straight lines nor are high and low pressure centers neat circles. Instead, highs and lows are surrounded by an abundance of irregular dips and bulges. Bulges that extend from an area of high pressure, such as the one in the Dakotas is a ridge of high pressure associated with the high in central Utah. The pronounced irregularity extending southward from the low pressure center near northern Lake Huron into the Ohio valley is a trough of low pressure. An annotated version of the surface chart showing the ridge and trough can be found here. An additional trough of low pressure is located in Washington and Oregon and marked with a dashed line. Generally, ridges of high pressure are associated with calm weather while troughs of low pressure typically promote unsettled conditions and thunderstorms.

Surface analysis chart for 18Z on April 4, 2011. Courtesy of NOAA.

In addition to isobars, lows and highs, surface weather charts identify a number of other features such as cold, warm and stationary fronts. These features are identified on an annotated version of the April 4, 2011 surface chart.

Upper-Air Charts
The layer of our atmosphere in which our weather occurs is approximately six miles thick. Although we experience weather as a surface phenomena, it is the characteristics and associated dynamics within this six mile layer that produces our weather. If you wish to predict the weather, you must analyze and understand this three dimensional picture.

Above the surface, barometric pressure on weather charts is represented as constant pressure surfaces or the height (in meters) at which a particular pressure value is located. Below is a constant pressure chart for 500 mb, a pressure level at the half way point of the six-mile layer located at an average height of 5,500 meters (18,044 feet). Similar to surface pressure observations, the actual height of the 500 mb pressure reading varies considerably across the country and from day to day.

Contours of constant height appear as solid black lines and are plotted in 60 meter increments (annotated version). For example, the contour bisecting the southeastern United States identifies those locations where the 500 mb level is measured at 5,880 meters above the surface. The next contour immediately to the northeast is the 5,820 contour indicating those locations where 500 mb was found at a height of 5,820 meters. The height of the 500 mb level is related to the density of the air column below and above the observation point. Essentially, the height of the 500 mb level will be higher in a warmer, less dense air column and relatively lower in a colder, denser column. It may be helpful to view a constant pressure chart as a topographic map of the atmosphere.

500 mb analysis at 2200Z on June 22, 2011. Courtesy of the SPC.
 

It can be very confusing if this is your first experience examining upper air charts. Fortunately, there is a relatively straightforward way to glean useful information from them even in the absence of a complete understanding. Essentially, low heights equate to low pressure at the level being depicted on the chart -- 500 mb in this example -- and high heights indicate high pressure at the particular level. Similar to surface charts, upper air charts depict troughs and ridges (annotated version), although in this context these terms refer to heights of constant pressure. In the example from June 22, 2011 (above), a broad upper air trough extends over Missouri, Illinois and Indiana, and a prominent upper air ridge is located over the northern US Rockies. Upper air troughs typically promote unsettled weather while upper air ridges are associated with quiet conditions.

Transitions of Upper Air Patterns
Similar to the features on a surface weather map, contours of constant height are always in a state of transition. Upper air ridges and troughs typically move from west to east, although in some instances they defy convention and drift from east to west (known as retrograding). In addition to their journey across the country, the amplitude of ridges and troughs can change dramatically or completely flatten out a short period of time.

Transitions of 500 mb Patterns
Imagery Courtesy of the Storm Prediction Center
500 mb analysis at 00Z on April 4, 2011 (Larger Image)
500 mb Analysis at 12Z on April 4, 2011.
(Larger Image)
500 mb Analysis at 00Z on April 5, 2011. (Larger Image)

The 500 mb analyses for the twenty-four hour period beginning at 00Z on April 4, 2011 and ending at 00Z on April 5, 2011 are presented above. At 00Z on April 4, the 500 mb trough was oriented southwest to northeast over the western United States. Just twelve hours later, the trough had traveled east to the Great Plains and had taken on a more north-south orientation. By 00Z on April 5, had continued the transition to a north-south orientation had nearly reached the Mississippi River valley.