This surface meteorological chart shows the temperature and sea level pressure pattern over the continental United States. The superimposed wind vectors depict the surface flow pattern. This chart is updated every hour.
The temperature is contoured every ten degrees Fahrenheit. Cold areas (near and below freezing) are shaded blue and purple while warm and hot regions are shaded orange and red. The overlaid light contours, called isobars, depict sea level pressure (contoured every four millibars). The wind vectors represent the horizontal wind at the surface. The arrows point in the direction in which the wind blows. The winds are in meters per second, with the scale vector of 10 m/s.
Areas of sharp temperature gradients (several contours close to each other) tend to be associated with the position of surface fronts, which separate airmasses of different temperature and moisture (and therefore density) characteristics. Fronts are also often characterized by a strong shift in wind direction and sometimes speed. Note that surface winds tend to flow out of the areas of High pressure as they rotate clockwise around its center, and into the areas of Low pressure, as they rotate counter-clockwise around its center. This is associated with the role surface friction plays in the force balance.
Areas on the map where the wind blows approximately at a perpendicular angle to the temperature contours are regions of strongest temperature advection Wind vectors pointing from colder to warmer temperatures represent cold advection and wind vectors pointing from warmer to colder temperatures represent warm advection. Typically strongest warm advection occurs behind the warm front and ahead of the cold front (southerly flow) and strongest cold advection occurs behind the cold front (northerly flow).