Atmospheric pressure is measured with an instrument on the ground called a barometer, and these measurements are collected at many locations across the U.S. by the National Weather Service. On weather maps, these readings are represented as a blue “H” for high pressure or a red “L” for low pressure.

The circular lines you see on the chart are isobars, which join areas of the same barometric pressure. The pressure pattern is important because we can use it to tell us where the wind is coming from and how strong it is. It also shows areas of high and low pressure.

On some weather maps, you may notice lines surrounding and encircling the "highs" and "lows." These lines are called isobars because they connect areas where the air pressure is the same ("iso-" meaning equal and "-bar" meaning pressure).

On weather maps, troughs are typically portrayed as dashed lines. Unless you're an amateur meteorologist, the significance of those lines probably hasn't been apparent. Which is too bad, because troughs are major drivers in predicting weather, often telling us if colder temperatures and rain or snow is expected.

The colors are typically a spectrum. The greens indicate light or moderate rainfall. Yellow, orange, and red indicate moderate to heavy rainfall and can also indicate hail. Whites or blues mean snowfall and pink indicates freezing rain, sleet, or a wintry mix.

The colors on a weather map indicate the amount of precipitation an area is, or will be, receiving. Generally speaking the darker the color the more precipitation indicated. Light and dark blue usually indicate snow, while dark red and purple mean strong to severe thunder storms.

The blue lines with triangles are called cold fronts. They represent the leading edge of colder air. The triangles are pointed in the direction the cold air is going. Red lines with semi-circles are warm fronts, the leading edge of warmer air.