Millibars is the unit used most by meteorologists.
Since there are fewer air molecules at higher levels to exert a force on a surface, the pressure at higher levels is less than at lower levels. A low pressure center should be labeled beneath the left column because there are fewer air molecules in that column to exert a force on the surface below. The green contours are isobars and they are lines of constant pressure. Working with current weather data, so no answer key is available. Credits and Acknowledgments for WW2010.