Sea Level Pressure and 1000-500mb Thickness

• The colored contours indicate sea level pressure in millibars. High pressure is red, low pressure in green or blue. Only the last 2 digits shown -- sea level pressure is usually around 1000 millibars, so add 1000 to values in the range of 00-50, and add 900 to values in the range of 50-98.
  • Low sea level pressure indicates cyclones or storms near the surface of the earth. High sea level pressure indicates calm weather.
• The shaded contours indicate the vertical distance, or thickness, between the 1000 millibar surface and the 500 millibar surface, measured in tens of meters.
  • Since air behaves nearly as an ideal gas, and vertical distance is proportional to volume over a specified surface area, the thickness between two pressure levels is proportional to the mean temperature of the air between those levels. Thus, low values of thickness mean relatively cold air.
  • The 540 line is highlighted in black, since this line is often used as a rule of thumb to indicate the division between rain and snow for low terrain. When there is precipitation where the thickness is below 540dam, it is generally snow. If the thickness is above 540dam, it is usually rain (or sleet if the air next to the surface is below freezing).

850mb Temperature, Humidity and Winds

• Colored contours indicate the air temperature at the 850 millibar level, in degrees Celsius. The 0?C contour is highlighted, as this is also often used as a divider between rain and snow.
• The green shading indicates the relative humidity percentage at the 850 millibar level. High values indicate the availability of moisture. When large rates of ascent (in panel 3) are located with high moisture availability, heavy rainfall will likely occur.
• The streamlines indicate the wind direction.
• Advection of moisture by the wind can be inferred by noticing the direction and rate at which moist areas appear to be blown. Similarly, temperature advection can be inferred by noticing whether the wind is blowing cold air toward a warm region, or warm air toward a cold region.