Any surface of equal pressure in the atmosphere is a constant pressure surface. A Constant Pressure Analysis Chart is an upper air weather map where all information depicted is at the specified pressure of the chart. From these charts, a pilot can approximate the observed air temperature, wind, and temperature/dewpoint spread along a proposed route. Twice daily, five computer-prepared constant pressure charts are issued from observed data: 850 mb (5000 ft); 700 mb (10,000 ft); 500 mb (18,000 ft); 300 mb (30,000 ft); 200 mb (39,000 ft). 

Contour lines on a Constant Pressure chart depict a height pattern by showing the heights of the specified pressure for each station. The contours depict highs, lows, troughs, and ridges aloft in the same manner as isobars on a surface chart. Just like closely-spaced isobars, closely-spaced contours mean strong winds.

Isotherms (dashed lines drawn at 5-degree C intervals) on a Constant Pressure chart show horizontal temperature variations at chart altitude. By inspecting isotherms, you can determine if your flight will be toward colder or warmer air. Subfreezing temperatures and a temperature/dewpoint spread of 5 degrees C or less suggest possible icing. 

Isotachs are lines of constant wind speed analyzed on the 300 and 200 mb charts; they separate higher wind speeds from lower wind speeds and are used to map wind speed variations over a surface. Isotachs are drawn at 20-knot intervals and begin at 10 knots. Isotach gradients identify the magnitude of wind speed variations. Loosely spaced isotachs depict weak gradients which represent small wind speed variations. Zones of very strong winds are highlighted by hatches.

If the constant pressure level is high, then it has warm air beneath it. Weather will tend to be good when there is a warm upper high (except for the possibility of restricted visibility), since warm air will not usually rise through air that is already warm. If the constant pressure level is low, then it has cool air beneath it. Weather will tend to be poor when there is a cold upper low since a parcel of warm air at the surface will tend to keep rising, creating unstable conditions.

When planning a flight from west to east in the northern hemisphere, tailwinds will be found along the northern side of a high pressure system or the southern side of a low pressure system. Going from east to west, tailwinds will be found along the southern side of a high pressure system or the northern side of a low pressure system.