Radio occultation measurements of temperature versus pressure and longitude at 66 deg N latitude on Mars. The figure was constructed from a set of 36 vertical profiles with roughly uniform spacing in longitude. The season was late spring (Ls = 75 deg) and the local time of all measurements was about 0400. Contours of constant temperature are labeled in kelvins. The dashed line shows the pressure at the surface. This pattern of thermal structure remained stationary relative to the surface during the one-week interval of the measurements. The largest temperature variations occur at low altitudes in the eastern hemisphere (0-180 deg E), where peak-to-peak variations on the 800-Pa pressure level exceed 12 K. Several characteristics of these data point to planetary waves as the source. Planetary waves, which are also known as Rossby waves, involve atmospheric motion at very low frequencies and large horizontal scales.