Retrieved temperature (top panel) and water abundance (bottom panel) as a function of the transition angle between the day and nightside of a planet with the radius andsurface gravity of GJ 1214 b. The colours correspond to the temperature range (blue: 300–650 K; black: 500–1000 K; red: 1000–1500 K). To put all cases on a single diagram we show the relative retrieved temperature, Θ ≡ (T_ret − T_night)/(T_day − T_night), so that in all cases Θ = 0.5 at the terminator. The retrieved temperature is systematically biased toward the dayside temperature (Θ ≥ 0.5), especially when the transition is sharp. For comparison the estimate of the width of the limb given by Eq. (3) is shown by the vertical dotted line.