We have measured the mobility spectra of small ions in air for temperatures between 20 and -40 degrees C at constant relative humidity of about 85%. Drift times were varied between 800 and 50 ms. The mobility ratio k(-)/k(+) was used to compare the ion mobility of negative and positive small ions. The average electric mobility ratio k(-)/k(+) decreases between 20 and -40 degrees C, reaching a minimum value around -20 degrees C. Below -20 degrees C the ratio for fast ions increases slightly, resulting in a very small decrease of the average mobility ratio. Variations in ambient air quality slightly affect the electric mobility spectra of the small ions but the overall influence on the electric mobility ratio is small (+/-5% or less) compared to the temperature effect and the associated absolute humidity changes.