Abstract
During winter daylight hours changes in ƒ 0F2 are almost continuous due to the presence of travelling ionospheric disturbances. It is suggested that variation in hourly values of ƒ 0F2 from day to day on magnetically quiet days is due mainly to this cause and it is shown that these values are only reliable to ± 0.5 Mc/s. The resultant error in estimated maximum usable frequencies has been determined. Critical frequencies as read from ionogram records are not true vertical incidence values but are reflected at an angle θ from the vertical because of sloping isoionic contours. This angle has been estimated as well as the amount by which average measured values of ƒ 0F2 must be decreased to give real vertical incidence values of ƒ 0F2 .
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