The HF Area Prediction chart is a guide to frequency selection for HF communications between mobiles and a specified base station, in this case San Francisco. The recommended frequency for communications with the base at a given hour is read off from the point in the map corresponding to the mobile’s location.The predictions are made from real time data which are updated each hour.
If present, dashed lines delineate areas of unlikely communication. Poor communication areas which can result from one or more of the following conditions.
- the Absorption Limiting Frequency (ALF) is higher than the Optimum Working Frequency (OWF)
- the ALF exceeds the highest frequency in the user’s frequency set
- there is no suitable frequency from the user’s set that is between the OWF and ALF
Space Weather Forecasting Centre
Bureau of Meteorology Australia – source