In the last article on lift we will discuss Convergence. Spotting convergence lines or zones can be very helpful to Paraglider pilots as this means that you will go further on your XC flight and stay in the air for longer.
The boundaries where two air masses meet are known as Convergence zones. The image below illustrates the classic convergence line that sets up at our local XC paragliding site in Porterville.
The convergence zone moves around and it depends on a number of factors like, the ambient temperature, the lapse rate and the strength of the Southerly wind in Cape Town.
The red arrows on the right indicate the prevailing SW airflow. The red arrows on the left illustrates the NW airflow that often is associated with the Seabreeze. Cumulus clouds or cloud streets are often a very good indicator of the presence of this convergence zone
A Seabreeze is a wind from the sea that develops over land near the coast. In a Seabreeze front, cold air from the sea meets the warm air from the land and creates a boundary like a shallow cold front along a shear line. This creates a narrow band of soarable lift with winds as light as 10 knots (19 km/h). These permit the gaining of altitude by flying along the intersection as if it were a ridge of land.
Convergence may occur over considerable distances and so may permit virtually straight flight while climbing or gaining altitude.
The image below displays the airflow associated with a Seabreeze front. The red arrows indicate the airflow and the big Cumulus cloud the Convergence zone.