Possible Clues:

• Line connecting points of equal atmospheric pressure
• Pressure indication on a weather map
• Line in answer penned by Boris, strangely
• Line offered by Boris, panicked about answer?
• Weather map marking
• Cold front line?
• Boris angry about answer - as the forecasters see it, it's bound to bring its own pressure
• Chart pressure line
• Evidence of pressure or bias, strangely
• A line connecting points of equal pressure