One of the common misunderstandings regarding Bernoulli's principle is that, in general, 'faster moving air has lower pressure'. Bernoulli's principle can only be applied to a single streamline in the absence of friction, and measurements of streams generated by fans, hairdryers or simply air blown from one's lungs show that static pressure inside those streams is exactly equal to atmospheric pressure. Equivalently, 'a free air stream in the atmosphere is exclusively decelerated by friction'. While I understand that Bernoulli's principle can't be applied in these situations, I'm not clear on why a free air stream would always have exactly atmospheric pressure. Could someone explain that to me?