Venturi Effect MET
As you may know from Principles of Flight, air pressure is made up of two components (static and dynamic) and the total of these is constant. So if dynamic pressure increases (because it is moving faster, such as through a venturi) than static pressure falls as a consequence. Because it is static pressure that is measured by the altimeter, if the static pressure at the surface falls, so do all the pressure levels. Consequently, the true altitude will be lower than your indicated altitude. This can potentially be unsafe: you are lower than you think you are.