# How come in these Bernoulli problems, the Pressures on both sides cancel?

If you have an open tank filled with water, and you poke a hole half way down the tank, you can solve for things like the speed of the water coming out of the hole with the Bernoulli's equation, $P_1 + \rho gh + \frac{1}{2}\rho v_1^2=P_2 + \rho gh_2 + \frac{1}{2}\rho v_2^2$

Why do these pressures cancel out?