Above the interface between the liquid and the surface, the pressure is equal to the atmospheric pressure . At the meniscus interface, due to the surface tension, there is a pressure difference of , where is the pressure on the convex side; and  is known as Laplace pressure. If the tube has a circular section of radius ,  and the meniscus has a spherical shape, the radius of curvature is  , where  is the contact angle. The Laplace pressure is then calculated according to the Young-Laplace equation:where  is the surface tension.