The radius of curvature of the sound path is inversely proportional to the gradient.
[2] When the sun warms the Earth's surface, there is a negative temperature gradient in atmosphere.
[4] The opposite effect happens when the ground is covered with snow, or in the morning over water, when the sound speed gradient is positive.
In this case, sound waves can be refracted from the upper levels down to the surface.
[3] In underwater acoustics, speed of sound depends on pressure (hence depth), temperature, and salinity of seawater, thus leading to vertical speed gradients similar to those that exist in atmospheric acoustics.