The surface map below displays Barometric Pressure isobars as white lines. Green wind barbs/flags indicate wind speed and direction. Areas of blue forecast light rain, while yellow and red are heavier rain.
The surface map, taken from the CaribWX site on March 26, 2004, is a good example of how friction effects the relationship between the direction of the winds and the isobars.
Note that, out at sea, the surface wind flags are closer to parallel to the isobars than perpendicular to them. Now look at the wind flags on the southern continental US and Central America, where the winds are more oblique to the isobars. Why?
Recall that the pressure gradient force tries to drive winds from high towards low pressure, perpendicularly across the isobars. Friction acts in the opposite direction of flow, and the Coriolis force deflects wind to the right of the motion, in the northern hemisphere. It is important to remember that the Coriolis force increases with increasing wind speed. Since friction slows the wind speed, it plays an indirect role in which way the winds blow.
The sea surface generally offers less friction than land. So the wind speeds will be higher for a given pressure gradient. Higher wind speeds means more Coriolis force deflecting the air to the right, which turns it more parallel to the isobars. In fact, if there was absolutely no friction, the gradient force would accelerate the wind, increasing the speed, and thus increasing the deflecting Coriolis force. These two forces would come into equilbrium as the wind direction comes parallel to the isobars. This is very close to what happens in the middle and upper troposhere. Such winds are called geostrophic winds.
Now back to the surface. On land, the greater friction keeps the wind from achieving the higher speeds, hence the Coroilis force never gets high enough to deflect the winds to nearly parallel to the isobars. Instead, the slower winds are directed more diagonally across the isobars.