THE OROGRAPHIC EFFECT

How is the top of a mountain different from sea level? Temperatures are colder Pressure decreases

Why is the vegetation so different in California, compared to Nevada?

What separates California from Nevada? The Rocky Mountains

Why is California green and lush, while Nevada is dry and arid? Coastal cities receive moisture from the ocean.

Why does the air have to come from the Pacific and not from the western plains?

air molecules at sea level air molecules on the mountain top At sea level the molecules are closer together so they bump into each other more – this causes friction. Friction causes HEAT

Can air go through the mountain? Since air cannot go through the mountain it is forced to go up and over it. Since there is less friction the air becomes cooler. Cooling due to expansion.

As the air rises it cools eventually reaching the dew point and forming CLOUDS

The overall result is temperature will Decrease Humidity Increases And it will rain Windward side of the mountain.

The air will expand, cool to the dew point temperature and condense as long as condensation nuclei are present. The moist air rises

As the air mass travels down over the Rockies, it is now dry, since all of the rain fell on the west coast. The air molecules are pushed closer together as the pressure increases with the decrease in altitude.

As the air sinks pressure will Increase. This forces the molecules closer together again increasing friction. Temperature will Warming due to compression.

Deserts are on the leeward side.

This is known as the OROGRAPHIC EFFECT Windward side is Wet Leeward side is Dry

Windward vs. Leeward LEEWARD WINDWARD

Orographic Effect – Rain Shadow

orographic effect in action

The air mass moves in towards the mountain carrying with it evaporated water vapor. The slope of the mountain forces the air mass to move up into an area with less air so the molecules can expand. Due to the decrease in friction it cools. This is known as adiabatic cooling.

Once the air reaches the dew point the water molecules condense and clouds begin to form. As the air continues to cool, but not as fast as before since condensation releases energy, the drops continue to grow and when they are large enough will fall as rain, sleet or snow.

Once the air is over the mountain it descends and warms as the molecules are compressed back together with the increase in pressure. This is known as adiabatic warming. The air is dry since the moisture was left on the windward side of the mountain and also warmer. The heat added to the air from the condensation can make the air up to 50 degrees warmer as it sinks and compresses.