Weather changes are often seen as random and most people would think that the affairs of the air are only affected by the air alone.
However, the shape of the land also affects the air in very dramatic ways.
If you live in a valley or near a mountain, you might be more keenly aware of how the weather changes around these areas.
The fact that hot and cold air pressure has to rise and fall over large bodies of land affects what exactly happens to the weather.
So what is the orographic effect and how does it affect the way the weather changes?
Well, after reading this article, you should be a little bit more aware of what happens when this effect really kicks in and you can observe it yourself.
The Orographic Effect – What Is It?
In short, the orographic effect is when air rises over an elevated surface, usually a mountain, causing a rise in temperature which then leads to altered weather.
This rise in temperature then changes the weather, usually resulting in cloud coverage as well as precipitation down the other side of the mountain.
Normally, air will cool down as it rises in temperature. There are many reasons why heat might rise, one of the main ones being if the ground is warmed up by the sun.
Also, if a cold front comes in, it will usually force any warm air to rise.
However, when the orthographic effect comes into play, there are no atmospheric reasons for the air to rise.
Instead, the air is raised by the land itself, which is also known as ‘artificial rising’. This sharp increase soon causes the air to rise and change drastically.
This is why rain happens at the top or down the side of a mountain. The air that rises cools down, which then creates precipitation.
Orographic Rainfall – How Does It Happen?
As warm air starts to rise with the mountain, then it will cool down. This is because air gradually gets cooler the quicker it rises.
Once the temperature drops then it changes its form. This is done through a process called Adiabatic cooling.
This is when the air starts to expand and, as a result, heat is lost. The pressure of the air lowers, which causes it to expand and cool down even further.
No heat is added or subtracted from this process as it is all done ‘artificially’.
The air will soon reach its dew point, which is the maximum level it can reach before it has to change from a gaseous form to a liquid form.
This is when condensation starts to occur. Moisture gathers in these clouds that start to form on the slopes.
The more that the moisture accumulates, the more it turns into drops of water. This is what then leads to rainfall.
The rainfall on a mountain can take the form of a light drizzle or a very heavy downpour.
Again, this all depends on the amount of moisture that has gathered in this area.
What Happens When Cold Air Descends The Mountain?
Once the air reaches the top of the mountain it will be very cold and very dry.
However, once the air travels down the side of the mountain on the side that is shielded from the wind then it will start to change form once again.
As the air goes down the mountain, the pressure will increase and the air will once again start to warm up.
Again, through the process of adiabatic heating, the air will regain a lot of moisture and the temperature will rise again.
This combination of moist and dry air on the top of the mountainside and the warm and moist air at the bottom, as well as the rainfall that occurs during this transition, will affect vegetation in very unique and interesting ways.
The Atmosphere And Topography Of The Windward Slope
If you have a mountain near a body of water, then this will usually lead to an increase in moisture that is traveling up the windward slope.
As a result, the orographic effect kicks in and there’s often a lot of heavy rainfall on this side.
As a result, any seaside mountain will usually have some lush vegetation growing on the side.
This is due to a combination of prevailing winds that are coming in from the sea and moist air particles.
A lot of farmers throughout history have set up their crops on the sides of these mountains to take advantage of this effect.
They’ll usually have a higher yield of crops that year as well as quicker regrowth.
You can often find dense forest areas on the sides of mountains, especially in areas such as the Amazon.
This is because of the orographic effect and the constant levels of heavy precipitation down the mountainside.
The Atmosphere And Topography Of The Leeward Slope
This is the part of the mountain that is facing away from the wind. This is the side of the mountain where the air travels downwards, gathering more warmth.
As a result, the air on the leeward side of the mountain is very warm and dry.
You can expect as little as 10 inches of rainfall on this side of the mountain, which is pretty meager compared to the 100 inches that can fall on the windward side.
As a result of this warm and dry air and the lack of moisture, there is very little plant or vegetation growth on this side of the mountain.
This can get so extreme, that you can even experience desert-like conditions on the leeward side of a mountain.
This other side of the orographic effect is what is known as a rain shadow.
Most farmers will be able to spot the difference between these two types of effects and will not establish any farms on the leeward side of the mountain.
This type of desert-like effect is very noticeable e.g. the Chinook Winds in the USA.
Where Can You Witness The Orographic Effect?
There are many places in the world where the orographic effect is very noticeable.
In fact, wherever you see a mountain, the orographic effect will be occurring. However, here are a few of the more famous examples:
- Sierra Mountains California (USA)
- The Great Dividing Range Mountain (Australia)
- The Southern Andes Mountains (Europe)
- The Northwestern face of Table Mountain (Africa)
- The Oppland Mountain (Norway)
These areas are known to experience the orographic effect on a regular basis.
Where Can You Witness Rain Shadowing?
As we have mentioned above, the downward side of the mountain where the air is completely dry owing to zero moisture is what is called rain shadow.
Here are some places where that effect is most noticeable:
- The Himalayas – this mountain range will block moisture that is coming from the Tibetan Plateau.
- Rhone Valley – this is a mountain range and valley in Switzerland.
- The Olympic Mountains – this is in Washington State and occurs mainly in the areas east of that region.
- Cascade Range – this is in the Pacific Northwest in areas such as Washington and Oregon and can be found in the east.
- The Prairies in Canada – with their rolling valleys and peaks, the orographic effect happens often.
- The Hawaiian Islands – the island of Kaho’olawe is entirely in rain the shadow. This is due to a prevailing wind that comes from Britain in the South West. (Also see ‘When Is Monsoon Season In Hawaii?‘).
- Grampian Mountains – this is in places like Herefordshire and along the border of Wales and England.
If you are mountain climbing or hiking, then you might have to be extra aware of this phenomenon, as it will dry out the area and cause you to become dehydrated.
What Clouds Form From The Orographic Effect?
There are various types of clouds that are only associated with this kind of weather phenomenon.
Here is a list of a few of them as well as their physical characteristics, so that you can spot them next time:
- Orographic fog – this is when the air will rise up the mountain and start transitioning to water molecules. This usually takes the form of grey fog that clings to the mountainside.
- Banner cloud – This type of cloud is usually found on the upper side of steep slopes and will be very dense. This type of cloud can be frequently seen on the side of Matterhorn mountain.
- Foehn wall – the air flowing down the side of the leeward mountain (known as a foehn wind) starts to get cooler. At this point, the air develops a visible wall, which is often known as the Foehn wall.
- Rotor cloud – this is when a pocket of rough air forms downwind at the edge of the ridge. This cloud is circular in shape and quite ragged.
- Lenticular clouds – This is a stationary dew cloud that usually forms downwind. In terms of direction, this often takes a strange angle, usually a 90-degree turn from the direction of the wind.
- Cap cloud – this is a circular cloud that forms over the peak of a mountain. This will form a sort of ‘cap’ on the top of a lot of mountain peaks where the orographic effect is occurring.
- Chinook arch cloud – this is a wave cloud that usually forms over the peaks of a mountain range. This forms an arch over the downward side of the mountain. There is usually a space that separates this from the mountain that is purely clear air.
You can attach this effect occurring from a close distance or you can view it through a telescope.
You can even make a note of the different types of clouds to see how the orographic effect is working.
Frequently Asked Questions
How Often Does The Orographic Effect Occur?
This effect occurs daily and you can often see it happening over many mountaintops and the upper portion of downwind mountainsides.
Can The Orographic Effect Be Dangerous?
This effect can produce torrential rain that can cause mudslides down the sides of mountains.
If you live near the bottom of a mountain, then you might need to keep a frequent eye on the local weather.
This effect can also be dangerous if you are mountain climbing. Make sure that you are aware of the weather forecast before you go hiking up a mountain.
The orographic effect can also be dangerous if you are scaling down the downwind face, as this is where a lot of dry air will be.
Make sure that you carry plenty of water with you when you go hiking down these faces.
We hope that our guide to the orographic effect has helped you to understand this phenomenon and encouraged you to view it in real life.
Make sure that if you are hiking up a mountain you check the weather forecast and keep an eye on the mountain peak for signs of torrential rain.