# How far will rain clouds travel inland?

I am writing a story that takes place in a big desert. I've figured that I could make my desert as extensive as possible by building a Pangea-like supercontinent, since I've learned here that, given the distance, eventually the clouds from the surrounding seas will exhaust their water.

However, I want to know how far from the coast I may extend my desert. Let's assume a planet with the same conditions as Earth and let's assume that the continent is as plain as possible (so as not to have rain shadows as a confounding variable).

How far will the rain clouds travel inland? What's the distance from the coast where I may place the desertic climate?

• For clarification, is it okay if there is an occasional rainstorm? I have a benchmark, but it does allow rainstorms every two to five years. – Mikey Dec 7 '16 at 1:04
• Yes, that's OK. – Pedro Gabriel Dec 7 '16 at 9:43

# Wet places stay wet

Rainfall happens in an area because of prevailing atmospheric conditions. For example, around the equator there is the Intertropical Convergence zone (ITCZ), a zone of rising humid air that releases rain. This band is around 10 degrees of latitude wide and moves back and forth across the equator. Directly on the equator, there is heavy rainfall all year round, as the ITCZ sits on top of this area for most of the year.

For the tropical rainforests, this is the proximate cause of their enormous moisture. Rising, cooling air releases moisture over the forests. The forests themselves, that great mass of trees and vegetation, then respires the water at night, 'recycling' the water back into the air only for it to fall again. For the Amazon, Congo, and Southeast Asia this is what causes rainfall and is independent of distance from the coast.

# Dry places stay dry

On the other hand, around 20-30 degrees north and south, there is a band of descending dry high altitude air. This air is has little to no moisture and thus the land below it is dry year-round. At these latitudes around the world there are the deserts of Southwest US and northern Mexico, the Atamaca and Cuyo in South America, the Sahara and Kalahari in Africa, the Middle East and Thar in Asia, and nearly the whole of Australia.

In these regions, the land is always dry...even if there is a sea breeze! This is especially true in places with cold ocean currents. A desert with a cold ocean current is dry indeed, as you can see from the exceedingly dry conditions in Lima, Peru (Peru current) or Namibe, Angola (Benguela current).

If there is a warmer current, then there is hope for wetter conditions; but only erratically. The best you can hope for is a seasonal burst of rain which leads to a semi-arid climate. Examples here would include scattered summer rains in Mogadishu, Somalia; or scattered winter rains in Benghazi, Libya.

# Rain doesn't make it very far inland if it isn't supposed to be wet

The best example for how far rains can penetrate inland would be from Egypt. Here is a map of wind patterns in the Mediterranean.

Egypt sees oncoming winds from the Mediterranean all year round (the ancient Egyptians used this since they could sail up the Nile, but drift with the current back down).

Here (from en.climate-data.org) is a climate breakdown for Alexandria, directly on the coast.

Now here is one for Cairo, about 175 km inland.

Where did all the rain go? As little rain as there was in Alexandria, which is directly on the coast of a warm sea, with oncoming sea breeze all year long, it is all gone by the time the winds get to Cairo. You can tell that Alexandria is getting the sea breeze from a warm sea, because it is slightly warmer than Cairo in the winter, but much cooler in the summer. But wonderful climate aside, its still not getting any rain.

# Conclusion

Wet and dry bands are determined by rising and falling air patterns, and by mountains. Places far from the ocean or sea breezes like the middle of the Amazon Basin, or St. Louis, or the center of the Congo, or Chonquing can get plenty of rain if they are located at the right latitudes or on the right side of a continent (the east side is generally wetter than the west; Africa is the big exception). The dryest places in the world are right on the coast in the Atamaca and Namibe deserts.

• I'd disagree about the east side of continents generally being wetter than the west. The western part of Europe is pretty wet, as is the western edge of North America, north of roughly San Francisco. Think the Redwood Coast, western, Oregon, the Olympic Peninsula up through Alaska. It's just that the mountains keep much of the moisture from moving far inland. – jamesqf Dec 5 '16 at 2:32
• @jamesqf There may be specific wet areas on West sides of continents, but as as generalization, East costs are much wetter. On North America, West Coast cities with rainfall in mm [all numbers from Wikipedia] are Seattle (952), San Francisco (600), and Los Angeles (325). Compare rainfall to Boston (1112), Washington (1009), and Miami (1572). For Afro-Eurasia compare Dublin (714), Lisbon (714), and Casablanca (415) to Vladivostok (840), Tokyo (1524), and Shanghai (1166). West coast mountains concentrate moisture in small areas and create wet spots, not the other way around. – kingledion Dec 5 '16 at 3:17
• I think you have it backwards. It's the west coast mountains that keep the precipitation from getting inland. For instance, here's a map of US rainfall: wrcc.dri.edu/pcpn/us_precip.gif Much wetter on the west coast, no? Of course there are global scale patterns, which is why it's dry south of San Francisco. Also note that a lot of the 'eastern' precipitation is actually from the southern coast. Bottom line is that topography and latitude are more important than sheer distance inland. – jamesqf Dec 5 '16 at 19:43
• @jamesqf The rainfall is caused by the mountains, and falls only in the mountains on the West Coast; just look at your linked map to see it. Even 'wet' Seattle isn't as wet as any of the major cities of the Eastern US. Neither is Portland (915 mm) or any other large west coast city. You have to compare like to like. There are no mountains the size of the Cascades in the Eastern US to compare to, so you have to compare flatlands at sea level. And even in the Pacific Northwest, the flatlands at sea level get less rain than the East coast. – kingledion Dec 5 '16 at 20:01
• @jamesqf The interior east is wetter than the west. Take an example of the two largest rivers in each region. The Columbia drains 668000 km$^2$ and discharges 7500 m$^3$/s. The Ohio drains 490601 km$^2$ and discharges 7957 m$^3$/s. The Ohio discharges 511mm per sq km per year; the Columbia is 354mm. The East coast is wetter than the West coast. – kingledion Jan 27 '17 at 17:08

It's going to depend on the prevailing winds (which depend on your latitude) and topography. For instance, the Atacama desert of South America, the Namib desert of Africa, and many others are directly on the coast. The Great Basin and contiguous areas of North America are further inland, but shielded from most precipitation by the Sierra Nevada and Cascade mountain ranges. Then there's the Sahara, which extends clear across North Africa, with desert continuing in the same latitudes through the Arabian Peninsula and into the northwest of the Indian subcontinent.

OTOH, the Amazon Basin extends inland for a long distance, and is quite rainy, as is the Congo Basin.

• Thank you for your comment. Could you please tell me how far does the rainfall extends into the Amazon Basin and the Congo Basin? And what are the reasons for such an extension of inland rainfall on these two specific instances? – Pedro Gabriel Dec 5 '16 at 20:08

Australia is a pretty flat continent with a lot of desert. The north is tropical, but there is a watershed (quite a subtle one, given the flatness of the continent) that the rain no longer reaches, so south of that watershed it is desert. The desert goes pretty much all the way to the coast in the west of the country, but it is not bare sand dunes. It has spinifex grass and little, sturdy shrubs. Along watercourses there is greenery and a fair bit of animal activity.

Bear this in mind when you place mountains - water that falls on mountains drains off the back side as well as the coastal side. Australia has a low mountain range along the east coast, and the rivers flowing west from those mountains support the entire Murray-Darling basin, which is the bread basket of the nation even though the rains are erratic. The water eventually reaches the sea in Adelaide, on average 1500 miles from where it fell.

As an idea of scale, Australia is about the same size as continental USA - it takes 5 hours to fly from one side to the other.

The best way to make a desert is to put a big mountain range on the coast which gets the predominant winds. As moisture-laden air is blown inland it will rise, cool, and dump its moisture as rain, producing a rain forest on the rising slope. The air which makes it over the mountains will have very little moisture if the mountains are fairly high, and thus produce a desert. You can see this in Oregon, where the Cascade Mountains produce very dry country to the east.