Why do cold coastal currents create deserts?

[Professor (male)] Q6 So today I thought we’d focus our discussion on deserts--different types
   of deserts.., how they form...that sort of thing. I suppose we should probably
   start by defining deserts. You no doubt already know that deserts are tracts
   of land that receive meager amounts of annual rainfall. On average, they’re
   places that get less than 250 millimeters of rain a year.That...or they lose more
   water than they receive. Some other typical features of deserts include sand
   dunes and ergs--sand "seas" full of shifting sand dunes. They also have rocky
   plateaus. And mineral deposits that were created by evaporation. A general
   term for these is evaporites...minerals that are left behind when surface water
   evaporates. Uh, some typical evaporites would be...gypsum, and different salts.
   OK. Did you realize that deserts account for about a third of the Earth’s land
   surface? Pretty amazing, isn’t it? It makes sense if you think about it, though.
   Q11 Look at a map...you’ll see deserts covering huge sections of
   Africa and Asia--and then there’s Antarctica...
[Student A (female)] That’s a desert?
[Professor] Well...yes. But I can understand why you might be confused. There’re actually
   quite a variety of deserts...not just the sandy ones that you might think of first.
   Typically, we classify all these different types of deserts according to how they
   form. And this can be the result of their geographical location...meaning the
   major features of the surrounding land. Or their dominant weather patterns.
   Let’s take that second idea...weather patterns...and I’ll give you an example.
   Q7 Trade winds. It’ll be good to start with trade winds, because they’re
   actually what’s caused the formation of the world’s best-known desert. Do
   you know which one I’m referring to?
[Student B (male)] It’s gotta be the Sahara. I’m pretty sure that’s the largest desert on Earth.
[Professor] You’re right about the Sahara being the desert I’m thinking of, but it’s actually
   the world’s second-largest desert...next to Antarctica. Q10(A) The Sahara is a
   trade-wind desert that was created by the movement of trade winds across
   northern Africa.
[Student B] Urn, could you just quickly explain what you mean by trade winds? I’m familiar
   with the term, urn, from other geography classes I’ve taken, but I don’t really
   know much about them.
[Professor] surprised Oh. Uh, this isn’t something you’ve studied in detail before? Uh,
   OK. Sure. Trade winds are the winds that you find near the equator...blowing
   almost constantly through the tropics. From the subtropical regions, actually,
   to the tropics. In the northern hemisphere, they blow from the northeast. In the
   southern hemisphere, they blow from the southeast.
   OK. Back to what I was saying about the Sahara.Trade winds traveling toward
   the equator cross the Sahara, picking up every last bit of moisture in the land.
   And, since they’re moving toward the equator and warming up, their capacity
   for holding water is increasing. Because hot air has the capacity to hold a lot of
   water. At the same time, though, these trade winds are very dry. When they pass
   over the Sahara, they pick up what little water there is.The result is, the land is
   left with almost no water at all. It can’t support plants...it becomes a desert.
[Student A] So you’re saying the Sahara turned into a desert not just because it doesn’t get
   much rain, but...um, also because these trade winds blow across it and suck out
   the moisture?
[Professor] Absolutely right. See how that works? All right. So trade winds are one of the
   causes of deserts. Let’s move on to another weather pattern that causes deserts.
   The "rain-shadow effect." Q8(B) The name "rain-shadow" has to do with the
   location of these deserts...in the shadow of huge mountain ranges. Uh, here’s
   how it works. Moist air currents blowing from the sea sweep across land, until
   they run into a mountain range. The shape of the mountains basically forces
   the moist air to move upward, where it cools, its moisture condenses, and
   rain falls. Q8(D) All of the moisture is released on the ocean-facing side of the
   mountain range. And by the time it reaches the opposite side of the range, the
   air is almost totally dry--no more rain. Q10(C) In extreme cases, this rain-shadow
   causes the opposite side of the mountains to turn into a desert. The Mojave
   Desert in the United States is affected by this phenomenon.
   Moving on...let’s consider coastal deserts. Like rain-shadow deserts, they’re
   influenced by the ocean, though the method is somewhat different. They most
   often occur along the western edges of continents, where huge, cold currents
   in the ocean run parallel to the coast. Q9 What happens is that cold ocean
   currents keep rain clouds from forming. They don’t provide much moisture, and
   as a result, the coastal region becomes very dry. Q10(B) A good example of a coastal
   desert is the Atacama in Chile, which is actually one of the driest places on Earth.
   OK, class. I think we’re about out of time for today, but next time we’ll begin
   with polar deserts and discuss Antarctica in greater detail.