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I was looking at this website and surprised to see that a cold area is surrounded by much hotter area. My understanding is that from the equator the more north/south direction one goes, the colder area one would encounter.

But why this colder place is surrounded by hotter area?

enter image description here

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    $\begingroup$ Try to find a map of temperatures in the upper troposphere, and check if you still see this pattern! $\endgroup$ – gerrit Jul 11 '17 at 9:50
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The Himalayas and the Tibetan Plateau have high elevation and temperature drops with altitude up until the tropopause. The temperature on the map is measured and modeled on the land surface, regardless of elevation. You can also see that the temperatures in e.g. Ethiopia and the Caucasus Mountains are lower than surrounding lowlands.

At latitudes around 30 degrees (north and south), the air is dry and hot due to global circulation pattern. The weather patterns are also controlled by the topography, humid air can't pass the mountain range as it's forced to an altitude where the temperature force it to condensate. That is why the Gobi desert is hot and warm in the summer.

I hope this answers your question.

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Look at how similar the elevation map is of the region:

enter image description here From Wikipedia

And that goes really well with Why Is It Colder in the Mountains Than at Sea Level

As tbb noted, the band near 30 north/south tends overall (climatologically) to be drier and hotter due to descending air caused by the Hadley circulation.

As to why this area stands out in comparison with other locations at the same latitude, the moisture in the air has quite a bit to do with it:

dewpoint 071117
Today's dew points, courtesy of Pivotal Weather

Indeed, you can see it is fairly dry over parts of central and western China compared to many other places, such as eastern China and back nearer Europe... due in part to the affects of the mountains and distance from large bodies of water... though you may also notice other spots to the west do show comparable moisture for this date despite producing lower temperatures.

Ground cover differences are part of the reason (more plants = more area the solar energy is distributed into, plus typically store moisture, equating to slower temperature changes).

And then wind direction gives one final explanation why it's not as warm further to the west:

winds 071117

The northerly winds have been in place for a while back in Kazakhstan and Uzbekistan have brought slightly cooler temperatures down from the north, whereas the flow into China is less consistent and has only recently started shifting. Even in the summer, wind changes and even fronts are still often very important in the temperature layout.

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