In extreme situations, heat and humidity can have disastrous effects on PCs. An overheating computer might slow down or turn off unexpectedly, and if computer components get wet, they could short circuit or corrode. In day-to-day use, however, you don't need to worry too much about heat or humidity; your computer won't break down simply due to a hot and humid summer day.
Heat
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Methods of Cooling Components
Computers generate a lot of heat. Most of that heat comes from the CPU and graphics processor, but the hard drive motor and other various mechanisms also contribute to the temperature inside the chassis. To regulate the temperature, computers use fans or alternative cooling systems to cool the processors and to exhaust hot air from the chassis.
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Risk Factors for Overheating
The harder you push your computer, the more heat it generates, but a properly built system won't overheat just because of the temperature in the room, within reason. Close a running computer in a small closet on a 110-degree day and you might have a problem, but in normal situations, the fans can handle the load. More likely, a fan is broken, blocked or dust-clogged. Keep the fans clean and uncovered -- and have them replaced if they break -- to keep your system running cool. Laptop vents are usually on the bottom of the machine, so rest it on a breathable surface. A quick way to tell if your resting spot is blocking the vents is to lift the laptop up when you hear the fan start running. If it stops each time you do this, then your vents aren't getting proper ventilation.
Temperature Limits
Computer components have differing heat safety thresholds, often listed in the part's specifications. The most likely part to overheat, the CPU, automatically throttles its speed if it approaches its maximum temperature. If it continues to overheat, it triggers a failsafe which shuts down the computer rather than risk permanent damage. Maximum temperatures -- sometimes called Tcase or Tjunction max -- vary widely from model to model. A temperature of 90 degrees Celsius -- around 200 Fahrenheit -- might be safe for one processor but well over the limit for another. Look up your processor on the Intel or AMD website to find its maximum temperature, and then monitor your processor's temperature if you're concerned about overheating.
Humidity
During normal use, humidity doesn't pose as likely a threat as heat. Everyone knows that electronics and water don't mix, but as long as you don't spill liquid into your computer, humidity is unlikely to break it. That said, both extremely humid and extremely dry climates can cause problems.
High Humidity
When the air entering your computer case is very humid, such as near a humidifier or in a humid climate, like a tropical jungle, the water in the air can corrode components over time, causing them to break. Water might also condense inside the computer, leading to a short circuit. Keep the humidity near your computer below 80 percent, and if possible, closer to 45 to 50 percent.
Low Humidity
Extremely low humidity poses a less obvious risk: static discharge. As humidity drops, static builds up more easily. If a static shock hits a computer component, it can destroy it. Below 35 percent humidity, the risk worsens, so keep your computer -- and any rooms where you handle electronic components, even while unpowered -- above this point.