Other premium smartphones -- notably, Apple's iPhone 7 (£41 at uSwitch) and 7 Plus (£47 at uSwitch) and Samsung's Galaxy S7 and Galaxy Edge S7 -- can survive a sudden rainstorm without issue, but the Pixel and Pixel XL are only certified to withstand a small amount of dust and moisture.
Related: How water-resistant is the iPhone 7?
What does IP53 really mean?
The Pixel and the Pixel XL are IP53 certified. IP stands for International Protection (sometimes called Ingress Protection), and the numbers after IP indicate how protected a device is against intrusion by foreign objects such as dust particles and water. The first number (5) indicates how protected the product is against dust, while the second number (3) indicates how protected the product is against water.
A dust rating of 5 means that the Pixel is protected against dust. It's not completely sealed (that would be a rating of 6), but it is protected enough that any dust that happens to get inside will not affect the operation of the device. In other words, the Pixel should continue to operate well even if you end up trapped in a sandstorm in the middle of the Sahara (though it will get some dust and sand inside it).
But the dust rating isn't the issue -- the water rating is. A water rating of 3 means that the Pixel is just barely waterproof -- it should be able to withstand "water falling as a spray at any angle up to 60 degrees from the vertical," and it must pass a test that involves being sprayed for at least five minutes. In other words, you'll be fine if you're caught in a light shower with this phone, but you shouldn't go running in the rain with it. Dropping it in the sink or toilet is a no-go, and you should definitely not have it anywhere near a pool.
To compare: The Samsung Galaxy S7 (£35 at uSwitch) and Galaxy Edge S7 are both rated IP68, which means that they should be able to withstand full immersion in one to three meters of water for more than 30 minutes; and the iPhone 7 and 7 Plus are both rated IP67, which means they should be able to withstand full immersion in one meter of water for up to 30 minutes. (Though this is not always the case in practice.)
Waterproof your Pixel
Option 1: A waterproof case
Good news: If you happen to be a clumsy aquatic ape like the rest of us, you can still pick up the Pixel or the Pixel XL and waterproof it yourself...with a case.
Lifeproof has already tossed up versions of its waterproof cases for both the Pixel and the Pixel XL on its website -- Lifeproof's cases, when properly applied, will make your device submersible in 2 meters of water for up to one hour.
The cases cost $90 each, but they also add drop protection for drops of up to two meters.
Option 2: A waterproof pouch
If you're looking for a quick and cheap waterproofing solution, you can pick up a waterproof phone pouch like this $8 universal phone bag from Moko. It's not as attractive as the Lifeproof case, but it'll get the job done. And if you're really in a pinch, you can just use a Ziploc bag (though I suggest you check it for leaks and only use it once or twice before recycling it).