Climate normals can teach us a lot about what a place is like on any given day, but they also help us examine why that is the case. This information, for a subset of about 450 stations across the United States, and is available through this tool. Jump in and play with the data yourself!
The Hottest and Coldest Places in America
No surprises, right? The warmest places are in the south and the coldest places are in Alaska. This really underscores our first climate adage of the day: All else being equal, places closer to the equator are warmer and places closer to the poles are cooler. The underlying reason is straightforward: over the course of a year, the equator catches more of the sun’s energy than the poles.
Notice that not all of the top ten coldest are in Alaska. Oh, that’s right. There’s Mt. Washington, New Hampshire, which reminds of us a climate second adage: All else being equal, places at higher elevation are cooler, and places at lower elevation are warmer. What’s the underlying physics behind this? It’s simple: the atmosphere – at least the bottom two-thirds or so of it – cools with height. Mt. Washington combines elements of the first and second adages to sneak its way onto the list.
Triple Digits and Frequent Freezers
Another variation involves how often a place gets really hot or really cold. One way to examine that is by counting the number of days each year a place reaches triple digits, or on the other extreme, drops below freezing. We’ve listed the top ten of each below. For the cold list, we’ve excluded Alaska to explore some details in the CONUS.
Again, looks like southern places are warm and northern are cold. Wait, Redding, California has the sixth-most number of days over 100F? It was 99th in the “warmest places in America” list. Flagstaff, Arizona makes the list of frequent freezers? Looks like we may need to tag a few more truisms to explain these.
The triple-digit list emphasizes the following adage: When the sun is high, and the ground is dry, it gets hot. Redding, like most of California, and much of the Southwest, has very dry summers. It’s no coincidence that the triple-digit list is chock full of places with dry summers.
The frequent-freezer list evokes the first two truisms about temperature, and Flagstaff is certainly cool thanks to its elevation, but it also conjures up an additional adage: All else being equal, coastal places don’t experience the temperature extremes of inland places. The oceans have a moderating effect, while inland places will exercise more of the thermometer over the course of a year.
The Wettest and Driest Places in America
The tables below list the ten wettest and ten driest locations in the United States. Again, these are annual normals, or what would be expected in the mythical “average” year.
Unlike extreme temperature stations, which were dominated by where you are on a map, the “ten wettest” list is dominated by what you see when you look out the window on a clear day. It’s no accident that all of these stations are within a few miles of some pretty substantial summits: mountains play a huge role in local precipitation.
That cements our first precipitation truism: the windward sides of mountain ranges tend to be significantly wetter than the leeward sides.
How does that work? The mountains provide the mechanism to lift moist air until it cools enough to condense cloud droplets (or form snowflakes) that are big enough to fall to earth. Unlike other weather mechanisms, like cold fronts and warm ocean waters, the mountains don’t go away. Basically, all that’s needed is some moist air being driven toward higher elevations.
In most cases, the warmer the air, the more water vapor it can “hold”, and therefore deliver. But, as this list clearly shows, the cool, damp air of the Gulf of Alaska is plenty moist to dump tubs of rain (and snow!) onto Alaska’s southern mountains. Each of the listed Alaskan stations sits along Alaska’s southern coast, where the ocean, the mountains, and a persistent low-pressure feature – the “Alaskan low” – work together to produce prolific rainmaking.
The dry side of the table underscores the corollary of the first point: the leeward reaches of mountain ranges tend to be much drier than the windward side. Many of the dry stations sit just east of a significant mountain range. After the moisture is wrung out on the windward side, the air will warm as it descends. This warming brings the air out of saturation – it is plenty capable of “holding” the remaining water vapor – which means precipitation doesn’t happen in most situations.
A map of the 10 places with the lowest annual precipitation in the United States. Map produced with NOAA NCEI U.S. Climate Extremes Tool.
It’s also worth noting, looking across these lists, that the wet stations are comfortably in the far northern regions of the country. And many of the dry stations, especially those that aren’t obviously in the “rain shadow” of a mountain range, are at sub-tropical latitudes. The world’s major deserts sit at around 25 to 30 degrees latitude.
The exceptions to the above are: 1) Barrow, Alaska, which is so far north and so cold, that there’s just not a great deal of precipitation, and 2) Hilo, Hawaii, which is so far south that it’s tropical. Hilo also sits on the east side of a major summit, the windward side in the tropics, where the prevailing winds are easterly.
Or, to sum it up in a truism: all else being equal, subtropical latitudes tend dry.
The following table lists the places with the most, and the least, days per year with precipitation. What do these tables tell us? They tell us that the places that get the most and least precipitation per year also get the most and fewest days with precipitation. Let’s not overthink this.
What does this list tell us? As you’d expect, the more northern and higher-elevation places make the list. But the influence of the Great Lakes really pops in this list. Large, relatively warm bodies of water, especially in the early winter, can juice snow-making storms with even more moisture and atmospheric instability. Being downwind of a Great Lake means you experience this early and often in the winter. Or, presented as a truism: The Great Lakes Make Great Flakes!