You probably already have a baseline understanding of simple temperature and precipitation forecasts. Those are the ones you check every morning to decide which jacket to grab as you’re heading out the door.
But the details of a weather forecast become much more important when you’re going to be out in the elements over several days on a backpacking trip. When you’re considering what to add to your backpacking checklist, you need to know a bit more than just temperature and rain forecasts.
That’s why I compiled this simple guide to the most important weather report details for backpackers and hikers.
Relative humidity is a very important concept for understanding other weather-related measurements. The basic idea behind relative humidity is simple: air can have a certain amount of water dissolved into it, based on the temperature. Warmer air can hold much more moisture than colder air can.
When we talk about relative humidity, we’re talking about how much water is currently dissolved in the air, relative to how much water the air could potentially hold at its current temperature.
When the relative humidity is around 80% or higher, the air starts to feel “moist” or “sticky” and also tends to feel a bit warmer. When relative humidity is low, people refer to the air as feeling “dry” and it generally feels a bit colder.
Relative humidity has a big impact on how you feel at a certain temperature, and how your body controls its moisture. You’ll have problems drying out your gear or clothing in environments that have high humidity, and you may feel like you’re constantly covered in sweat.
Conversely, low humidities will make you feel more exposed, and will also likely make you more dehydrated, even thought you might feel less sweaty.
I already mentioned how warmer air can hold more moisture than colder air. So what happens if the relative humidity is high, and then the temperature drops? The air can no longer hold all of that moisture, so it condenses into something like dew or frost.
The temperature at which water will condense out of the air, for the given humidity, is called the “dew point.” That means that if the temperature drops below this number, you can expect condensation to start forming everywhere.
Condensation happens first around colder surfaces like the ground or metal objects. You might also find condensation happening above the dew point if you’re in an especially humid area, like a poorly ventilated tent.
If you want to avoid waking up to a wet sleeping bag, make sure your shelter is well-ventilated to avoid excessive humidity building up. It also helps to be in an area that might hold heat better – like under thick, low foliage – so that the air stays warmer and doesn’t drop below the dew point.
Atmospheric Pressure (aka "Barometric Pressure")
Atmospheric pressure is measured with a barometer, so it’s sometimes referred to as “barometric pressure.” It basically measures the “weight” of a column of air.
At higher altitudes, pressure will generally be lower, since there’s less air above you. This can make it hard to track both atmospheric pressure and altitude on the same device, since you usually need to know one value to calibrate and control for the other.
There are a whole bunch of different ways atmospheric pressure is measured and reported, but knowing the current reading isn’t that important. Instead, you should focus on the trend – is the pressure generally going up or down?
If the air pressure is increasing, you can usually expect the skies to clear up. Since the air is getting heavier and sinking down, it’s warming which prevents clouds from forming.
If the air pressure is decreasing, you can usually expect clouds to start forming. Since the air is generally rising, it will start to cool and any moisture it’s holding will condense (crossing below the dew point) forming clouds. More information about air pressure here.
In addition to bringing precipitation, clouds have an important function in regulating the surface’s air temperature.
When skies are clear, temperatures will fluctuate a lot between the day and night. Sunlight will heat the earth’s surface more effectively when there aren’t clouds. But at night, all of that warm air will quickly rise into the sky and dissipate, making it much colder on the ground.
When skies are overcast, it tends to be cooler during the day without the sunshine. But the clouds also functions as a thick blanket that holds the warm air down towards the earth, meaning it won’t get as cold at night.
While it might be nice to lay out under the stars on a clear night, expect it to get much colder before the morning sun starts to rewarm things. Also, with that sharp drop in temperature overnight, you’re more likely to dip below the dew point and wake up with a wet sleeping bag.
The UV Index is a measure of the strength of the sun’s ultraviolet rays. While the units can be a bit tricky to understand, higher value generally mean that your skin and eyes will start to burn faster.
Normal values of 3-6 mean that you should try to find shade around “solar noon,” which is generally around noon or 1pm and tends to be the hottest part of day.
Anything higher than 6 means it’s definitely time to put on sunscreen or clothing with a UPF rating. You also probably want to be wearing sunglasses and a wide-brimmed hat.
Make sure you check the UV Index anytime you’re hiking above treeline in an alpine area, or anywhere else that doesn’t have adequate tree cover or shade. It’s easy to underestimate how badly you can get burned when you’re hiking with no shade for hours a day.
“Visibility” refers to the transparency of the air. It’s not literally how far you can see, which depends on lighting, vantage points, obstructions, and many other factors.
In cities, visibility can be affected by smog and other air pollution. In the backcountry, visibility is more likely to be affected by fog, mist or haze.
Generally, a “clear” reading tops out at something around 10 miles. The number can be a bit misleading, since you can often see much farther from high vantage points on clear days.
With a light haze, you might see a visibility rating around 2-3 miles, and with mist or fog, it’ll usually be less than one mile.
It’s important to know the visibility reading if you’re planning on doing a trip that relies on big views or sight seeing.
If you’d like to learn more about the conditions for an upcoming backcountry trip, check out a simple tool I built that shows you detailed weather history for an area.