Lightning is one of the greatest hazards to outdoor recreation in the summer months, especially in mountain ranges such as the Rockies where thunderstorms are frequent and the terrain is often exposed.
Even if you aren't a peak bagger, there will always be some element of risk when it comes to any summer outdoor activity, including hiking, biking, climbing, camping, and water sports, and this is because no place outside is 100% safe from lightning.
However, with proper planning and preparation, you can significantly reduce your risk of exposure to lightning.
This article will explore the basics of lightning science, followed by our recommended strategies for minimizing lightning risk, both ahead of time and while you're outside.
Lightning Science Basics
Updrafts and downdrafts in thunderstorms result in an ongoing collision between water droplets in the updraft and ice particles (hail or graupel) in the downdraft. This process results in the creation of an electrical field, in which positively and negatively charged particles become separated in a storm cloud with positive charges gathering near the top of the storm cloud and negatively charged particles near the bottom.
Cloud-to-cloud lightning strikes are common during thunderstorms as a result of the in-cloud separation between positive and negative charges.
However, an electrical field also develops underneath a storm with positively charged particles gathering near the surface of the earth, and rising up tall objects such as lone trees and mountain tops or ridge tops.
This electric field results in dangerous cloud-to-ground lightning that we are most concerned with, and also explains why higher elevation areas (and objects such as lone trees or buildings) are more susceptible to lightning strikes as they are closer to the negative charges at the base of the storm cloud.
However, it should also be noted that areas away from tall objects can still be struck by lightning, as it all depends on where the charges "accumulate". The unpredictable nature of individual lightning strikes is what makes lightning so dangerous.
Most cloud-to-ground lightning forms under the negative region of a storm cloud, but this is not always the case. Positive lightning strikes originate from the top of a storm cloud, typically in the flat anvil that extends outward in a mushroom shape.
Positive lightning strikes are less common, only making up 5% of all lightning strikes, but they are more dangerous for two reasons: 1) Since these strikes originate from the upper region of a storm cloud, they are much more powerful and contain a much higher electrical charge compared to typical cloud-to-ground lightning strikes. 2) Typical "negative" lightning strikes usually occur within 5-10 miles of a thunderstorm, whereas positive strikes can extend much further out, in some cases as far as 25 miles away from a thunderstorm.
Lightning Preparation Ahead of Time
In most cases, the earlier in the day you plan your outing, the less likely you are to be caught in a thunderstorm.
Thunderstorms are most common during the afternoon and evening hours after the sun has sufficiently heated up the lower atmosphere, though upslope winds which force moist air parcels to rise with terrain often result in storms developing by midday to early afternoon in the mountains. The advice to "be off the summit by noon" is not a bad rule of thumb as a starting point when planning.
However, rules of thumb do not always apply when it comes to thunderstorms, as they sometimes occur during the overnight and morning hours as well. These "off-hour" storm situations are more likely during especially moist and unstable airmasses, and/or when frontal systems or strong upper air disturbances are approaching.
Therefore, it helps to view a reliable weather forecast beforehand to get a good handle on the situation.
Also, if you have a big day planned where you will be spending an extended amount of time at high altitude or above treeline, then picking a day with little to no thunderstorm risk would be wise.
If you're camping, you may not be able to avoid thunderstorm risk altogether, but by planning ahead of time, you can at least pick a location to set up camp that is less vulnerable to lightning depending on the forecast.
1) OpenSummit Forecast:
The OpenSummit 5-day and hourly forecast available for hiking and mountain locations throughout the U.S. is a great first-check resource to get an idea of the lightning threat on a particular day and to see which hours of the day that lightning potential will be greatest.
The image below is an example for Mt. Sneffels in Colorado, with the lightning icon providing the hour-by-hour probability of lightning for a given day.
In this particular example, Friday is highlighted as "green" for a low, but not a zero risk of lightning, whereas the lightning risk is higher on Saturday, Sunday, and Monday.
Therefore, if you had to choose any day from Friday to Monday to climb Mt. Sneffels, this forecast indicates that Friday would be your best bet, but that you'd still want to get an early start and be alert for the potential for afternoon storms past noon.
2) OpenSummit Forecast Radar
Our Forecast Radar extends out to 48 hours and is a useful tool for getting an idea of what might happen when evaluating the day before or the morning of.
The forecast radar is based on a high-resolution weather model, so it's a projection for how thunderstorm activity could transpire. This should be used to get a general idea of how things could play out, but not be taken literally.
Also, when looking at the forecast radar, take note of which direction the storm cells are moving. For example, if you notice southwest to northeast storm motions, then once you're outside, you should keep an eye out for possible thunderstorms approaching from the southwest.
Pro tip: Compare what you are seeing on the forecast radar versus what our hourly precipitation and lightning forecasts are showing. If there is a relative agreement (i.e. the forecast radar is projecting thunderstorms during the periods when our hourly forecast is showing higher chances of rain/lightning), then this should give you more confidence in the forecast.
If the forecast radar and hourly forecasts are not in good agreement, then you'll want to give yourself a greater margin for error when planning you're outing.
3) Read the latest Western U.S. Daily Summit
From May through the end of September, we write a Western U.S. Daily Summit which is similar to our wintertime Daily Snows. The Daily Summit is an in-depth discussion on weather patterns across the Western U.S. which is typically updated 3 times/week on Mondays, Wednesdays, and Fridays.
This is a great resource to develop an understanding of how the weather pattern is evolving across the West as a whole over the next 3-5 days and beyond, with more active areas and ranges highlighted. These discussions also contain weather maps to help you visualize what's happening.
We do not have a Daily Summit for the Appalachians/Eastern U.S. at this time, but perhaps we will sometime in the future.
4) Storm Prediction Center Convective Outlook
NOAA's Storm Prediction Center is a great quick-look resource to see whether or not there will be a chance of thunderstorms on a given day over an area. This will not tell you how likely or unlikely storms are, just whether or not a thunderstorm could occur.
The areas highlighted in light green on these outlooks depict where thunderstorms could occur across the U.S. on a given day, and these outlooks go out 3 days.
Another advantage of the Storm Prediction Center Outlook is that it also indicates when severe thunderstorms are possible, and if so, the potential hazards that could occur.
This is important if you're planning an adventure because even if your area is listed under a Marginal Risk (level 1 out of 5) for severe weather, it should factor into your planning as thunderstorm hazards are more pronounced when you're recreating in the mountains versus when you're at home.
Severe weather, which includes hail of 1" or more in diameter, wind gusts in excess of 58 mph, or tornadoes, is more common in the Appalachians compared to the Rockies, but it occasionally occurs in the Rockies as well, and even in the Sierra and Cascades on rare occasions.
In the image above, a large portion of the Appalachians is labeled under a Marginal (level 1 out of 5) risk for severe weather, while Northwest Montana and Eastern Montana/Wyoming are also listed under a Marginal Risk. On the Storm Prediction Center website, you can also view the specific weather hazards (including hail, damaging wind, and tornadoes) and the probabilities for each hazard on a given day.
5. Pack a NOAA Weather Radio
In the age of smartphones, a NOAA radio might sound old school, but it's a valuable tool to pack with you in the backcountry where cell service is usually unreliable.
NOAA Weather Radio covers more localized geographic zones, including mountain regions, and provides comprehensive forecasts as well as alerts for severe or inclement weather.
Often, the National Weather Service will issue "special weather statements" when impactful thunderstorms are developing or approaching an area, even for non-severe thunderstorms. This can give you a heads up about an approaching thunderstorm while you're in the backcountry, though you shouldn't rely on it solely and should also use your own judgment (more on that below).
6. OpenSummit Lightning Map
Our Lightning Map displays approximate locations of lightning strikes in near real-time. This is a valuable tool when you happen to have cell service, but you should use this with caution when out in the field so that you don't get a false sense of security.
For one, it often takes several minutes for lightning strike data from the National Lightning Data Network to transfer out to data feeds such as what we use, so the lightning data you are seeing is not instantaneous.
Also, if a storm is developing overhead, prior lightning strikes are not going to alert you to the upcoming lightning risk. If a storm is developing right on top of you or very close by, then the first lightning strike could occur dangerously close.
So to sum it up, use the lightning map when you have service to get an idea of what's going on in the general area and also prior to heading outside if you are still at home. But you shouldn't rely on lightning detection as your #1 go-to tool once you're outside.
Once You're Outside: Looking for Visual Clues of Thunderstorm Development
Once you're out on an adventure in the mountains, you're basically on your own as far as watching the weather goes since consistent cell service can't be counted in most mountain environments. Fortunately, the atmosphere usually offers clues that can warn you of approaching or developing thunderstorms ahead of time.
Altocumulus clouds are small, layered patchy clouds that form in the mid-levels of the atmosphere. If you see these clouds in the morning, it's typically a sign of instability in the atmosphere and is often a precursor to afternoon thunderstorms.
Cumulus clouds are puffy non-layered clouds that often begin to form over the course of the day as the sun heats up the lower levels of the atmosphere. Cumulus clouds sort of act as an "advance notice" that thunderstorms could possibly develop in the next few hours. During active thunderstorm patterns, cumulus clouds may begin to form as early as mid to late morning.
On days with less available moisture and limited thunderstorm potential, cumulus clouds may not form until later in the day, and they may also not develop into anything more than "fair weather" cumulus clouds.
Towering cumulus clouds that show obvious vertical growth are ones that you should really start to pay attention to. These clouds indicate that warm, moist air parcels are beginning to rise vertically in the form of updrafts, and are the first signs that thunderstorms could be developing.
In some cases, towering cumulus will grow and then fall apart with the cycle repeating itself a few times, before eventually a towering cumulus will "break through" and develop into a thunderstorm cloud, also known as a cumulonimbus cloud.
However, in other cases when the atmosphere is more unstable, the first towering cumulus clouds that develop will quickly grow into thunderstorms.
It's important to monitor the development of towering cumulus clouds carefully, and if these clouds start to become thicker and taller and develop dark bases, then it's time to start descending from a summit or exposed ridgeline to give yourself time to reach safer terrain before a thunderstorm develops.
Cumulonimbus clouds are tall and thick bubbling clouds that exhibit impressive vertical growth, often reaching 30,000 to 40,000 feet in altitude. Dark/gray bases will appear underneath cumulonimbus clouds as well. A flat anvils will often extend outward from the top of a cumulonimbus cloud in a mushroom-like shape once a thunderstorm has reached maturity.
The image below is a visual of a cumulonimbus cloud with red arrows indicating the thunderstorm's updraft, and blue arrows indicating the storm's downdraft, which includes, rain, hail, wind, and cooler air.
A cumulonimbus cloud is a sign of trouble and indicates that a thunderstorm is imminent. If you are above treeline or on an exposed summit or ridgeline, then it's time to start heading down unless the cloud is way off in the distance and you are sure it's moving away from you (and even then, you should be on guard for rapid new thunderstorm development).
In addition to thunderstorms developing overhead or in the vicinity, you must also monitor the skies for ongoing thunderstorms that could be approaching you from a distance.
If previously sunny or partly cloudy skies overhead give way to a dark high cloud layer moving overhead, this may indicate the anvil from the top of a mature thunderstorm that could be heading your way. Pay attention to these signs, especially if your distant views are obscured by mountains or haze. If clouds continue to darken and approach your direction then there is a good chance a thunderstorm is approaching.
Once you hear thunder, then that means a thunderstorm is close enough so that you are in danger of lightning, and that means it's time to start heading down toward safer terrain. This is true in the case of both developing thunderstorms and approaching thunderstorms.
If you can hear thunder and see lightning, then you can count between the lightning flash and the resulting thunderclap to estimate how far away the lightning strike occurred. Every 5 seconds in between lightning and thunder is approximately one mile. Also, keep in mind that surrounding mountains can both obscure your ability to hear thunder and/or see lightning in certain situations.
Keep in mind that when it comes to making decisions based on thunderstorm signs, it's a combination of evaluating changing weather conditions, and also your exposure to lightning risk based on where you're located and the time it will take you to retreat safely.
Another sign that you should descend immediately from a mountain is if you start to feel, hear, or see static or any crackling noises. This could include the hair raising on your skin, the feeling of static, or buzzing/cracking noises in the rocks around you. If you experience this spooky sensation, then you need to start descending quickly and immediately.
And finally, one sign of stability in the atmosphere is the presence of widespread flat-layer clouds resulting in overcast skies during the morning hours. These flat cloud layers act as a stable layer in the atmosphere and limit the amount of solar radiation reaching the surface that can heat up and destabilize in the atmosphere.
This can often happen when there is a lot of moisture in the atmosphere (sometimes in the wake of previous days' thunderstorms), but which manifests itself into cloud cover at the mid-levels of the atmosphere.
Overcast, flat-layered cloud cover in the morning usually indicates that thunderstorm development will be more delayed, and this could buy you some time to complete your activity before storms develop. Occasionally, these cloud layers can persist all day and prevent thunderstorms. However, if and when the cloud cover breaks and the sun comes out, it might not take long for thunderstorm development to begin if moisture levels are high.
What To Do When You Are Caught in a Thunderstorm
The simple rule of thumb when you are caught in a thunderstorm in an exposed area is to descend to or move to a less exposed location as quickly as you can in a safe manner.
In the past, the "lightning crouch" was a common practice when getting caught in a storm above treeline. However, the National Weather Service and other outdoor safety and leadership organizations no longer recommend this as it doesn't really offer any added degree of safety.
Instead, the recommendation now is to move quickly out of harm's way rather than waiting for a storm to pass if you're already in an exposed area.
The National Weather Service in Salt Lake City put together this awesome graphic detailing the most and least exposed terrain features to lightning danger when spending time outside. In the diagram below, areas labeled with "0" are the highest risk areas and "3" are the lowest risk areas outside. The diagram also notes a "10" rating as being the safest location of all, which is indoors.
The most dangerous places to be caught in a thunderstorm include the top of a mountain summit, ridgeline, or any other isolated high points on a ridge or mountain. If you are caught on or near a high point, then descent immediately.
The next most dangerous areas include isolated trees, bodies of water, and wide-open areas. Move away from these features as quickly as you can if caught in a storm. However, it should be noted that a wide open field in a valley or lower elevation area is preferable to being caught above treeline or even next to a lone tree or body of water.
Contrary to popular belief, taking refuge in a cave or rock overhang is also a bad idea in a thunderstorm as ground currents from lightning strikes can travel over, around, and through caves and overhangs.
The least risky areas outdoors during a thunderstorm include heavily forested areas with relatively uniform tree height, and any lower-lying terrain depressions such as valleys or gullies. Keep in mind that no place outside is 100% safe from lightning, but your risk of being struck by lightning in these areas is much lower.
Anyone who spends enough time outside in the mountains is almost certain to run into thunderstorms every now and then, and sometimes even the most weather savvy among us get caught by surprise. However, with proper planning and evaluation, you can minimize your risk to lightning while you're out exploring in the mountains.
Three Key Ingredients for Mountain Thunderstorms