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 including mountaintops along with isolated tall objects such as trees and buildings are more susceptible to lightning strikes as they are closer to the negative charges at the base of the storm cloud.

Even areas away from tall objects can still be struck by lightning, though, 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

Thunderstorms are most common during the afternoon and evening hours after the sun has sufficiently heated up the lower atmosphere. However, the combination of solar heating and orographic lift often results in storms developing by midday to early afternoon over mountain ranges.

In most cases, the earlier in the day you plan your outing, the less likely you are to be caught in a thunderstorm. For peak-bagging in the Rockies, summiting by mid-morning and descending to treeline by no later than noon is a good general rule of thumb.

Occasionally, thunderstorms can develop during the overnight hours or in the morning hours prior to noon. These "off-hour" storm situations are more likely when high levels of moisture and instability are present, and/or when low pressure troughs or shortwaves approach. 

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.

Hourly Lightning Forecasts for the Next 10 Days

A great place to start is with our 10-day and hourly forecasts, which are available with an All-Access subscription. 

First, you can search for any peak, trail, park, town, or point of interest in our Search Bar. If you can't find the location you're looking for, then you can use our Forecast Anywhere feature to view a forecast for any location and elevation in the world.

The hourly lightning forecast graphs show the relative likelihood of lightning for each hour. A small yellow bar indicates there is at least a minor risk of lightning for a given hour, while taller bars and orange and red colors indicate higher lightning chances.

Forecast Radar

Our Forecast Radar Map 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 of 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. 

View → Forecast Radar

Pro tip: Compare what you are seeing on the forecast radar versus what our hourly precipitation and lightning forecasts are showing.

If there is 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.

Western U.S. "Daily Snow"

From late May through early September, we write a Western U.S. Daily Snow which is similar to our wintertime Daily Snows. These in-depth discussions on weather patterns across the Western U.S. are updated 3 times/week on Mondays, Wednesdays, and Fridays during the summer. 

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 week, with more active areas and ranges highlighted. These discussions also contain weather maps to help you visualize what's happening.

Storm Prediction Center Convective Outlook

The Storm Prediction Center's Convective Outlook is a good supplemental resource to hour point forecasts 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 cover the next 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. 

The Storm Prediction Center also publishes a General Thunderstorm Outlook which denotes areas with 10%, 40%, and 70% thunderstorm probability over 4 to 8-hour increments for the next day. This product uses Z-time, so during the summer months, subtract 4 hours from Z time for Eastern time, 5 hours for Central time, 6 hours for Mountain time, and 7 hours for Pacific time.

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).

Thunderstorm Tracking When You're in the Mountains

When it comes to making decisions in the mountains, it's a combination of 1) evaluating changing weather conditions, and 2) your exposure to lightning risk based on where you're located and the time it will take you to retreat safely.

Here are some tools you can use to evaluate current conditions to help you make decisions on whether or not to turn back.

Lightning Risk Map

We just added a new tool that is very useful for tracking thunderstorms and thunderstorm risk when you're high up on a mountain (where cell service tends to be better, though this varies by location).

Our Lightning Risk Map shows the real-time lightning risk for the next 60 minutes and recent lightning strikes over the past hour. This is an incredibly useful tool to help you determine if you should turn around due to lightning risk ramping up, and also if there have been any recent strikes nearby.

The lightning risk element of this map uses satellite data to determine the likelihood of new thunderstorm development within the next 60 minutes and can help you make a decision to head down before a thunderstorm arrives or develops over your location.

View → Lightning Risk Map

Looking for Visual Clues of Thunderstorm Development

Once you're out on an adventure in the mountains, you must keep a close eye on the skies as cell service is not always reliable outdoors, and there is no better data than what you can see or hear. 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 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-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/bulkier, 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 40,000 feet or higher in altitude. Dark/gray bases will appear underneath cumulonimbus clouds as well. A flat anvil will 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, that means a thunderstorm is close enough so that you are in danger of lightning, and you should start heading downhill to 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 a lightning flash and a clap of thunder is approximately one mile. So if you see a flash of lightning, and hear a clap of thunder 15 seconds later, you can estimate that the lightning strike was about 3 miles from your location.

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 rising 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 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. 

In summary, here are the visual cues you should look for in the sky that could indicate a developing or approaching thunderstorm:

• Vertically towering puffy cumulus clouds 
  
  
• Vertically towering clouds that are becoming wide/bulky
  
  
• Dark and ominous-looking cloud bases
  
  
• Flat anvils extending from the top of a cloud in a mushroom shape
  
  
• You can hear thunder or see lightning

An example of a developing thunderstorm exhibiting vertical growth, dark cloud bases, and becoming thicker/bulkier in width. 

Time, Distance, and Complexity of Your Route

It's important to think about your intended hiking or climbing route and how quickly you can descend to your car or at least to safer terrain if you are summiting a peak or high mountain pass.

Routes that involve long distances and vertical gain above treeline or on exposed ridges mean that the time it takes to retreat to safer terrain could be significant. Also, any routes that involve scrambling over rocky terrain can mean slow descents.

Your intended route and the type of terrain you will be covering should be factored into your planning and decision-making when it comes to avoiding thunderstorms.

The Most and Least Safe Terrain During Thunderstorms

The National Weather Service in Salt Lake City put together this excellent 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 descend 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, 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.

What To Do If 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.

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. You're better off utilizing your time to descend to less vulnerable terrain even if a thunderstorm is overhead, rather than to sit and wait out the storm in more exposed terrain. 

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.

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Alan Smith