And Now for a Look at the Weather: A Little Conversation with the Clouds

Words & photos by Michael DeYoung

Former Air Force meteorologist and professional photographer Michael DeYoung continues his series about the weather with a deep dive into the clouds. They hold a lot of useful information you can use in the midst of an adventure if you know what to look for. Grab a favorite bevvie or two and buckle up for a real education about the friendly and sometimes not-so-friendly skies!

What are the clouds telling you?

As I sat down to write this blog I realized no matter how much I tried, that there was no way to make this a short blog and have it be useful in any way. So right off the bat I’m giving up on that idea. This post is long but hopefully informative and interesting.

I follow several hiking groups on social media. Too many discussions are about gear. Do I have the right shoes? Best tent? On and on. I’m a knowledge guy more than a gear junkie. I see this in photography circles too. I constantly preach and remind people that getting better cameras doesn’t translate to better photos. Knowing weather, light and design skills do. No different with other outdoor pursuits. Good gear is baseline. Skills, knowledge and judgement are what carries the day. The more informed you are about your environment including clouds and weather patterns among other things the better your judgement and wilderness skills become.

Let’s start with a quick analogy. Thinking back on learning whitewater canoeing years ago there are three primary skill sets involved. The first is boat balance and feel. It took a while to learn how a whitewater canoe behaved with balance and leaning into the chine. The second is learning paddle stokes and boat control. It takes a few months of regular practice to be able to turn the boat anyway you want without having to think about the stroke you need. Sort of like driving a manual transmission without having to look at the tach or the diagram on the gear knob to know when to shift gears. You just do it while thinking about something else. The skill that takes the longest is reading water and identifying hazardous features, picking safe routes and knowing how the current will react with your boat. This skill can take years to master.

Like reading water, reading the sky and what it’s telling you takes practice and time. On many of my extended day trips weather was no issue at all and that’s great. But there were also trips where accurately reading atmospheric indicators influenced decisions that averted unnecessary misery or mishap.

Identifying cloud types, their height above the ground, their formation, dissipation and movement and what they mean, as well as understanding other basic meteorological elements can prove to be quite beneficial for backcountry planning and on-location decision making. We can’t do this all in one blog or even a series but at least we can plant seeds and get interested backcountry adventurers started on the path to becoming better weather observers and forecasters. Here, we will start with precipitation producing clouds, temperature, dew point and pressure and wind. This post is an overview. In the next post (May) I will go more in depth on certain clouds and mountain weather.

Clouds aren’t just clouds. Different cloud types at different levels in the atmosphere mean different things. Rather than reinvent the wheel and describe all cloud types one might see in this I will cherry pick a few significant clouds. To learn more about all cloud types you can check out the National Weather Service’s on-line meteorological tutorials, Jetstream, at: https://www.weather.gov/jetstream/clouds_intro There is some great information here and I would spend some time going through their well written and illustrated tutorials.

Altostratus clouds lit by the rising sun over Lost Lake near Seward, Alaska will otherwise be featureless and grey when the sun rises higher in the sky. The gradual thickening of these clouds from the top down were indicative of an approaching widespread and prolonged rain event. This event was accurately forecasted which motivated us to camp here the night before. If a storm system approaches at sunrise, the leading edge of mid and high clouds often produce warm sunrise colors. I was able to capture what I was there for.

Most outdoor folk are most concerned with precipitation and temperature. Will it rain/snow/hail or sleet? How cold will it get?

Significant precipitation falls from low clouds. Low clouds are those whose bases are 6500’ or lower above ground level. Low clouds that produce precipitation are at least 4000’ thick. There are always exceptions of course. In the Southwest, cumulus clouds, the kind that produce rain showers and thunderstorms can have bases as high as 12,000’ above the ground during the dry summer months but they still take on the same characteristics and properties of cumulus clouds that form at much lower elevations elsewhere. Upsloping winds into major mountain ranges (orographic lift) can produce precipitation with cloud depths of only 2,000’ thick.

There are cool middle and high clouds worth talking about as well. More on those in a later post.

The first cloud type discussed here are cumuliform clouds or convective clouds. The cumulus cloud in various stages of development is the most common cloud across much of the CONUS especially in the summer months and all of us have seen these many times. They form from moist columns of air at the surface are heated by the sun, or mechanically lifted by a front, disturbance or terrain that rise until they cool and condense into a cumulus cloud base. In an unstable atmosphere these clouds can grow to great heights sometimes 50,000 to 60,000 feet high. Cumulus clouds are cellular and resemble popcorn or cauliflower that is easily recognizable at a distance or when they are widely scattered. When they are on top of you or packed close it may be a little harder to recognize what stage of development they are in.

Cumulus that reach a mature stage are called cumulonimbus. Mature cumulonimbus clouds often have anvil shaped heads that are composed of high cirrus clouds. The more vertical depth cumulonimbus have the more capable they are at producing severe weather.

This is a mature cumulus cloud called cumulonimbus over the far western plains of northern New Mexico seen from near 100 miles to the west. These clouds are easily identifiable from a distance. You can't see the base but you can see the significant vertical development and the signature anvil head. The presence of billowing development extending above the anvil head indicates the potential for severe weather. Cumulus clouds always build from the bottom up until solid clouds builds high enough in the sky to produce precipitation. Lightning and tornadic activity ONLY come from cumulonimbus clouds.

Cumulonimbus clouds are the ONLY clouds that produce severe weather defined as thunder and lighting, hail, damaging microbursts and tornadic activity which includes funnel clouds (tornados that don’t reach the ground) and waterspouts (tornados over bodies of water). They represent the most potentially dangerous cloud in the backcountry everywhere. Not all cumulonimbus clouds produce severe weather but all are capable of lightning. Additionally, heavy rain showers and snow showers can fall from cumulus clouds that never reach the cumulonimbus stage.

Precipitation from cumulus clouds can be heavy, start and stop abruptly and or have a sharply defined delineation of where precipitation is falling or not. In the summer time when cumulus activity is common across much of the US, they form when the temperature inversion is erased by daily heating and lift begins. In mountain regions especially when valley inversions break one can see clear skies to mature cumulonimbus in less than an hour. (In the following blog post, I will talk about the difference between airmass thunderstorms and frontal thunderstorms and address the model biases concerning thunderstorms also referred to convective activity.)

8AM Thunderstorm over the Continental Divide, Continental Divide Trail, San Juan Mountains, Colorado in mid-September. Sometimes cumulonimbus clouds, the only clouds that produce thunderstorms are hard to identify when you are underneath them. Water droplets in a cumulonimbus are large and block a lot of light and thus give that foreboding dark sky not seen with other cloud types. In the absence of visible lightning or audible thunder, the darkening sky with a billowing cloud base is your best clue as to the presence of these clouds. Sometimes, precipitation shields obscure any defined cloud base. Precipitation from large moist cells such as this comes fast and furious with large water droplets, hail or large snow flakes. The temperature drops fast, as much as 30 degrees in 10 minutes not from the blockage of the sun or passage of a cold front but simply from vigorous evaporative cooling that causes super adiabatic lapse rates. We tried to out run this cell not expecting to see one this intense so early in the day. We did get down into some near timberline shelter before the brunt of this storm hit us.

Cumulus clouds develop in the winter too as seen here over the Grand Canyon on the South Kaibab Trail. This large cell is producing snowshoers seen by the white ragged shafts. The cellular nature of the cloud base and the fact that snow started and stopped pretty abruptly several times that day is characteristic of cumulus clouds and showery precipitation.

The second chief precipitation cloud is nimbostratus. They from top-down moistening where moisture streams in from the upper and middle levels and the atmosphere saturates and clouds thicken downward. When this process of cloud thickening continues into the lower levels (below 6500’ AGL) significant precipitation can occur. This happens in a stable atmosphere and characterized by widespread continuous precipitation. The cloud character of nimbostratus and other stratiform clouds resembles sheets and blankets and can lack definitive form in their bases. They are almost always featureless and blah grey.

Featureless grey base of nimbostratus producing an all day rain in the Glacier Peak Wilderness in the Washington Cascades.

A watchful observer may notice a day starting with an alluvial fan of thin white cirrus clouds (high clouds composed of all ice crystals) on the horizon. The leading edge of advancing high clouds is ragged and can be characterized by hooks and filament shapes. Cirrus and cirrostratus clouds are always white when the sun is up. Except in the polar regions they are usually above 20,000’.

This is the first stage of nimbostratus formation and widespread steady precipitation from an occluded front. Cirrus (high level) clouds at sunrise over the Fairweather Range of Glacier Bay National Park, Alaska. There are many instances of cirrus clouds floating across the sky that mean absolutely nothing weatherize except for being pretty and photogenic. In this case, the clouds were in the shape of an alluvial fan which represented a concern at the time. This cloud shield kept advancing and the thin whisps seen here turned into an overcast veil of a white and featureless cloud ceiling of cirrostratus. Often, and advancing alluvial fan of high clouds represent the leading edge of an approaching frontal system and widespread steady precipitation. The moisture thickened from the top down and rain began later that day. I didn't take more photos of the advancing stages because at that time the clouds were not photogenic anymore, and I didn't know that 10 years later I would be teaching weather classes.

When hooks and filaments becomes a bleak white sheet this is cirrostratus, still high in the atmosphere. A telltale sign of cirrostratus is a complete halo around the sun or moon. This alone doesn’t mean imminent bad weather coming but it merits careful observation. When cirrostratus thickens into the mid levels of the atmosphere turning grey it is now altostratus with bases usually around 10,000-12,000 feet.

This is the second stage of nimbostratus development with cirrostratus clouds in the highest levels of the atmosphere continuing to thicken from the top down into very high level altostratus clouds near 15000 feet above the Rio Grande Valley in northern New Mexico. There is a very shallow and small deck of stratocumulus clouds hugging the peaks but those are insignificant. The higher featureless cloud deck covering most of the sky started as cirrostratus and is gradually thickening and lowering into the mid levels of the atmosphere. This alone doesn't mean rain is coming but is sure is an indicator worth watching.

If the clouds continue to thicken one might see virga which is rain/snow falling but evaporating before reaching the ground which can give some texture to the base of altostratus. In the absence of virga or wind shear, the bases of altostratus are mainly bleak featureless grey with sometimes thin cellular altocumulus forming beneath them.

This is the third stage of nimbostratus development as altostratus continues to thicken int the lower levels of the atmosphere. This image shows 3 stable cloud layers over Lost Lake Trail near Seward, Alaska on the Kenai Peninsula. The upper most layer is altostratus that is continually thickening from the top down that ended up resulting in a 2-day steady rain. Hugging the peaks on the left are stratocumulus clouds forming as moist air in the lower levels of the atmosphere is forced up the mountain. The long narrow cloud behind the backpacker is stratus, a low cloud that is advancing inland from the nearby and very cold fjord of Resurrection Bay. That thin low cloud isn't much of a concern for precipitation. The main indicator of impending rain is the thickening high clouds with a much deeper and more widespread area of moisture it is bringing in.

Very light rain or snow can begin from altostratus but significant precipitation doesn’t usually begin until the cloud layer continues to thicken and lower its bases to less than 6500’ above ground. This signifies a likely widespread continuous precipitation event. In mountain regions after a long precipitation event, (there is no hard and fast rule here but generally longer than 6 hours) one can expect low clouds obscuring peaks and ridges or trapped in valleys to linger long after the precipitation ends.

This is the final stage of nimbostratus development with a ragged featureless cloud base below 6500' and producing steady rain. Our air taxi leaving Kilik Lake, in Lake Clark National Park. Alaska. This shows a typical nimbostratus cloud producing a long rain event who's base is near the mountain top levels and below 6500' above the ground. The lower white clouds below the rain producing nimbostratus are ragged stratus and are caused by evaporative cooling as not all rain that falls reaches the ground. This is a very typical development after a long rain event.

Even in a steady rain/snow and a saturated atmosphere, not all precipitation that falls reaches the ground. Some of it evaporates and we know that evaporation is a cooling process that takes heat away from the atmosphere. When the temperature drops during rainfall, it is not necessarily from a cold front or lack of sun heat, but simply the product of evaporative cooling. After prolonged precipitation of several hours or more, low clouds, commonly referred to scud, cumulus fractus or stratus fractus form below the base of the solid cloud producing the precipitation as a result of evaporative cooling. This may be hard to recognize to the untrained weather observer especially in flat areas but in mountain and canyon terrain, these clouds can be seen clinging to ridges or partially up mountain slopes and can take on a beauty of their own adding depth and definition to the landscape.

The featureless base of nimbostratus over Zion National Park in late May that resulted in an all night and near all day steady rain. The ragged stratus clouds hugging the canyon walls are the result of evaporative cooling with air in contact with the canyon walls reaching saturation. It is very common in lower latitudes such as southern Utah and in late spring for a steady prolonged rain in a stable atmosphere to transition into an unstable rain shower and cumulus cloud scenario caused by strong sun heating.

Nimbostratus and stratiform clouds are more common in the winter months especially in northern latitudes. Again, prominent mountain ranges can enhance and suppress precipitation in a stable atmosphere with top down moistening. Systems that produce widespread steady precipitation events often leave very low clouds in their wake after the solid precipitation producing clouds have moved on.

As always, nothing in the atmosphere is always simple or cut and dry. Frontal systems and even weak upper air disturbances can produce both types of precipitation at the same time. Long steady rain from stratiform clouds can follow a heavy rain shower and thunderstorm. An all day stable atmosphere rain can be interrupted by embedded heavy rain showers and thunderstorms. Skillful cloud identification and observation can help identify when cloud types are changing.

This is a rapidly developing GTFO situation on the summit of Angel's Landing in Zion National Park, Utah. Looking south toward Zion Canyon entrance are the telltale signs of a cumulonimbus cloud-darkening bases, shafts of rain and the formation of ragged low clouds beneath the base that signify strong evaporative cooling. Significant low level moisture remained over the area left from an all night rain from nimbostratus. A little daily heating set the stage for thunderstorm development as the atmosphere transitioned from stable to unstable. This thunderstorm did not move over us but expended and built over us so we were under one massive cell. We got off the summit in time to avert exposure to lightning and heavy rain. Slow moving cells with very light steering flow aloft can be quite dangerous as they unload their moisture over a very concentrated area vs moving through your location.

Generally speaking, cumulus clouds and thunderstorms not linked to a front or disturbance reach maximum intensities in the afternoon and early evening. Precipitation from nimbostratus can have a nighttime and morning maximum intensity somewhat abating in the afternoon. The bottom line is precipitation in a stable atmosphere from nimbostratus starts and ends more gradually and is more widespread than precipitation from cumulus clouds not linked to a front or disturbance.

Sunrise on the eastern face of Denali and the Alaska Range reveals itself after a typical prolonged period of clouds and rain. The only significant cloud left in the wake of this wet system is fog and stratus trapped on the valley floor under a temperature inversion. Alaska has an abundance of low level moisture from lakes, rivers, wetlands and the ocean.

Stratus: Stratus clouds are the lowest of low clouds and usually form below 2000’ above ground level (AGL). Fog is just stratus on the ground. Stratus is very common in the wake of long precipitation events but it can form without being linked to any front or precipitation event. Stratus is a stable atmosphere cloud, always trapped under a temperature inversion and is common with Pacific high pressure and Arctic high pressure. If you wake up at a valley camp and it’s foggy or very low clouds, grey and damp this doesn’t mean bad weather or an approaching storm, especially if there is no precipitation occurring.

These clouds over Park City, Utah seem dark an ominous but they are of little concern for any outdoor activity and don't mean a storm is approaching. This is a shallow layer of moisture trapped below a temperature inversion caused by strong radiational cooling in the long winter night. On this mid January morning it was several degrees below zero in town. Low stratus and fog is very characteristic of cold high pressure especially in high elevation valleys where moisture, sometimes human caused by car and home heater exhaust can remain trapped for days under an inversion until cleared out by the next front. The air above the fog layer is bone dry and it was a nice day in the mountains. The edges of the sun are crisp and sharply defined and are a clear indication of fog and stratus. If the sun's edges were soft and diffuse that would indicate a higher cloud layer above the fog.

Moisture trapped under a temperature inversion and cooled to the dew point (more on that down below) simply from nighttime cooling can cause stratus and fog to form in a valley. Chances are, with valley fog and no precipitation, you can hike or drive above it into warmer and drier air.

If you are hiking a ridge top and it is precipitating, this is a different story than valley fog and stratus trapped under an inversion. It might appear like fog but it is most likely that you are in a precipitation producing cloud more than 4000 feet thick where the base of the cloud happens to be below the ridge top level.

Camp along the remote Pacific Coast in Olympic National Park, Washington. The clouds you see off shore are shallow stratus clouds and fog with its ragged front edge seen the upper half of this image. This is a very shallow layer of moist air trapped under a persistent temperature inversion in a stable atmosphere. What does this tell you? No storm and nothing threatening at all. This fog/stratus layer sometimes referred to as a marine layer is too shallow with cloud tops less than 1000 feet high to produce any precipitation. At times this fog/stratus layer reduced visibility. For mariners or kayakers around rocks this could be an issue. For backpackers and hikers it was a non-issue doing nothing more than blocking the sun. The edges of the sun always appear sharp and crisp when seen through fog and stratus clouds.

Temperature and Elevation. Most people are aware of temperature changes with elevation. Generally speaking, temperature decreases with height and elevation but not always! Most people know that especially in western North America, elevation drives temperature and weather much more than latitude. Proximity to ocean air also has a profound influence on temperatures and precipitation. This effect is perhaps most pronounced across the Washington Cascades and the Sierra Nevada. At identical elevation and latitude, the eastern slopes of the Cascades are drier and much colder in winter than the western slopes.

Hiking in very warm air on Harding Icefield Trail in Kenai Fjords National Park high above the Resurrection River Valley on Alaska's Kenai Peninsula. The clouds below are stratus clouds and are capped by a temperature inversion. Cold, moist air caused the cold and large body of water, Resurrection Bay, remains trapped below while mountain slopes above bake in the July sun. These clouds below the hiker and the absence of any cumulus clouds above the mountains indicate a very stable atmosphere in where temperatures with increasing elevation change at a very slow rate and in this case even increase as the hiker is in an inversion layer.

The rate at which temperature decreases with altitude is called the environmental lapse rate. This rate is highly variable influenced by atmospheric stability, predominate wind flow, and reflectivity of the surface such as snow and vegetation cover. An average environmental lapse rate is 3.3 Fahrenheit degrees per 1000 feet. In a wet stable atmosphere, it is near 2F degrees and in a dry an atmosphere it is 5 Fahrenheit degrees per 1000 feet. In the Grand Canyon it is nearly 5000’ difference between the rim and river and the temperature difference can be 30 degrees warmer at the river, exceeding the dry lapse rate when you factor in radiant heat and thermal mass at the bottom of the canyon.

Four backpackers descend Bright Angel Trail in Grand Canyon National Park just below the rim. An early winter Pacific cold front brought 5-6

In vigorous thunderstorm activity, especially with dry and high based cells one can see what’s called super adiabatic lapse rates where temperatures can cool as much as 15-20 Fahrenheit degrees per 1000 feet. In monsoonal weather in the San Juan Mountains the Rio Grande Valley can see 80 degrees and under a thunderstorm cell 4000 feet above at elevations of 12000 feet can see snow, even in July! This catches many unprepared hikers/backpackers off guard.

Dicey Call. Southbound Pacific Crest Trail hikers ascend Fire Creek Pass in July in the Glacier Peak Wilderness in the Washington Cascades. Precipitation from cumulus is a cold rain process. Even in summer, precipitation from cumulus cells starts as snow that melts to rain in the column of warm air below. At high elevation, 6000' here, all the precipitation fell as snow even in the 3rd week of July. Evaporative cooling from large cumulus cells can be strong enough to drop temperatures 15-20 degrees in a 1000'. An hour before crossing this pass we were in shorts and T-shirts. When reaching the pass, evaporative cooling created a stratus cloud and was accompanied by strong winds funneling through the pass to where the trail was totally lost in the snow with visibility of around 100'. Had it not been for navigational skills we could have been lost for a period of time in a dangerous hypothermic environment.

Temperature inversions are a common and almost daily occurrence in all mountain valleys in a stable atmosphere and light winds. In winter, cold high pressure entrenched in western valleys can mean extreme temperature inversions that last for days unbroken. In cold high pressure regimes the coldest and heaviest air settles into the lowest spots. Even on clear summer days, in a high elevation and sharply pronounced mountain valley it is often colder in the morning than it would be 1000-1500 feet up slope.

The presence of fog and low clouds in a valley signify small temperature falls with height and even warming. For example it is common to see warmer and more pleasant air on the ski slopes above Park City than you see in town under high pressure patterns. Arctic high pressure that plows into the front range of the Rockies often means nicer conditions on Continental Divide ski areas than in Denver, Ft. Collins and other areas on the high plains.

Vigorous cumulus cloud presence over a high plateau or mountain peaks means a steeper lapse rate and stronger cooling with elevation than the standard 3F degrees per 1000’. If you are hiking into the mountains and there is near zero cumulus cloud development in the afternoon, chances are the environmental cooling with elevation is not as steep as normal.

Wind: If you stand at the edge of a whitewater river or creek, the direction of the current is clearly defined. However if you look more closely you will find that the direction of flow below rocks and in eddies can be 180 degrees opposite from the prevailing current. Keep in mind that atmospheric currents behave similarly. Surface winds are frequently from a different direction than winds aloft. If clouds are present it’s better to determine the prevailing wind flow aloft. That will give you more useful information about where weather might come from more so than surface winds. Wind is directly connected to pressure and thermal contrast.

Moving air is created by pressure differential and blows from high pressure to low pressure. This is the pressure gradient. If you are good at reading topo maps then reading surface weather maps will be easy. Surface pressure maps show us highs, lows, ridges and troughs. Instead of contour lines of equal elevation on a topo map, a surface weather map consists of isobars-lines of equal pressure. Stronger winds exist where lines are close together.

A stiff west wind at sunrise blows across Convict Lake in the Eastern Sierra, CA. This is a downslopping wind. Strong winds blowing across a mountain range flow downward on the lee side of the range as seen here. Downsloping winds are drying winds and the western slope of the Sierra which would be on the windward side is probably experiencing considerable cloud cover and precipitation. Winds accelerate when they descend and accelerate even more when they blow across a surface with reduced friction such as a lake. The high Sierra is the most prominent and formidable terrain barrier in the CONUS. They are the reason why the Owen's Valley in their eastern shadow is a desert.

Winds spiral clockwise (anticyclonic) out of high pressure and spiral counterclockwise (cyclonic) into low pressure. This is opposite in the southern hemisphere. This ideal flow pattern only exists over large areas of even terrain with minimal friction like oceans and the Great Plains and Prairie. In complex terrain, the friction coefficient changes things drastically and wind direction can be strait across isobars much like a skier going strait down the fall line. Winds can accelerate through passes and at openings of canyons and valleys. A common mountain wind pattern is a Chinook wind where winds accelerate and warm by compression when flowing downhill.

As mentioned above pressure and wind is also connected to temperature. Cold air is heavier and denser than warm air and this density difference alone can drive wind direction and speed from heavy air to lighter air. Density difference can be created by unequal heating alone. A large landmass both heats and cools more quickly than a large body of water. This is what creates sea breezes as well as mountain and valley breezes. Very large lakes across the north, not just the Great Lakes, can generate their own land and sea breeze patterns Winds accelerate over water when they leave land simply from reduced friction. Large glaciers on clear hot days can kick out very strong down-glacier (katabatic) winds.

In mountainous regions, strong winds can occur particularly from late morning to early evening even with a light surface pressure gradient. Strong winds aloft linked to the lower depths of a jet stream often mix down to the surface during the day when the sun’s heat erodes a temperature inversion. So if you you look up at ridge tops from your valley/river/lake camp and you hear winds blowing in the trees on ridges above you it might not be the terrain that’s protecting you from the wind but an inversion that deflects the wind aloft. When the inversion breaks the winds aloft mix down to the valley level. In the evening with sufficient clear skies and radiational cooling, winds aloft decouple from valleys again when a temperature inversion sets up.

Ridge top camping can be tricky. Even in fair dry weather, winds aloft that have mixed all the way down to valley and plains can mean only moderate winds at exposed ridges during mid-day. When valley inversions set up forcing winds aloft again, ridge top winds often increase at night and calm down mid morning- the opposite of what can happen at lower elevations.

Winds blowing all night in valley location signify an approaching system and night winds also prevent temperature inversions thus keeping conditions warmer not considering wind chill. Winds that begin after a calm period at night and result in significantly warmer temperatures in the morning frequently mean the approach of an organized weather maker.

I remember before GPS devices were common some folks would carry an altimeter in the backcountry thinking that taking pressure readings might be meaningful. Unfortunately, barometer readings from one location don’t tell you much. You need many over a large area to get the big weather picture. Even under the same pressure pattern (i.e. no approaching highs, lows, fronts, etc.) there are 2 diurnal cycles of pressure falls and rises per day. So seeing your altimeter/barometer fall at the same elevation does not necessarily signify an incoming low.

Dew Point and Humidity. Outdoor adventurers will get more useful information by paying more attention to dew point temperatures than relative humidity. Relative humidity is just a ratio of moisture in the air relative to how much moisture the air can hold at that temperature. Dew points are a more actual measure of water vapor present at the surface. Warm air holds more moisture than cold air. Dew point temperature is the temperature the air would need to be cooled to reach for 100% saturation to occur. Dew points can be equal to but NEVER higher than ambient temperature. They also fluctuate much more slowly than ambient temperature does. When dew points continue to rise and/or fall over many hours bucking the normal daily trend, this can mean a changing weather pattern. Tracking surface fronts across the intermountain west can be difficult. Sometimes our best indicator in tracking fronts across the West is tracking dew point changes on a large scale.

Consider this example: Ely, MN near the Boundary Waters can have a January temperature of -10F and a dew point of -11F yielding a relative humidity of 95% In July on a pleasant 75F day under a dry Canadian high pressure the dew point might be 45F yielding a relative humidity of 34%. But air at -10F can only hold a fraction of the moisture that air at 75F can hold. Even though the relative humidity in the January example is higher at 95% the actual amount of moisture in the air is so much less than the case in July. This is why skin and lips chap more in winter. Cold air simply holds less actual water vapor than warmer air regardless of the relative humidity.

Sometimes people wonder, how can it be raining so hard and the RH is only 65%? And other times, the RH is 100% and it’s clear. How can this make sense? To start with, relative humidity at the cloud base and above where the rain is coming from is near 100% If it rains long enough, remembering from the discussion above, some evaporation will take place the surface humidity will definitely increase after prolonged rain. On a clear or hazy morning across the Midwest or Appalachians why is it clear with 100% humidity? First, the moist layer of 100% humidity may only be a few hundred feet thick and the air above is much drier. Remember, with radiational cooling at night, the air is cooling toward the dew point temperature sometimes reaching saturation. Second, saturated air doesn’t always result in clouds unless there is some mixing usually from a little wind, or, lift. On a typical clear, calm valley morning there is no lift or mixing and thus you can have an atmosphere with 100% saturation with no clouds.

Under the same airmass, again, no fronts or tropical systems approaching, dew points and relative humidities reach a morning max and afternoon minimum. We all know that the southeastern US is hot and sticky in summer. Growing up in central Florida it’s hard to forget that. However, 90 degrees with 100% humidity is an exaggeration and these conditions are rarely if ever observed in the US. When the eastern US reaches temperatures in the upper 80’s and higher under the persistent maritime tropical airmass (aka Bermuda High) afternoon dew points hover around 70 which puts your typical afternoon RH in the 50-60% range. Sticky and oppressive yes, but not 90-100%. Those levels only occur around the minimum temperature cycle.

It’s not very practical for backcountry travelers to measure dew point. You would need a psychrometer to measure the wet bulb temperature and an app to calculate the dew point temperature. However you can be aware of current and forecast dew points before you go out. Monitoring nearby weather stations can tell you a lot. In the summer monsoon season, pretty much every day across most of the Four-Corner states is going to be in an unstable atmosphere. The determining factor in thunderstorm coverage and intensity can be largely a function of available low level moisture as indicated by dew point temperatures. For example, in August, if I see dew point temperatures consistently in the high 40’s and low 50’s in places like Moab, Grand Junction, Albuquerque that tells me thunderstorms over the mountains can hold a lot of moisture, be more widespread in coverage and hold much more flash flood potential than seeing dew points in the 20’s and 30’s with similar ambient temperatures. The drier dew points indicate it’s a much safer time to venture in mountains and canyons.

Lower dew points may indicate much less cumulus and thunderstorm potential but cells that do form have higher bases in drier low level air. In an airmass with a light pressure gradient, high based cells are more likely to kick out stronger and erratic winds, especially when they dissipate. All that energy lifted high into the atmosphere during cumulus development has to come back down to earth at sometime. So if you are camped on a ridge top with calm winds and there are high based cells around that existed most of the afternoon, expect a period of potential high erratic winds indicating the dissipation of cells.

It looks like this southbound Pacific Crest Trail hiker on the Kendall Catwalk in the Alpine Lakes Wilderness, Washington is in the fog but she is not. She is engulfed in a stratocumulus cloud in a stable atmosphere, who's base is below ridge top level. A shallow, low level onshore flow being lifted by the abrupt rise of the Cascades frequently causes this on the west facing slopes of the range. This is a high pressure cloud with little vertical development capped by a temperature inversion at the top of the cloud layer. There was no precipitation falling from this cloud and we descended well below the base with unlimited visibility within an hour of making this image.

Most of us go into any extended backcountry trip hoping for the best but prepared for the worst. Most people also factor in a basic knowledge of the area’s climatology to determine what warm clothing to bring or what temperature rating your sleeping bag should have. The ability to read clouds on location and what they might mean for temperatures, wind and/or precipitation can influence where you camp, (valley, mid slope, ridge top) when to spend half a day descending a narrow canyon or when you decide to do a high and exposed traverse.

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And Now for a Look at the Weather: A Little Conversation with the Clouds

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