By Alan Smith, Meteorologist Posted 5 months ago June 17, 2023
Three Ways That Precipitation Forms In The Mountains
Precipitation occurs when rising air parcels containing water vapor cool and condense into clouds, and condensation particles become too heavy for the rising air to support, therefore falling to earth in the form of snow, rain, sleet, or hail.
Moisture sources for precipitation typically originate in the oceans, while other large bodies of water and smaller-scale features (such as moisture transpiration from plants) can contribute as well.
This water cycle diagram is a good visual depiction of the entire precipitation process.
While there are numerous ways in which moist air parcels can be lifted vertically to result in precipitation, there are three primary methods of precipitation generation when it comes to mountain weather: 1) convective precipitation, 2) frontal (or cyclonic) precipitation, and 3) orographic precipitation.
These methods vary by season, location, and topography, and overlap between methods also happens, especially in the mountains.
1) Convective Precipitation
The process of convection occurs when warm, moist air parcels in the lower atmosphere become buoyant and rise vertically into cooler air aloft.
Warm air is less dense than cold air, so when a warm, moist air parcel is lifted upward, it will continue to rise into the cooler air aloft until reaching its condensation level, at which point towering cumulus clouds form and continue to expand vertically.
Convection is largely a function of atmospheric instability, which is greatest when warm, moist air exists in the lower levels of the atmosphere and colder air simultaneously exists in the upper levels of the atmosphere. Sharper declines in temperature with increasing altitude typically indicate an unstable atmosphere.
Convection can be shallow or deep, depending on the level of instability and amount of moisture. Shallow convection typically results in towering cumulus clouds that reach altitudes of less than 30,000 feet before producing rain or snow showers.
Deep convection involves strong updrafts that typically result in thunderstorms with towering cumulonimbus clouds that can reach 40,000 feet or higher.
Nature of Precipitation:
Convective precipitation is variable, showery, and often hit-or-miss in nature as opposed to steady/widespread precipitation.
Forecasters and weather models can determine the likelihood of showers (or thunderstorms) occurring in an area during a convective precipitation regime. But in a localized sense, where exactly showers occur and how often they occur in an exact spot are often random in nature.
One area can receive a heavy downpour under a convective shower or thunderstorm, while another area just a few miles away can miss out completely.
In the mid-latitudes, convective precipitation is most common during the spring and summer months (~ March to September) when solar radiation is strongest and temperatures at the surface are warm. However, it can occur at any time of year when there is a significant difference between surface temperatures and upper atmosphere temperatures.
Time of Day:
Convective precipitation is most common during the afternoon and early evening hours once the sun has sufficiently heated the lower atmosphere to allow moist air parcels to rise. Widespread, flat cloud cover in the warmer months can often delay or inhibit convection from occurring.
Occasionally, convective precipitation can also occur during the overnight and morning hours when there is unstable air aloft and processes in the upper atmosphere are able to initiate convection. This is known as elevated convection, as opposed to the more typical surface-based convection.
Warm Season Impacts:
Convective rain showers are common during the spring months in mountainous regions, and occasional thunderstorms occur during early/mid-spring as well.
From late spring through summer, thunderstorms are more common as this period coincides with the warmest temperatures while solar radiation is also very strong, resulting in greater atmospheric instability. Warmer air can also hold more moisture, and latent heat from abundant moisture further contributes to instability.
Lightning is the primary hazard with convective precipitation, but locally heavy rainfall can also occur, which can result in flash flooding. Hail also occurs frequently with mountain thunderstorms, especially in spring and early summer when freezing levels are lower.
Cool Season Impacts:
We might think of convective precipitation as a summertime regime, but it is common during the latter part of ski season in the late winter and spring months once the sun is getting stronger and the days longer.
While instability in the late winter/spring months is less than what we see during the summer, resulting in a lower likelihood of lightning, convective snow showers are quite common late in the ski season.
The nature of convective snow showers is similar to summertime showers/thunderstorms, meaning they are often hit-or-miss in nature and can lead to highly variable snow totals over short distances that are not always explained by orographics/terrain differences.
Occasionally, convective snow showers can lead to snow squalls, resulting in very heavy snowfall rates and gusty winds along with whiteout conditions.
Convective snow showers are most common behind cold fronts, in which colder air aloft arrives while the stronger late-season sun can counteract the cooling effect at low altitudes behind the front and lead to relatively warm temperatures at the bases of mountain ranges.
Also, post-frontal convective snow showers are more common during mid-winter across coastal ranges such as the Cascades and Sierra compared to the Rockies.
This is because cold air aloft moves over the Pacific lowlands and into these mountain ranges, while the Pacific Ocean has a moderating effect on low elevation temperatures west of these mountain ranges, resulting in a steeper decrease in temperature rate with altitude which contributes to instability.
2) Frontal (or Cyclonic) Precipitation:
Frontal precipitation is what we think of when a large trough of low pressure impacts a region, resulting in widespread and often heavy precipitation. OpenSnow forecasters will also refer to these systems as "storms" or "storm systems".
Frontal precipitation covers larger geographical areas compared to convective or orographic precipitation. It develops as a result of opposing airmasses (warm and cold) that develop in the vicinity of a low pressure trough, and dynamic processes in the upper atmosphere also play a significant role.
The interaction of airmasses results in warm, low density air rising over the top of colder, denser air. This rising motion leads to precipitation developing, which tends to be longer lasting and more widespread compared to convective precipitation.
Nature of Precipitation:
Frontal precipitation is steady and widespread, often covers large areas, and can last anywhere from a few hours to a few days depending on the strength and speed of the low-pressure system.
Precipitation rates, while steady overall, can vary between light and heavy over the course of an event.
Frontal precipitation is most common during the fall, winter, and early spring when the jet stream is strongest and the atmosphere is more stable (meaning precipitation is less likely to be convective). However, frontal precipitation can occasionally happen during the warmer months, especially in northerly maritime regions.
Time of Day:
Unlike convective precipitation, frontal precipitation is not dependent on solar radiation and can occur at any time of day or night.
Cool Season Impacts:
Frontal precipitation and winter storms are synonymous, and therefore the type of precipitation you want to see if you are chasing powder. The orographic effect can also enhance frontal precipitation to result in heavy snow for favored ski areas depending on wind direction (more on that in the next section).
At the same time, valley areas in between mountain ranges that typically receive lighter precipitation compared to surrounding higher terrain can see periods of heavy snowfall rates thanks to lifting mechanisms in the upper atmosphere that can be strong enough to overcome the orographic effect.
Strong frontal systems can also produce high winds and poor visibility, which is a consideration when it comes to travel, lift operations, skiing conditions, and avalanche danger.
While frontal precipitation is usually good when it comes to powder during the winter months, "unwanted" precipitation types can also occur – i.e. rain, sleet, or freezing rain.
Warm Season Impacts:
Frontal precipitation is less common during the warm season (~ May to September) as convective precipitation tends to be the dominant precipitation mechanism for many areas. However, it does happen on occasion, with the most likely locations being the Pacific Northwest (Cascades and BC Coast Range) and New England.
Warm season frontal precipitation events typically involve consistent cloud cover, steady rain (often light to moderate in intensity), and cooler-than-average temperatures.
3) Orographic Precipitation:
When moving air encounters a mountain range and is forced to rise up and over the range, it is called orographic lift. This process is responsible for a large portion of annual precipitation that falls across mountainous terrain and is also responsible for large differences in precipitation between high and low elevations.
As winds encounter a mountain barrier at a roughly perpendicular angle, air is displaced up the windward side of the mountain range.
When enough moisture is available, this upward-moving air cools and condenses into precipitation on the windward and upper leeward (downwind) side of a mountain range. Precipitation rates tend to increase up the windward side with increasing elevation.
The opposite side of the mountain is called the leeward side and usually sees much less precipitation, except for upper portions of the leeward slopes where heavy precipitation often spills over. The reason is that air is descending on the leeward side of the mountain, and descending air is warmer and drier, which is the opposite of ascending air.
The orographic effect and resulting impacts around a mountain range can vary depending on the wind direction of a given event, and the orientation of the mountain range relative to the wind direction. Winds hitting a mountain range at a perpendicular angle maximize the orographic effect, while winds blowing parallel to a mountain range minimize the effect.
Nature of Precipitation and Seasonality:
Orographic precipitation can occur entirely on its own, or it can occur in conjunction with frontal/cyclonic or convective processes. As a result, orographics are a factor year-round though the greatest influence on precipitation amounts (and differences between mountain/valley precipitation) is during the winter months.
When a frontal system is impacting an area, a favorable wind direction relative to a mountain range will enhance the orographic effect, often resulting in heavy precipitation.
However, a frontal system is not necessary for orographic precipitation to occur.
In the absence of a larger storm system, consistent moderate winds, blowing from a favorable direction and containing sufficient moisture can result in orographic precipitation on the windward and upper lee slopes of a mountain range while the valley on the lee side of the range is dry and sunny. This process can be aided by subtle shortwaves as well.
During the warm season, the orographic effect can aid the process of convection by giving a "boost" to warm, moist air parcels which are forced to rise as they encounter a mountain range. As a result, it's common for showers and thunderstorms to develop directly over mountain ranges.
Time of Day:
Similar to frontal precipitation, orographic precipitation can occur at any time of day or night.
Cool Season Impacts:
Orographics play a significant role when it comes to skiing and chasing powder. Forecasters pay close attention to wind directions when a storm is looming to determine which ski areas or mountain ranges will be most favored for snowfall relative to other areas.
Ski resorts that receive higher annual snowfall amounts compared to their neighbors are often located in an orographically favored area – meaning they are in a prime spot based on topography and prevailing winds.
Warm Season Impacts:
Mountain ranges are often a focal point for summertime showers and thunderstorms. As a result, hikers and climbers should pay attention to vertically growing popcorn-like cumulus and cumulonimbus clouds, which could very well develop right overhead or in the vicinity.
Lightning is a serious threat for high-elevation hikers and climbers, not only because higher treeless terrain is more exposed to lightning strikes, but because thunderstorm initiation is more likely to occur over high terrain.
Thanks so much for reading, and we hope this is something you can add to your weather knowledge base as an outdoor enthusiast.