Friday, July 14, 2017

Rainier Weather 101- Stratus Clouds

Stratus clouds are layered clouds that are very common in the northwest during the summer and impact Rainier climbing frequently.  Weather geeks subdivide these clouds into three sub-groups depending on elevation:  cirrostratus are found above 20,000 ft and are composed of ice particles. Altostratus tend to form in the 10-20,000 ft elevation band and can be either water droplets or ice particles.  Below 10,000 ft marine stratus, or just stratus are composed of water droplets (during the summer).

Low-level stratus as seen from Paradise looking west.

The 'marine' portion comes from the fact that these clouds form over the eastern Pacific and then move onshore when the winds are out of the west.  Large areas of clouds (thousands of square miles) form over the ocean because the cold water temps cool the lower atmosphere causing water vapor to condense into very small water droplets which then forms the clouds.  What is ironic about marine stratus formation is that they occur under a ridge of high pressure, which in general is associated with clear skies and warm temps.  Once a marine stratus deck forms it will have a well defined top.  This occurs because the air in the upper and middle atmosphere is slowly sinking (subsidence), this produces an inversion (temps increase with height over a short distance) which acts like a lid on any further cloud formation.
Typical stratus top as seen from Muir Snowfield.

When there is moderate to strong onshore flow across western Washington marine stratus move inland; how far inland these clouds move depends on the strength of the wind at various elevations.  When the winds are light clouds may only reach Puget Sound and the western foothills of the Cascades, cloud tops are typically 4-5,000 ft.  When the low-level winds are stronger a stratus deck can move across the crest of the Cascades and down along the eastern slopes.  When this occurs cloud tops are in the vicinity of 7-8,000 ft.  This means that Paradise and Sunrise are enveloped within the clouds.  In these situations visibility on the Muir Snowfield or Inter Glacier is greatly reduced. During summer these events do not produce much more than drizzle if any precipitation at all.

The intrusion of marine stratus into the Cascades can display a broad spectrum of behavior.  For example, at times the cloud layer breaks apart during the later afternoon (due to local heating and the mixing of drier air), only to reform overnight.  This may occur over a number of consecutive days (3 to 5 is typical).  At other times the atmospheric conditions are such that the stratus remain intact for days with no afternoon clearing.  I recall climbing the Tahoma Glacier years ago during one of these events- the top of the stratus were around 7,500 ft for three days, above was nothing but blue skies.

Evening stratus breaking-up near Cougar Rock CG.
The moral of this story is that stratus clouds on Mt. Rainier are common during the summer season and can at times produce low visibility conditions, especially at lower to middle-elevations.  In general, the Recreational Forecast does not reflect the nature of stratus clouds on the mountain.  Here is an recent example when it did: "Thursday...Cloudy on slopes below 7000 feet through mid afternoon, otherwise partly sunny."  Often the forecast will just say "mostly cloud", which in reality is only true for the lower half of the mountain- much of the time during this events the upper mountain is in the clear.  Hence if you know that Puget Sound is under the influence of marine stratus and the Rec. Forecast says "mostly cloudy"or "cloudy", there is a good chance that the upper mountain is going to be in the clear or partly cloudy.

The best way to predict whether a given stratus event is going to impact Rainier is by looking at a weather model and in particular an upstream sounding (vertical profile of winds, temps, moisture).  You can check-out the following website (Univ. of Wash.) for forecast soundings:
Keep in mind that forecasters tend to emphasize viable weather: clouds, precip, strong winds, etc.   Hence the forecast highlights these elements at the expense of more benign conditions like clear skies and light winds. With over 12,000 ft of relief within the park, Rainier can have a number of different weather scenarios at various elevations at the same time, so it is important to keep this in mind when planning and when climbing.

The next Rainier Weather 101 topic will cover lenticulars and mountain cloud caps.