Sunday, May 17, 2015

Summer Recreational Weather Outlook

We have had an unusual winter, with warmth and drought over the western U.S. and record-breaking cold and snow over the Northeast.  For over a year, the weather pattern was locked up with high pressure over the West Coast and low pressure over the eastern U.S.   We are now close enough to the summer to get a clearer view of what might be in store;  let me tell you about it.

The most powerful tool for forecasting months or seasons ahead available to U.S. meteorologists is called the Climate Forecast System (CFS), which is run by the National Weather Service.   Its prediction for surface air temperature for June through August (see below) indicates warmer that normal temperatures over the western U.S.  Such warmth will increase the danger of wildfires over the Sierra Nevada, the Cascades, and  the mountains of British Columbia.  The opposite is true of the High Plains and the Colorado, with below-normal temperature are predicted from Texas to the Dakotas.

Surface air temperatures forecast by the National Weather Service Climate Forecast System for June through August.  The colors show the difference of temperature from normal (°C), with red being above normal and blue below normal.

What about precipitation?   Generally near normal along the West Coast, but substantially wetter than normal over the Rockies and High Plains.  Most of this precipitation will be associated with convection:  thunderstorms, which also bring lightning.

Precipitation predicted by the same modeling system, with the colors showing differences from normal (green and blue are wetter than normal, orange and red drier than normal, units are in mm)

Lightning is a major initiator of wildfires, and with warmer than normal conditions drying the land and surface vegetation, the potential for wildfires will be substantially increased this summer.   Lightning is also a danger when ascending the high terrain, so it is always a good idea to get an early start, so that one is off peaks and crests by early afternoon, when thunderstorms tend to be most frequent.   If cumulus clouds are developing rapidly around you when ascending a ridge, you should descend as quickly as possible to minimize your risk.

So if plan on hiking along the West Coast be ready for sun and heat.  Be careful with fire.   In contrast, an excursion in the Rockies might require some rain gear and a watchful eye for thunderstorms.  Enjoy.

Thursday, March 26, 2015

Pressure in the Mountains

After hiking a few thousand feet up a slope or taking a lift at some ski area, you open you water bottle and hear a momentary rush of air.   Or after staying at a high-altitude resort and returning home your shampoo container is partially crushed.  On higher peaks we have less energy and move a bit slower.  Even higher and altitude sickness strikes.  Boiling water comes easier and at lower temperatures at high elevations,  causing cooking times to lengthen.

Pressure decreases with height and the effects are sometimes quite noticeable.

Atmospheric pressure decreases with height for a good reason:  it is dependent on the weight of the air above you.  If you move higher, there is less air above you and thus pressure declines.

Interestingly, pressure decreases with height at different rates, depending on your elevation.
Specifically,, pressure falls more rapidly near the surface where the air is dense and less rapidly aloft where the air is thinner (see figure).

Let's explore how pressure declines with elevation.

At sea level, the average pressure is about 1013 hPa.   hPa is a unit of pressure that is also known as a millibar (mb).

By roughly 5000 ft, the pressure has declined about 15% to approximately 850 hPa.

Ascend to about 10,000 ft and the pressure drops to roughly 700 hPa, with 30% less air pressure and oxygen.  No wonder such elevations sap our strength and sometimes lead to headaches and dizziness.

Reach 18,000 ft above sea level and pressure drops to about 500 hPa, about half the pressure measured at sea level.  Few of us can function at this altitude, which is close to the elevation of the highest permanent human settlements.

Altimeters are popular hiking accessories and essentially small barometers.  They make use of the normal change of pressure with height and should be calibrated with a known elevation at the start of a hike. Since the actual change in pressure will typically be a slightly different, altimeters often possess small errors (generally no more than 25-50 ft) over a hike of a few thousand feet in the vertical.  Interestingly, pressure altimeters often provide more accurate elevations than GPS units, which frequently are off by 50-100 feet and are sometimes inoperable in heavy trees.

Many smartphones (such as the Samsung Galaxy series and the Iphone 6) now have pressure sensors and there are a number of altimeter apps available of little or no cost.

Saturday, January 3, 2015

Freezing Level Versus Snow Level

Anyone interested in winter recreation cares deeply whether they are in rain or snow, snow generally being preferred, of course.

So it is important to know the current and future elevations of the snow level, the height separating snow from rain.

The snow level can be pretty obvious, as seen in this image of Capitol Hill in Seattle.

And there is another closely related term that is used in weather forecasts:  the freezing level, the altitude at which the temperature drops to freezing.

So let's get educated about these important levels.  What exactly do they mean?  How are they related? And how do they change in time?

In most midlatitude locations, particularly in winter, precipitation starts aloft as snow.    As the snow falls from the colder upper atmosphere into the warmer air below, it often reaches a level at which temperature warms to freezing (32F), the freezing level.   Below that level the snow starts to melt, but it takes a while to do so--on average about 1000 ft (300 meters).    Wet snow, but still snow.   Since melting snow stays at freezing, the melting layer is often at a uniform temperature of 32F.

Eventually the snow melts completely and we reach the snow level, below which only rain is observed.  
The snow level in the mountains is often obvious, with white-clad trees 
above and bare trees below.

Both the freezing level and snow level can change in time as precipitation falls, and the direction is usually down.  The reason?  Cooling due to evaporation and melting.

First evaporation.   The air below the cloud is often unsaturated, which means the relative humidity is less than 100%   As the snow falls into that layer there is evaporation (actually sublimation), which results in cooling.   If the snow turns into rain there still can be evaporation and cooling.   Such cooling continues until the air is saturated, and can cause the freezing and snow levels to drop quickly and substantially (hundreds to even thousands of feet).

And then there is melting.   When snow falls into air warmer than freezing, it melts.  But it takes energy to melt the snow, and thus as melting occurs the surrounding air cools.   Heavier precipitation results in  more melting and more cooling.   Such cooling can occur even after evaporation has stopped (because the air becomes saturated).  Melting thus causes the freezing and snow levels to fall.

The good news about this:  if you are up in the mountains and it starts to rain on you, there is a good chance, particularly if you are near the snow level, for the rain to turn into snow!

Freezing and snow levels can also rise as warm air floods a region, particularly as a warm front approaches.  The highest freezing levels in the western U.S. are generally associated with atmospheric rivers: warm, moist currents of air originating in the tropics and subtropics.  In such events the freezing level can rise to 5000-8000 ft, even in the winter!

 The National Weather Service forecasts often talk about freezing and snow levels and how they will change in time:  information worth being aware of.