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M O UN T AI N
WEAT HER
Part I: Weather Patterns
TO BE A SAFE, EFFICIENT, AND
DECISIVE CLIMBER it is essential to understand
and identify weather patterns. The climber who
understands the weather will choose appropriate objectives
before leaving home, will select the best campsite for
protection or a good view, will schedule an alpine start at
just the right time, and will most likely have a safe and
successful day in the mountains.
To forecast mountain weather on a daily basis, one must
first learn how weather is generated on a global scale and
how large-scale weather patterns influence regional
weather events.
The Atmosphere
The atmosphere of earth is an envelope of gases that
surround the planet. To understand the relative size of
the atmosphere, imagine earth is the size of an apple.
The atmosphere would have a thickness equivalent to the
skin of the apple. This is a pretty thin layer, but it is very
important for life as we know it on our planet. The gases
that make up the atmosphere are Nitrogen (78%), Oxygen
(21%) and other trace gases like Argon and Carbon
Dioxide (1%).
Making Observations
Observing the current
weather pattern and
keeping notes in a field
notebook will enable a
climber to track changes in
temperature, cloud cover,
wind speed, rates of
precipitation and many
other parameters. Based
on these finding one can
begin to forecast future
weather changes.
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The atmosphere is divided into four layers: the
Troposphere, the Stratosphere, the Mesosphere, and the
Thermosphere.
The Troposphere - where weather happens
Of the four layers, only the first - the troposphere- has any
significant influence on earth’s weather. This 10 mile high layer
of atmosphere contains moisture and high density air that
moves around as it is heated and cooled. The remaining three
layers of the atmosphere protect the planet from harmful
radiation, but have little influence on earth’s weather.
Energy from the sun
The sun emits energy in the form of electromagnetic
waves. This energy travels through space and earth’s
atmosphere, and eventually to earth’s surface, where it is
absorbed as heat. In areas where there is very direct sunlight at
the earth surface, such as the equator, there will be strong
heating of land, water, and the atmosphere. Conversely, areas
that receive little direct sunlight, such as the poles, will
experience little heating. See figure 1.
Air that is heated becomes less dense than surrounding air,
causing it to rise. When air rises due to heating, we call this
form of lifting convection. Areas around the globe where
warm air is rising are called areas of low pressure. Once a
parcel of warm air rises to a certain height in the atmosphere it
will cool, become more dense, and will sink back down. Areas
around the globe where dense, cool air is descending are called
areas of high pressure. See figure 2.
Since the most intense solar radiation reaches the globe at
the equator, this region of the earth has predominately rising
warm air and therefore, low pressure. The poles receive the
Figure 1. Differential heating of the earth’s
surface due to direct solar radiation at the
equator vs. indirect solar radiation at the poles.
Figure 2. Global high and low pressure patterns
least intense solar radiation and limited warming, therefore there is
predominately cool, sinking air and high pressure. Areas of low and high
pressure exist in cells around the earth. These areas account for
tremendous movement of air and are the driving force behind earth’s
weather.
Global Winds
Wind is the horizontal movement of air from an area of high
pressure to an area of low pressure. Wind is described by the direction it
is coming from. For example, a wind that is blowing from the north and
toward the south would be called a northerly wind. Global winds are
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winds that blow steadily from specific directions over long
distances. They are created by unequal heating of the earth’s
surface, and are influenced by the spinning of earth on its axis.
Major global winds include the trade winds, the prevailing
westerlies, and the polar easterlies. These winds blow
consistently and define large-scale weather patterns. For
example, weather in the US always flows from west to east due
to the prevailing westerlies. See figure 3.
The Coriolis Effect
The movement of air around the globe is greatly affected
by the rotation of the earth. As earth spins on its axis, at land
speeds of nearly 1000 miles per hour, air in areas of high
pressure that is sinking to earth and then spreading across
earth’s surface, gets deflected from its normal trajectory. The
air veers to the right of its normal trajectory in the northern
hemisphere and to the left in the southern hemisphere. This
phenomenon is called the Coriolis Effect, and it results in
global wind patterns such as the trade winds, the prevailing
westerlies, and the polar easterlies.
Regional Weather
As we have seen so far, the uneven heating of earth by the
sun causes areas of rising air and areas of falling air. The
spinning of earth causes the movement of air to veer into
easterly and westerly patterns. The result is global wind
patterns that move storms across the earth on predictable paths.
Storms are regional events that are generated by the
interaction of different air masses that have different
characteristics. Regional air masses have unique properties as a
result of their formation over warm water, cold water, warm
land, or cold land. An air mass that is warm and wet will not
Figure 3. Illustration of global wind patterns.
Note that moving air in the northern
hemisphere is deflected to the right by the
spinning of the earth, a phenomenon called the
Coriolis
Effect.
Figure 4.
Regional air
masses and
common
frontal
boundaries.
easily mix with an air mass that is cold and dry, much like oil does not mix with
water. For example, when a mass of warm and wet air from the Pacific Ocean
is blown west and interacts with a mass of cold and dry air from the Arctic, the
two will have difficulty mixing due to their differences in temperature and
humidity. The boundary where two air masses meet is called a front, and it is
the spawning ground for storms. Global wind patterns create common frontal
boundaries, such as the Polar Front or the Arctic Front, that indicate where
storms will be born over wide regions. See figure 4.
The jet stream also plays a role in defining regional weather. The jet
stream is a band of high-speed wind about 10km above earth’s surface in the
top of the troposphere. It can be 100’s of kilometers wide but is only a few
kilometers deep. The jet stream blows from west to east at speeds of 200-400
km/hr and will wander north and south in a wavy path as it travels around the
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The two most common
types of weather fronts
are warm fronts and
cold fronts.
Warm Fronts
Warm fronts are typically slow
moving and will linger over an
area bringing dull and drizzly
weather. As a warm front
passes, skies will clear slowly
and warm temps will remain.
Cold Fronts
Cold fronts typically arrive
quickly and bring dramatic
changes to the weather such as
falling temperatures, gusty
winds, and periods of heavy
precipitation. A cold front will
usually pass within 24 hours,
yielding clearing skies and
cooler temperatures. Cold
northerly winds signal the
passing of a cold front.
globe. The jet stream will accelerate the
collision of air masses, intensifying the
resultant storms. It will also accelerate the
collision of an air mass with a mountain
range, causing powerful lifting and
potentially intense precipitation.
Whenever weather maps indicate the jet
stream is nearing your area
be prepared for powerful
changes in the weather.
pressure, a change in the wind direction
(usually to the southwest in the US), and
gradual increase in cloud cover that begins
with high clouds and progresses to thick,
dark low-level clouds.
Cold Fronts
Cold fronts occur when a fast moving
mass of polar cold air
overtakes a slower moving
mass of tropical warm air.
The more dense cold air
slides under the lighter
Types of weather
warm air, pushing the warm
Fronts
air upward along the leading
How a quickly a
edge of the front. The
storm develops, how long
warm air is lifted up
it lingers, and what type
turbulently, causing rapid
and rate of precipitation
cloud formation and periods
occur is dependent on
of heavy precipitation.
the type of weather front
Gusty winds often
that is passing through. All fronts cause air accompany the instability of a cold front.
to be lifted, and we know that lifted air gets Cold fronts tend to pass as rapidly as they
cooled and loses moisture as rain or snow.
arrive, often lasting 18-24 hours from
However, the rate and duration of lifting
arrival to departure. Clues to the
varies with different types of fronts. It
approach of a cold front are rapid falling
pays to understand the characteristics of
air pressure, a change in wind direction
different fronts so you can make an
(usually to the northwest in the US), a
educated guess about how long it will last
significant drop in air temperature, and a
and how intense the storm will be.
rapid build-up of high, towering clouds.
Following is a description of the four main
Stationary Fronts
types of fronts that occur on a regional
Stationary fronts occur when cold and
scale.
warm air masses meet but neither is able
Warm Fronts
to move the other. Water vapor in the
Warm fronts occur when a fastwarm air may condense into precipitation
moving mass of warm tropical air
as it swirls along the frontal boundary. A
overtakes a slower moving mass of polar
stationary front can remain stalled over an
cold air. The lighter, more buoyant warm area, causing cloudy conditions and light
air slides over the mass of cold air. This
to moderate precipitation for days.
creates a long frontal boundary and results
Occluded Fronts
in prolonged light to moderate
Occluded fronts occur when a mass of
precipitation. Warm fronts move slowly
warm
air is caught between two masses of
and can last for days. Clues to the
cooler air. The cold air masses force the
approach of a warm front are falling air
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warm air up, causing it to be cut off from
the ground, or “occluded”. If the warm air
is moist, it may reach the dew point and
cause rain or snow. Precipitation that
occurs will usually be light to moderate,
much like a warm front. The approach and
departure of an Occluded front is very
similar to a warm front.
Other types of Lifting
In addition to weather fronts, there are
other types of lifting that cause air to rise.
As we saw in an earlier discussion,
convection is a form of lifting that occurs
when daytime heating from the sun causes
air to warm and rise. If the air contains
water vapor it will precipitate as the air rises
and cools, and the dew point is reached. As
mentioned earlier, convection drives the
perpetual rain showers around the equator;
it also drives the afternoon thundershowers
in the North American Rockies.
Convection is evident anytime fluffy, white
cumulus clouds begin to grow into towering
thunderheads, called cumulonimbus clouds.
Cumulonimbus clouds can grow miles into
the atmosphere. Strong convection can
produce dangerous lightning during a
thunderstorm.
A form of lifting unique to the
mountains, orographic lifting, occurs
when an air mass crashes into a mountain
range and is forced upward by the
topography. It is often said the mountains
are “wringing the moisture out” of these air
masses as they pass over a range. Once an
air mass crosses the range and descends
down the other side, it warms and absorbs
moisture, creating a dry area on the lee side
of the range called a rain shadow.
The last form of lifting occurs when the
boundary at a front often becomes distorted
by mountains, surface features, or strong
winds such as the jet stream. When this
happens bends develop along the front
causing air to swirl and resulting in a low
pressure center. The swirling center of low
pressure is called a cyclone, and the lifting it
causes is called cyclonic lifting. Cyclonic
lifting can be very powerful and can result
in heavy precipitation and high winds.
Figure 5. Orographic Lifting
by a mountain range. As air
descends down the lee side of
the range it warms and
absorbs moisture, creating a
rain shadow.
Lightning
During a thunderstorm, areas of
positive and negative electrical
charges build up in storm
clouds. Lightning is a sudden
spark, or electrical discharge, as
charges jump between parts of a
cloud, between nearby clouds,
or between a cloud and the
ground. A lightning bolt can
heat the air near it to as much as
30,000 degrees C. The rapidly
heated air expands suddenly
and explosively. Thunder is the
sound of the explosion. Because
light travels much faster than
sound, you see lightning before
you hear thunder.
To calculate your distance from
a storm, count the number of
seconds between the flash of the
lightning and the boom of the
thunder. Divide the number of
seconds counted by five to get
the approximate distance in
miles.
15 s = 3 miles
5 s/mi
Wait for another lightning flash
and calculate the distance
again. Is the storm moving
toward you or away from you?
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Clouds
Clouds form when water vapor in the air condenses to form liquid water or
ice crystals. When you look at a cloud you are actually seeing millions of tiny
water droplets or ice crystals. Scientists classify clouds into three main types
based on their shape: cirrus, cumulus, and stratus. Clouds can be further
classified based on their elevation. Each type of cloud can be associated with a
different type of weather.
Cirrus Clouds
Cirrus clouds appear thin, wispy or feathery. Cirrus is latin for “curl of
hair”. They are composed of ice crystals because they form at high elevations
where temperatures are low. Cirrus clouds are often the lead clouds in
approaching fronts, and may signal the arrival of a front within 24 hours.
Cirrus clouds
Cumulus Clouds
Cumulus clouds appear fluffy and round, like puffs of cotton candy.
Cumulus is latin for “mass” or “heap”. They are mid-level clouds (around 2km)
and usually indicate fair weather. If convection occurs, cumulus clouds can
grow into towering, gray thunderheads called cumulonimbus clouds. “nimbus”
is latin for “rain” and that’s usually what cumulonimbus clouds will bring. They
most commonly form at the leading edge of a cold front or during summer
convection thunderstorms.
Cumulus clouds
Stratus Clouds
Stratus clouds appear dull and gray and form in uniform flat layers across
the sky. Stratus is latin for “spread out”. Thick stratus clouds are called
nimbostatus and commonly produce drizzle, rain, or snow. Stratus and
nimbostratus clouds are often associated with the development of a warm front.
Other Cloud Types
Lenticular clouds appear as a cap over a mountain peak, or as waves
than run over summits and ridges. These clouds usually signal high winds aloft.
The photo below is of a lenticular cloud that formed over Mt. Rainier.
Stratus clouds
Observing Cloud Formation Over Time
Much can be gained by observing the progression of cloud formation over time. For
example, it is important to note when high cirrus clouds are present. It is more important to
recognize the high cirrus are followed by a lenticular cloud over a nearby peak 6 hours later,
and then stratus clouds that gradually build into the valley the following day. This
progression illustrates the approach of a warm front, and it should be noted that any one
cloud type on its own did not confirm the weather change. The combination of cloud types
in sequence signaled the approach of the oncoming front. As another example, a summer
thunderstorm begins with benign cumulus clouds that gradually grow into cumulonimbus,
which become dark gray on the bottom, and finally gusty winds indicate the strong
convection that is developing and the resultant low pressure zone that is created.
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MO UNT A IN W E A THE R PAR T I QUIZ
Quiz Questions
1. Name the lowest layer of the atmosphere, the layer where weather occurs.
2. Name the two gases that make up 99% of the earth’s atmosphere.
3. Describe how the sun heats the earth. What effect does this have on earth’s weather?
4. What direction is the air in a low pressure center moving? What type of weather is commonly associated with low pressure?
5. What direction is the air in a high pressure center moving? What type of weather is commonly associated with high pressure?
6. How does the Coriolis Effect contribute to prevailing weather patterns in the continental United States?
7. What type of weather results from a cold front? How long will it last?
8. How does the jet stream influence regional weather?
9. If the time counted between flash of lightning and rumble of thunder is 20 seconds, how far away is the storm?
10. What type of lifting explains why mountains receive more precipitation than the plains?
Application of knowledge
Scenario 1: After driving most of the night from out of state, you and your climbing partner arrive in Rocky Mountain National
Park with plans to climb the Petit Grepon, a classic 5.8 alpine rock route. Four hours of good sleep in the parking lot leave you feeling
somewhat refreshed, and you depart the car at 5:00 am under clear, starry skies. On the approach, the day dawns clear and brilliant.
There is not a hint of wind. You reach the base, rack your gear, and are leading the first pitch by 9:00 am. While belaying your
partner an hour later, you notice puffy white clouds forming in the distance. The two of you discuss this development, wondering if
the weather is changing. During your discussion you notice the a steady breeze beginning to blow. “It doesn’t look too serious. One
more pitch, then we’ll re-evaluate”, you agree. While your partner leads the next pitch, now 10:30, you are sure the clouds are growing
taller, into towering columns, and you note they are becoming gray on the bottom. The temperature has cooled, but you’re not sure if
the air is actually cooler or it is just chilly because the sun is mostly obscured now. “Belay on!”, your partner yells down to you.
Quickly, you disassemble the anchor and climb to reach the next belay. You can tell the weather is changing, and fast. By the time you
reach the belay, the sky is completely overcast, the wind is gusting, and you hear a distant rumbling. The look on your partner’s face
confirms your plan: descent. The next 3 hours involve rappeling in the rain, running down the talus field amid flashes of lightning, and
reaching the car wet and worn out from the narrow escape. Later that evening the weather clears as quickly as it deteriorated, only 6
hours ago.
1.
2.
3.
4.
What type of storm occurred and what type of lifting was responsible?
What did the sudden increase in wind signal?
What type of clouds were developing and what did they indicate?
How could the storm have been avoided?
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