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Meteorologist Jim Woodmencey |
(Note: The content in this post first appeared in the MountainWeather column in the Jackson Hole News & Guide on July 6th, 2016).
July begins the heart of the “Thunderstorm Season” in the Rockies. It is also the peak of the tourist season, climbing, backpacking, & baseball seasons, etc. etc. . That means more
people will be outdoors enjoying the weather, and potentially getting caught in
thunderstorms.
In this post I will describe how thunderstorms work,
and what to watch for when you are outside during the day. Those telltale
signs that dangerous thunderstorms may be developing. I will
follow this post with a subsequent one on lightning safety.
Moist and
Unstable Air
Most of us would
probably describe a “typical summer day” as: Clear blue skies in the morning, followed
by some puffy cumulus clouds in the afternoon, and maybe a chance of some
afternoon thunderstorms.
How big or
ominous those cumulus clouds become will depend on two things:
1) How much moisture is in the air. 2) How unstable the air is.
1) How much moisture is in the air. 2) How unstable the air is.
That clear
blue sky that you see in the morning actually contains some moisture, in the
form of invisible water vapor. As the day goes on, and the ground heats up, the
air begins to rise. As the warmer air rises higher into the atmosphere it
encounters colder temperatures, and the invisible water vapor condenses into visible
water droplets.
If the air
keeps rising, cooling, and condensing, then water droplets start to gather
together, and clouds appear.
Relative
humidity (RH) is one way to measure how moist the atmosphere is. For
thunderstorm development, it can still be very dry in the lower levels of the
atmosphere, with low RH. But at the same time, high RH may exist in the upper
levels of the atmosphere, which is enough to perk up thunderstorms once there
is enough instability.
How
unstable the atmosphere is will depend on the difference in temperature between
the ground and the upper levels, like up around 20 to 25,000-feet. The bigger
the difference in temperature, the more unstable the air is, that is, it must
be warmer down low and colder up high to be unstable. Think, “hot air rises”,
that’s instability.
If the air
isn’t rising, or if temperatures aren’t very cold aloft, or the atmosphere is
staying bone dry up through 25,000-feet, then all we see is clear blue skies,
all day.
Be Observant
There are a
few things to keep an eye on to determine if those little white puffy clouds
are going to develop into full-blown thunderstorms on any given day. Watch the
sky for these signs of impending thunderstorms:
1) The first puffs of cumulus appear before noontime.
2) The individual
cumulus clouds start gathering together and growing vertically.
3) The base
of the cloud gets darker, indicating the clouds are growing taller.
4) Taller
clouds, darker bases and more of the sky covered by those clouds, the greater
the potential for heavy rainfall, hail, and/or strong gusty downdraft winds.
And of course, lightning.
However, don’t
be lulled into thinking that clouds and thunderstorms can only develop during
the afternoon hours. There are many days that don’t fit this “typical” summertime
thunderstorm scenario.
Storm
systems, otherwise known as low-pressure systems, will periodically roll across
the Rockies in the summer, and these will add additional moisture &
instability to the atmosphere.
Often associated
with a low-pressure system, is a “cold front”. Cold fronts produce more lift
and give a boost to the instability. Some of the more potent and violent
thunderstorms we see are directly related to the passage of a cold front.
The last
thing to be aware of is, the “summer monsoon”. In brief, the Desert Southwest
Monsoon is a seasonal wind flow pattern that brings very moist and unstable air
up from central Mexico, across the Four-Corners Region, and sometimes that
monsoon moisture gets circulated as far north as Northwest Wyoming.
Thunderstorms
associated with the monsoon are some of the most random that we will see in the
summer, and can occur any time of the day or night.
Whenever
you experience a thunderstorm in the middle of the night or just after sunrise
– when the afternoon heating is not really a factor– it is very likely that a
storm system, a cold front, or the monsoon is triggering the thunderstorm activity.
Keeping an
eye on thunderstorm development is of paramount importance to anyone who is
spending the day outside. If the clouds develop into lightning-producing
thunderstorms, then you are at risk of being injured or killed by a lightning
strike; whether you are climbing the Grand, playing a around of golf, fishing the
river, or out on the lake or ball field.
Jim is the
chief meteorologist at mountainweather.com and has been forecasting the weather
in Jackson Hole and the Teton Mountains for almost 25 years.
The moisture is the air had the main hand on happening this thunderstrom as it had droplet of water in it which make it happened at the end of the day with the gathering of cumulus clouds over there. If you want to keep things safe then use gears from www.outdoorgearonly.com and stay safe and focus on the nature.
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