weather / T-storms

[arghman]

Hmm, I've been watching the NOAA forecast for Whitefield & Berlin NH closely for this wkend, it seems to predict the same for Fri - Tues: 30-40% chance of rain + thunderstorms.

I don't mind rain but don't want to be hiking anywhere near treeline during a T-storm (walking around the woods in Derryfield Park in Manchester yesterday during the approaching storm was exciting enough)... do T-storms have a propensity for late afternoon / early evening? or do they occur equally at any time?

edit: and when is the right time to call off a hike due to T-storm risk?

 

[Puck]

From my understanding they can occur anytime. However, they are more probable in the afternoon. I always try to come down from the peaks between 12 and 1.

 

[giggy]

agree 100% with puck. - below treeline prior to 1pm - you should be good. I typically plan day hikes this way.

 

[Rick]

I seem to remember back when I lived in GA & AL in the Army, and then later in PA in high heat, that it was the high humidity and hot superheated air that built up during the day and caused the big thunder clouds to come rushing in.

Hot air rises and as it does, it causes some sort of updraft and more air is sucked in causing turbulance. As the air rises it turns cold quickly. Once the air turns cold, it loses it's abiltiy to hold moisture - hence the heavy rain.

With the clash of hot air and cold air, you would have negative and positive Ions clashing which would cause lightening.

I could be totally wrong, (and if I am, I don't mean to mislead) but this is how I understood it years ago - And it seems to make sense that heavy
thunderstorms always come through quickly in late afternoon leaving a colder drier air for a short period of time. The more intense the Thunderstorm, the faster it comes on.

Normal periods of rain take longer to develeop and usually stay for a day or more.
I'd love to know more though, if anyonee can add to or correct me.!!
Rick

 

[sp1936]

I got caught in a thunderstorm above treeline once (btw, in the morning). Interesting experience; not as frightening as the upstate NY tornado in the 60s, but once was definitely enough.

CMIIW, but I do not believe that there has ever been a death due to lightning in the White Mountains. Which means that either we shouldn't worry about it, or it's overdue...

Steve

 

[Artex]

One of the summit interns at the Mt. Washington Observatory posted an interesting story on a sudden storm last week. It's a real eye opener. Blow is a cut and paste of part of the story and the link (you have to click on "past comments" to view it).


Written by Amanda Waylett (Summit Intern):

05:21 PM Mon Jun 06, 2005 EDT

The phrase “Bolt from the Blue” has long been used to describe lightning strikes with no apparent source. They usually occur off of the anvil tops of strong thunderhead clouds, and can strike the Earth as far as 10 miles or more away from the rest of the storm. Though in the fog yesterday, and thus no 'blue,' a random flash instantly followed by the snap of thunder caught the observers, whom for hours closely monitoring the radar screen, completely by surprise. There was much confusion, as the closest storm was over 20 miles away. Where the heck did THAT come from…it was a bit frightening actually?!?! The next updated radar image suddenly showed a small, strong cell directly on top of us, forming in less than 5 minutes time!

Mt. Washington Observatory

 

[weatherman]

overdue?

I've always considered myself lucky to have come through thunderstorms on Washington (x2), Moosilauke, Old Speck (at 7:30 AM!!), and the entire Franconia Ridge unscathed, but those statistics are right... the absolute risk is pretty low. Good thing that lightning doesn't care if you are there or not. You're really only around for a little while. Having said that, I wouldn't seek it out, and below treeline by 1 PM as others have mentioned is a great idea.

I'd call off a hike depending more on the feelings of others, e.g., would someone significant to you hate you for years for bringing them along in a t-storm, or would everyone laugh it off and say "well, we all decided to come outside in the summer, didn't we?" You can always turn around, and you are seldom far from treeline except in the Presi's. They don't call it ridgerunning for nothing...

Good luck.
Weatherman

 

[Puck]

Hot air rises and as it does, it causes some sort of updraft and more air is sucked in causing turbulance. As the air rises it turns cold quickly. Once the air turns cold, it loses it's abiltiy to hold moisture - hence the heavy rain.

With the clash of hot air and cold air, you would have negative and positive Ions clashing which would cause lightening.
Rick

The turbulance can come from fronts clashing or from convection currents. The turbulance acts to create a charge polarity in the clound and also between the cloud and the ground. The top of the thunderhead will be positively charged the bottom negatively charged. The ground has a positve charge. A positive trailer rises from the ground through something...a tree, a golf cart or a hiker. That is why people sometimes report thier hair rising before a lightning strike. Remeber if you hear thunder you are at risk.

 

[DougPaul]

Bolt from the Blue

Lightning ground strikes have occurred with no visible clouds. Rare, but possible.

Doug

 

[DougPaul]

A positive trailer rises from the ground through something...a tree, a golf cart or a hiker. That is why people sometimes report thier hair rising before a lightning strike.

Not quite. The hair rising is caused by the electric field--no trailers required.

There is always an electric field in the air--normally it is not strong enough for you to sense. When it is strong enough for you to sense, there is risk of lightning.

Doug

 

[peakbagger]

The previous advise for planning on heading down off the summits before 1PM has served me well for years in the whites. Granted its difficult for some folks to make the transition to start hiking early in the day but its rare that there is any significant lightning activity in the whites before 2 PM. On more than a few occasions I have been caught in major downpours and thuderstorms on my way down off the mountain.


The bonus to early hiking is that I normally have the summits to myself and the hardest hiking is done when its the coolest.

 

[Puck]

Not quite. The hair rising is caused by the electric field--no trailers required.

There is always an electric field in the air--normally it is not strong enough for you to sense. When it is strong enough for you to sense, there is risk of lightning.

Doug

Perhaps the use the word "trailers" is misleading as is your use of the term "electrical field." Simply put, postive charge moves upward from the earth through objects. The air is an insulator and wend the electric differential exceeds the airs resistence lightining forms. An electric field comes from a flow. http://www.lightningsafety.noaa.gov/science.htm

Doug your advise about "sensing" is great because as others have pointed out a random strike can hit from 10 miles away even when the sky over head can be blue.

 

[DougPaul]

Perhaps the use the word "trailers" is misleading as is your use of the term "electrical field." Simply put, postive charge moves upward from the earth through objects. The air is an insulator and wend the electric differential exceeds the airs resistence lightining forms. An electric field comes from a flow.

Actually I believe the preferred term is "leader" not "trailer". (I used trailer because the previous poster used it and I wanted to avoid confusion.)

Uhh. Sorry, I used the term "electrical field" correctly--I'm an electrical engineer. And electrical field is caused by a charge separation (no flow
required)--a flow is a current (charge driven by voltage through a conductive medium) and any current causes a _magnetic_ field.
Electricity and magnitism 101--basic Maxwell's equations.

And air is an insulator (ie has a high resistance) until it breaks down, ionizes (electrons ripped from the molecules), and becomes a conductor (ie has a low resistance). It is NOT an issue of overcoming resistance.

A more accurate description of what happens is:
* Strong charge separations occur within the storm clouds. The mechanisms are not entirely understood.
* Positive sections of cloud attract electrons in the ground and cause a negative "mirror" charge in the ground. Negative sections of cloud repel electrons in the ground and cause a positive "mirror" charge in the ground.
* Lightning can occur from either positive or negative clouds.
* When the electric field gets high enough (electric field = voltage/distance), the air begins to break down. (IIRC, the breakdown of air occurs around 20KV/in.) This breakdown occurs in short steps or leaders.
* Once the sequence of leaders has completed a complete path, one or several power (high current) strokes follow.
* Most lightning strokes are cloud-to-cloud.

There are charged layers high in or above the atomosphere. These layers cause the imperceptable electric field in the absence of storm activity.

Doug

 

[Puck]

Doug
I was the one who used the word "trailer", which confused matters. Thanks for the clarifications. I don't mind being wrong, it means I am learning/relearning something. A slight bump to the male ego isn't bad for tuition. It has been awhile since college physics.

A few things; (not to beat a dead horse, some of this is pertinent to Mountain safety)
*overcoming air resistance. A simple statement used in lay literature, over course the mechanism is ionization. Is the ionization occuring in particles or are electrons getting stipped from O2, N2, CO2?
*The NOAA link I posted mentioned the role of ice particles in the cloud being central in creating charge within the cloud.
* The charge differential exists within a cloud and not between clouds. The positive part of the cloud is the cirrus "anvil" top. The negative is the lowest part of the cloud. The ground becomes positive as you mentioned. The bolts that travel for miles and can strike 10 miles a way are from the positive part of the cloud.
*I recall the "mirroring" of charge that you also mentioned. So if I am right does that mean that the negative charge of the bottom of the cloud would be equal and opposite the sum of the positive charges from the ground and the top of the cloud?

 

[CatskillsYeti]

The types of storms we've been having around here (Long Island and up the Hudson Valley into the Catskills) the past couple of days are the typical localized afternoon-evening storms caused by local convection currents rising into unstable, moisture-laden air. The kind of T-storms that lead the passage of a cold front in late spring and summer are generally more intense, greater in extent and are caused by the accelerated uplift of warm, moist air by the wedge of colder air preceding the passage of the front. They can happen any time of day or night, but their strength is determined by the temperature differential across the front.

So whether you have to worry about T-storms on a hike depends on what kind of storm is forecast. If it's the localized type (typical forecasts will say "40% chance of late afternoon or evening thunderstorms"), then getting below treeline by 1 PM should work fine, but always keep your eyes and ears peeled for sudden changes before that magic hour. If the forecast is for the passage of a cold front with embedded storms, try to get the best estimate of when the front is scheduled to pass the area you're in and plan to be below tree line at least two hours before.

My personal rule is if the forecast is below a 50% chance of "localized" storms, I go, taking sufficient rain gear and getting down before the stuff arrives.

 

[dropack]

Western T-Storms

My wife and I are going to be hiking on the north rim of the grand canyon in the second week of July. We both used to hike in Az. years ago before we meet. We have always wanted to hike some of our old favorites out there and act as guide for each other. If any of you have hiked in northen Az. in July and Aug. you will know about monsoon season. At about three pm every day the bomers start and you can have some real down pours.
One time out side of Boulder Co. two years ago we were with a guide and were above tree line and in a electrical storm, we had St. Elmos fire and bolts droping all around it was hairy and the hair really does stand up on your head. Tom the leader had us all dropack and get prone, there was nothing else to do, We laughed about it later that night around the camp fire.

 

[SAR-EMT40]

Remeber if you hear thunder you are at risk.

Absolutely. Bolts can travel 20 miles. Like Doug said. It is unusual, but it definitely does happen and it definitely does kill people.

Keith

 

[Neil]

Surviving an electrical storm.

This is off the top of my head a couple of years after reading up on it so please correct any mistakes.
The section in "Freedom of the Hills" that deals with lightening recommends (if you are above treeline) sitting on a small rise (ie. like 2-3 feet) or on a rock in order to avoid the most lethal threat from lightening: ground currents. Sitting on one's pack provides some insulation. Wet depressions are electrical current sinks supposedly and should be avoided. Also, they say to avoid the ends of ridges which attract strikes. Below treeline, sitting under the tallest trees is not good as the taller trees are the best targets.
Contrary to what I use to believe, having metal on one's person does not increase one's chances of getting struck by lightening.

 

[arghman]

thanks all for the advice, I'm just going to play it safe and not go to a peak w/ significant above-treeline.

Contrary to what I use to believe, having metal on one's person does not increase one's chances of getting struck by lightening.

electric fields are greater around metal conductors with "sharp" features (radius of curvature small), you're helping the air ionize more easily at those points. A long piece of metal with sharp points at the end is a bad thing to carry around: I wouldn't go around above treeline in a thunderstorm holding my metal hiking stick above my head. But I wouldn't worry much about carrying metal coins, they're not very large.

edit: i.e. length is bad because it shortens the path the lightning has to travel through nonconducting air. sharpness is bad because it concentrates the field locally & causes ionization (which then shortens the path when the air can conduct through ions). (this field concentration is called "corona discharge" I think, it's used in copiers & laser printers) Van de Graaf generators you see in science museums have large spherical electrodes so they don't cause corona discharge

 

[DougPaul]

The section in "Freedom of the Hills" that deals with lightening recommends (if you are above treeline) sitting on a small rise (ie. like 2-3 feet) <snip>

Sounds like an iffy trade-off to me--chance of a direct hit vs chance of ground currents. One reason for not sitting in a depression is that it may soon become a pool.

Contrary to what I use to believe, having metal on one's person does not increase one's chances of getting struck by lightening.

Small pieces of metal which do not extend any distance beyond the body may not increase the chance of strike. However, any piece of metal has the potential for concentrating the current near it, so a less harmful sheet flow on the skin can be concentrated near the corners of the metal causing localized greater injury.

Sticking a metal tent pole, trekking pole, or ice axe out in any direction is ill advised...

Doug