Happy Monday and welcome to another edition of Beyond The Forecast!
June started with storms, and as we approach the end of the month, there are still storms in the forecast. Last week’s storms brought plenty of lightning (which you can always send pictures of to us with the Pin It feature on our website). One bolt, in particular, caused major damage in Covington. Let’s look at how lightning happens, how intense it is and when you can expect it.
Lightning is another example of nature’s balancing act. A lot of weather is moving amounts of mass or energy from one location that has a high concentration to another with a low concentration. On a large scale, hurricanes act as a balance of energy, moving heat from the tropics further north. Storms bring heat from the surface higher into the air to balance the atmosphere, and even a mild breeze starts with more air molecules present in one spot than another. In lightning’s case, the imbalance is between electric charges (typically) between the ground and the sky.
The roots of lightning come from another balancing process: when there is a large difference in temperature between air at the ground and air higher in the atmosphere. Warmer air from the surface rises to even out temperatures with the colder air aloft and carries moisture with it. Once water molecules reach temperatures below the freezing point in the sky, they can join with more water molecules to form ice crystals. The ice crystals have a little bit of charge each, and as more and more form they form a difference in charge between the top and bottom of a cloud. Winds increase with more warm air rising from the surface that in turn blows around the ice crystals within the cloud. Impacts between the crystals break them into smaller pieces and increase the charge difference in the cloud.
When the difference in charge builds to a great enough point (which can vary depending on the size of the cloud, the distance between the positive and negative charges, and tons of other factors) some negative charges concentrated at the bottom of the cloud travel in a line to find the positive charges higher in the cloud. As soon as the line of charge, the stepped leader, makes contact with the positive charge at the top of the cloud the positive charge rushes down.
It happens quickly enough to give off a massive amount of heat and light, which we see as a lightning bolt within a cloud. Lightning travels between clouds or between clouds and the ground in much the same way. Positive charge slowly builds on the ground while negative charges build at the bottom of the cloud and when a stepped leader meets positive charges in the earth the energy transfer begins. Lightning tends to strike the tallest object in an area because that is the easiest spot for the stepped leader to find.
Thunder coincides with lightning strikes, but depending on distance and terrain, you might see lightning without hearing anything. When the huge amount of heat traveling along a lightning bolt is released, it expands the air surrounding the bolt to keep a balance of heat in the atmosphere. Lightning can be as hot as 50,000 degrees, which requires a massive change in the atmosphere to balance. When the charge transfer is done, that air comes rushing back together. This huge change in pressure is detected by our ears as thunder. The consistent rumble heard after some lighting strikes is the pressure wave bouncing off terrain and buildings that warp the wave.
This week is Lightning Awareness Week. Here are some key tips for staying safe during storms this summer. The best thing you can do to avoid lightning when a storm builds is to go inside when you hear thunder.
There are storm chances in the forecast for a few days this week, so be sure to stay up to date on what your zone will experience each day. You can download our weather app for information on storms as they develop and get Meteorologist Chris Michaels’ latest updates online.
You can always get specific forecast details for your zone, whether it’s the Roanoke Valley, Lynchburg area, the New River Valley or elsewhere around Southwest and Central Virginia, anytime at WSLS.com/weather. Know your zone!
In case you missed it, we have great weather and science content on WSLS.com. Here are some featured stories from the past week:
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-- Marshall Downing