14.4: Lightning
- Page ID
- 46277
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)Lightning is caused by a discharge of electricity that typically occurs in mature thunderstorms. Lightning can strike within the same cloud, from one cloud to another, from a cloud to open air, or from cloud to ground. Only about 20% of lightning strikes are cloud-to-ground (CG) strikes, while the majority of strikes occur within the cloud (CC). As a lightning stroke travels through the atmosphere, it heats the surrounding air to a staggering 30,000°C, which is roughly five times hotter than the surface of the sun. This incredible heating causes a sudden, explosive expansion of the air, generating a shock wave in all directions from the flash. This booms outward in the form of a sound wave called thunder.
Due to the speed that light travels, we see lightning instantly as it happens. Thunder travels much more slowly at about 330 m·s-1 so it takes much longer to reach our ears. You can estimate how distant a lightning stroke is by counting the seconds after you see the flash until you hear the thunder. It takes sound about 5 seconds to travel one mile. If you hear thunder roughly 10 seconds after you see the lightning flash, then the strike occurred about 2 miles away. However, if the lightning is too far, you may not hear any thunder, because sound waves attenuate and bend as they travel through the atmosphere, especially through layers of differing air temperature.
Why do we have lightning? During ordinary fair weather conditions, the earth’s surface has negative charge, while the upper atmosphere is positively charged. Lightning requires differing regions of opposite charge inside a cumulonimbus cloud. There are several theories as to how this separation of charge may come about.
One theory is that clouds can become electrified when graupel and hailstones fall through areas of supercooled droplets, causing the droplets to freeze and release latent heat. This release of latent heat keeps the hailstone surface warmer than the surrounding ice particles. A net transfer of positive ions (molecules that carry a charge) occurs from the warmer hailstone to the colder surrounding ice crystals when they come in contact. Therefore, the larger, warmer hailstone becomes negatively charged and the smaller, cooler ice crystal becomes positively charged. This also happens when supercooled liquid droplets freeze when they come into contact with a warmer hailstone and little fragments of positively charged ice break off. These tiny positively-charged particles are easily swept to the upper regions of the cloud through updrafts. The larger, heavier, negatively-charged hailstones tend to remain suspended at the same level in the updraft or fall to the lower parts of the cloud. This causes the colder upper regions of the cloud to have a net positive charge, while the middle level of the cloud has negative charge. The lowest portion of the cloud generally has a mixture of negative and positive charges, with a positively charged region occasionally present in precipitation bands near the melting level.
In short, thunderstorms must have ice phase processes to produce thunder and lightning—lightning is produced from a discharge of electricity and thunder is simply produced as a result of the lightning.