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Lightning basics - 2. Conditions for lightning

 
Lightning becomes likely to occur when powerful updrafts carry moisture up into the air, and when the upper air temperature has fallen to below minus 10ºC.
 

One of the most common features of clouds that produce lightning is the presence of power updrafts extending to an altitude of several kilometers. The upper ranges of these updrafts reach altitudes where the temperature has fallen to below minus 20ºC. Temperatures of minus 20ºC occur in the Japanese summer at between 7 and 8 kilometers above the ground. In the northern part of Japan, in winter, these temperatures occur at between 3 and 5 kilometers above the ground.

In the Japanese summer, heat lightning occurs in clouds whose tops are at between 8 and 16km high, and in winter in the north in clouds whose tops are between 4 and 6km high. In summer, in order for updrafts to occur, there needs to be hot and humid air near the ground, with a comparatively cold air mass above it. Summer days with strong sunshine are likely to cause heat lightning, and particularly when a cold front moves across the Japanese archipelago, heavy thunderstorms can occur. In winter along the Japan Sea coast, a great temperature difference is generated between the cold air mass coming from Siberia and the air temperature close to the surface of the sea, causing frequent lightning in the snow clouds during November and December.
 
 
 

Meteorological conditions for “lightning”

 
Meteorological conditions  

If the following three conditions are satisfied, lightning can occur anywhere and in any season.


(1) Fast air movement (updrafts, strong winds, etc.)
(2) Large amounts of water vapor in the air
(3) Temperatures of between minus 10ºC and minus 20ºC in the upper atmosphere
 
 
 

How thunderclouds form, grow and then disperse

 

hunderclouds are large cloud masses with a diameter of several kilometers. These are called cells. In fact, a thundercloud is very rarely only one cell, and usually comprises a cluster of several cells, and has a very complex overall structure.

Figure 1 shows a model that demonstrates how a single cell develops and eventually disperses.
 
 
(a)Cumulus stage (b)Mature stage (c)Dissipating stage
 
 

Cells in the cumulus stage are observed as vertically developing clouds that can attain a diameter of between 5 and 10 kilometers within 10 to 15 minutes, with a height of between 7 and 9 kilometers.

All of the air currents inside the cloud are updrafts. Cloud droplets grow within the cloud and large droplets of water and ice are formed and the cell continues to enlarge. This is the beginning of the mature stage, and clouds in this form are called cumulonimbus. Part of the cloud is dragged downwards by precipitation, creating downdrafts. During the mature stage, convection occurs due to the action of the updrafts and downdrafts.

The updrafts are as strong as the upper layer, reaching speeds of up to 30m/s. The top of the cloud can often be as high as around 12 kilometers, and some have been as high as 16 kilometers. Lightning discharge is most common during this period. The mature period can last between 15 to 30 minutes, after which the updrafts begin to fade away, leaving only the downdrafts. The cloud then enters the dissipating stage. The rain becomes weaker than in the mature stage, and stops after about 20 minutes.
 
 
 
 
 

Lightning basics

 

1. How does lightning happen?

●2. Conditions for lightning

3. Types of thunder

4. Structure of thunderclouds

 

5. Conditions for lightning

6. How to protect yourself from lightning

7. The scale of lightning and lightning damage (power lines)

 
 

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