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March 31, 2020

A guide to understand thunderstorms

Thunderstorms and lightning over sea
Summary
Everything you need to know to understand this powerful phenomenon.

Thunderstorms can have a large impact on offshore operations. It is therefore vital to understand the different factors that affect the development and movement of thunderstorms and have knowledge about how different synoptic weather situations affect the risk of lightning. In this article we run through these topics:

  • The basics of the formation of thunderstorms
  • Isolated vs. embedded showers
  • The movement of thunderstorms
  • The clouds associated with a building thunderstorm
  • Night instability
  • Weather radar and satellite images as tools
  • Signs of an approaching thunderstorm - thunderstorm characteristics
  • Difference between showers and normal rain
  • The risk of thunderstorms and lightning in connection with different types of precipitation
  • The risk of thunderstorms and lightning in different types of weather
     
The basics of the formation of thunderstorms

There are three main ingredients in the formation of thunderstorms: Moisture, instability and lifting mechanisms.
The vertical temperature distribution in the atmosphere can be divided into two main types: Stable and unstable air.

  • Stable air: The vertical temperature is increasing with height and there will be limited vertical movement as the colder air is heavier than the warmer air above.
  • Unstable air: The opposite is true and there is a large vertical movement as the warm and lighter air will rise (the process that is called convection). The rising air will gradually cool down until reaching saturation and the water content in the air will condensate and form clouds.
    Showers and thunderstorms don’t necessary develop spontaneously in an unstable airmass, as this development also depends on the amount of moisture in the air, but it is good indicator of the possibility for formation of showers and thunderstorms. However, thunderstorms will never form in stable conditions.

Although an airmass is unstable, it often needs a lifting mechanism in order for showers and thunderstorms to form. In summertime, the rising surface temperature is the most common lifting mechanism. However, front zones, terrain and the advection of warmer air could for example also trigger a lifting motion of the air.
Normally during the summer, there is a temperature difference between land and sea, and the thunderstorms will weaken when moving out over the colder sea. However, depending on the speed of the showers and how large the temperature difference is between land and sea, the thunderstorms can still have an impact on offshore sites.
Read our article for more about the different lightning seasons at sea.

 

Isolated vs. embedded showers

When shower and thunderstorm clouds are surrounded by clear skies or just a few other clouds, they are called isolated showers/thunderstorms. If they are closely surrounded by a large amount of other clouds, they are called embedded showers/thunderstorms.

  • Isolated showers or thunderstorms typically appear in relative cold air after a cold front has passed or forming during the day over land in the summertime and are easy to spot.
  • Embedded showers are often associated with front zones (and therefore impossible to spot) but can sometimes be isolated showers that are embedded in a thinner layer of clouds (possible to spot, depending on the height and amount of the other clouds). In the case of embedded showers in fronts, the precipitation often is referred to as showery rain.
     
Isolated shower over sea
Isolated shower over sea. Falling rain (also called a precipitation shaft) can be seen below a part of the dark cloud
 
The movement of thunderstorms

The wind speed in a height of around 10.000ft (around 3.0km), is often a good indicator of both the direction and the speed of isolated showers and thunderstorms.
The best way to measure the speed of fronts with embedded thunderstorms, is to look at weather radar images or satellite images and compare the location of the front at two different times and then calculate the speed based on the distance and time.

The clouds associated with a building thunderstorm
  • Cumulus clouds: Convective clouds that typically will develop during a sunny summer day. They are typically as wide as they a high. They will rarely produce any precipitation.
     
  • Towering cumulus (cumulus congestus clouds): If the atmosphere is unstable enough, the cumulus clouds will develop further into towering cumulus with a height that will exceed the width. They can produce some light to moderate showers.
     
  • Cumulonimbus clouds: The towering cumulus will often develop further into cumulonimbus clouds and get the typical thunderstorm cloud appearance with an anvil formed cloud.

Read this article if you want to know how to differentiate between dark thunderstorms and dark stratocumulus clouds. By learning some simple rules of thumb, you can sometimes continue working offshore although the clouds are looking threatening.

Night instability

As there will be a net flux of radiation outwards during the nighttime, the temperature in clouds will fall in the top of the clouds. This will cause more unstable conditions, especially during the hours around sunset. It is therefore not uncommon to see lightning during the late night and early morning, and then no lightning detected a few hours after sunset.

Weather radar and satellite images as tools

Weather radar data and satellite images are vital in order to monitor thunderstorm formation, development and movement. If an offshore location is far from the coast, there could be problems with both the radar coverage and that heavy showers could be displayed with the same intensity as light or moderate showers closer to the radar location.
It is therefore important to use satellite images as an additional tool. However, it is easiest to spot potential thunderstorms on a visual satellite image which could cause problems during the winter months when the sun is rising late and setting early. In that situation, satellite images that displays the cloud top temperature can be used. A cold cloud top temperature could indicate a large vertical extent of the cloud and hence risk of thunderstorms. Read our
article for more about how to use satellite images.

Satellite image and weather radar image
Thunderstorms over Central Europe – displayed by a satellite image and a weather radar image
 
Difference between showers and normal rain

Showers are typically defined as precipitation with a rather short duration and rapid changes in intensity. When the showers are embedded in fronts, there will be more consistent rain and with less variation in intensity compared to isolated showers. Normal front precipitation (also called stratiform precipitation) is rather continuous and with small variations in intensity.

Signs of an approaching thunderstorm - thunderstorm characteristics
  • Wind shift: There will often be sudden wind shifts in connection with thunderstorms, both in direction and wind speed. Sometimes a heavy thunderstorm can create a gust front ahead of the actual thunderstorm with strong gusty winds.
     
  • Temperature: The temperature will often drop rapidly in connection with a passing thunderstorm. This is due to the great vertical movement within the thunderstorm clouds that will bring down cold air from aloft.
     
  • Clouds: The anvil cloud is typical for thunderstorm formation but could also be heavy showers with no lightning. However, the risk of lightning will be high.
     
  • Due to the turbulent nature of thunderstorms, the clouds will be more nonuniform compared to clouds in connection with stratiform precipitation. Below are two examples.

 

An anvil cloud (Cumulonimbus Incus)
An anvil cloud (Cumulonimbus Incus) in connection with the last stage of the formation of a thunderstorm. To the right and in front of the anvil cloud, towering cumulus (Cumulus Congestus) can be seen

 

A shelf cloud associated with a gust front
A shelf cloud associated with a gust front at the base of a heavy thunderstorm
 
The risk of thunderstorms and lightning in connection with different types of precipitation
  • Drizzle: Is light rain with small water droplets. It is often associated with warm air and stable conditions, and there is no risk of lightning. The small size of the water droplets indicates that there is a small vertical movement in the cloud.
     
  • Snow: Lightning can in some rare cases occur in connection with snow showers but is rather uncommon (at least in northwestern Europe).
     
  • Hail: If the showers are accompanied by hail, then there is a high risk of lightning.
    However, there are two types of hail that typically is not associated with thunderstorms; graupel (also called soft hail or snow pellets) and ice pellets. Graupel typically occur in connection with other wintry precipitation and is not as hard as normal hail (therefore the name soft hail).
    The ice pellets will often occur when the surface temperature is around or below the freezing point and is smaller than regular hail.
  • Heavy rain: Could be an indicator of embedded showers and hence a risk of lightning, especially in cold fronts and occluded fronts.
  • Heavy showers: Heavy showers will form when there is a large vertical extent of the cloud, and there is therefore also often a high risk of lightning.
 
The risk of thunderstorms and lightning in different types of weather

Below we have stated the general risk of thunderstorms in different weather systems and air masses. The examples are “schoolbook” examples, but there will always be different factors that will change the risk for any given situation. 

  • High pressure:
    In a high pressure, the vertical motion will be downwards (descending air), and the air will become warmer and drier as it sinks. This will prevent any significant development of clouds. In the summer it often means sunny weather, while it in the winter sometimes can cause mist, fog and low clouds.

    Thunderstorm risk: None.
     
  • Low pressure:
    In connection with lows, there will be generally be ascending air causing unstable conditions.  However, the exact vertical profile depends on the distribution of different air masses - see how below.  
A satellite image with an overlay
A satellite image with an overlay of highs (H), lows (L), warm front (red), cold front (blue) and occluded front (purple). A cold air mass is located northwest of the displayed cold fronts. Two warm sectors can also be seen between the warm fronts and cold fronts
  • Warm front:
    In connection with a warm front, the warm air will rise above the colder air ahead of the front – this means stable conditions. This together with the typically slow movement of the warm front, gives typically a very low risk of thunderstorm formation.

    Thunderstorm risk: Very low.
  • Warm sector - between warm front and cold front:
    The air, after a warm front has passed, is typically warm and moist. In the winter, this often means misty weather and no showers and thunderstorms. However, in the summer really warm and moist air can move in from the south, triggering thunderstorm formations. This is especially true in a so called open warm sector, when there is a relatively large distance between the warm front and cold front.
    The thunderstorms will be a mix of isolated and embedded showers, and often be more frequent and heavier compared to thunderstorms in a cold air mass. The thunderstorms often appear in larger groups, with blue skies (but sometimes somewhat misty conditions) between the areas with showers. When the cold front later approaches, a line with showers and thunderstorms often will form just ahead of the cold front.

    Thunderstorm risk winter: None.
    Thunderstorm risk summer: Low, but in open warm sector with high dewpoints and just before cold fronts, there is a high risk.
    Difficulties in forecasting: As the atmosphere will be very unstable, just small differences in temperatures could trigger thunderstorms. The development will also be very rapid, and sometimes there will be detected lightning before you can spot the showers on a radar image.
    Sometimes the weather forecasting models do not have any precipitation throughout the day, but due to slightly different temperature/moisture, heavy thunderstorms can form in reality.
    Movement: Often slow, 5-10 knots.
     
  • Cold front:
    In a cold front, the cold air in connection with the front will lift the warmer air ahead of the front. The air mass will be somewhat more unstable compared to the warm front, and there can sometimes be embedded cumulonimbus clouds which will cause showery rain and, in few occasions, also embedded thunderstorms.

    Thunderstorm risk: Low.
  • Cold air/after a passing cold front:
    The cold air after a passing cold front is always unstable, and there will often be showers. However, the number of showers and the intensity are depending on a lot of different variables. Thunderstorms are rarely widespread, but it is difficult to determine which showers that will develop into thunderstorms.
    There will also often be a large difference between land and sea (depending on the season).
    Thunderstorm risk: Medium
    Difficulties in forecasting: If there are moderate to heavy showers in a given area, it could be difficult to forecast which of the showers that could develop into thunderstorms.
    Movement: Often rather fast (15-30 knots, sometimes faster).
  • Occluded front:
    Occluded fronts are harder to define than cold fronts and warm fronts, and the risk of thunderstorms do therefore differ a lot between different occluded fronts. In general, the risk of thunderstorms is lower in occluded fronts that appear east and south of the low (for east moving lows on the Northern Hemisphere), and higher in occluded fronts north and west of the low.
    Thunderstorm risk: Low to medium risk.
  • Trough/line of showers:
    Sometimes a line of showers could develop in the cold air after a cold front has passed. These lines will sometimes resemble fronts, with a shift in wind direction in connection with the passage of the line. They are often referred to as troughs or squall lines and will often be associated with moderate to heavy showers and many times also thunderstorms.

    Thunderstorm risk: High