four main components of severe weather – cutting, lifting, instability, moisture

BISMARCK, ND (KFYR) – SLIM is an acronym for the four main ingredients we usually need to form severe weather. Wind is the change in wind speed or direction with altitude. A source of lift is what pushes air up to form large thunderstorm clouds. The instability helps the air to rise quickly when it is pushed. And moisture provides fuel for severe thunderstorms.

Extreme Weather 101: Basic Ingredients(National Weather Service)

Milk is probably the most important component when forecasting severe weather and there are two types: velocity shear and direction shear. A wind speed curve is when the wind speed is different at different altitudes in the atmosphere. This usually occurs when the winds are relatively calm near the ground and very strong a few thousand feet up. The speed of movement is critical to the development of severe thunderstorms because it pushes the updrafts, or rapidly rising air, that form the clouds.

Wind Circulation Explainer
Wind Circulation Explainer(KFYR)

Updrafts are important for the development of severe thunderstorms because they allow rain to fall before the updrafts. If the rain falls in the same place where the air is rising rapidly, the thunderstorm can “self-rain”, that is, it will slow down quickly because the rising air cannot maintain itself. But with the change in speed in the atmosphere due to the difference in wind speed on the ground compared to the height, our updrafts allow the warm and rising air to replace the cold and sinking air, where the rain causes additional strengthening of the air. thunder and lightning

A sheared environment pushes updrafts so that rain falls before upstream
A sheared environment pushes updrafts so that rain falls before upstream(none)

Velocity can also create swirling air in the atmosphere due to changes in velocity with height. These horizontal columns of air circulation can be converted into vertical columns of air circulation when it is in an area of ​​rising air. This can allow large rotating thunderstorms called supercells to form and even eventually hurricanes to form thanks to this wind rotation.

The speed pattern creates circulating air
The speed pattern creates circulating air(KFYR)

The second form of flow is directional shear, when there are different wind directions with height in the atmosphere that can increase the speed of movement in the atmosphere. In the cross section example below, winds blow northwesterly near the ground and become northeasterly. This creates vertical columns of air circulation that can promote severe thunderstorm development and is an important component of low-level circulation that can produce tornadoes.

Directional flow creates circulating air
Directional flow creates circulating air(KFYR)

To estimate the amount of wind circulation in the atmosphere, meteorologists can look at two different areas. The plot on the lower left shows the wind (which shows the speed and direction of the wind) at different heights in the atmosphere. In this example, near the ground at the bottom of the continent, the wind is relatively calm from the south-southwest. Then, the winds increase to such an extent that they are very strong at an altitude of 6000 km above the ground and blow mostly from the west-southwest direction. This combination of different wind speeds and directions with height (speed and cross section) is powerful enough to create severe thunderstorms.

The plot on the right is a hodograph that depicts the wind as it rises into the atmosphere. The longer and more curved this section is, the greater the shear in the atmosphere.

Wind barbs with the height of the vertical wind shear, a hodograph plot showing the wind shear
Wind barbs with the height of the vertical wind shear, a hodograph plot showing the wind shear(KFYR)

Elevator is the mechanism we need to force moist, unstable air aloft to form thunderclouds in the first place. There are four main lift mechanisms that can help push air vertically. Convergence with low pressure areas occurs when winds from different directions come together, forcing air to rise in the center of the low pressure area. Convective upwelling is when the sun heats the earth during the day, it does so unevenly depending on the exact composition of the earth. Due to the differential heating of the earth, the air near the earth is warmer in different places compared to cooler in other places, which allows it to rise. Orographic lift is when the wind encounters a physical terrain barrier, such as a mountain range, and forces air to rise on the downwind side of the mountain. And finally, a front (such as a warm front or a cold front) is a lifting mechanism because when two air masses collide, air is forced to rise across that boundary.

With all these lifting mechanisms that force the air to rise, as the air rises it cools and condenses into clouds at a certain level of the atmosphere.

Mechanisms of atmospheric lifting
Mechanisms of atmospheric lifting(none)

Instability (or unstable atmosphere) is the next component of severe weather that we need to allow our air that has been driven upward by our lift mechanism to rise quickly. Air is considered unstable when the air is warmer or more humid than the surrounding air. When this happens, the air rises because it is warmer than the surrounding air. In an unstable environment, the weather can change rapidly and can be severe. Meteorologists measure atmospheric instability with CAPE, or Convective Potential Energy. CAPE is determined by meteorologists in sounding plots, as shown on the left below, and the higher the CAPE, the faster our air rises, leading to stronger updrafts and more severe thunderstorms.

Convective potential energy
Convective potential energy(KFYR)

Moisture is the fuel for thunderstorms and must be high enough to form clouds in the first place, but the higher the dew point, the stronger our thunderstorms will be in general. If there is moisture in the lower atmosphere closer to the ground, tornadoes can form. With a dew point scale of 60 degrees, the ground feels a little muddy and wet, and that’s generally the threshold we need for severe weather to develop. One of the main sources of this moisture that feeds thunderstorms, even in northern Canada and North Dakota, is the Gulf of Mexico. Often strong low pressure systems move from west to east to pull this key component north out of the bay.

Dew point scale
Dew point scale(KFYR)

All of these ingredients come together to form our powerful Super Cell Thunder with built-in mesocycle. This means that our storm moves quickly throughout the atmosphere due to the different types of wind. And it is very strong and big thanks to other main ingredients: lift, stability and moisture. Hail is common with these strong and intense supercells due to their strong and rotating updrafts. If there is enough low-level circulation, a tornado is possible.

Mesocycle, the structure of the mature supercell
Mesocycle, the structure of the mature supercell(NOAA)

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