Atmospheric Instability

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In meteorology, the atmosphere is often described as “stable” or “unstable”. The degree to which the atmosphere is stable/unstable determines most of the weather phenomena experienced both at the surface and in the upper levels of the atmosphere. Stability is assessed in the atmosphere by looking at how temperature and humidity change with altitude.

In the atmosphere, air that is warmer and more humid (i.e. less dense) than its surroundings will have the tendency to rise until it reaches surroundings of similar density. Air that is cooler and dryer (i.e. denser) than its surroundings, on the other hand, will tend to sink.

The graph on the left is a depiction of an unstable atmosphere, where temperature decreases with height. On the right, the graph depicts a stable atmosphere, where temperature increases with height. A section of the atmosphere where temperature decreases with height (denser air over less dense air, left-hand side graph) is considered unstable, whereas an atmosphere where temperature increases with height is considered stable (less dense air over denser air, right-hand side graph).

Importantly, the atmosphere is never entirely stable or unstable, but made up of several stable and unstable layers, meaning different kinds of vertical motion can occur at different heights.

Image source: Environment and Climate Change Canada

A layer will become more stable as the air parcel becomes colder and drier than the surrounding air. This GIF is an example of a stable, neutral and unstable equilibrium using a balloon, which can be applied to the atmosphere as well. In example 1, the temperature of the parcel of air is warmer than the air surrounding it. It will rise until it reaches air that is equal to or warmer than it. In example 2, the parcel of air is considered “neutral” and doesn’t move. It is the same temperature as the surrounding air. In example 3, the parcel of air is colder than the air surrounding it and sinks.

Stable Atmosphere

A stable atmosphere deters or inhibits vertical motion, that is to say it is an atmosphere where air “sinks” or remains at the “same level” or neutral. A stable layer at the surface (usually referred to as an inversion) often means calm winds and clear skies but can also trap smoke and pollutants in the low levels and prevent them from being dispersed into the upper atmosphere. When humidity is high, this stability can trap moisture near the surface and promote the formation of fog and drizzle.

Unstable Atmosphere

An unstable atmosphere is one that leads to air rising, the formation of convective clouds, precipitation and turbulence. It implies that air in lower levels becomes warmer than the surrounding air, and begins to freely rise. In general, an atmosphere where temperature decreases with height is considered unstable.

Instability will intensify by increasing the temperature or humidity (or both) of the air parcel, with respect to surrounding air.

Image source: Environment and Climate Change Canada

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Convective Available Potential Energy, or CAPE is a measure of energy, used to determine how much energy a parcel has to rise through the atmosphere. CAPE is one of the main indicators used in meteorology to predict thunderstorm potential and severity.

This table shows the approximate CAPE values and the corresponding atmospheric instability assessment. If no CAPE exists, the atmosphere is considered stable. As CAPE values increase, the atmosphere is considered increasingly unstable. It is important to note that these values are approximative and should be used in conjunction with other meteorological parameters (such as the presence of CIN, a potential trigger, and wind shear) when assessing the potential for thunderstorm development and their associated hazards.

CAPE is assessed using a tephigram by assessing how a parcel of air would cool and condense as it rises in the atmosphere relative to its surrounding environmental air. A warm, humid air mass at the surface and colder air in the upper atmosphere contributes to more atmospheric CAPE. Additionally, as air becomes cooler and dryer in the upper levels of the atmosphere, or when moisture and heat is added to the lower levels of the atmosphere (for example during peak heating hours on a hot summer day) this adds to CAPE values. Typically, a CAPE value of 1500 J/kg or greater is enough to produce severe thunderstorms in Canada, however general thunderstorms can form with as little as 500 J/kg of CAPE (for more information, NWS explains CAPE visually hereOpen a new window).

Image source: Environment and Climate Change Canada

Convective Inhibition (CIN) works against the buoyancy of a parcel of air in the opposite direction of CAPE, and is in essence “negative CAPE”. CIN can be described as the energy exerted on a parcel of air to decelerate or slow down its rising motion. When assessing thunderstorm potential, the amount of CIN is another indicator to assess potential thunderstorms development.

In this example, we see an atmospheric profile with an inversion in the mid level. Parcel 1 is slightly warmer than the surrounding air and rises to the inversion level (CIN). It does not have enough energy to break through the inversion and overcome the CIN acting on it. Parcel number two is much warmer than the surrounding air and starts to rise. Parcel two has more energy (CAPE) and is able to overcome the inversion level. Once it overcomes the inversion, the air surrounding the parcel is cooler and the parcel accelerates vertically even more and rises into the upper atmosphere. If external forcing (such as a trough in this example) acts upon parcel one, it may be able to overcome the inversion (CIN) if the force is strong enough.

CIN is also assessed via a tephigram by determining when the environmental air is warmer than a parcel of air as it rises through the atmosphere. Often CIN is present when there is an atmospheric inversion in the mid levels, meaning temperature increases with height. A parcel of air that is initially warmer than its surroundings rises from the surface and hits a “ceiling” of warmer air in the mid levels and stops its vertical motion.

When CAPE and CIN coexist, the atmosphere can be considered “conditionally unstable”. This means that while the atmosphere is largely unstable, there is a barrier that air parcels must overcome in order to freely rise beyond a certain height. An external force acting on the air parcel (such as a front/trough, a change in topographical elevation, or additional surface heating) is required to provide the extra energy it needs to overcome this barrier and rise freely.

Image source: Environment and Climate Change Canada

Areas of instability are relatively easy to forecast in most situations by analyzing atmospheric profiles and tephigrams and using scientific reasoning to determine likely areas of instability. Determining exact values of instability is trickier, as they are dependent on the accuracy of the modelled soundings. If the modelled profiles are off even by one degree, it could be the difference between convection developing and a clear sky in situations where convective initiation is suppressed due to an inversion, or the presence of CIN.

In situations where forecasters have assessed that models are not adequately capturing all factors that will affect instability, they must mentally adjust the model output in order to account for those factors. While this typically gives better forecasts, capturing exact details of the forecast is often challenging.

Additionally, there are certain days when the atmosphere is “conditionally unstable” that are difficult to forecast. Typically on these days, both CAPE and CIN are present and forecasting whether there will be enough forcing to overcome the CIN and lead to thunderstorm development is particularly difficult. While this can be observed through upper air soundings (weather balloons), the Canadian observation network is not dense enough geographically or in time in order to see real time atmospheric conditions.

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The GFA valid at 1200Z on July 1, 2023 captures the atmospheric instability present that morning across much of Ontario and Quebec. Areas of more organized convective showers are associated with the trough extending from the weak low just southwest of James Bay. This surface trough serves as a trigger for thunderstorms. The effect of the terrain serves as another trigger for thunderstorm development over central-northern Quebec. The orographic lift forces unstable air up and generates isolated thunderstorms over higher terrain.

Atmospheric Instability for surface-based thunderstorms tends to peak late in the day at the cusp of maximum daytime heating. This effect is shown here on the 0000Z GFA from July 2, 2023 with thunderstorms possible across almost the entire GFA area. Their extent of coverage, tops, and potential severity has also increased relative to the 1200Z GFA with tops possible to 42,000ft and hail possible across southern Quebec.

The TAF for CYUL valid between July 1 at 1500Z and July 2 at 1200Z depicts the level of atmospheric instability forecast that day. Convective showers are expected intermittently throughout the forecast period, while a risk of thunderstorms is forecast between the hours of 1500Z and 0300Z. After 0300Z, thunderstorms are no longer in the forecast, which implies that the atmosphere has stabilized and there is no longer enough instability to support thunderstorm development.

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12Z Atmospheric Profile

Modeled atmospheric profiles at CYUL from the HRRROpen a new window, valid between 1200Z on July 1 and 0000Z July 2 capture the generalized instability present throughout the day. Simply click through the image carousel to see the various images. At 1200Z on July 1 there is evidence of a cap at the surface (yellow circle), which results in CIN giving stability through the lowest layers of the atmosphere (see science explained section for an explanation of a “cap”). This stability inhibits surface-based convection unless a trigger exists (like orographic lift and/or the trough in this case) that could lift air parcels up into more unstable air where upward motion is supported. This was already on-going as per the GFA and as shown in the various products in the observations section.