How dynamics and thermodynamics create weather

As a pilot, you know that the atmosphere is constantly evolving. The changes in precipitation, cloud types, and hazards you see all link back to changes in temperature, pressure, and forces. Understanding weather means understanding the two main meteorological processes behind weather changes: dynamics and thermodynamics.

Dynamic meteorology can be thought of as the branch of meteorology dealing with atmospheric motion and its relation to key atmospheric forces. These forces include the Pressure Gradient Force, gravity, the Coriolis Force, and friction. The imbalance and evolution of these forces often cause weather systems to form, strengthen, and weaken. The math and science behind dynamic meteorology is very complex, but pilots can benefit from what these equations reveal.

Dynamic meteorology can be understood more easily by following the evolution of the wind and the changes it creates. One of the weather features with which you’re likely familiar and is closely related to dynamic meteorology is a front. While a front can be thought of as a division between air masses, it can also be thought of as an area with a low-level convergence of air, which in turn leads to rising air and an increased coverage in clouds and precipitation. While areas of surface low-pressure – such as troughs – can be weak, stronger areas of low pressure are often linked to upper-level wind changes.

500mb chart
When the upper air analysis shows a deep trough, you can expect clouds and precipitation.

Let’s travel aloft to a constant pressure surface of 500 millibars (or approximately 18,000 feet above ground level for much of the United States). At this level of the atmosphere, we find troughs and ridges or zones of the country where the upper-level flow is dipping southward or soaring northward. Let’s focus on the base of the upper-level trough. On large scales and due to the conservation of angular momentum, air at the base of the trough will move faster compared to air north of it. This creates a counter-clockwise spin – or more formally, positive shear vorticity – in the base of the trough. Additionally, the north to south motion of air through the trough creates positive Earth vorticity. We care about these two forms of vorticity because there is a horizontal convergence of air – and thus rising air – ahead of the area of vorticity. This rising air often creates clouds or precipitation and is often why you’ll find a surface area of low pressure downstream of an upper-level trough.

If we go even higher in the atmosphere to 250 millibars (or approximately 35,000 feet above ground level), we find the jet stream. Within the jet stream we can find regions with wind speeds in excess of 100 knots, called jet streaks. Due in part to air accelerating and decelerating as it moves through the jet streak, rising air occurs in the right front and left rear quadrants of the jet streak. A strengthening jet streak often means air is rising more rapidly in these right front and left rear quadrants, so a change in the jet stream forecast can have significant surface weather implications.

Thermodynamics is the branch of meteorology that involves the movement of air and the changes to water within it as a result of temperature and dewpoint changes in the atmosphere. Thermodynamics is more tangible than dynamics because we can perceive the temperature and humidity of air. An experienced pilot will be alert for thunderstorms on a hot, humid summer day and for ice and snow on a sub-freezing day with saturated low levels of the atmosphere. This same pilot has also watched this warm, moisture-laden air cool and condense as it rises through the atmosphere, evolving from a cumulus to a cumulonimbus cloud.

Thunderstorm
A storm like that requires more than just heat – it also needs moisture.

Thermodynamics tells us that air will be able to rise as long as it remains warmer than the air around it and considering the rate at which that air cools and condenses. The amount of moisture in the air – or the dewpoint – can greatly affect atmospheric temperatures while also making the process of convection more efficient. The dewpoint can also have a large impact on the height of cloud bases and whether thunderstorms will be able to form.

Ultimately, it is the combination of both atmospheric dynamics and thermodynamics that drives the weather we see. Just like a review of your pre-flight checklist, a meteorologist often reviews a checklist of ingredients to assess the current weather situation and plan for future changes. Strong thermodynamic support in the form of heat and humidity coupled with a lack of dynamics – such as the absence of front or positive vorticity aloft – will usually support storms, but they often won’t be long-lived. Add in large amounts of positive vorticity and position the right front quadrant of a jetstreak overhead, and severe convection is increasingly likely. Remove the warm, humid air near the ground, and storms are likely to be traded for showers. Adjust temperatures so that they increase with increasing altitude, and precipitation may not form at all.

Understanding the processes of meteorology will not only help you better anticipate changes in the weather. It will also help get you to your destination safely.

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