Note: This article uses measurements with units of degrees Celsius (°C) and kilometers (km). To convert, 1°C = 1.8°F, and 1 km = 0.62 miles = 5280 feet.

Weather, while only being felt on the ground by the average person, is studied all across Earth’s atmosphere. Rain that falls on the surface comes from clouds miles high in the sky, formed from atmospheric processes that are well away from our towns and cities.

There are several measurements aloft we can take to determine how the atmosphere will act on any given day. Lapse rate focuses on the interplay between height and temperature, and we will take a deep dive in this article.

Lapse rate is the rate at which air temperature falls with increasing altitude. As one goes up in the air, the temperature will almost always decrease. This is why it is typically colder and snowier on top of mountains versus down below, because surface heat wanes as you increase in altitude. These lapse rates can vary depending on humidity and other atmospheric conditions.

Meteorologists tend to follow two lapse rates. The first is the dry adiabatic lapse rate, which is the typical decline in temperature in an idealized, water-vapor-free environment, with a rate of 9.8°C per kilometer. Once moisture is added to the atmosphere, you get the moist adiabatic lapse rate, which can vary, but is generally around 6°C per kilometer. 

Putting this in the real world, let’s say you have two different environments. One dry, and one humid, both with a surface temperature of 22°C. In the dry environment, the temperature will decrease faster, with the air temperature at 1 kilometer at 12°C. In the humid atmosphere, the same conditions otherwise would lead to the air being 4°C warmer.

Lapse rate’s role in forecasting is wide-ranging. We can deduce the stability of air using this value, with different impacts depending on whether the air is either very stable or unstable. Stable air, or when the lapse rate is between 0 and the moist adiabatic lapse rate, can be attributed to good weather with very little risk for precipitation. When air is unstable, for simplicity, temperature is declining above the dry adiabatic lapse rate, thunderstorms can most easily develop as parcels of air will become buoyant and lift upwards, satisfying a key step for storm formation. 

 Lapse rates can also be negative, as well! This uncommon case is known as a temperature inversion, where temperatures increase as altitude also increases. This is most common under wintertime high-pressure systems, and doesn’t usually last for a very long time. In cases where inversions do last, this poses risks for air pollution, as air is usually trapped within the inversion.

Within the Valley, lapse rates have the highest importance during thunderstorms and lake-effect events. This week, high lapse rates are to blame for lake-effect rain showers.

Lake-effect events typically occur in the winter when cold, Canadian air blows over the still-mild Great Lakes, which then form narrow bands of heavy snow. With the introduction of the first true fall-like airmass in our area, suddenly cold air 1km aloft (about 10°C, or 55°F) has blown over the warmer-than-average lakes, which are currently 23°C (75°F). With some simple math, the lapse rate in this instance is 13°C, well above the 9.8°C dry adiabatic lapse rate necessary to make air sufficiently unstable. All that’s needed is some wind coming off the lake, which will give the precipitation its movement towards communities on land.

What has resulted since then has been a blend of scattered showers across Trumbull and Mercer counties, with the rest of the Mahoning and Shenango Valleys rather dry, a classic wintertime pattern- just a couple months early.