What are mountain-valley wind circulations?

I very much enjoy how on any given day when stepping outside it's possible you can point to all kinds of interesting weather phenomena. A lot of attention will always be captured by severe thunderstorms, hurricanes, tornadoes, and blizzards. However, in this explainer I want to bring attention to the science behind a benign, yet prevalent, weather process creating mountain-valley circulations, which could have more impact on your everyday life then you realize. Let’s first establish why mountain-valley circulations exist and are often a dominant localized wind pattern over complex topography. Since the Phoenix metropolitan is in my backyard, I will zoom in on the physical geography of south-central Arizona to help explain, but please keep in mind that mountain-valley circulations of varying strength and spatial impact are widely observed across our planet.

The main premise for how localized mountain-valley winds develop starts with the uneven heating and cooling of landscapes having significant elevation contrasts in relative close proximity. The essentials for this concept are that exposed mountainous terrain, relative to adjacent flatter lower valley floors, heat quicker with the rising sun and result in sustained rising thermals of warm and buoyant air concentrated over these higher elevations as the day progresses. Rising motion creates low pressure and air from the surrounding valleys must move towards and up mountain slopes to replace the air mass leaving the surface. These are called anabatic or upslope winds.

Applying that to the Phoenix area, it now makes sense why westerly winds by late morning and afternoon are commonly observed, since elevations near downtown Phoenix are around 1,100 feet, or so, but within 30-40 miles north and east elevations rise dramatically up to 7,000 feet.

After sunset, it’s a different ball game! Mountain tops now lose heat quicker being exposed to higher altitudes. Cooler air, having greater density with negative buoyancy, now reverse daytime wind directions by flowing downslope back into the lower valleys (now called katabatic winds). For Phoenix, this why easterly morning winds are typical, especially along natural drainages where colder air tends to collect and gain momentum.

Now that we got the fundamentals down for why and how a mountain-valley wind circulation works, let’s explore more what that means for us living in locations supportive of these prevailing local wind circulations.

Assuming ideal conditions for a mountain-valley wind setup in Phoenix, Arizona, here are a couple influences for consideration:

1) In aviation, planes want to fly into the wind during takeoff and landing (helps reduce aircraft speeds necessary to get off the ground or required runway distance coming to a full stop). For this reason, the direction you face on the runway taking off (or point when landing) during the overnight and early morning hours at Phoenix Sky Harbor airport is likely to the east (into the cooler air flowing down out of the higher terrain). If your flight is scheduled for takeoff (or to land) later in the morning, afternoon, or early evening, then your plane likely points west against the daytime westerly flow being pulled towards the mountains behind you.

2) Both daytime upslope and overnight/early morning downslope winds can be breezy, however, reviewing daily air parcel trajectories caught in mountain-valley circulations may actually reveal little net movement of air in the local region. This is a critical aspect to consider for air quality meteorologists forecasting pollutant concentrations in areas susceptible to recurring mountain-valley wind regimes! The consequence is gradual pollution buildup over time until larger weather systems passing through disperse trapped urban and natural emissions. 

Indeed, mountain-valley winds are ideally expected during what we might consider “quiet” weather days. Passing dynamic and large weather systems do override local mountain-valley induced pressure gradients to maintain a region-wide general wind direction regardless of it being daytime or nighttime or a mountain versus valley environment. For example, an approaching strong cold front from the west would support persistent southerly winds throughout the region ahead of the frontal boundary and then as the cold front passes the prevailing wind direction would turn northwesterly. Once the unsettled weather pattern exits the area, localized mountain-valley circulations can then reestablish themselves.

-JWM

Photo Credit: Jonny William Malloy