Lesson 1. Meeting the Mesoscale

This is a sample lesson page from the Certificate of Achievement in Weather Forecasting offered by the Penn State Department of Meteorology. Any questions about this program can be directed to: Steve Seman


Heading into our examination of mesoscale forecasting, it's possible that some folks might not be familiar with the term "mesoscale." For starters, what exactly is mesoscale meteorology? Let's break the word "mesoscale" into its components. First, the prefix, "meso", means "intermediate." The root, "scale," refers to spatial scales, or the extent of a weather system in a specified horizontal direction. So, mesoscale meteorology pertains to weather features with an "intermediate" spatial scale.

That's a simple (although vague) definition. We'll get into more specifics soon enough, But for now, it suffices to say that mesoscale weather features are smaller than most of the large-scale weather features (high- and low-pressure systems, etc.), but larger than really small features that span only a few kilometers. What kinds of weather features fit into the mesoscale? Thunderstorms, lake-effect snow, terrain-induced wind circulations, and sea / lake breezes all fall under the umbrella of mesoscale meteorology. That's right: Whether you live near the beach, in the mountains, or anywhere that thunder occasionally rumbles, mesoscale meteorology is part of your life!

Radar image showing tornadic thunderstorms
The mosaic of composite reflectivity at 00Z on May 11, 2010 (the evening of May 10). At the time, there was at least one tornadic thunderstorm over Oklahoma. Such a storm qualifies as a small mesoscale feature.
Credit: WSI Corporation

Furthermore, many types of dangerous and destructive weather occur on the mesoscale.  Thunderstorms can spawn destructive hail, damaging wind gusts, flooding rains, and even tornadoes. These phenomena can be a threat to both life and property, and understanding mesoscale meteorology is critical to making accurate short-term weather forecasts and for assessing potentially life-threatening risks.

While this course focuses on the mesoscale, one recurring theme you'll encounter is the strong connection between mesoscale weather and the larger-scale weather pattern. Your ability to analyze the "big picture" will be critical in this course, because the large-scale weather pattern determines what types of mesoscale weather can occur. However, as you'll learn, some critical aspects of mesoscale weather differ from larger weather features. Briefly consider two important contrasts:

  • Mesoscale weather features tend to have much shorter life spans than larger weather features.
  • Recall that vertical motions on the large scale tend to be very slow (a few centimeters per second or less), but on the mesoscale, that's not always true! In extreme cases, vertical motions on the mesoscale can be upwards of 50 meters per second -- hundreds of times faster, in comparison!

You'll see these ideas at work throughout the course, but in this lesson, we'll explore (and distinguish between) the spatial scales associated with weather features of various sizes, all the way from the very smallest (less than a few kilometers) to the largest, which span huge portions of the globe. Of course, along the way we'll focus on the mesoscale.  We'll also take a brief look at a few of the mesoscale models that weather forecasters use as guidance.

If you're ready to meet the mesoscale, let's get started!