If you have watched the Weather 101 on wind you will recall that I mentioned that it was important to pay attention to the isobars on the surface weather map. If you haven’t watched that segment you can HERE. What exactly are isobars? In this week’s episode of Weather Word of the Week we are going to explain what they are and why they are important to the world of weather forecasting.

Isobar

The word can be understood better when we break it down into two parts. Iso means equal and bar is a metric unit of pressure. Place the two together and you get isobar defined as an equal metric unit of pressure. Simply put, an isobar is a line of equal pressure on a surface map.

Isobars or Isohypse?

Surface Map Example. The Black Contours are Isobars.

Isobars only exist on a surface map. If you’re looking at an upper air map, then your are examining isohypses or lines of equal heights NOT isobars. This will be explored in a separate digital piece. Both of these types of contours do indicate how strong the wind speeds are across an area at different levels in the atmosphere.

Example of a Upper Air Chart at 500mb. Black Lines are Isohypses.

One thing that will help you identify a surface map from an upper air map is the numbers on the contours. If the numbers start with a 8,9, or 10 then you are most likely looking at a surface map. This would then mean you are also looking at isobars, If the start of the number is anything else it most likely is an upper air chart. The wind also tends to strengthen with height. If you are seeing wind speeds of well over 50 knots, then that is also a good hint that you are looking at an upper air chart.

How to Interpret Isobars

Northern Plains Surface Map Showing an Area of Strong Winds.

Now that you understand a little more about isobars, let’s dive into how to interpret them. It is actually much easier than you might think. The closer the isobars are to each other the stronger the wind is. This is because a tight packing indicates the presence of a stronger pressure change over an area.

Examining the surface map from the Northern Plains, you can see a tighter isobar packing located in northern Colorado into southern Wyoming (black circle). Surface winds are anywhere from 25-30 knots. Let’s contrast that with the image below.

9Northern Plains Surface Map Showing an Area of Weak Winds.

Notice in this image the much calmer winds across the state of Iowa (black circle). this is because the pressure gradient is much weaker under the influence of a surface high pressure system. The winds are at most are 5 knots! If you look at the isobars across Iowa they are much farther apart.

The next time the wind comes sweeping down the plains into NW Arkansas and the River Valley, make sure to glance at the surface chart. (You can find one HERE!) I wouldn’t be surprised if you find the isobars are packed together like sardines!

Check out other episodes of Weather Word of the Week HERE!