February 16, 1998

Introduction to winds
Introduction to Weather Map Analysis - Isoplething

Winds - The atmosphere in motion

We have taken a microscopic look at the air pressure by examining the behavior of gases in a container. We established that wind is created by horizontal pressure gradients, which are changes in pressure across a horizontal distance. The bigger the difference in pressure between 2 locations, the stronger the wind.

Beaufort Wind Scale

In 1806, Admiral Sir Francis Beaufort devised a simple scale that coastal observers could use to report the state of the sea to the Admiralty. It was officially adopted in 1838. It was primarily used by sailors, navigators, and travelers as a means of estimating wind force based on observations.

Below is the Beaufort Wind Scale in its original form complete with the classifications 19th Century marine operators used:

Effect of friction on the wind

Terrestrial objects such as trees, buildings, soil, and mountains slow the wind down by friction. With increasing elevation, these effects of friciton diminish, thus wind speed increases with elevation.

Force of the wind

The wind has a force to it. It is capable of accelerating objects. The force of the wind is proportional to the square of the velocity of the wind and also the density of the air. Since the force of the wind is proportional to the square of the wind velocity, small increases in wind speed are accompanied by large changes in the destructive power of the wind. A 5 mph wind has 25 times more force than a 1 mph wind.

Also, since density decreases with height in the atmosphere, a 25 mph wind in Tallahassee would have more force than a 25 mph wind in Denver. Intuitively, it is apparent that the force of the wind is a result of the transport of mass of the atoms and molecules of the air. In a less dense fluid, there is a lower concentration of mass thus a lower force imparted on objects in the moving air.

This is one reason why forecasting the intensity of a hurricane with precision is quite important. There is a very big difference in the destructive power of a 150 mph wind versus a 160 mph wind.

Weather Map Analysis - Isoplething

Maps are an extremely critical part of meteorology. Forecasters rely on weather maps to make forecasts and to understand and describe the behavior of the atmosphere. We will learn in today's lecture the practice of isoplething, which is a very important principle in meteorology and many other natural sciences as well.

An isopleth is a line connecting equal values.

Consider the map below. It is an isotherm analysis. Isotherms are lines connecting points of equal temperature.

The circles represent weather stations and the numbers to the left of each circle are the temperature readings in degrees Farenheit. The red lines are isotherms. There are 3 isotherms drawn on the map: 35 F, 40 F, and 45 F. Let's take a closer look at the 40 F isotherm. All points on this line are assumed to have a temperature of 40 F. Look at the 40 F isotherm where is passes between the city with a 39 F reading and a 44 F reading. The isotherm passes much closer to the 39 F reading. In actuality, there is no data available from this location, but it can be said with almost certainty that the 40 F reading would be closer the 39 F station than the 44 F station. Thus the line is drawn closer to the 39 F station. This process of fitting curves to discontinuous data is called interpolation.

The next type of isopleth we will consider are isobars which are lines of equal pressure. Consider the map below.

The readings to the right of the stations this time are pressure readings in millibars (mb). Isobars are drawn on this map in 4 mb intervals. The practice of interpolation should be apparent here. From what we have learned, can you pick out locations on the map where you would suspect winds would be strongest?

The closer together the isobars are, the stronger of a horizontal pressure gradient exists in that area. Wind speed is directly proportional to horizontal pressure gradient.

When we overlay isotherms on top of isobars, we can get a good idea as what type of temperature advection we will experience. Consider the map below:

Consider that as a good approximation, wind will blow parallel to isobars with high pressure to the right of the direction of flow. Look at the map and try and pick out whether there will likely be warm advection or cold advection occurring.

Weather stations

Across the country, there are hundreds of weather stations that are sites of forecast preparation and weather recording and monitoring. Typically, weather stations are nearby airports because of the importance of the weather to aviation. Weather stations have varying amounts of meteorologists and equipment. Generally, the bigger the airport, the more meteorologists and equipment will be onhand.

Each airport weather station has a 3-letter station identified code. For instance, Tallahassee is TLH. They are not always as obvious however. For instance, Orlando International Airport is MCO.

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