500-mb chart

Become familiar with this essential briefing tool. Thunderstorms frequently move proportionately to the direction and speed of the 500-mb winds.

By Karsten Shein
Climate Scientist

500-mb absolute vorticity chart. Areas of red indicate greater positive spin of the atmosphere, which is an indicator of rising air and potential convection. Black lines (in decameters) show roughly the altitude at which the pressure is 500 mb.

In the early days of flight, pilots would look to the sky to study the character and movement of the clouds, and perhaps consult a barometer to discern what weather they might face on their flight that day.

But advances in meteorology over the past century ushered in the concept of a flight briefing, where a trained forecaster would provide a digest of weather information to the pilot about the route of flight.

Now there are many online services that bring the latest weather maps and charts directly into the hands of pilots. A pilot can see the winds aloft and larger-scale weather patterns that were previously just being described by the briefer. Being able to visualize the weather through which one is about to fly is a very important part of flight planning, and one most pilots welcomed.

However, this evolution of weather services for pilots also came at a price. As aviation weather services evolved, the reliance on briefings from professional forecasters stifled a corresponding development of meteorological education for pilots at the click of a mouse.
In most countries, the requirements for a pilot license are limited to knowledge of airspace rules and the operation of the aircraft. There are a few weather-related questions on the written tests, but these are primarily to test knowledge of weather minima and severe weather avoidance, not interpretation of weather behavior.

Instrument ratings require basic knowledge for recognizing critical weather conditions, but as with licenses, emphasis is placed on knowledge of the rules for operating in IFR conditions. Many aviation ground schools do provide basic instruction in meteorology such as how to read a surface weather map, but the degree of meteorological knowledge varies greatly among the pilot community. For many pilots, weather maps and charts remain a simple visual aid, when in reality, they each contain a wealth of information that can help a pilot understand and take advantage of favorable conditions. One such map is the 500-mb (500-hPa) chart.

Troposphere is where most aircraft fly

The severity, rate of development, and movement of thunderstorms and other convective systems are largely determined by conditions found in the middle troposphere – at around 500 millibars. Meteorological charts of this level provide meteorologists and pilots a wealth of knowledge about the atmosphere.

Though our atmosphere has no precise upper boundary, the Kármán line (edge of space where air density is no longer sufficient for aerodynamic lift) occurs at around 100 km (62 mi). A trace atmosphere (meaningful from a physics perspective) extends to around 10,000 km (6200 mi) from the surface. The atmosphere is divided into 6 distinct layers based on energy and temperature behavior. However, except for a few select aircraft, almost all of our flight takes place in the lowest layer – the troposphere.

The troposphere extends from the surface to, on average, 33,000 ft (10,000 m), ranging between around 26,000 ft (8000 m) over the poles in winter to around 60,000 ft (18,000 m) above the tropics. It is powered by solar heating of the Earth's surface and as a result, air temperature decreases with increasing altitude. At the top of the troposphere, air temperature levels off for several thousand feet before increasing with altitude in the stratosphere. It is the temperature profile of the troposphere, coupled with an ample source of water, that makes this layer the home to a majority of all of the atmosphere's weather.

Gravity ensures that around 75 to 80% of the overall mass of the atmosphere is in the troposphere. We can measure this mass in terms of the pressure it exerts. Given an average sea level pressure of 1000 millibars (1000 hectopascals), this means that the top of the troposphere occurs at around 250 to 200 mb. And given the exponential decrease in pressure with altitude, 500 mb occurs around halfway up in the troposphere (around 18,000 ft or 5500 m).

500-mb significance

Although meteorologists look at many levels of the troposphere when they model and forecast the weather, there are several atmospheric levels that are arguably of greater importance than others. The 1st is near the top of the troposphere. Conditions at around 300 mb describe the jet streams and the upper air support for vertical movement of air. Patterns at this level drive large-scale storm systems. The 2nd level is the surface layer. Heat and water availability at the surface determines the energy available to drive the weather machine. The 3rd critical level is the halfway point in the troposphere, or around 500 mb.

The properties of the atmosphere at around 500 mb are essential for describing the dynamic motion of the troposphere. Thunderstorms and midlatitude cyclones frequently move proportionately to the direction and speed of the 500-mb winds. The spin of the air, or vorticity, is a measure of how much energy is being moved upward or downward, and therefore provides an indication of the instability of the troposphere.

Reading the charts

There are 2 kinds of upper air charts: constant altitude and constant pressure charts. Both types of charts are contour maps. In a constant altitude chart, the contour lines are lines of equal pressure (isobars). Every place on the chart is the same altitude, and the isobars tell you the pressure at that altitude.

A constant pressure chart is the opposite. Everywhere on the map is the same pressure and the contour lines are lines of equal altitude that describe the height above sea level (in meters or decameters) of that pressure.

Since pressure is the force applied by the mass of the atmosphere, pressure and altitude are intrinsically connected. Visually there is little difference between a 500-mb chart and an 18,000 ft chart, but because most of the meteorological equations governing dynamics rely on pressure, constant pressure charts are of more use in weather forecasting.

The connection between pressure and height means that the height contours on a 500-mb chart will reveal the pressure patterns in the middle troposphere. High altitudes on the chart are connected to high surface pressure (more air above that point), while low altitude contours signify lower surface pressure. Through this relationship, it is straightforward to see the atmospheric pressure patterns and positions of major highs and lows. Aligning them with the highs and lows at the surface and at 300-mb will provide an indication of the likelihood of growth or decay of midlatitude storm systems.


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