The Science of Meteorology Online

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**More about weather maps.**

On surface maps meteorologists use these weather map symbols to analyze the weather. Here's an example. An analytical map usually includes these elements.

They display the data consistently around the world so it's never confusing. How about upper air symbols and patterns on the 500 millibar map? When preparing a weather forecast, meteorologists often start with this map.

Why bother? **Weather map symbols help people understand** and prepare for potential weather-related hazards by providing a visual representation of weather conditions. Forecasters and meteorologists use these symbols to convey complex weather information.

- People care about weather map symbols because they give them valuable information to **plan **their activities and **take precautions**. When a person sees a thunderstorm symbol on a weather map, they know to take shelter and avoid outdoor activities. As well, if they see a snowflake symbol, they know to dress warmly.

- Additionally, emergency management officials use weather map symbols to make evacuation **decisions,** school closures, and other safety measures. During severe weather, members of the public can stay informed and take appropriate action by understanding these symbols. Weather map symbols keep the public informed about weather conditions and help them make informed decisions to protect themselves and their communities.

We're going to explore a specific set of symbols embedded in a map for analysis. There may be atmospheric motions contributing to intense rain or snow.

There are faint whirlpools in a quasi-concentric pattern on maps which display these values. Integers mark their central values, ranging from -20 to +40. What do the numbers and these symbols mean? Circulation. Vortex-like movement.

**Vorticity**. When a fluid element rotates in a fluid flow, it's called vorticity. The curl of a fluid's velocity vector at a particular point. Mathematically, it's the component of the local rotation vector that's perpendicular to the flow plane.

Weather patterns such as cyclones, anticyclones, and tornadoes are formed by vorticity. These weather phenomena can be caused by vorticity in a fluid flow.

Numerical weather prediction models use vorticity as a key parameter to forecast weather patterns. Storm fronts, low-pressure systems, and tropical cyclones can be predicted by analyzing the vorticity field. Understanding vorticity is crucial for forecasting because it explains fluid flow behavior and helps predict weather patterns.

To visualize vorticity, think about ceiling fans rotating. We plot it in circle radians per hundred thousand seconds using weather map symbols. It's a little longer than a day. Here's a bit about angles.

A complete circle takes a little over six radians. Actually, Pi multiplied by two (2π), or 6.28318530717958 to be a bit more precise.

Vorticity has two definitions. In fluid dynamics, including atmospheric and oceanic science, relative vorticity and absolute vorticity are related concepts. There's a small but important difference between them.

**1. Relative vorticity** - Air above the earth rotates horizontally due to relative vorticity. The relative vorticity of a fluid element is how much it rotates relative to the rotating Earth. Flows around corners, swirls, and eddies. These weather map symbols give that effect.

The curl of the relative velocity vector of a fluid at a particular point is the difference between the fluid's velocity and the Earth's rotation at that point. Relative vorticity is how much rotation you'd see if you were moving with the fluid.

Changes in speed over a horizontal distance, also called wind shear, create the same result. Look at a fast-flowing creek to see this relationship. Near the edges, the water flows slower than in the middle. There are little eddies.

In the shear zone, they look like whirling vortices. They redistribute water and energy.

Also, the earth rotates. By adding the two rotation vectors together, we get...

**2. Absolute vorticity**.

Absolute vorticity takes the Earth's rotation into account. At a particular point, it's the sum of relative vorticity and Earth's rotation rate. Thus, rotation due to Earth's rotation is included in absolute vorticity.

In atmospheric and oceanic sciences, relative vorticity describes the development of weather patterns, like cyclones and anticyclones, while absolute vorticity describes large-scale circulation. So, the difference between relative and absolute vorticity is whether the Earth's rotation is taken into account.

Imagine a pencil sticking straight through the ground beside you. Your latitude determines how fast the earth rotates relative to the pencil.

The earth's circulation wouldn't circle the equator. The earth would spin around your pencil if you were at the North Pole. About seven radians per 100,000 seconds. I'll help you visualize this.

Where is the equator? Northern and southern hemispheres are divided by this circle. If our pencil were inserted into the ground here instead, the earth's rotation would carry it. Do you get it? Since the stick moves with the earth, none of the globe goes around it.

...and the line of longitude and latitude. What if we put our post at the North and South Poles? Everything else moves around while it stays static. Multiply by a fraction.

For points between the equator and the poles, it's the cosine of latitude. That's pretty much the whole world.

We use degrees per second for vorticity symbols. We use radians per second, where 1 radian is about 57°.

Typical values are like earth rotation speed. There's usually less than a ten-thousandth of a radian per second. Here's the velocity formula.

An edge's speed is just the radius times the angular velocity in **radians per second**. Multiply that by the cosine of the latitude for a twirling sphere like the earth. You'll get your rotation speed at any point. About 6400 km is the radius of our planet.

Upward vertical motion in the atmosphere is caused by positive relative vorticity. Clouds and precipitation are associated with positive vorticity, while clear skies and dry conditions are associated with negative vorticity. Cloud, drought, and rain forecasts can be improved by looking at vorticity patterns.

What's the deal with POP - probability of precipitation? Read the answer.

I mentioned the curl of a vector earlier. What does that mean? The relative vorticity is calculated from wind data using this idea. Calculate the difference between:

1) the change in southerly component of wind as you go east and

2) the change in westerly component of wind as you go north.

Meteorologists at Environment Canada try to pinpoint highs and lows. Winds at upper levels stretch or transport them intact based on current vorticity weather map symbols.

**Vorticity shear** and advection are the two mechanisms. The maximum and minimum weather map symbols can appear and disappear quickly. This makes it harder to track them from one map to the next. There's vorticity shear and advection.

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