# What incites meteorologists to forecast hourly weather?

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When do we forecast hourly weather? Patterns can change quickly and require meteorologists to make sudden changes to their predictions at times.

How can weather changes and variations occur so unexpectedly? Sudden changes can make conditions change dramatically between hourly observations. This is it become interesting to forecast hourly weather. It can be fun to see systems moving in on an animated radar display and anticipate their arrival.

Then you can relate the observed conditions with your expectations.

See how much the weather can change in this one-minute video:

## Deformation

In the formula below, the size or mass of the air parcel does not change. Only the shape. When an elastic object changes shape without altering its size or colour, it deforms.

In engineering, they use words like malleability, the ability to be squished, and ductility, the ability to be stretched. These things happen to soft metals, doughy substances and masses of fluids such as air and clouds. And they're all forms of deformation.

In meteorology, deformation primarily means stretching and reshaping of air blobs due to applied stresses. It's reversible, so we can consider it elastic deformation. One of the simplest mathematics and physics formulae for this type of change in shapes goes like this:

Deformation = du/dx – dv/dy

...where we want d to mean partial derivative. The letters u and v are components of velocity. Deformation looks at the differences in changes in each direction's velocity component as you move along that same direction.

What does that mean? For example, if you face east, is the wind at your back? Does it blow at your back stronger as you move forward? Now, ask the same questions while facing north.

As a result of this process, our maps and recorders would then show tightly packed temperature or humidity gradients. These vectors lead to sudden transformations in the air that can surprise us. And might need us to forecast hourly weather.

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Then one of the forecaster's science jobs is to amend our projections frequently and even predict hourly weather.

Want one of those jobs? Check the weather forecast jobs on the right.

## Where is deformation found?

You can find areas of deformation on weather maps. A trained observer would find one near a saddle point or col in the vorticity field, or the streamline field if you prefer. Like this.

What is a col? Think of this position as the low point between two mountains going one way. At the same time, the col would be the high point between two valleys on your left and right. Look at a two-dimensional plan plot, such as a map. The col would exist between a pair of areas marked high and two marked low.

A simple example. Maybe the highs would lie directly north and south of our middle point. Meanwhile the lows show up to the east and west to make the col.

## How does deformation lead us to forecast hourly weather?

We have two types of deformation zones leading us to suddenly adjust our weather forecasts.

1) Longitudinal – stretched along the wind, and
2) Transverse – across the wind and advected (transported) by the wind.

A primary place Northern Hemisphere analysts look on the map for these is on the north side of cyclones. Areas of high vorticity and low pressure.

When meteorologists have to forecast hourly weather, we use this principle because it often shows us where the edge of cloud is. We can compare with satellite pics, and our knowledge of the meteorological processes behind these cloud formations.

Go back from Forecast Hourly Weather to the Forecast Map Weather web page.

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