SymbolMeaning 2. What is barometric pressure? What does it mean? 3. What are isobars? What do they show? 4. Why is forecasting important?

 Forecasting: predicting weather before it happens ◦ Sometimes it works, sometimes it doesn’t ◦ It’s not an exact science!

Before the 1950s, all forecasting was based on observations – People would look for changes in: Air pressure Wind speed/direction Temperatures Current weather conditions

Forecast models from computers are now used to predict the weather. Each forecast model will create several different outcomes – Humans are still required to pick the best possible forecast model – The atmosphere is unstable and constantly changing, which means that forecasts become less accurate the farther out they are made

How to draw/read weather symbols First, look at the symbol in the middle and match up weather conditions Look to see if there’s a line that means wind – Figure out the speed of the wind and the direction it’s blowing

SymbolMeaning Clear skies Cloud cover 25% Cloud cover 50% (partly cloudy) Cloud Cover 75% Cloudy Wind (will be different speeds and directions) SymbolMeaning Thunderstorms Snow Fog Freezing rain Rain Hurricane T S f F R

Barometric pressure (air pressure): The amount of force the air is pressing down on the Earth. – About 15lbs per square inch at sea level – Measured in inches of mercury (mb)

 Barometer- The instrument that measures air pressure

 Used for forecasting  High air pressure, or air pressure increasing— nice, calm weather  Low air pressure, or air pressure decreasing— weather changing, storms more likely

 Isobars—Lines on a weather map that connect areas of equal pressure ◦ Areas that are connected on the line have similar weather *Weather in the USA moves from west to east

 Isobars show differences in air pressure. Air flows from areas of high pressure to areas of low pressure. ◦ The more tightly packed the isobars the faster the air moves across them.  Just as water flows faster downhill where there is a steep slope, air moves faster "downhill" (down pressure) faster where there is a steep slope. Closely spaced isobars means a steep slope.  Therefore, tightly packed isobars would mean strong winds and widely spaced ones would mean slow winds

How to draw isobaric maps 1. Find all the areas on a map that have similar numbers – Generally go by increases of 4 mb 2. Draw a line around the similar numbers 3. Lines cannot cross or touch 4. Find areas of high and low pressure H =high pressure (biggest numbers) L =low pressure (lowest numbers)

(10)00 97, 98, 99, 00 (10)04 01, 02, 03, 04 (10)08 05, 06, 07, 08, (10)12 09, 10, 11, 12 (10)16 13, 14, 15, 16 (10)20 17, 18, 19, 20 (10)24 21, 22, 23, 24

 Forecasting is important for multiple reasons. ◦ Weather warnings are used to protect life and property. ◦ Forecasts based on temperature and precipitation are important to farmers, and may cause food prices to increase/decrease

 Forecasting is important for multiple reasons. ◦ Temperature forecasts are used by electric companies to estimate demand over coming days. ◦ People use weather forecasts to determine what to wear on a given day. ◦ Forecasts can be used to plan activities around outdoor events

How to Forecast Weather in the USA generally moves from west to east – Take a look at the different fronts and how they’ve been interacting in the past few days – Pay attention to the barometer Falling pressure: bad weather Rising pressure: good weather – Make predictions based on the above information, and the time of year