AIR PRESSURE

AIR PRESSURE is an important part of the weather. Air pressure happens because air molecules in the atmosphere have mass and weight. We know Air takes up space and it has weight We weighed a balloon filled with air to find it weighed more than an empty balloon. We pushed air down into water using a cup to “trap” it – it takes up space!

HIGH PRESSURE An air mass that has A LOT of air molecules – dense and close together – is a HIGH PRESSURE SYSTEM. It presses down with more force because the amount of and thickness of air molecules weigh more. Pressing down – HIGH PRESSURE LOW PRESSURE An air mass that has fewer air molecules – spread out – is a LOW PRESSURE SYSTEM. Fewer molecules, less dense, spread out – less weight, not pressing down – LOW PRESSURE

HIGH PRESSURE  High Pressure Systems can be 1000s of miles across and cover a huge area of land - covering more land than Low Pressure Systems.  Air near the center of a High Pressure System moves outward towards areas of low pressure. “HIGH TO LOW” – high pressure moves towards low pressure  CLOCK-WISE - In the northern hemisphere the air in a high pressure system moves in a clockwise direction.

 “HIGH & DRY” -- High Pressure Systems contain CLEAR Skies and DRY Skies (Fair Weather). As the air is moving out towards lower pressure, the air at the top of the system sinks from above to replace it. The air gets warmer as it sinks and vaporizes the clouds. The sky tends to be clear in a high pressure system because of this. High pressure systems typically mean fair weather.

 LIGHT WINDS - Winds are usually light and calm in a high pressure system because the air pressure varies only a little over the expanse.

 High pressure systems are shown on a weather map with a blue H.

LOW PRESSURE  Not a lot of air molecules in Low Pressure Systems – not as much as there are in High Pressure Systems.  Low Pressure Systems tend to be SMALLER than High Pressure Systems.  Form along a boundary between air masses in contrast (dry/moist or cold/warm).  Air flows TOWARDS the CENTER of the Low Pressure System – from High Pressure  INWARD SPIRAL – in a counter-clockwise direction

 “LOW & WET” Rising air cools and the water vapor in the air condenses to form CLOUDS. Those clouds usually result in RAIN or STORMS  STRONG WINDS - since the pressure difference in a low pressure system is typically greater than in a high pressure system, the wind tends to be stronger.

 Low pressure systems are shown on a weather map with a red L.

Remember:

HIGH moves to LOW /// HIGH to LOW --- this causes WIND!

HIGH & DRY - High Pressure brings FAIR WEATHER, CLEAR SKIES, LIGHT WINDS

LOW & WET - Low Pressure brings CLOUDS, PRECIPITATION (Rain,Snow), WINDY

Increasing pressure means HIGH Pressure System is coming!

Decreasing pressure means LOW Pressure System is coming!

Look at the arrows below: Air moves OUT Air moves IN and UP Clockwise Counter-Clockwise HIGH is on the left LOW is on the right

LOW Pressure Systems very often bring EXTREME weather! On land – low pressure forced by a large drop in pressure will cause a TORNADO! At sea – over very warm waters, low pressure will grow into HURRICANES / CYCLONES!

SOURCE: https://www.ck12.org/user%3Aynjpyw4ubwfjy2fyzwxsaubiy3nlbwfpbc5vcmc./book/Buncombe-County-Schools-5th-Grade-Science-Flexbook/r198/section/5.6/