When checking the weather what does the pressure tell you?
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High pressure == stable. Low pressure == unstable. When it’s very high pressure, expect a beautiful sunny day. Very low pressure, expect bad weather. Lowest in the center of hurricanes. Now here’s the thing…. high pressure wants to fill low pressure to equalize pressure. This causes winds to move. The rotation of hi vs lo pressure systems are opposite- so the winds from each affect neighboring weather patterns. That’s how they forecast the direction of weather systems.
Here’s more [info](https://scied.ucar.edu/learning-zone/how-weather-works/highs-and-lows-air-pressure)
The atmosphere at any given time can hold a certain amount of water. The largest determining factor aa to *how much* water is the atmospheric pressure.
Higher pressures can hold more water. If a lot of water is IN the atmosphere and all of a sudden the conditions change such that there is more water than it can hold, well… I’m sure you can imagine what happens next.
Now two pockets or bubbles of atmosphere with different conditions, let’s say differing in pressure – they are like oil and water. They do not want to mix. Generally for a high pressure and a low pressure system to equalize something violent tends to happen. The boundary and movement of these pressure systems is how you can predict when and where weather happens.
Usually, the change in pressure is the key (going down=bad weather and going up means decent weather). As a general thing, high pressure means air is descending from high up, so is dry (can’t hold much moisture at the colder temperatures of the higher atmosphere), whereas low pressure means the air is rising, so it usually fairly humid and water will condense as it cools, meaning clouds will form, and rain will happen.
Also, low pressure tends to be the front, the zone of contact between two zones of high pressure. The air coming down and spreading out from a high pressure zone runs into air from another zone doing the same general thing from somewhere else, and there is no room, so some of the air MUST rise (meaning low pressure), and rain tends to happen.
A lot of weather tends to work by large blobs of air moving across the face of the earth. Nastier weather is found where separate blobs come into contact and have a “battle” of sorts for the space. Usually, the battle involves warm air on one side and cool air on the other, and depending on which side is “winning”‘ it will be either a cold front, or a warm front.
Sometimes, like with tropical storms, so much heat energy is coming from the warm ocean below that the air is rising and a big zone of low pressure develops, perhaps becoming a hurricane. These are not being caused by war between two blobs of air (frontal systems), they form because the earth surface is too warm and makes the air rise. They weaken and eventually die when they go over land and the heat from below is less powerful, slowing the upward movement of near-surface air. Also dry out because air from land is dryer than air from ocean.
Just like warm water can hold more salt or sugar in solution than cool water, “high pressure” air can hold more moisture than “low pressure”. High pressure days therefore have lower humidity (more room to hold more liquid) and low pressure days will have higher humidity, and might even rain since the air can’t hold as much moisture in it, with excess falling out of the sky as rain.
Everything tends towards homeostasis. Everything tends to uniformity (mountains erode, water cools etc)
Differential heating of the earths surface by the sun warms the atmosphere. The air becomes warmer near the equator than at the poles. This energy gained flows towards the poles. It churns and mixes both horizontally and vertically. It’s messy and has lots of things in the way to affect the flow of energy. Continents, mountains, cold and warm ocean currents.
Warmed air expands and rises. A taller column of air will have a higher weight at the earth’s surface (“pressure”). As you might expect this taller column of air wants to even out towards areas of lower pressure. It happens on a large scale (look up Hadley cell and the trade winds) and on small scales ( look up Foehn winds)
Generally speaking higher pressure means warmer air. Lower pressure is associated with cooling air where water vapor precipitates out as cloud, rain, snow etc.
As an old nurse this is what it tells me – people get migraines, flares of joint pain and labor and delivery nurses dread their upcoming shift.
High pressure is caused by air cooling and sinking towards the ground, and low pressure is caused by air warming and rising up into the atmosphere. Low pressure means that air masses will have to move in to replace the rising air, meaning wind and changing weather. High pressure means that the air is settling in and change is not likely.
The exact number for the air pressure doesn’t mean much, but relative pressure (i.e., high pressure and low pressure systems) tells us a lot.
Low pressure systems tend to be warmer and moisture air, so storms tend to form. High pressure systems tend to be cool and dry, so they push storms away.
A drop in pressure means rain is on the way, and a rise in pressure means that nice weather is on the way.
There’s a lot more going on here, but that’s the basics.
I knew a navy seal who had a fancy watch with a bunch of readouts on it. I asked him, other than looking cool, what use they were. He said that knowing the pressure let him know what the weather they would be operating in would be like.
Pressure? Nothing really.
CHANGE IN PRESSURE? now you got something.
It’s going to tell you if a warm or cold front is moving in (which usually brings storms with it) depending on if pressure rises or falls.
So that’s why a lot of analog barometers have a second needle you adjust by hand. You set this to the current pressure and you can easily see if it raises or lowers.
Digital barometers usually have a chart, which can trend pressure change over time
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