Atmospheric Pressure

Question 1

What are the equivalents of 29.92

Answer 1

29.92 =

• 1 Atmosphere
• 101.325 kpa (kilopascals)
• 1013.25 mb

Question 2

What is Station Pressure?

Answer 2

The actual weight of the column of air extending upwards from the station to the outer limit of the atmosphere of the observing station

It is what you read without any corrections.

Question 3

Why can’t meteorologists use station pressures when drawing out maps for areas and high and low pressure?

Answer 3

Because as the elevation of the terrain rises from sea level, the pressure changes. So a station at sea level could be in a high pressure area while a station at 2000 ft, in the same high pressure area, would be under half as much pressure.

It would make it kind of impossible to have defined borders around areas of pressure because of all the fluctuations in elevation changing that pressure.

So meteorologists correct for elevation, to bring all areas to Mean Sea Level/Sea Level (MSL)

Question 4

How do meteorologists correct for elevation in pressure altitudes/systems?

Answer 4

Remember that:

• One inch of mercury is lost for every 1000ft of altitude gained
• Air density/pressure is affected by temperature. An imaginary column of warm air between SL and the station will exert less pressure than one of cold air

They take the station pressure and add one inch of mercury x the elevation of the station from sea level. For the temperature correction, they use the average surface temperature over the last 12 hours to create this imaginaaaary column of air

Question 5

If a station is located at 5000ft elevation with a pressure of 25.00 inches, what would it’s MSL be?

Answer 5

30.00”

Question 6

What are Isobars?

Answer 6

If we were to take a plotted chart of sea level pressures, we could then connect the areas of equal pressure

Iso = the same
bar = connecting line

Isobars are usually spaced 4 millibars apart

Question 7

In the picture below, in the high pressure system, the pressure is indicated as 1013mb, which is quite close to the standard (1013.25mb). Why is it marked as high pressure if its almost at standard?

Answer 7

Because on weather maps the pressure regions are relative to one another.

In the picture, the low pressure area is 1001mb, which compared to 1013, is quite a bit lower. This makes the other one higher in comparrison

High and Low pressure systems are not compared against ISA, they are based off of each other

Question 8

What way do the winds move around high and low pressure areas?

Answer 8

Low: Counter-clockwise
High: Clockwise

Winds also move from high pressure to low pressure to tryto equalize. From the top down view this kind of looks like turning gears

Question 9

What are Pressure Gradients on a weather map and why are some closer together or farther apart?

Answer 9

Pressure gradient is how the pressure changes between pressure systems.

When isobars are closer together (strong pressure gradient): the pressure is changing faster, which means the air is moving faster/its more windy

When isobars are farther apart (weak pressure gradient): the pressure is changing less quickly and the winds are less strong

Question 10

Pressure Gradients can be thought of like topographical maps.

Answer 10

Question 11

Why does pressure change horizontally?

Answer 11

High pressure areas are heavy, they sink and spread out along the ground and shove in to lower pressure areas, pushing them up. Which is why low pressure areas ‘rise’.

Conversely, warm are that is rising will tend to create areas of low pressure, while areas of cooler and denser air will sink and compact, creating areas of higher pressure.

Question 12

What is the air doing in and around an area of high pressure?

Answer 12

• Air is sinking in high pressure areas.
• Air is rotating clockwise and flowing outward from the center (only in the norther hemisphere, in the soutern it would be the opposite)

** Areas of High Pressure are also sometimes called anticyclones

Question 13

Some more info about Highs

Answer 13

• High pressure areas can be caused by a drop in temperature. As temp drops, the air density increases and will tend to sink.
• This will tend to push clouds and weather and shit out of the way. So areas of high pressures are associated wit clearer weather/skies
• High pressure systems tend to stick around for several days

**Rememeber that a high pressure system has descending air flowing in a clockwise direction

Question 14

Some more info about Lows

Answer 14

• Lows are areas of rising air that is flowing counter-clockwise and inward towards the center.
• Also called cyclones
• Tend to bring crappy weather, as the air is rising and cooling, creating all kinds of bullshit
• Lows are also assimilating air from the highs, producing stronger winds
• Unlike Highs, Lows tend to move on/blow by after a day or two. Faster in the summer than the winter.

Question 15

What is a trough?

Answer 15

• An elongated area of low pressure
• Indicated by a purple line on a weather map, usually sitting in the V of isobars (see picture below)
• Troughs are likely to bring about a wind shift at the surface as well as clouds and weather, particularly following the passage of the trough
• Fronts always lie in troughs

Question 16

What are ridges?

Answer 16

• Areas of elongated high pressure
• Indicated by a zigzag/sawtooth line (see pic below)
• Associated with fairer weather
• Not always included in weather maps

Question 17

What is a Col?

Answer 17

• A neutral region between two highs and two lows
• Weather in cols tends to be unsettled
• Expect fog in a col in the winter
• Expect showers and thunderstoms in a col in the summer

Question 18

How does temperature effect pressure?

Answer 18

If columns of air were sealed and could not expand, higher temperatures would mean higher pressure (because higher temps, mean faster moving molecules)

Because columns of air are easier to expand up than out, they expand upwards as temperature is increased.

Though a column of warm air has the same number of melocules as a column of cold air, the molecules will be spaced much farther apart, lowering the pressure.

Question 19

Moving the same distance upward or downward through a warm olumn of air will result in a _______ pressure change as compared to colder air. Why is this?

Answer 19

Moving veritcally within a column of warm air will result in a smaller pressure change than in cold air because the molecules are so much father apart than in close air.

In cold air the molecules are all packed in tight, so the difference in pressure in say 1000’ is a lot more than in warm air where the molecules are so far apart.

Question 20

Quiz Question:
According to standard atmosphere the station pressure as measured by two stations in the same reagion but at different altitudes will be:

Answer 20

• Higher as measured by the lower station

A lower altitude station will have a higher pressure given matching conditions to those at the higher altitude station.

The question is saying two stations at different elevations but both having ISA conditions.

The station that is higher is in thinner, therefor less dense, air.

Therefore the pressure measuered at the lower station is higher

Question 21

What is another name for Anticyclones?

Answer 21

Area of High pressure

Question 22

Cyclones/Low pressure systems in the Northern Hemisphere rotate counter-clockwise and the air rises as it flows inward toward the center.

How do cyclones behave in the Southern Hemisphere?

Answer 22

The air still rises and moves toward the center, but it spins clockwise instead of counterclockwise

The coriolis effect only impacts the direction that highs and low spin, but nothing else