Pressure & Density

What is the ideal gas concept?

- Theory to explain how the atmosphere works

- Atmosphere is homogenous perfectly compressible gaseous volume

- We can make assumptions about how gases behave (all the same way regardless of composition)

What are the 6 assumptions behind the ideal gas concept?

1. No or entirely negligible attractive intermolecular forces between molecules

2. Volume of space occupied by molecules is negligible, relative to container volume

3. Gas molecules are in constant motion

4. Molecules are rigid spheres

5. Pressure from collisions between molecules (and with the exterior)

6. No kinetic energy lost (from molecular collisions)

Why is it assumed no kinetic energy is lost (from molecular collisions)?

- Volume would cool over time

- Energy loss

- May change state

Why is it assumed pressure is from collisions between molecules (and with the exterior)?

- Protects the volume

- Allows it to keep its shape

Why is it assumed molecules are rigid spheres?

Prevents molecules sticking together

Why is it assumed gas molecules are in constant motion?

- Wouldn't be a gas then

- They slow down

- Colder

- Now a solid

Why is it assumed the volume of space occupied by molecules is negligible, relative to container volume?

- For expansion and contraction willingly

- Room to do so

Why is it assumed there no or entirely negligible attractive intermolecular forces between molecules?

- Expansion and contraction can occur

- Easier to change volume

- Molecules can move at will and more occupy the space

What is boyle's law?

- Constant temperature

- Volume inversely proportional to pressure

What is charles' law?

- Constant pressure

- Volume proportional to temperature

What is dalton's law?

Total pressure of a gas is the sum of its partial pressures

How are boyle's and charles' laws used to explain the behaviour of the atmosphere?

Applied separately, but they work together

Example

- Volume of air at surface warmed through conduction and convection

- Temperature changes

- Apply charles' law

- Constant pressure

- Volume increases

- New temperature reached

- Apply boyle's law

- Pressure reduced as volume is increased

How are the partial pressure of gases in a volume calculated?

Assume atmospheric conditions so 78% nitrogen, 21% oxygen

What is atmospheric pressure?

Weight of a column of air above a given horizontal area

What are the three measures of pressure and where are they used?

- Hectopascals (NZ/AUS)

- Inches of mercury (US)

- Pounds per square inch

What are the conversions for units of pressure?

14.7 PSI = 29.92 Hg = 1013.25 mb/hPa

What causes the vertical variations of pressure?

Concentration and compression of the atmosphere near the surface

What causes horizontal variations of pressure?

Advection of large amounts of air relative to given points

What causes diurnal variations of pressure?

- Natural atmospheric cycles varying with latitude

- Two minima at 0400/1600

- Two maxima at 1000/2200

- Small variations (equator has greater than poles)

- Transition period as temperature changes, changing pressure

- High altitude atmospheric motion from mountain ranges, etc slowing air, changing its movement

- Also gravitational effect of sun and moon

What is the effect of density on pressure?

Increasing density increases pressure

What is the effect of water vapour on pressure?

Inversely proportional (water vapour has lower molecular weight)

What is the effect of latitude on pressure?

Increasing latitude increases pressure (colder, more dense, greater pressure)

What is atmospheric density?

Mass per unit volume

What is the equation for atmospheric density?

⍴ = 0.3484 x P/T

*T in Kelvin

What is the impact of temperature on atmospheric density?

Density inversely proportional to temperature

What is the impact of pressure on atmospheric density?

Density proportional to pressure

What is the impact of water vapour on atmospheric density?

Density inversely proportional to moisture content

How does density change with increasing altitude?

Reduces, though temperature decreases, the decrease in pressure has a greater effect

What is the pressure lapse rate equation?

Pressure lapse rate = 96T/P

*T in Kelvin

What is the relationship between temperature and pressure lapse rates?

Lapse rates decrease in warm air and increase in cold air (warm air has slower PLR, greater distance to lose 1hPa of pressure)

What is the pressure lapse rate from 0 to 5,000ft?

30ft/hPa (34hPa/1,000ft)

What is the pressure lapse rate from 5,000 to 10,000ft?

34ft/hPa (29hPa/1,000ft)

What is the pressure lapse rate from 10,000 to 20,000ft?

43ft/hPa (23hPa/1,000ft)

What is the pressure lapse rate from 20,000 to 40,000ft?

72ft/hPa (14hPa/1,000ft)

What is the effect of flying from warm to cold air at a constant altitude?

Assuming surface pressure remains constant, pressure will decrease

How do patterns of convergence and divergence affect pressure and meteorological conditions?

- Dual atmospheric processes

- From movement and speed of movement relative to points

- Always in opposition

- Pressure changes when it is unbalanced

- Net outflow (more upper level divergence than surface convergence, low)

or

- Net inflow (more upper level convergence than surface divergence, high)

What is speed convergence?

Air enters a region faster than it leaves

What is speed divergence?

Air leaves a region faster than it enters

What is directional convergence?

Air moves towards a certain point

What is directional divergence?

Air moves away from a certain point

What is mass convergence/divergence?

Speed and directional occur simultaneously

What is confluence?

Opposing patterns of speed and directional occur

What are the meteorological conditions associated with low pressure systems?

- Air spins cyclonically

- Naturally turns towards poles

- Surface to upper troposphere

- Net outflow of air

- Unstable air

- Cumulus type cloud and rain

What are the meteorological conditions associated with high pressure systems?

- Spins anti-cyclonically

- Upper troposphere to surface

- Net inflow

- Mass subsidence

- Poor visibility and low level layered clouds

How do highs and lows form?

- Net inflow in upper troposphere

- Surface pressure rise

- Upper-level ridges, troughs and jet streams create absolute vorticity

- Changes to upper-level convergence and divergence

- Updrafts and downdrafts form, adding/removing air to column

- Highs/lows weaken when not enough difference of inflow and outflow

- Friction spirals air into low

- Influenced by jet stream complex (positions of convergence and divergence)

What is the effect of temperature on pressure?

At temperature increases, pressure decreases but volume also must change (if it doesn't, different relationship)