INTERPRETING ASKEW-T LOG-P

AEROLOGICAL DIAGRAM

• Locate pressure level (and matching ICAO Height)

• 400 m ASL

• 970 hPa

• Read off T and Tdp from that level

• T = 32°C

• Tdp = 7°C

• Follow Tdp upwards along line of constant mixing ratio.

• Mixing ratio = 6.5 g/kg

• Follow T upwards along dry adiabat.

• Potential temperature = 307 K= 34°C

• Lines cross at Lifting Condensation Level = cloud base.

• LCL = 670 hPa

• LCL = 3400 m (ICAO)

• Dry adiabat is cooler than Environmental Lapse Rate (right-hand red line) at all levels.

• Air is stable

• Between the surface and 900hPa the reversed ELR shows the mixing layer.

• As the day heats up this will move to the right, to the grey line.

• Now…Let’s add heat to the surface air with a fire (no additional moisture).

• T = 50°C

• Potential temperature = 323 K

• LCL = 540 hPa

• LCL = 5000 m (ICAO)

• T @ LCL= -2°C

• Dry adiabat is warmer than ELR – air is unstable.

• Above LCL air rises along Saturated Adiabat.

• Dynamic inversion when saturated adiabat and ELR cross.

• P = 250 hPa

• Height = 10500 m (ICAO)

• T = -42°C

• Winds at surface

• 35 kts (60 km/hr) from N

• Winds at LCL

• 30 kts (50 km/hr) from NW

• Winds at dynamic (plume) inversion.

• 55 kts(100 km/hr)from W

• So under these conditions a hot backburn would move to the south, but its plume, rising to 10km height, would increasingly rotate to the left as it rose.

• Additionally, pyro-cumulus clouds would form within the plume at 5000m.

• We have no information on fuels, and so cannot assess how much cloud.

• CALCULATING HAINES INDEX

• T850=27°C

• T700=12°C

• DP850=-1°C

• Stability term = 3 (range of 15°)

• Moisture term = 3 (range of 28°)

• Index = 3+3 = 6

• CALCULATING CONTINUOUS HAINES INDEX (CH)

• CA = (T850-T700)/2-2

• CA = 5.5

• CB = (T850-DP850)/3-1

• If(CB > 9) then CB = 9• If(CB > 5) then CB = 5+(CB-5)/2• CH=CA+CB• CB=6.7• CH=12.2• 95% HI = 8.5

• CALCULATING FUEL MOISTURE CONTENT

• FMI = 21-(2*T-Tdp)/4

• FMI=7%• FMC=7*FMI/DF• If DF = 10,

FMC = 5%