INTERPRETING A SKEW-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%