Soaring Guidance
Issued by NWS Denver/Boulder, CO
Issued by NWS Denver/Boulder, CO
808
UXUS97 KBOU 181233
SRGBOU
Soaring Forecast
National Weather Service Denver/Boulder, Colorado
0632 MDT Friday, September 18, 2026
This forecast is for Friday, September 18, 2026:
If the trigger temperature of 76.9 F/24.9 C is reached...then
Thermal Soaring Index....................... Good
Maximum rate of lift........................ 590 ft/min (3.0 m/s)
Maximum height of thermals.................. 12859 ft MSL (7563 ft AGL)
Forecast maximum temperature................... 84.0 F/29.3 C
Time of trigger temperature.................... 1100 MDT
Time of overdevelopment........................ 1500 MDT
Middle/high clouds during soaring window....... Broken/overcast thin middle
Surface winds during soaring window............ 20 mph or less
Height of the -3 thermal index................. 10350 ft MSL (5054 ft AGL)
Thermal soaring outlook for Saturday 09/19..... Good
Wave Soaring Index............................. Poor
Wave Soaring Index trend (to 1800 MDT).........
Height of stable layer (12-18K ft MSL)......... None
Weak PVA/NVA (through 1800 MDT)................ Neither
Potential height of wave....................... 15144 ft MSL (9848 ft AGL)
Wave soaring outlook for Saturday 09/19........ Poor
Remarks...
Sunrise/Sunset.................... 06:43:45 / 19:03:20 MDT
Total possible sunshine........... 12 hr 19 min 35 sec (739 min 35 sec)
Altitude of sun at 12:53:32 MDT... 51.84 degrees
Upper air data from numerical model forecast valid on 09/18/2026 at 0600 MDT
Freezing level.................. 15987 ft MSL (10691 ft AGL)
Convective condensation level... 12413 ft MSL (7117 ft AGL)
Lifted condensation level....... 12693 ft MSL (7397 ft AGL)
Lifted index.................... -4.0
K index......................... +28.5
Height Temperature Wind Wind Spd Lapse Rate ConvectionT Thermal Lift Rate
ft MSL deg C deg F Dir kt m/s C/km F/kft deg C deg F Index fpm m/s
--------------------------------------------------------------------------------
50000 -65.2 -85.4 215 25 13 2.3 1.3 M M M M M
45000 -59.9 -75.8 215 37 19 5.3 2.9 M M M M M
40000 -51.6 -60.9 225 54 28 6.9 3.8 M M M M M
38000 -47.5 -53.5 220 53 27 6.8 3.8 M M M M M
36000 -43.4 -46.1 215 53 27 7.1 3.9 M M M M M
34000 -38.4 -37.1 215 51 26 8.5 4.6 M M M M M
32000 -32.6 -26.7 210 48 24 9.0 4.9 M M M M M
30000 -28.1 -18.6 210 41 21 7.7 4.2 48.2 118.7 14.6 M M
29000 -25.8 -14.4 210 38 19 7.7 4.2 47.5 117.4 14.2 M M
28000 -23.6 -10.5 210 35 18 6.4 3.5 46.6 115.9 13.7 M M
27000 -21.7 -7.1 215 33 17 6.5 3.6 45.3 113.5 12.8 M M
26000 -19.7 -3.5 220 31 16 6.5 3.6 44.1 111.4 11.9 M M
25000 -17.8 -0.0 225 30 15 6.4 3.5 43.0 109.5 11.2 M M
24000 -16.0 3.2 225 26 14 6.2 3.4 41.6 106.9 10.1 M M
23000 -14.1 6.6 230 23 12 6.2 3.4 40.3 104.5 9.1 M M
22000 -12.3 9.9 230 20 10 7.0 3.8 39.1 102.4 8.2 M M
21000 -10.0 14.0 240 16 8 7.9 4.3 38.3 101.0 7.6 M M
20000 -7.7 18.1 255 12 6 8.0 4.4 37.7 99.9 7.0 M M
19500 -6.6 20.1 265 11 6 8.2 4.5 37.4 99.4 6.8 M M
19000 -5.7 21.7 275 9 5 3.8 2.1 36.8 98.2 6.2 M M
18500 -5.1 22.8 285 7 4 4.3 2.4 35.8 96.5 5.4 M M
18000 -4.5 23.9 305 6 3 5.5 3.0 34.9 94.8 4.7 M M
17500 -3.5 25.7 310 5 2 6.6 3.6 34.4 93.9 4.2 M M
17000 -2.5 27.5 315 4 2 6.6 3.6 33.8 92.9 3.8 M M
16500 -1.4 29.5 310 3 2 7.7 4.2 33.4 92.1 3.4 M M
16000 0.2 32.4 285 3 2 7.6 4.2 32.9 91.1 2.9 M M
15500 1.3 34.3 270 4 2 7.4 4.1 32.5 90.4 2.5 M M
15000 2.3 36.1 260 4 2 7.5 4.1 32.1 89.8 2.1 M M
14500 3.3 37.9 260 4 2 7.0 3.8 31.7 89.0 1.6 M M
14000 4.3 39.7 265 4 2 7.1 3.9 31.2 88.2 1.2 M M
13500 5.3 41.5 270 3 2 6.4 3.5 30.8 87.4 0.7 M M
13000 6.2 43.2 280 3 1 6.2 3.4 30.2 86.3 0.1 M M
12500 7.0 44.6 290 2 1 6.5 3.6 29.6 85.3 -0.5 555 2.8
12000 7.9 46.2 290 2 1 6.8 3.7 29.1 84.4 -1.1 513 2.6
11500 8.9 48.0 290 2 1 6.7 3.7 28.6 83.6 -1.6 470 2.4
11000 9.8 49.6 290 1 1 6.5 3.6 28.2 82.7 -2.2 428 2.2
10500 10.7 51.3 290 1 1 3.4 1.9 27.7 81.8 -2.7 386 2.0
10250 10.8 51.4 285 1 1 4.3 2.4 27.1 80.7 -3.3 375 1.9
10000 11.4 52.5 265 1 1 10.0 5.5 27.0 80.5 -3.5 350 1.8
9750 12.1 53.8 245 1 1 10.1 5.5 27.0 80.5 -3.5 322 1.6
9500 12.7 54.9 230 1 1 8.0 4.4 27.0 80.5 -3.6 295 1.5
9250 13.2 55.8 220 1 1 5.6 3.0 26.7 80.0 -3.9 274 1.4
9000 13.7 56.7 210 2 1 7.8 4.3 26.4 79.6 -4.1 252 1.3
8750 14.3 57.7 200 2 1 9.1 5.0 26.4 79.4 -4.2 225 1.1
8500 14.9 58.8 195 2 1 7.9 4.3 26.3 79.3 -4.3 199 1.0
8250 15.5 59.9 195 3 1 7.2 3.9 26.1 78.9 -4.5 175 0.9
8000 16.1 61.0 190 3 1 7.5 4.1 25.9 78.6 -4.7 150 0.8
7750 16.6 61.9 190 3 2 7.3 4.0 25.7 78.3 -4.9 126 0.6
7500 17.1 62.8 195 4 2 6.5 3.5 25.5 77.9 -5.1 103 0.5
7250 17.6 63.7 200 4 2 5.9 3.3 25.2 77.4 -5.4 82 0.4
7000 18.1 64.6 205 4 2 6.5 3.6 25.0 76.9 -5.6 61 0.3
6750 18.5 65.3 210 4 2 5.8 3.2 24.7 76.5 -5.9 39 0.2
6500 18.9 66.0 220 5 2 4.4 2.4 24.3 75.8 -6.3 22 0.1
6250 19.2 66.6 225 5 3 4.7 2.6 23.9 75.1 -6.8 6 0.0
6000 19.5 67.1 230 6 3 3.2 1.7 23.5 74.4 -7.2 M M
5750 19.5 67.1 240 5 3 -1.0 -0.5 22.9 73.2 -7.9 M M
5500 19.2 66.6 240 4 2 -3.2 -1.7 22.0 71.5 -8.9 M M
* * * * * * Numerical weather prediction model forecast data valid * * * * * *
09/18/2026 at 0900 MDT | 09/18/2026 at 1200 MDT
|
CAPE... +282.3 LI... -2.6 | CAPE... +405.5 LI... -2.2
CINH... -94.2 K Index... +30.2 | CINH... -19.5 K Index... +31.7
|
Height Temperature Wnd WndSpd Lapse Rate | Temperature Wnd WndSpd Lapse Rate
ft MSL deg C deg F Dir kt m/s C/km F/kft | deg C deg F Dir kt m/s C/km F/kft
--------------------------------------------------------------------------------
50000 -65.0 -85.0 215 25 13 3.1 1.7 | -64.3 -83.7 210 31 16 2.3 1.2
45000 -59.1 -74.4 215 37 19 4.8 2.6 | -59.3 -74.7 210 43 22 4.9 2.7
40000 -51.5 -60.7 225 52 27 6.8 3.8 | -51.6 -60.9 225 52 27 6.7 3.7
38000 -47.4 -53.3 220 53 27 6.8 3.7 | -47.6 -53.7 215 53 27 6.7 3.6
36000 -43.3 -45.9 220 54 28 7.2 3.9 | -43.6 -46.5 210 54 28 7.1 3.9
34000 -38.1 -36.6 215 50 26 8.8 4.8 | -38.2 -36.8 210 53 27 9.2 5.0
32000 -32.8 -27.0 215 46 24 8.7 4.8 | -32.7 -26.9 210 51 26 9.2 5.1
30000 -28.1 -18.6 215 41 21 6.8 3.7 | -28.2 -18.8 215 48 24 6.2 3.4
29000 -26.1 -15.0 215 39 20 6.8 3.7 | -26.4 -15.5 220 46 24 6.2 3.4
28000 -24.1 -11.4 215 37 19 6.5 3.5 | -24.5 -12.1 225 44 23 7.0 3.8
27000 -22.2 -8.0 215 33 17 6.5 3.6 | -22.4 -8.3 220 40 20 6.9 3.8
26000 -20.3 -4.5 220 30 15 6.5 3.6 | -20.3 -4.5 220 35 18 6.9 3.8
25000 -18.3 -0.9 225 27 14 6.5 3.6 | -18.3 -0.9 220 31 16 6.9 3.8
24000 -16.4 2.5 225 23 12 6.4 3.5 | -16.2 2.8 220 28 14 7.0 3.8
23000 -14.5 5.9 225 20 11 6.4 3.5 | -14.1 6.6 215 25 13 7.0 3.8
22000 -12.6 9.3 225 17 9 6.8 3.7 | -12.1 10.2 215 22 12 6.8 3.8
21000 -10.5 13.1 220 14 7 7.2 4.0 | -10.1 13.8 210 20 10 6.7 3.7
20000 -8.5 16.7 220 10 5 7.4 4.1 | -8.2 17.2 210 17 9 6.8 3.7
19500 -7.5 18.5 215 8 4 7.6 4.2 | -7.2 19.0 210 16 8 6.8 3.7
19000 -6.6 20.1 210 6 3 4.5 2.5 | -6.3 20.7 210 15 8 6.0 3.3
18500 -5.9 21.4 200 5 2 4.9 2.7 | -5.5 22.1 210 13 7 6.2 3.4
18000 -5.1 22.8 180 3 2 6.3 3.4 | -4.7 23.5 210 12 6 6.3 3.5
17500 -4.0 24.8 180 4 2 7.4 4.1 | -3.8 25.2 210 11 6 6.4 3.5
17000 -2.9 26.8 185 4 2 7.5 4.1 | -2.9 26.8 205 10 5 6.4 3.5
16500 -1.8 28.8 195 4 2 7.4 4.1 | -1.9 28.6 205 10 5 6.8 3.7
16000 -0.8 30.6 205 3 2 7.4 4.1 | -1.0 30.2 205 9 5 6.8 3.7
15500 0.3 32.5 210 3 2 7.4 4.1 | -0.0 32.0 205 8 4 6.0 3.3
15000 1.8 35.2 225 3 1 7.6 4.2 | 1.2 34.2 205 6 3 5.6 3.1
14500 2.9 37.2 235 2 1 6.8 3.7 | 2.1 35.8 210 5 2 6.4 3.5
14000 3.8 38.8 260 1 1 6.9 3.8 | 3.0 37.4 215 3 2 6.4 3.5
13500 4.8 40.6 300 1 1 6.7 3.7 | 3.9 39.0 230 2 1 7.0 3.9
13000 5.7 42.3 330 2 1 6.7 3.7 | 4.9 40.8 270 1 1 7.2 4.0
12500 6.7 44.1 340 2 1 7.2 3.9 | 5.9 42.6 315 1 1 7.2 3.9
12000 7.7 45.9 340 3 1 7.4 4.1 | 6.9 44.4 330 2 1 7.1 3.9
11500 8.8 47.8 340 3 2 7.2 3.9 | 7.9 46.2 335 2 1 7.2 3.9
11000 9.7 49.5 335 4 2 6.7 3.7 | 9.0 48.2 340 3 1 7.0 3.9
10500 10.7 51.3 335 4 2 3.2 1.7 | 10.1 50.2 350 3 2 3.1 1.7
10250 10.8 51.4 335 4 2 4.0 2.2 | 10.2 50.4 350 3 2 4.5 2.5
10000 11.4 52.5 330 4 2 10.4 5.7 | 10.9 51.6 350 3 2 11.3 6.2
9750 12.1 53.8 330 4 2 10.2 5.6 | 11.6 52.9 350 3 2 10.6 5.8
9500 12.8 55.0 330 4 2 7.4 4.1 | 12.4 54.3 350 4 2 8.6 4.7
9250 13.2 55.8 330 4 2 4.7 2.6 | 12.9 55.2 350 3 2 6.4 3.5
9000 13.6 56.5 325 4 2 7.5 4.1 | 13.4 56.1 345 3 2 8.7 4.8
8750 14.2 57.6 325 4 2 8.8 4.8 | 14.1 57.4 345 3 1 9.7 5.3
8500 14.8 58.6 325 3 2 7.5 4.1 | 14.8 58.6 340 2 1 9.2 5.0
8250 15.4 59.7 320 3 2 6.9 3.8 | 15.5 59.9 340 2 1 8.8 4.8
8000 15.9 60.6 320 3 1 7.6 4.2 | 16.1 61.0 340 2 1 9.8 5.4
7750 16.5 61.7 310 3 1 8.0 4.4 | 16.9 62.4 340 2 1 10.6 5.8
7500 17.1 62.8 305 3 1 7.2 4.0 | 17.6 63.7 335 2 1 9.8 5.4
7250 17.6 63.7 300 3 2 7.0 3.8 | 18.4 65.1 335 2 1 9.3 5.1
7000 18.1 64.6 295 3 2 6.8 3.7 | 19.1 66.4 340 2 1 10.3 5.6
6750 18.6 65.5 290 4 2 5.8 3.2 | 19.9 67.8 335 2 1 10.9 6.0
6500 19.0 66.2 295 4 2 5.4 2.9 | 20.7 69.3 335 2 1 10.3 5.6
6250 19.4 66.9 300 4 2 5.4 3.0 | 21.4 70.5 335 2 1 10.2 5.6
6000 19.7 67.5 305 5 2 5.7 3.1 | 22.2 72.0 335 2 1 10.1 5.5
5750 20.2 68.4 305 4 2 8.5 4.7 | 23.1 73.6 335 2 1 15.5 8.5
5500 20.9 69.6 305 2 1 12.2 6.7 | 24.6 76.3 340 2 1 24.9 13.7
09/18/2026 at 1500 MDT | 09/18/2026 at 1800 MDT
|
CAPE... +470.6 LI... -2.4 | CAPE... +212.0 LI... -1.6
CINH... -6.0 K Index... +31.7 | CINH... -41.7 K Index... +30.4
|
Height Temperature Wnd WndSpd Lapse Rate | Temperature Wnd WndSpd Lapse Rate
ft MSL deg C deg F Dir kt m/s C/km F/kft | deg C deg F Dir kt m/s C/km F/kft
--------------------------------------------------------------------------------
50000 -64.4 -83.9 215 33 17 1.6 0.9 | -64.5 -84.1 215 31 16 1.4 0.7
45000 -59.7 -75.5 215 47 24 5.5 3.0 | -60.7 -77.3 215 46 24 5.4 2.9
40000 -51.2 -60.2 225 53 27 6.7 3.7 | -51.8 -61.2 220 52 27 7.3 4.0
38000 -47.2 -53.0 215 53 27 6.7 3.7 | -47.5 -53.5 215 54 28 7.3 4.0
36000 -43.2 -45.8 210 55 28 6.9 3.8 | -43.1 -45.6 210 57 29 7.4 4.0
34000 -38.2 -36.8 210 53 27 8.6 4.7 | -38.2 -36.8 210 55 28 8.2 4.5
32000 -33.0 -27.4 210 51 26 8.5 4.7 | -33.3 -27.9 210 52 27 8.2 4.5
30000 -28.1 -18.6 215 46 24 7.1 3.9 | -28.3 -18.9 215 43 22 7.3 4.0
29000 -25.9 -14.6 220 44 22 7.1 3.9 | -26.1 -15.0 215 38 20 7.3 4.0
28000 -23.8 -10.8 220 41 21 6.4 3.5 | -23.9 -11.0 215 34 17 7.0 3.8
27000 -21.9 -7.4 215 37 19 6.5 3.6 | -21.9 -7.4 215 29 15 7.1 3.9
26000 -20.0 -4.0 210 34 17 6.5 3.6 | -19.8 -3.6 210 25 13 7.3 4.0
25000 -18.1 -0.6 205 30 16 6.9 3.8 | -17.7 0.1 205 20 10 7.2 3.9
24000 -15.9 3.4 205 29 15 7.6 4.2 | -16.0 3.2 210 21 11 6.0 3.3
23000 -13.6 7.5 210 28 14 7.6 4.2 | -14.2 6.4 210 22 11 6.0 3.3
22000 -11.4 11.5 210 27 14 7.0 3.8 | -12.5 9.5 215 23 12 6.2 3.4
21000 -9.7 14.5 210 24 12 6.1 3.3 | -10.6 12.9 220 23 12 6.6 3.6
20000 -8.0 17.6 210 21 11 6.1 3.4 | -8.7 16.3 230 23 12 6.6 3.6
19500 -7.1 19.2 215 20 10 6.2 3.4 | -7.7 18.1 235 24 12 6.6 3.6
19000 -6.3 20.7 215 19 10 5.7 3.1 | -6.7 19.9 235 23 12 6.7 3.7
18500 -5.5 22.1 215 18 9 5.8 3.2 | -5.8 21.6 235 23 12 6.7 3.7
18000 -4.6 23.7 220 17 9 5.7 3.1 | -4.8 23.4 240 22 11 6.3 3.5
17500 -3.8 25.2 220 17 9 5.7 3.1 | -4.0 24.8 240 20 10 5.6 3.1
17000 -3.0 26.6 220 16 8 5.7 3.1 | -3.2 26.2 240 18 9 5.6 3.1
16500 -2.1 28.2 215 15 8 6.0 3.3 | -2.3 27.9 240 16 8 5.9 3.2
16000 -1.3 29.7 215 14 7 5.9 3.3 | -1.1 30.0 235 15 8 5.8 3.2
15500 -0.4 31.3 215 13 7 6.3 3.5 | -0.2 31.6 230 13 7 6.5 3.6
15000 0.5 32.9 215 11 6 6.5 3.5 | 0.7 33.3 230 12 6 6.4 3.5
14500 1.9 35.4 215 9 5 6.6 3.6 | 1.7 35.1 230 10 5 7.2 3.9
14000 2.8 37.0 215 7 4 6.7 3.7 | 2.7 36.9 230 9 5 7.3 4.0
13500 3.8 38.8 220 6 3 7.2 4.0 | 3.8 38.8 235 8 4 7.0 3.8
13000 4.8 40.6 225 5 2 7.4 4.1 | 4.7 40.5 245 6 3 6.9 3.8
12500 5.9 42.6 240 4 2 7.3 4.0 | 5.7 42.3 260 4 2 7.1 3.9
12000 6.9 44.4 240 2 1 7.3 4.0 | 6.7 44.1 280 4 2 7.4 4.1
11500 7.9 46.2 240 1 0 7.9 4.4 | 7.8 46.0 300 4 2 7.7 4.2
11000 9.2 48.6 000 0 0 8.5 4.7 | 9.0 48.2 310 4 2 8.2 4.5
10500 10.5 50.9 330 1 0 3.5 1.9 | 10.2 50.4 315 5 2 4.2 2.3
10250 10.6 51.1 335 1 0 5.3 2.9 | 10.4 50.7 320 5 2 5.4 3.0
10000 11.4 52.5 335 1 0 13.4 7.4 | 11.2 52.2 320 5 3 12.0 6.6
9750 12.3 54.1 335 1 0 11.8 6.5 | 12.0 53.6 320 6 3 11.3 6.2
9500 13.1 55.6 335 1 0 10.3 5.6 | 12.8 55.0 325 6 3 9.6 5.3
9250 13.8 56.8 330 1 0 8.3 4.5 | 13.4 56.1 325 7 3 6.8 3.7
9000 14.5 58.1 330 1 0 10.4 5.7 | 13.9 57.0 320 7 4 7.9 4.3
8750 15.3 59.5 325 1 0 11.2 6.2 | 14.5 58.1 320 8 4 9.0 4.9
8500 16.1 61.0 325 1 1 10.2 5.6 | 15.2 59.4 320 8 4 7.9 4.3
8250 16.8 62.2 325 1 1 9.4 5.2 | 15.7 60.3 315 9 5 7.2 4.0
8000 17.5 63.5 325 1 1 10.0 5.5 | 16.3 61.3 315 10 5 7.1 3.9
7750 18.3 64.9 320 1 1 10.4 5.7 | 16.8 62.2 315 11 6 6.6 3.6
7500 19.0 66.2 320 1 1 10.1 5.5 | 17.3 63.1 310 12 6 6.8 3.7
7250 19.8 67.6 320 1 1 9.9 5.4 | 17.8 64.0 310 12 6 7.3 4.0
7000 20.5 68.9 320 1 1 9.9 5.4 | 18.3 64.9 305 13 7 6.4 3.5
6750 21.3 70.3 315 1 1 10.1 5.5 | 18.7 65.7 305 14 7 5.7 3.1
6500 22.0 71.6 315 1 1 10.5 5.8 | 19.1 66.4 300 15 8 7.7 4.2
6250 22.8 73.0 315 1 1 10.5 5.8 | 19.7 67.5 300 15 8 9.4 5.2
6000 23.6 74.5 315 2 1 10.6 5.8 | 20.3 68.5 295 15 8 7.1 3.9
5750 24.5 76.1 315 2 1 16.0 8.8 | 20.8 69.4 295 15 8 10.4 5.7
5500 26.1 79.0 320 1 1 24.6 13.5 | 22.0 71.6 295 13 6 21.3 11.7
________________________________________________________________________________
This product is issued twice per day, once by approximately 0630 MST/0730 MDT
(1330 UTC) and again by approximately 1830 MST/1930 MDT (0130 UTC). It is not
continuously monitored nor updated after its initial issuance.
The information contained herein is based on rawinsonde observation and/or
numerical weather prediction model data taken near the old Stapleton
Airport site in Denver, Colorado at
North Latitude: 39 deg 46 min 5.016 sec
West Longitude: 104 deg 52 min 9.984 sec
Elevation: 5285 feet (1611 meters)
and may not be representative of other areas along the Front Range of the
Colorado Rocky Mountains. Note that some elevations in numerical weather
prediction models differ from actual station elevations, which can lead to
data which appear to be below ground. Erroneous data such as these should
not be used.
The content and format of this report as well as the issuance times are subject
to change without prior notice. Comments and suggestions are welcome and should
be directed to one of the addresses or phone numbers shown at the bottom of this
page. To expedite a response to comments, be sure to mention your interest in
the soaring forecast.
DEFINITIONS:
CAPE (Convective Available Potential Energy) - An integrated measure of the
energy available for convective development, also known as the positive
area on a thermodynamic diagram. Units are Joules per kilogram. Larger
values are indicative of greater instability and the possibility of
stronger convective activity, including thunderstorms. Values around or
greater than 1000 suggest the possibility of severe weather should
convective activity develop.
CINH (Convective Inhibition) - An integrated measure of the amount of energy
needed to initiate convective activity, also known as the negative area on
a thermodynamic diagram. Units are Joules per kilogram. The more negative
this number, the more energy is required to initiate convection. This
inhibitive energy can be overcome through surface heating, cooling aloft,
lifting mechanisms (orographic, frontal, gravity waves, etc.)
Convective Condensation Level - The height to which an air parcel possessing the
average saturation mixing ratio in the lowest 4000 feet of the airmass,
if heated sufficiently from below, will rise dry adiabatically until it
just becomes saturated. It estimates the base of cumulus clouds that are
produced by surface heating only.
Convection Temperature (ConvectionT) - The surface temperature required to make
the airmass dry adiabatic up to the given level. It can be considered a
"trigger temperature" for that level.
Freezing Level - The height where the temperature is zero degrees Celsius.
Height of Stable Layer - The height (between 12,000 and 18,000 feet above
mean sea level) where the smallest lapse rate exists. The location and
existence of this feature is important in the generation of mountain
waves.
K Index - A measure of stability which combines the temperature difference
between approximately 5,000 and 18,000 feet above the surface, the amount
of moisture at approximately 5,000 feet above the surface, and a measure
of the dryness at approximately 10,000 feet above the surface. Larger
positive numbers indicate more instability and a greater likelihood of
thunderstorm development. One interpretation of K index values regarding
soaring in the western United States is given in WMO Technical Note 158
and is reproduced in the following table:
below -10 no or weak thermals
-10 to 5 dry thermals or 1/8 cumulus with moderate thermals
5 to 15 good soaring conditions
15 to 20 good soaring conditions with occasional showers
20 to 30 excellent soaring conditions, but increasing
probability of showers and thunderstorms
above 30 more than 60 percent probability of thunderstorms
Lapse Rate - The change with height of the temperature. Negative values indicate
inversions.
Lifted Condensation Level - The height to which an air parcel possessing the
average dew point in the lowest 4000 feet of the airmass and the forecast
maximum temperature must be lifted dry adiabatically to attain saturation.
Lifted Index (LI) - The difference between the environmental temperature at a
level approximately 18,000 feet above the surface and the temperature of
an air parcel lifted dry adiabatically from the surface to its lifted
condensation level and then pseudoadiabatically thereafter to this same
level. The parcel`s initial temperature is the forecast maximum
temperature and its dew point is the average dew point in the lowest 4000
feet of the airmass. Negative values are indicative of instability with
positive values showing stable conditions.
Lift Rate - An experimental estimate of the strength of thermals. It is
computed the same way as the maximum rate of lift but uses the actual
level rather than the maximum height of thermals in the calculation.
Also, none of the empirical adjustments based on cloudiness and K-index
are applied to these calculations.
Maximum Height of Thermals - The height where the dry adiabat through the
forecast maximum temperature intersects the environmental temperature.
Maximum Rate of Lift - An estimate of the maximum strength of thermals. It
is computed from an empirical formula which combines the expected maximum
height of thermals with the difference in the environmental temperatures
between the maximum height of thermals and the temperature 4,000 feet
above the ground. After this computation, further empirical adjustments
are made based on the value of the K-index and the amount and opacity of
middle and high level cloudiness expected between the time of trigger
temperature and the time of overdevelopment.
Middle/High Clouds - The amount and opacity of middle (altostratus, altocumulus)
or high (cirrus, cirrostratus, cirrocumulus) clouds. Broken means that
between 60% and 90% of the sky is covered by the cloud, with overcast
conditions occurring when more than 90% of the sky is covered by the cloud.
Thin implies that the clouds are predominantly transparent, meaning that
some sunlight is reaching the ground, in contrast to opaque which suggests
that little sunlight is reaching the ground.
Potential Height of Wave - The minimum of the following two heights:
1. Level above the height of stable layer (or 14,000 feet if none exists)
where the wind direction changes by 30 degrees or more
2. Level above the height of stable layer (or 14,000 feet if none exists)
where the wind speed no longer increases with height
PVA/NVA - Positive vorticity advection (PVA)/negative vorticity advection (NVA)
on the 500 millibar isobaric surface (approximately 18,000 feet above mean
sea level). Weak PVA has been shown to assist in mountain wave soaring.
Soaring Window - The time between the time the trigger temperature is reached
and the time of overdevelopment.
Thermal Index - The difference between the environmental temperature and the
temperature at a particular level determined by following the dry adiabat
through the forecast maximum temperature up to that level. Negative values
are indicative of thermal lift.
Thermal Soaring Index - An adjective rating (for sailplanes) based on the
computed maximum rate of lift, and the wind speed and middle and high
cloud cover expected during the soaring window (the time of the trigger
temperature and the time of overdevelopment) according to the following:
Maximum rate of lift Adjective Rating
>= 800 fpm Excellent
>= 400 and < 800 fpm Good
>= 200 and < 400 fpm Fair
< 200 fpm Poor
Time of Overdevelopment - The time one or more of the following phenomena,
which essentially shut off thermal lift, is expected to occur:
1. formation of broken to overcast convective cloud cover
2. formation of scattered to numerous downbursts
3. initiation of widespread precipitation
Time of Trigger Temperature - The time the surface temperature is expected to
reach the trigger temperature.
Trigger Temperature - The surface temperature required to make the first 4000
feet of the atmosphere dry adiabatic.
Wave Soaring Index - An empirical, adjective rating (for sailplanes) which
attempts to combine a variety of phenomena important in mountain wave
soaring into a single index number. Objective points are assigned to
these phenomena: wind speed and direction at 14,000 ft MSL, the static
stability in the 12,000-18,000 ft MSL layer, the wind speed gradient above
the stable layer, jet stream location and frontal and upper trough
movements.
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