High Level Airport
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the High Level Airport clear sky chart from 2005-11-06 to 2026-04-02.

This page is a summary of past forecasts. Even though it's based only on forecasts, some people use these pages as clue to a site's climate. But climate data based on actual observation should be preferred (if you can find it!).

Cloud, Transparency and Seeing forecasts are summarized individually. But because good observing conditions require a combination of minimum cloud cover, transparency and seeing, the individual forecast summaries can be difficult to interpret. The observing conditions section counts the combinations of conditions needed for common types of astronomical observation.

Observing Conditions

The following tables count the occurrence of the combinations of forecast and darkness conditions needed for different observations. Conditions are rated by by what percentage of the hours the observation would likely be attempted are usable.

Observation Type	Minimum conditions for hour to be usable				Out of hours
	Clear of Clouds	Transparency	Seeing	Sky Brightness
Planetary:	50%	(no limit)	Average	(no limit)	Nighttime Hours
Deep Sky:	70%	Average	(no limit)	5.9	Nighttime Hours
CCD Imaging:	70%	Below Average	Poor	(no limit)	Nighttime Hours

Percentages are of nighttime hours per month, or per year, when a particular observing conditions was met. Deep Sky percentages never exceed 50% because the moon is up half the time. Deep sky percentages are much lower for charts at high latitudes because of fewer dark hours in the summer.

Overall

By Month

Individual Forecasts

The length of each colored bar in the "occurrence of each Forecast" column is proportional to the number of times the chart forecast that color. The percentages are of the total number of hours per month, or per year, with a particular forecast. The colors as the same as those used in clear sky charts.

Note: Transparency probabilities are only counted for those hours that have 30% or less cloud cover. Seeing probabilities are only counted for those hours that have 80% or less cloud cover. If that's too tricky, try Observing Conditions instead.

Overall

		Occurrence of each Forecast	Data Points
Cloud Cover	Day	7% 9% 3 9% 6% 6% 6% 5% 7% 8% 34%	134573
	Night	6% 10% 4 11% 7% 6% 6% 5% 7% 8% 30%	135934
Transparency	Day	45% 8% 17% 23% 7%	134238
	Night	50% 7% 15% 21% 7%	135851
Seeing	Day	13% 9% 26% 30% 18% 5%	18729
	Night	13% 8% 27% 34% 17% 2	26836

By Month

Percentage Cloud Cover NW (2006-)
Color Meanings: Overcast 90% covered 80% covered 70% covered 60% covered 50% covered 40% covered 30% covered 20% covered 10% covered Clear
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	4 12% 6% 17% 10% 8% 8% 8% 8% 6% 13%	3466
	11th to 20th	Day	4 8% 4 10% 5% 5% 8% 5% 8% 8% 33%	3522
	21st to end	Day	6% 14% 7% 19% 9% 7% 5% 3 5% 6% 20%	3966
February	1st to 10th	Day	6% 14% 3 12% 7% 7% 5% 5% 7% 7% 27%	1665
	11th to 20th	Day	5% 13% 3 9% 9% 6% 6% 5% 7% 8% 29%	1640
	21st to end	Day	3 14% 4 13% 8% 6% 7% 5% 7% 8% 25%	1557
March	1st to 10th	Day	2 7% 5% 15% 7% 7% 6% 6% 8% 9% 28%	4618
	11th to 20th	Day	4 10% 6% 15% 9% 7% 6% 4 5% 6% 29%	4784
	21st to end	Day	3 9% 4 9% 6% 6% 6% 4 6% 8% 39%	6056
April	1st to 10th	Day	5% 11% 3 10% 6% 6% 5% 4 7% 7% 35%	2463
	11th to 20th	Day	5% 8% 3 8% 7% 6% 6% 5% 7% 7% 38%	2642
	21st to end	Day	10% 11% 4 10% 6% 6% 5% 5% 6% 8% 30%	2806
May	1st to 10th	Day	6% 5% 2 5% 4 4 5% 4 7% 9% 48%	6780
	11th to 20th	Day	8% 8% 3 9% 6% 6% 6% 5% 8% 10% 32%	7071
	21st to end	Day	8% 7% 3 7% 6% 5% 5% 5% 7% 9% 37%	7997
June	1st to 10th	Day	12% 9% 3 9% 5% 6% 5% 5% 6% 8% 32%	3400
	11th to 20th	Day	15% 8% 3 8% 5% 5% 5% 5% 7% 8% 29%	3400
	21st to end	Day	11% 7% 2 7% 5% 5% 6% 6% 7% 10% 35%	3400
July	1st to 10th	Day	8% 5% 2 7% 6% 6% 7% 6% 9% 11% 32%	7566
	11th to 20th	Day	6% 4 2 6% 6% 6% 6% 5% 7% 9% 43%	7240
	21st to end	Day	10% 5% 2 5% 5% 5% 5% 4 7% 9% 42%	7877
August	1st to 10th	Day	7% 5% 2 5% 4 5% 5% 5% 7% 7% 49%	4890
	11th to 20th	Day	7% 6% 2 7% 5% 5% 7% 5% 7% 8% 40%	4470
	21st to end	Day	13% 6% 2 7% 6% 5% 6% 5% 7% 8% 37%	4866
September	1st to 10th	Day	13% 6% 3 7% 5% 5% 5% 4 5% 7% 41%	2480
	11th to 20th	Day	9% 7% 3 7% 5% 4 4 4 6% 9% 42%	2331
	21st to end	Day	11% 7% 3 8% 5% 5% 5% 5% 6% 8% 38%	2200
October	1st to 10th	Day	8% 10% 5% 10% 6% 5% 5% 5% 6% 6% 35%	3227
	11th to 20th	Day	9% 14% 3 8% 4 4 4 4 6% 8% 35%	2932
	21st to end	Day	9% 15% 7% 12% 5% 5% 6% 5% 5% 6% 25%	2986
November	1st to 10th	Day	10% 15% 5% 11% 9% 6% 4 4 5% 6% 25%	1442
	11th to 20th	Day	10% 17% 5% 13% 9% 5% 6% 4 5% 7% 19%	1470
	21st to end	Day	6% 11% 6% 13% 9% 5% 6% 5% 7% 7% 25%	1213
December	1st to 10th	Day	5% 16% 8% 16% 10% 8% 5% 4 7% 6% 17%	1680
	11th to 20th	Day	6% 19% 7% 15% 6% 6% 5% 5% 5% 6% 21%	1929
	21st to end	Day	6% 12% 5% 15% 9% 8% 6% 4 4 4 27%	2541
January	1st to 10th	Night	5% 14% 6% 19% 9% 8% 8% 5% 7% 5% 13%	9425
	11th to 20th	Night	5% 7% 3 10% 7% 8% 7% 6% 8% 9% 30%	9376
	21st to end	Night	5% 14% 6% 16% 11% 7% 5% 4 5% 6% 22%	10270
February	1st to 10th	Night	5% 11% 5% 11% 7% 6% 6% 4 6% 6% 32%	2955
	11th to 20th	Night	5% 12% 4 10% 7% 7% 5% 4 7% 7% 31%	2866
	21st to end	Night	5% 11% 4 11% 8% 7% 7% 4 6% 7% 30%	2247
March	1st to 10th	Night	5% 7% 5% 13% 7% 6% 6% 5% 7% 8% 31%	5496
	11th to 20th	Night	4 11% 6% 14% 8% 6% 6% 5% 7% 8% 25%	5435
	21st to end	Night	3 7% 4 10% 5% 5% 6% 5% 7% 6% 41%	5147
April	1st to 10th	Night	6% 10% 4 8% 6% 6% 6% 4 7% 9% 34%	1959
	11th to 20th	Night	5% 9% 2 8% 7% 6% 6% 5% 7% 7% 36%	1707
	21st to end	Night	9% 11% 4 8% 6% 5% 5% 4 7% 9% 32%	1594
May	1st to 10th	Night	6% 7% 2 5% 4 5% 5% 5% 7% 8% 45%	3180
	11th to 20th	Night	8% 8% 3 8% 5% 5% 5% 5% 7% 9% 37%	2822
	21st to end	Night	7% 6% 2 7% 6% 5% 6% 6% 8% 9% 36%	2876
June	1st to 10th	Night	11% 7% 2 9% 5% 6% 4 5% 5% 8% 38%	1000
	11th to 20th	Night	12% 8% 2 9% 5% 5% 6% 5% 5% 8% 35%	1000
	21st to end	Night	10% 5% 3 8% 5% 5% 6% 6% 7% 9% 36%	1000
July	1st to 10th	Night	6% 7% 2 6% 6% 6% 7% 6% 9% 13% 32%	2391
	11th to 20th	Night	8% 5% 2 6% 5% 5% 5% 4 8% 9% 42%	2720
	21st to end	Night	9% 3 1 4 4 5% 5% 5% 7% 11% 44%	2966
August	1st to 10th	Night	5% 3 1 4 4 3 5% 4 6% 9% 55%	2258
	11th to 20th	Night	6% 7% 2 6% 4 5% 6% 5% 7% 9% 43%	2577
	21st to end	Night	9% 6% 2 6% 5% 4 5% 5% 7% 10% 41%	3037
September	1st to 10th	Night	11% 6% 3 7% 4 4 5% 4 6% 8% 41%	1917
	11th to 20th	Night	9% 7% 2 7% 5% 6% 5% 5% 7% 8% 40%	2068
	21st to end	Night	9% 7% 2 7% 6% 6% 5% 5% 8% 9% 36%	2200
October	1st to 10th	Night	8% 6% 3 7% 6% 5% 5% 5% 7% 10% 39%	3966
	11th to 20th	Night	9% 11% 4 7% 5% 5% 5% 4 6% 8% 37%	4216
	21st to end	Night	7% 13% 5% 13% 7% 6% 5% 4 6% 7% 27%	4777
November	1st to 10th	Night	12% 16% 5% 11% 6% 5% 4 4 5% 6% 26%	3075
	11th to 20th	Night	10% 16% 5% 13% 8% 6% 5% 4 6% 6% 21%	3150
	21st to end	Night	8% 13% 5% 13% 8% 6% 5% 4 6% 6% 25%	3401
December	1st to 10th	Night	7% 13% 5% 15% 10% 8% 7% 5% 6% 6% 18%	5720
	11th to 20th	Night	8% 18% 6% 13% 7% 6% 6% 4 5% 7% 20%	6483
	21st to end	Night	4 11% 5% 14% 9% 7% 6% 4 6% 7% 26%	8657

Transparency (when cloud cover <= 30%)
Color Meanings: Too cloudy to forecast Poor Below Average Average Above average Transparent
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	67% 7% 9% 13% 4	3466
	11th to 20th	Day	45% 8% 15% 17% 13% 1	3540
	21st to end	Day	65% 5% 15% 10% 5%	3967
February	1st to 10th	Day	54% 7% 12% 18% 9%	1665
	11th to 20th	Day	51% 7% 17% 19% 6%	1648
	21st to end	Day	54% 9% 15% 17% 5%	1557
March	1st to 10th	Day	49% 8% 15% 23% 6%	4618
	11th to 20th	Day	55% 7% 14% 20% 4	4784
	21st to end	Day	40% 7% 14% 22% 16% 1	6056
April	1st to 10th	Day	44% 8% 14% 24% 9%	2463
	11th to 20th	Day	41% 8% 14% 23% 13% 1	2642
	21st to end	Day	50% 8% 15% 19% 8%	2806
May	1st to 10th	Day	30% 6% 14% 30% 18% 2	6780
	11th to 20th	Day	43% 8% 13% 24% 12% 1	7075
	21st to end	Day	40% 8% 16% 23% 12% 2	7998
June	1st to 10th	Day	47% 8% 14% 21% 9% 1	3400
	11th to 20th	Day	49% 9% 15% 21% 5%	3400
	21st to end	Day	41% 10% 19% 22% 7% 1	3400
July	1st to 10th	Day	39% 10% 26% 22% 2	7484
	11th to 20th	Day	34% 8% 18% 33% 7%	7153
	21st to end	Day	35% 8% 19% 35% 3	7656
August	1st to 10th	Day	31% 10% 19% 32% 8%	4871
	11th to 20th	Day	37% 10% 19% 21% 12% 1	4512
	21st to end	Day	43% 9% 26% 21% 2	4865
September	1st to 10th	Day	41% 9% 25% 20% 5%	2480
	11th to 20th	Day	39% 8% 25% 26% 2	2331
	21st to end	Day	42% 9% 21% 21% 6%	2200
October	1st to 10th	Day	48% 6% 19% 24% 3	3227
	11th to 20th	Day	47% 6% 21% 23% 3	2932
	21st to end	Day	59% 7% 15% 17% 2	2986
November	1st to 10th	Day	59% 8% 14% 16% 4	1442
	11th to 20th	Day	66% 7% 14% 11% 2	1470
	21st to end	Day	56% 8% 15% 17% 5%	1213
December	1st to 10th	Day	67% 5% 13% 13% 3	1680
	11th to 20th	Day	65% 4 10% 19% 2	1930
	21st to end	Day	58% 6% 12% 18% 6%	2541
January	1st to 10th	Night	68% 6% 8% 13% 5%	9426
	11th to 20th	Night	47% 7% 15% 18% 13%	9420
	21st to end	Night	63% 6% 14% 13% 4	10270
February	1st to 10th	Night	51% 6% 13% 21% 10%	2955
	11th to 20th	Night	50% 7% 14% 22% 8%	2868
	21st to end	Night	51% 8% 14% 22% 6%	2247
March	1st to 10th	Night	47% 7% 13% 27% 5%	5496
	11th to 20th	Night	53% 6% 15% 22% 4	5436
	21st to end	Night	38% 7% 12% 27% 14% 2	5147
April	1st to 10th	Night	44% 6% 14% 24% 10% 2	1959
	11th to 20th	Night	44% 7% 10% 23% 15% 1	1707
	21st to end	Night	46% 7% 13% 22% 11% 1	1594
May	1st to 10th	Night	31% 8% 14% 27% 17% 3	3180
	11th to 20th	Night	40% 8% 10% 24% 17% 1	2822
	21st to end	Night	38% 9% 16% 24% 13%	2876
June	1st to 10th	Night	44% 8% 12% 24% 11% 1	1000
	11th to 20th	Night	47% 9% 15% 22% 7%	1000
	21st to end	Night	40% 9% 17% 22% 10% 1	1000
July	1st to 10th	Night	39% 10% 25% 19% 7%	2364
	11th to 20th	Night	36% 7% 17% 33% 7%	2684
	21st to end	Night	30% 8% 20% 38% 4	2882
August	1st to 10th	Night	24% 8% 20% 41% 7%	2252
	11th to 20th	Night	34% 9% 20% 25% 11%	2601
	21st to end	Night	37% 10% 23% 26% 4	3037
September	1st to 10th	Night	39% 9% 20% 23% 7%	1917
	11th to 20th	Night	40% 9% 19% 27% 6%	2068
	21st to end	Night	41% 10% 18% 22% 9% 1	2200
October	1st to 10th	Night	38% 9% 19% 28% 6% 1	3966
	11th to 20th	Night	45% 6% 19% 25% 6%	4216
	21st to end	Night	56% 8% 17% 18% 2	4777
November	1st to 10th	Night	58% 8% 14% 17% 3	3075
	11th to 20th	Night	62% 8% 11% 15% 3	3150
	21st to end	Night	58% 7% 14% 17% 4 1	3401
December	1st to 10th	Night	65% 7% 13% 11% 4	5720
	11th to 20th	Night	64% 6% 12% 15% 3	6481
	21st to end	Night	56% 7% 15% 16% 6%	8657

Seeing (when cloud cover <= 80%)
Color Meanings: Too cloudy to forecast Bad 1/5 Poor 2/5 Average 3/5 Good 4/5 Excellent 5/5
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	9% 8% 26% 37% 18% 3	220
	11th to 20th	Day	18% 9% 30% 31% 11% 1	220
	21st to end	Day	14% 12% 31% 30% 12% 1	242
February	1st to 10th	Day	15% 7% 32% 28% 11% 6%	220
	11th to 20th	Day	10% 8% 34% 29% 15% 4	220
	21st to end	Day	8% 9% 36% 29% 15% 3	318
March	1st to 10th	Day	5% 11% 30% 31% 18% 4	540
	11th to 20th	Day	10% 11% 35% 28% 12% 4	540
	21st to end	Day	9% 10% 31% 36% 12% 3	594
April	1st to 10th	Day	8% 13% 36% 27% 14% 2	505
	11th to 20th	Day	10% 11% 30% 31% 15% 3	492
	21st to end	Day	13% 17% 26% 30% 13% 1	506
May	1st to 10th	Day	11% 14% 28% 29% 14% 5%	703
	11th to 20th	Day	12% 12% 25% 29% 19% 4	865
	21st to end	Day	12% 8% 21% 34% 20% 5%	1006
June	1st to 10th	Day	14% 9% 28% 29% 15% 5%	930
	11th to 20th	Day	19% 7% 21% 29% 20% 5%	930
	21st to end	Day	11% 6% 18% 33% 25% 7%	930
July	1st to 10th	Day	13% 6% 22% 28% 25% 6%	916
	11th to 20th	Day	13% 5% 19% 30% 24% 9%	912
	21st to end	Day	12% 4 18% 31% 27% 8%	1013
August	1st to 10th	Day	10% 5% 19% 28% 28% 10%	747
	11th to 20th	Day	12% 9% 33% 30% 13% 3	517
	21st to end	Day	18% 5% 26% 32% 15% 5%	583
September	1st to 10th	Day	15% 9% 30% 26% 15% 4	530
	11th to 20th	Day	12% 9% 25% 31% 19% 5%	530
	21st to end	Day	12% 13% 33% 26% 12% 4	530
October	1st to 10th	Day	17% 11% 30% 27% 11% 3	479
	11th to 20th	Day	18% 8% 31% 28% 12% 2	330
	21st to end	Day	20% 10% 34% 22% 12% 3	319
November	1st to 10th	Day	15% 11% 34% 25% 12% 3	220
	11th to 20th	Day	15% 9% 33% 27% 12% 3	220
	21st to end	Day	14% 8% 29% 32% 15% 2	220
December	1st to 10th	Day	16% 8% 21% 35% 17% 1	220
	11th to 20th	Day	14% 10% 32% 32% 9% 4	220
	21st to end	Day	12% 3 25% 35% 18% 7%	242
January	1st to 10th	Night	13% 8% 24% 38% 15% 2	1160
	11th to 20th	Night	14% 9% 31% 32% 11% 2	1160
	21st to end	Night	15% 12% 32% 31% 9% 1	1265
February	1st to 10th	Night	10% 9% 29% 34% 15% 3	847
	11th to 20th	Night	11% 9% 29% 34% 15% 2	824
	21st to end	Night	11% 7% 33% 32% 16% 3	692
March	1st to 10th	Night	8% 8% 28% 34% 19% 3	840
	11th to 20th	Night	10% 8% 32% 32% 15% 2	840
	21st to end	Night	8% 7% 24% 43% 16% 2	924
April	1st to 10th	Night	11% 9% 28% 33% 17% 2	753
	11th to 20th	Night	8% 8% 31% 35% 15% 2	519
	21st to end	Night	12% 12% 28% 33% 13% 2	400
May	1st to 10th	Night	11% 8% 30% 32% 18% 3	400
	11th to 20th	Night	9% 10% 24% 33% 21% 2	394
	21st to end	Night	11% 5% 21% 37% 24% 1	433
June	1st to 10th	Night	13% 4 25% 36% 20% 3	400
	11th to 20th	Night	16% 4 20% 31% 26% 4	400
	21st to end	Night	11% 3 19% 37% 27% 4	400
July	1st to 10th	Night	14% 4 20% 31% 30% 2	390
	11th to 20th	Night	13% 3 16% 35% 30% 4	388
	21st to end	Night	11% 2 14% 33% 34% 5%	438
August	1st to 10th	Night	10% 4 18% 32% 29% 8%	398
	11th to 20th	Night	9% 3 23% 41% 21% 2	396
	21st to end	Night	16% 3 19% 31% 28% 2	530
September	1st to 10th	Night	15% 5% 21% 35% 21% 3	717
	11th to 20th	Night	12% 7% 22% 38% 19% 3	800
	21st to end	Night	13% 9% 28% 31% 16% 3	800
October	1st to 10th	Night	14% 8% 27% 34% 15% 2	800
	11th to 20th	Night	13% 8% 30% 35% 14% 1	800
	21st to end	Night	18% 10% 29% 30% 11% 2	910
November	1st to 10th	Night	19% 8% 29% 29% 13% 2	1025
	11th to 20th	Night	19% 8% 26% 34% 11% 2	1050
	21st to end	Night	14% 9% 29% 32% 13% 2	1147
December	1st to 10th	Night	14% 8% 27% 35% 14% 1	1160
	11th to 20th	Night	14% 10% 28% 33% 12% 3	1160
	21st to end	Night	11% 6% 24% 38% 18% 3	1276

Page generated at 2026-03-30 00:18:45. Version ac07

Copyright © 2026 Attilla Danko.