Icefog Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Icefog Observatory clear sky chart from 2005-11-06 to 2026-05-02.

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	13% 11% 4 10% 6% 6% 6% 4 6% 7% 27%	134919
	Night	8% 13% 5% 11% 6% 6% 5% 4 6% 6% 30%	136388
Transparency	Day	53% 9% 16% 16% 5% 1	134521
	Night	52% 7% 14% 16% 9% 1	136366
Seeing	Day	15% 6% 16% 26% 26% 11%	20534
	Night	13% 4 16% 32% 27% 7%	25022

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 13% 5% 10% 7% 4 3 3 4 6% 42%	1674
	11th to 20th	Day	5% 18% 6% 12% 7% 6% 4 4 5% 5% 28%	2358
	21st to end	Day	5% 11% 4 11% 9% 9% 8% 5% 8% 6% 24%	3173
February	1st to 10th	Day	2 13% 6% 11% 5% 4 5% 5% 5% 5% 40%	1260
	11th to 20th	Day	4 13% 5% 16% 6% 7% 6% 5% 6% 8% 24%	1546
	21st to end	Day	4 18% 5% 11% 7% 6% 6% 5% 5% 6% 27%	1384
March	1st to 10th	Day	3 16% 7% 15% 6% 5% 4 3 5% 5% 30%	4473
	11th to 20th	Day	2 10% 6% 14% 7% 7% 6% 4 6% 7% 30%	4714
	21st to end	Day	5% 13% 6% 10% 5% 5% 5% 4 6% 7% 34%	6096
April	1st to 10th	Day	6% 17% 6% 11% 7% 7% 5% 5% 6% 7% 25%	2686
	11th to 20th	Day	5% 12% 4 10% 6% 5% 4 4 5% 7% 38%	2909
	21st to end	Day	5% 8% 3 8% 7% 6% 6% 5% 7% 7% 39%	3203
May	1st to 10th	Day	10% 13% 5% 12% 8% 6% 6% 5% 6% 7% 23%	7426
	11th to 20th	Day	5% 7% 3 10% 6% 6% 7% 4 6% 7% 38%	7870
	21st to end	Day	12% 10% 4 11% 7% 7% 8% 6% 8% 8% 21%	9183
June	1st to 10th	Day	12% 8% 4 10% 7% 7% 6% 5% 7% 8% 26%	3935
	11th to 20th	Day	12% 8% 3 9% 6% 6% 6% 5% 8% 8% 29%	4000
	21st to end	Day	14% 7% 2 7% 7% 5% 6% 5% 8% 10% 29%	4000
July	1st to 10th	Day	27% 7% 3 6% 5% 5% 5% 4 5% 6% 28%	9060
	11th to 20th	Day	15% 8% 3 8% 6% 6% 6% 5% 9% 10% 23%	8324
	21st to end	Day	23% 8% 3 8% 5% 5% 6% 5% 6% 7% 24%	8833
August	1st to 10th	Day	29% 10% 3 8% 5% 4 5% 4 5% 6% 22%	5242
	11th to 20th	Day	25% 11% 3 9% 6% 5% 5% 4 6% 7% 18%	4983
	21st to end	Day	27% 12% 4 10% 5% 5% 5% 4 5% 6% 17%	5038
September	1st to 10th	Day	20% 11% 4 9% 5% 5% 5% 4 6% 7% 24%	2605
	11th to 20th	Day	23% 14% 5% 10% 5% 5% 5% 4 5% 6% 19%	2363
	21st to end	Day	20% 15% 4 10% 6% 4 3 4 5% 4 24%	2172
October	1st to 10th	Day	12% 17% 5% 10% 5% 4 4 3 4 5% 30%	3078
	11th to 20th	Day	9% 18% 8% 13% 7% 6% 4 4 5% 4 22%	2846
	21st to end	Day	12% 18% 6% 14% 7% 5% 5% 3 5% 5% 20%	2517
November	1st to 10th	Day	9% 16% 6% 13% 7% 5% 5% 5% 6% 6% 23%	1272
	11th to 20th	Day	8% 12% 6% 14% 9% 7% 3 4 4 6% 28%	1050
	21st to end	Day	10% 15% 6% 11% 7% 7% 4 4 5% 6% 25%	776
December	1st to 10th	Day	10% 12% 4 7% 5% 5% 5% 3 6% 9% 33%	1010
	11th to 20th	Day	4 15% 7% 11% 7% 4 6% 4 7% 4 31%	826
	21st to end	Day	3 11% 5% 12% 6% 5% 6% 5% 6% 7% 35%	1034
January	1st to 10th	Night	3 10% 5% 11% 6% 4 4 3 5% 5% 45%	11306
	11th to 20th	Night	4 15% 5% 11% 7% 5% 5% 5% 7% 7% 29%	10570
	21st to end	Night	6% 11% 4 10% 7% 6% 7% 5% 7% 6% 32%	11041
February	1st to 10th	Night	3 13% 6% 14% 6% 4 4 3 4 6% 39%	3360
	11th to 20th	Night	4 12% 5% 11% 7% 7% 7% 5% 7% 7% 29%	2961
	21st to end	Night	4 16% 8% 12% 8% 6% 5% 4 5% 7% 26%	2422
March	1st to 10th	Night	3 15% 7% 15% 6% 6% 5% 4 5% 5% 30%	5601
	11th to 20th	Night	4 9% 5% 14% 8% 6% 6% 5% 6% 8% 29%	5464
	21st to end	Night	6% 14% 4 8% 4 5% 5% 4 6% 7% 37%	5075
April	1st to 10th	Night	6% 15% 5% 11% 6% 6% 6% 5% 6% 7% 26%	1931
	11th to 20th	Night	6% 10% 4 10% 7% 6% 5% 5% 6% 6% 35%	1660
	21st to end	Night	7% 9% 3 8% 6% 6% 6% 4 7% 7% 38%	1414
May	1st to 10th	Night	8% 10% 3 10% 8% 7% 7% 6% 8% 9% 24%	2570
	11th to 20th	Night	8% 7% 3 7% 5% 6% 5% 5% 7% 7% 41%	2033
	21st to end	Night	15% 10% 4 11% 7% 7% 7% 5% 6% 9% 21%	1714
June	1st to 10th	Night	8% 9% 3 8% 9% 5% 7% 5% 9% 10% 23%	464
	11th to 20th	Night	12% 8% 3 8% 7% 7% 5% 5% 6% 14% 26%	400
	21st to end	Night	14% 8% 1 10% 5% 4 8% 6% 9% 9% 27%	400
July	1st to 10th	Night	27% 11% 3 7% 4 4 4 4 5% 6% 25%	920
	11th to 20th	Night	18% 11% 4 9% 6% 6% 6% 4 7% 7% 24%	1641
	21st to end	Night	22% 9% 3 8% 6% 5% 5% 5% 5% 7% 25%	2029
August	1st to 10th	Night	26% 9% 4 9% 6% 6% 5% 3 6% 7% 19%	1916
	11th to 20th	Night	32% 12% 4 8% 5% 5% 4 3 4 5% 17%	2073
	21st to end	Night	25% 12% 4 9% 6% 6% 5% 4 5% 5% 18%	2871
September	1st to 10th	Night	18% 11% 4 9% 5% 6% 6% 3 6% 7% 25%	1794
	11th to 20th	Night	20% 14% 4 10% 6% 6% 5% 3 6% 6% 20%	2037
	21st to end	Night	20% 15% 4 9% 5% 5% 4 4 5% 5% 24%	2225
October	1st to 10th	Night	11% 16% 4 9% 5% 6% 6% 4 5% 7% 27%	4108
	11th to 20th	Night	10% 17% 6% 11% 7% 6% 5% 4 5% 6% 22%	4288
	21st to end	Night	10% 19% 7% 15% 8% 5% 3 3 4 5% 22%	5284
November	1st to 10th	Night	9% 18% 5% 12% 6% 5% 5% 4 5% 5% 26%	3245
	11th to 20th	Night	8% 14% 6% 14% 8% 5% 4 4 5% 6% 27%	3570
	21st to end	Night	8% 16% 5% 13% 7% 5% 5% 4 6% 6% 25%	3820
December	1st to 10th	Night	7% 12% 5% 11% 7% 6% 5% 4 6% 7% 30%	6400
	11th to 20th	Night	5% 15% 6% 12% 6% 6% 5% 4 5% 6% 30%	7595
	21st to end	Night	3 12% 6% 11% 7% 6% 6% 5% 6% 7% 31%	10186

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	45% 5% 10% 17% 20% 3	1674
	11th to 20th	Day	56% 6% 17% 14% 6% 2	2370
	21st to end	Day	54% 7% 13% 14% 9% 3	3174
February	1st to 10th	Day	45% 6% 8% 18% 18% 5%	1260
	11th to 20th	Day	54% 8% 12% 16% 9% 1	1553
	21st to end	Day	55% 7% 10% 16% 10% 1	1384
March	1st to 10th	Day	55% 5% 9% 17% 13% 1	4473
	11th to 20th	Day	48% 9% 10% 17% 12% 3	4714
	21st to end	Day	46% 8% 14% 19% 12% 1	6096
April	1st to 10th	Day	55% 7% 14% 18% 6% 1	2686
	11th to 20th	Day	45% 7% 15% 24% 7% 2	2909
	21st to end	Day	40% 8% 15% 28% 9%	3203
May	1st to 10th	Day	55% 8% 13% 19% 5%	7426
	11th to 20th	Day	39% 11% 13% 21% 12% 4	7873
	21st to end	Day	52% 11% 16% 16% 4	9184
June	1st to 10th	Day	51% 11% 17% 18% 3	3935
	11th to 20th	Day	47% 11% 19% 21% 3	4000
	21st to end	Day	47% 11% 22% 17% 2	4000
July	1st to 10th	Day	56% 8% 18% 14% 5%	8961
	11th to 20th	Day	51% 10% 23% 15%	8220
	21st to end	Day	55% 10% 21% 11% 3	8585
August	1st to 10th	Day	62% 8% 19% 9% 1	5222
	11th to 20th	Day	61% 11% 20% 7%	5031
	21st to end	Day	65% 9% 17% 8%	5037
September	1st to 10th	Day	58% 11% 20% 10% 1	2605
	11th to 20th	Day	64% 10% 16% 11%	2363
	21st to end	Day	63% 8% 13% 13% 3	2172
October	1st to 10th	Day	57% 6% 11% 20% 5%	3078
	11th to 20th	Day	64% 9% 12% 14% 2	2846
	21st to end	Day	65% 8% 12% 13% 2	2517
November	1st to 10th	Day	58% 9% 17% 12% 4	1272
	11th to 20th	Day	58% 8% 12% 17% 5%	1050
	21st to end	Day	57% 8% 12% 17% 6%	776
December	1st to 10th	Day	47% 7% 20% 20% 6% 1	1010
	11th to 20th	Day	51% 6% 19% 14% 4 6%	828
	21st to end	Day	43% 11% 18% 14% 13% 2	1034
January	1st to 10th	Night	40% 5% 11% 16% 22% 4	11307
	11th to 20th	Night	50% 6% 17% 17% 8%	10620
	21st to end	Night	48% 6% 13% 18% 11% 4	11041
February	1st to 10th	Night	47% 4 9% 15% 19% 5%	3360
	11th to 20th	Night	49% 9% 12% 19% 10% 1	2965
	21st to end	Night	57% 6% 12% 14% 9% 1	2422
March	1st to 10th	Night	55% 4 8% 22% 10% 1	5601
	11th to 20th	Night	50% 9% 11% 16% 12% 3	5466
	21st to end	Night	43% 8% 17% 17% 13% 1	5075
April	1st to 10th	Night	53% 9% 14% 17% 6% 1	1931
	11th to 20th	Night	46% 9% 18% 20% 6% 1	1660
	21st to end	Night	41% 7% 15% 28% 9%	1414
May	1st to 10th	Night	49% 12% 14% 21% 4	2567
	11th to 20th	Night	36% 10% 14% 24% 14% 3	2033
	21st to end	Night	54% 10% 12% 18% 6%	1714
June	1st to 10th	Night	49% 12% 18% 18% 3	464
	11th to 20th	Night	50% 11% 15% 20% 4	400
	21st to end	Night	48% 12% 23% 16% 1	400
July	1st to 10th	Night	60% 8% 14% 12% 5%	910
	11th to 20th	Night	57% 9% 18% 15%	1622
	21st to end	Night	58% 9% 19% 12% 3	1972
August	1st to 10th	Night	63% 8% 19% 9% 2	1911
	11th to 20th	Night	68% 8% 17% 7%	2091
	21st to end	Night	67% 8% 15% 9% 1	2871
September	1st to 10th	Night	56% 10% 21% 12% 2	1794
	11th to 20th	Night	64% 9% 14% 13% 1	2037
	21st to end	Night	60% 10% 13% 14% 3	2225
October	1st to 10th	Night	56% 7% 12% 20% 5%	4108
	11th to 20th	Night	61% 10% 16% 12% 2	4288
	21st to end	Night	66% 6% 11% 15% 2	5284
November	1st to 10th	Night	59% 8% 14% 15% 3 1	3245
	11th to 20th	Night	58% 8% 13% 16% 4	3570
	21st to end	Night	57% 8% 11% 18% 6%	3820
December	1st to 10th	Night	50% 7% 17% 18% 7% 1	6400
	11th to 20th	Night	53% 7% 17% 14% 6% 4	7592
	21st to end	Night	47% 8% 18% 13% 13% 1	10186

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	5% 4 16% 45% 25% 5%	110
	11th to 20th	Day	10% 2 19% 35% 30% 4	110
	21st to end	Day	10% 5% 14% 36% 23% 12%	230
February	1st to 10th	Day	5% 8% 13% 30% 26% 18%	320
	11th to 20th	Day	8% 7% 21% 30% 25% 10%	392
	21st to end	Day	9% 8% 20% 35% 22% 6%	355
March	1st to 10th	Day	6% 3 21% 37% 25% 8%	430
	11th to 20th	Day	7% 5% 21% 30% 25% 12%	430
	21st to end	Day	8% 6% 21% 28% 27% 10%	699
April	1st to 10th	Day	11% 9% 21% 30% 23% 7%	715
	11th to 20th	Day	10% 7% 24% 28% 25% 7%	740
	21st to end	Day	7% 5% 17% 32% 30% 9%	750
May	1st to 10th	Day	11% 7% 20% 30% 25% 7%	709
	11th to 20th	Day	7% 8% 21% 31% 26% 8%	815
	21st to end	Day	14% 8% 17% 30% 24% 7%	1122
June	1st to 10th	Day	11% 5% 18% 26% 26% 13%	1130
	11th to 20th	Day	12% 4 13% 23% 33% 15%	1130
	21st to end	Day	13% 3 9% 25% 33% 16%	1130
July	1st to 10th	Day	16% 4 12% 25% 32% 10%	1111
	11th to 20th	Day	18% 5% 14% 25% 26% 13%	982
	21st to end	Day	19% 4 13% 23% 24% 16%	993
August	1st to 10th	Day	27% 4 16% 22% 22% 9%	726
	11th to 20th	Day	23% 8% 16% 24% 22% 6%	715
	21st to end	Day	28% 7% 15% 21% 19% 9%	803
September	1st to 10th	Day	23% 7% 16% 21% 23% 11%	713
	11th to 20th	Day	29% 8% 15% 23% 15% 9%	583
	21st to end	Day	26% 8% 13% 21% 19% 13%	420
October	1st to 10th	Day	23% 10% 19% 20% 18% 10%	420
	11th to 20th	Day	17% 6% 20% 19% 26% 12%	420
	21st to end	Day	16% 7% 14% 21% 29% 14%	462
November	1st to 10th	Day	16% 6% 12% 26% 29% 12%	308
	11th to 20th	Day	10% 4 12% 25% 26% 23%	110
	21st to end	Day	15% 2 9% 35% 35% 5%	110
December	1st to 10th	Day	20% 5% 17% 32% 19% 6%	110
	11th to 20th	Day	7% 6% 25% 33% 22% 7%	110
	21st to end	Day	6% 8% 19% 33% 28% 6%	121
January	1st to 10th	Night	7% 2 16% 36% 28% 10%	1238
	11th to 20th	Night	10% 3 21% 38% 22% 6%	1060
	21st to end	Night	9% 4 19% 38% 25% 6%	1114
February	1st to 10th	Night	6% 5% 20% 33% 27% 9%	950
	11th to 20th	Night	6% 5% 21% 40% 24% 5%	934
	21st to end	Night	8% 6% 19% 39% 25% 3	783
March	1st to 10th	Night	8% 3 19% 40% 27% 3	761
	11th to 20th	Night	8% 3 18% 37% 30% 5%	718
	21st to end	Night	10% 3 19% 34% 26% 8%	693
April	1st to 10th	Night	10% 6% 19% 38% 24% 4	630
	11th to 20th	Night	8% 4 20% 37% 27% 4	623
	21st to end	Night	8% 3 13% 34% 36% 5%	574
May	1st to 10th	Night	10% 4 17% 29% 34% 6%	337
	11th to 20th	Night	6% 5% 19% 37% 31% 2	197
	21st to end	Night	15% 8% 9% 33% 29% 6%	217
June	1st to 10th	Night	11% 4 15% 26% 37% 8%	200
	11th to 20th	Night	12% 4 13% 30% 34% 9%	200
	21st to end	Night	15% 2 5% 28% 44% 8%	200
July	1st to 10th	Night	13% 4 12% 26% 39% 6%	195
	11th to 20th	Night	19% 4 12% 26% 31% 8%	194
	21st to end	Night	19% 3 9% 29% 33% 7%	219
August	1st to 10th	Night	27% 6% 9% 22% 32% 4	373
	11th to 20th	Night	26% 5% 12% 25% 28% 6%	472
	21st to end	Night	30% 5% 9% 21% 25% 10%	660
September	1st to 10th	Night	22% 5% 13% 23% 29% 9%	600
	11th to 20th	Night	23% 5% 14% 26% 28% 4	600
	21st to end	Night	25% 4 17% 23% 26% 5%	614
October	1st to 10th	Night	21% 8% 18% 26% 24% 4	820
	11th to 20th	Night	15% 5% 17% 29% 25% 10%	910
	21st to end	Night	13% 5% 13% 33% 29% 7%	1001
November	1st to 10th	Night	17% 5% 16% 31% 25% 7%	930
	11th to 20th	Night	13% 3 13% 29% 27% 13%	950
	21st to end	Night	14% 3 16% 31% 27% 10%	1118
December	1st to 10th	Night	13% 5% 18% 33% 23% 8%	1270
	11th to 20th	Night	10% 4 19% 31% 27% 8%	1270
	21st to end	Night	8% 5% 16% 34% 28% 9%	1397

Page generated at 2026-04-28 22:21:47. Version ac07

Copyright © 2026 Attilla Danko.