GKO North Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the GKO North Observatory clear sky chart from 2005-11-06 to 2026-04-17.

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% 5% 5% 5% 4 5% 7% 31%	130834
	Night	13% 12% 5% 11% 6% 5% 4 4 5% 6% 29%	131491
Transparency	Day	52% 5% 10% 21% 11% 1	134339
	Night	54% 5% 12% 19% 10% 1	136073
Seeing	Day	21% 16% 32% 20% 9% 3	17368
	Night	20% 13% 31% 26% 9% 1	27025

By Month

Percentage Cloud Cover NE (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	13% 17% 7% 21% 9% 4 4 3 4 4 14%	4691
	11th to 20th	Day	9% 20% 8% 22% 9% 9% 5% 3 3 3 9%	4679
	21st to end	Day	11% 17% 8% 15% 8% 6% 6% 3 5% 6% 16%	5156
February	1st to 10th	Day	10% 18% 5% 14% 12% 7% 4 3 5% 5% 17%	1882
	11th to 20th	Day	12% 15% 6% 15% 8% 5% 4 3 4 6% 22%	1958
	21st to end	Day	11% 18% 6% 14% 6% 5% 4 4 6% 5% 20%	1730
March	1st to 10th	Day	12% 11% 4 10% 5% 5% 4 4 6% 6% 33%	4578
	11th to 20th	Day	12% 14% 5% 10% 6% 5% 5% 4 6% 6% 26%	5090
	21st to end	Day	20% 13% 8% 11% 5% 5% 4 3 4 5% 23%	5586
April	1st to 10th	Day	17% 17% 4 9% 5% 5% 4 3 5% 5% 27%	2519
	11th to 20th	Day	19% 12% 3 7% 4 3 4 4 5% 5% 33%	2623
	21st to end	Day	16% 12% 3 8% 4 5% 4 3 5% 6% 33%	2600
May	1st to 10th	Day	17% 10% 2 7% 4 4 4 3 5% 6% 38%	5888
	11th to 20th	Day	12% 8% 3 7% 5% 4 5% 4 5% 6% 41%	5861
	21st to end	Day	14% 7% 3 7% 4 4 5% 4 5% 7% 40%	7217
June	1st to 10th	Day	12% 7% 3 6% 4 5% 4 4 6% 7% 42%	3000
	11th to 20th	Day	12% 8% 3 7% 5% 5% 5% 4 6% 9% 38%	3000
	21st to end	Day	14% 7% 2 7% 5% 5% 4 4 5% 8% 38%	3000
July	1st to 10th	Day	14% 6% 2 6% 4 4 5% 4 6% 8% 40%	6601
	11th to 20th	Day	10% 5% 2 6% 5% 5% 6% 4 6% 9% 44%	6360
	21st to end	Day	8% 5% 2 7% 4 4 5% 4 6% 10% 46%	6926
August	1st to 10th	Day	9% 6% 2 6% 5% 5% 6% 5% 8% 10% 38%	4249
	11th to 20th	Day	13% 6% 2 7% 4 5% 5% 4 6% 7% 42%	4174
	21st to end	Day	12% 6% 3 8% 5% 4 5% 4 5% 8% 41%	4649
September	1st to 10th	Day	12% 7% 3 7% 4 6% 5% 5% 6% 7% 38%	2315
	11th to 20th	Day	12% 5% 2 5% 4 5% 5% 3 6% 8% 45%	2200
	21st to end	Day	16% 9% 2 7% 5% 4 4 4 5% 9% 36%	2200
October	1st to 10th	Day	18% 8% 3 7% 5% 4 4 3 5% 7% 36%	2150
	11th to 20th	Day	21% 12% 4 9% 6% 5% 5% 3 6% 6% 23%	1960
	21st to end	Day	19% 15% 6% 10% 5% 5% 5% 3 5% 7% 19%	1980
November	1st to 10th	Day	14% 17% 4 9% 5% 5% 5% 4 5% 7% 25%	1852
	11th to 20th	Day	13% 16% 6% 12% 6% 6% 6% 4 6% 5% 20%	1837
	21st to end	Day	17% 20% 8% 12% 6% 4 5% 3 4 5% 16%	1680
December	1st to 10th	Day	12% 23% 8% 17% 7% 6% 5% 4 4 4 10%	2399
	11th to 20th	Day	16% 22% 5% 14% 8% 6% 4 3 4 5% 13%	2691
	21st to end	Day	22% 23% 9% 15% 5% 5% 3 2 3 3 7%	3553
January	1st to 10th	Night	11% 15% 6% 24% 9% 5% 5% 3 4 4 14%	8233
	11th to 20th	Night	10% 17% 8% 19% 11% 9% 6% 3 4 5% 9%	8236
	21st to end	Night	7% 14% 7% 16% 9% 7% 6% 4 5% 5% 19%	9058
February	1st to 10th	Night	9% 17% 6% 14% 9% 7% 5% 4 5% 5% 19%	2736
	11th to 20th	Night	11% 14% 6% 12% 7% 5% 5% 4 6% 7% 23%	2545
	21st to end	Night	14% 16% 5% 12% 7% 5% 5% 3 4 6% 23%	2076
March	1st to 10th	Night	14% 10% 4 8% 6% 4 4 3 5% 7% 36%	5482
	11th to 20th	Night	11% 12% 4 9% 4 5% 5% 4 7% 7% 33%	5089
	21st to end	Night	18% 13% 6% 10% 5% 4 4 3 3 5% 29%	5588
April	1st to 10th	Night	17% 13% 3 8% 4 4 4 5% 5% 6% 30%	2120
	11th to 20th	Night	19% 10% 4 6% 5% 4 3 4 5% 6% 34%	1857
	21st to end	Night	15% 11% 4 8% 5% 5% 5% 3 4 6% 35%	1800
May	1st to 10th	Night	13% 9% 3 7% 6% 5% 5% 4 5% 5% 37%	4053
	11th to 20th	Night	13% 10% 3 7% 4 4 4 3 6% 6% 40%	4015
	21st to end	Night	18% 8% 3 6% 4 4 3 2 5% 6% 42%	3679
June	1st to 10th	Night	14% 5% 3 6% 5% 4 4 4 5% 8% 42%	1400
	11th to 20th	Night	9% 6% 2 6% 6% 6% 5% 5% 8% 8% 40%	1400
	21st to end	Night	12% 8% 2 6% 4 3 4 3 6% 8% 44%	1400
July	1st to 10th	Night	11% 6% 2 6% 4 4 4 4 7% 8% 45%	3360
	11th to 20th	Night	9% 5% 2 5% 4 4 4 4 5% 6% 51%	3620
	21st to end	Night	8% 5% 2 5% 3 3 4 4 6% 8% 52%	3948
August	1st to 10th	Night	9% 6% 2 7% 5% 4 4 4 6% 7% 47%	2898
	11th to 20th	Night	14% 5% 2 4 3 3 4 3 5% 8% 49%	2875
	21st to end	Night	9% 6% 2 6% 4 4 4 4 5% 7% 50%	3246
September	1st to 10th	Night	10% 5% 2 6% 4 4 4 4 5% 7% 47%	2065
	11th to 20th	Night	12% 5% 2 5% 4 3 4 4 5% 7% 49%	2200
	21st to end	Night	16% 8% 2 7% 4 4 4 3 5% 7% 40%	2200
October	1st to 10th	Night	16% 7% 2 6% 4 3 4 4 5% 5% 43%	2250
	11th to 20th	Night	18% 13% 4 8% 5% 5% 4 3 5% 6% 30%	2437
	21st to end	Night	22% 11% 3 9% 5% 4 4 3 4 6% 28%	2860
November	1st to 10th	Night	14% 13% 5% 9% 5% 5% 5% 3 5% 7% 29%	2666
	11th to 20th	Night	14% 15% 5% 11% 6% 6% 5% 5% 5% 6% 23%	2782
	21st to end	Night	17% 18% 6% 11% 6% 4 4 3 5% 6% 19%	2940
December	1st to 10th	Night	12% 17% 7% 14% 7% 6% 5% 5% 6% 7% 16%	4992
	11th to 20th	Night	13% 23% 7% 16% 7% 5% 4 3 4 4 14%	5736
	21st to end	Night	22% 23% 11% 13% 6% 5% 4 3 3 3 9%	7649

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	73% 3 12% 8% 2 2	4692
	11th to 20th	Day	80% 4 5% 9% 3	4710
	21st to end	Day	68% 4 11% 11% 6%	5156
February	1st to 10th	Day	68% 5% 9% 15% 3	1882
	11th to 20th	Day	64% 4 9% 13% 8% 1	1967
	21st to end	Day	65% 4 9% 16% 5%	1730
March	1st to 10th	Day	50% 4 14% 18% 11% 3	4580
	11th to 20th	Day	57% 4 12% 19% 7% 2	5090
	21st to end	Day	63% 4 8% 17% 8% 1	5586
April	1st to 10th	Day	59% 4 8% 18% 10% 1	2519
	11th to 20th	Day	52% 4 9% 19% 15% 2	2623
	21st to end	Day	51% 4 10% 17% 15% 3	2600
May	1st to 10th	Day	45% 5% 8% 20% 18% 3	5897
	11th to 20th	Day	43% 5% 9% 22% 18% 3	5865
	21st to end	Day	43% 5% 9% 20% 19% 5%	7217
June	1st to 10th	Day	40% 5% 10% 28% 15% 3	3000
	11th to 20th	Day	42% 6% 11% 23% 17% 2	3000
	21st to end	Day	44% 6% 11% 24% 14% 2	3000
July	1st to 10th	Day	40% 6% 10% 26% 16% 1	6532
	11th to 20th	Day	35% 6% 10% 35% 13% 1	6285
	21st to end	Day	33% 6% 11% 30% 18% 1	6728
August	1st to 10th	Day	38% 7% 12% 27% 14% 1	4235
	11th to 20th	Day	41% 6% 12% 27% 14% 1	4213
	21st to end	Day	41% 6% 13% 29% 10% 1	4648
September	1st to 10th	Day	43% 6% 11% 24% 15% 1	2315
	11th to 20th	Day	37% 4 14% 30% 14% 1	2200
	21st to end	Day	46% 5% 14% 24% 11%	2200
October	1st to 10th	Day	36% 4 8% 34% 17% 1	3501
	11th to 20th	Day	55% 3 8% 21% 13%	3176
	21st to end	Day	61% 4 10% 19% 6%	3180
November	1st to 10th	Day	58% 5% 10% 21% 6%	1852
	11th to 20th	Day	62% 6% 15% 12% 5%	1837
	21st to end	Day	69% 4 14% 10% 2	1680
December	1st to 10th	Day	75% 6% 10% 8% 1	2399
	11th to 20th	Day	74% 4 9% 9% 4	2691
	21st to end	Day	83% 3 5% 6% 3	3553
January	1st to 10th	Night	72% 5% 9% 10% 4	8233
	11th to 20th	Night	76% 5% 8% 9% 3	8280
	21st to end	Night	64% 5% 9% 14% 8%	9058
February	1st to 10th	Night	66% 4 10% 14% 6%	2736
	11th to 20th	Night	59% 5% 12% 17% 6% 1	2547
	21st to end	Night	62% 4 11% 16% 6%	2076
March	1st to 10th	Night	49% 3 16% 20% 11% 1	5493
	11th to 20th	Night	49% 4 19% 20% 7% 1	5090
	21st to end	Night	58% 4 9% 17% 11% 1	5588
April	1st to 10th	Night	54% 4 9% 20% 12% 1	2120
	11th to 20th	Night	50% 4 7% 25% 13% 2	1857
	21st to end	Night	51% 5% 10% 17% 15% 3	1800
May	1st to 10th	Night	46% 5% 9% 18% 18% 3	4060
	11th to 20th	Night	43% 4 8% 26% 17% 1	4016
	21st to end	Night	44% 4 11% 22% 14% 5%	3680
June	1st to 10th	Night	39% 5% 13% 26% 17% 1	1400
	11th to 20th	Night	39% 5% 13% 26% 15% 2	1400
	21st to end	Night	37% 6% 13% 26% 14% 3	1400
July	1st to 10th	Night	36% 5% 14% 26% 16% 2	3321
	11th to 20th	Night	33% 4 10% 33% 18%	3572
	21st to end	Night	29% 5% 15% 32% 18%	3832
August	1st to 10th	Night	36% 4 15% 29% 14% 1	2888
	11th to 20th	Night	34% 5% 16% 32% 13% 1	2902
	21st to end	Night	33% 5% 16% 32% 14% 1	3246
September	1st to 10th	Night	34% 6% 14% 28% 16% 1	2065
	11th to 20th	Night	34% 5% 16% 32% 13%	2200
	21st to end	Night	44% 5% 16% 25% 10%	2200
October	1st to 10th	Night	32% 4 12% 35% 16% 1	3679
	11th to 20th	Night	46% 4 13% 24% 13%	3960
	21st to end	Night	52% 4 14% 24% 6%	4610
November	1st to 10th	Night	52% 7% 13% 21% 6% 1	2666
	11th to 20th	Night	59% 5% 14% 15% 6% 1	2782
	21st to end	Night	65% 6% 15% 12% 3	2940
December	1st to 10th	Night	64% 7% 15% 11% 2	4992
	11th to 20th	Night	72% 5% 10% 9% 4	5735
	21st to end	Night	81% 4 5% 6% 3	7649

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	25% 21% 38% 12% 4	330
	11th to 20th	Day	24% 19% 39% 13% 4	330
	21st to end	Day	27% 17% 37% 13% 5% 1	363
February	1st to 10th	Day	25% 26% 34% 11% 3 1	330
	11th to 20th	Day	25% 29% 33% 9% 3	330
	21st to end	Day	32% 26% 34% 7% 1	273
March	1st to 10th	Day	27% 25% 34% 12% 2	330
	11th to 20th	Day	26% 18% 33% 16% 5% 1	540
	21st to end	Day	31% 18% 30% 16% 4 1	594
April	1st to 10th	Day	30% 20% 27% 18% 5% 1	540
	11th to 20th	Day	32% 19% 26% 14% 8% 1	516
	21st to end	Day	27% 17% 29% 18% 6% 2	500
May	1st to 10th	Day	22% 22% 30% 15% 8% 3	503
	11th to 20th	Day	18% 13% 36% 21% 8% 3	527
	21st to end	Day	17% 16% 33% 24% 8% 3	789
June	1st to 10th	Day	15% 18% 39% 20% 7% 1	730
	11th to 20th	Day	18% 15% 35% 22% 8% 2	730
	21st to end	Day	17% 13% 37% 24% 8% 2	730
July	1st to 10th	Day	12% 10% 34% 29% 12% 4	721
	11th to 20th	Day	12% 13% 35% 25% 10% 4	718
	21st to end	Day	11% 9% 34% 29% 13% 4	794
August	1st to 10th	Day	11% 8% 28% 29% 16% 8%	541
	11th to 20th	Day	14% 8% 21% 29% 19% 10%	519
	21st to end	Day	15% 10% 28% 27% 14% 5%	583
September	1st to 10th	Day	16% 11% 28% 24% 15% 7%	530
	11th to 20th	Day	14% 7% 22% 29% 19% 9%	530
	21st to end	Day	20% 10% 23% 27% 13% 5%	530
October	1st to 10th	Day	25% 13% 27% 19% 13% 3	530
	11th to 20th	Day	24% 17% 34% 20% 4 1	490
	21st to end	Day	28% 25% 31% 12% 4	363
November	1st to 10th	Day	27% 19% 35% 15% 5%	330
	11th to 20th	Day	26% 25% 30% 14% 4 1	302
	21st to end	Day	26% 21% 35% 15% 2	220
December	1st to 10th	Day	27% 22% 34% 11% 5%	220
	11th to 20th	Day	29% 25% 33% 11% 2	220
	21st to end	Day	29% 27% 28% 14% 2 1	242
January	1st to 10th	Night	21% 20% 35% 19% 4	1050
	11th to 20th	Night	24% 17% 33% 21% 5% 1	1050
	21st to end	Night	16% 15% 37% 25% 5% 1	1155
February	1st to 10th	Night	19% 20% 37% 20% 4	846
	11th to 20th	Night	20% 20% 32% 23% 4	823
	21st to end	Night	23% 18% 33% 23% 3	692
March	1st to 10th	Night	18% 17% 36% 24% 5% 1	840
	11th to 20th	Night	22% 18% 33% 21% 6%	840
	21st to end	Night	22% 14% 32% 25% 5% 1	924
April	1st to 10th	Night	26% 13% 35% 21% 5%	650
	11th to 20th	Night	26% 13% 27% 26% 7% 1	611
	21st to end	Night	22% 11% 31% 27% 8%	600
May	1st to 10th	Night	15% 10% 36% 31% 8% 1	600
	11th to 20th	Night	20% 9% 28% 34% 9%	591
	21st to end	Night	13% 7% 32% 36% 13%	536
June	1st to 10th	Night	15% 9% 33% 36% 7%	400
	11th to 20th	Night	9% 10% 32% 38% 11% 1	400
	21st to end	Night	15% 7% 30% 36% 11% 1	400
July	1st to 10th	Night	7% 4 28% 46% 14% 1	469
	11th to 20th	Night	10% 4 25% 36% 22% 2	582
	21st to end	Night	9% 4 26% 43% 17% 1	657
August	1st to 10th	Night	10% 4 23% 39% 22% 1	597
	11th to 20th	Night	11% 5% 21% 34% 25% 4	594
	21st to end	Night	15% 5% 21% 37% 20% 1	660
September	1st to 10th	Night	12% 7% 28% 29% 20% 4	665
	11th to 20th	Night	14% 7% 24% 32% 21% 3	800
	21st to end	Night	20% 8% 24% 29% 16% 3	800
October	1st to 10th	Night	19% 9% 28% 29% 12% 1	800
	11th to 20th	Night	26% 13% 33% 22% 7%	800
	21st to end	Night	28% 14% 29% 23% 5% 1	880
November	1st to 10th	Night	24% 14% 32% 22% 7% 1	820
	11th to 20th	Night	22% 19% 35% 20% 4	840
	21st to end	Night	27% 15% 38% 17% 3	840
December	1st to 10th	Night	21% 20% 35% 19% 5%	1008
	11th to 20th	Night	22% 20% 37% 17% 4 1	1050
	21st to end	Night	28% 17% 33% 19% 3	1155

Page generated at 2026-04-14 02:44:26. Version ac07

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