GCC Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the GCC Observatory clear sky chart from 2005-11-06 to 2026-05-17.

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	17% 13% 5% 10% 5% 5% 4 3 5% 6% 27%	132010
	Night	16% 14% 6% 11% 5% 4 4 3 4 6% 26%	130817
Transparency	Day	56% 5% 10% 18% 9% 1	135609
	Night	59% 5% 10% 16% 9% 1	135288
Seeing	Day	26% 17% 29% 18% 8% 2	16478
	Night	25% 15% 27% 22% 9% 1	26170

By Month

Percentage Cloud Cover SE (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	16% 22% 9% 28% 5% 3 2 2 3 3 6%	4998
	11th to 20th	Day	10% 22% 11% 23% 9% 8% 3 1 2 3 8%	5280
	21st to end	Day	17% 16% 8% 16% 10% 8% 5% 3 3 3 11%	5780
February	1st to 10th	Day	15% 20% 7% 16% 9% 4 4 3 3 3 16%	1890
	11th to 20th	Day	15% 17% 8% 16% 9% 5% 4 3 3 4 15%	1843
	21st to end	Day	17% 20% 4 14% 6% 3 4 3 4 5% 20%	1691
March	1st to 10th	Day	17% 12% 8% 12% 6% 4 3 3 4 6% 27%	5045
	11th to 20th	Day	12% 16% 6% 10% 5% 4 3 3 4 6% 30%	5090
	21st to end	Day	21% 14% 7% 11% 4 4 3 2 3 5% 26%	5586
April	1st to 10th	Day	18% 14% 3 9% 5% 4 4 3 4 7% 28%	2431
	11th to 20th	Day	20% 13% 3 6% 5% 4 4 4 4 6% 32%	2569
	21st to end	Day	20% 13% 3 7% 4 4 3 3 4 6% 34%	2730
May	1st to 10th	Day	21% 10% 3 8% 5% 5% 4 4 5% 6% 29%	5992
	11th to 20th	Day	20% 10% 4 8% 4 4 4 4 6% 7% 29%	6016
	21st to end	Day	19% 9% 2 6% 4 5% 5% 4 6% 6% 33%	6934
June	1st to 10th	Day	16% 7% 2 7% 5% 5% 4 4 6% 8% 36%	2800
	11th to 20th	Day	15% 8% 3 7% 5% 5% 4 5% 5% 8% 35%	2800
	21st to end	Day	16% 6% 2 6% 4 5% 5% 5% 6% 9% 37%	2800
July	1st to 10th	Day	9% 6% 2 7% 5% 4 6% 4 6% 10% 40%	6360
	11th to 20th	Day	13% 6% 3 7% 5% 5% 5% 4 6% 9% 37%	5956
	21st to end	Day	13% 6% 2 6% 5% 5% 5% 4 5% 7% 43%	6424
August	1st to 10th	Day	14% 7% 2 8% 6% 5% 5% 5% 7% 9% 32%	4234
	11th to 20th	Day	15% 7% 3 8% 5% 5% 6% 5% 8% 8% 30%	4174
	21st to end	Day	11% 6% 3 7% 5% 5% 5% 5% 6% 9% 38%	4487
September	1st to 10th	Day	18% 7% 3 6% 5% 4 4 3 6% 8% 38%	2400
	11th to 20th	Day	12% 6% 2 7% 4 4 5% 4 6% 7% 43%	2216
	21st to end	Day	20% 9% 3 8% 4 5% 5% 3 6% 6% 32%	2195
October	1st to 10th	Day	17% 7% 2 7% 5% 4 5% 4 6% 7% 37%	2000
	11th to 20th	Day	21% 11% 3 9% 4 5% 5% 3 5% 6% 29%	2000
	21st to end	Day	25% 14% 3 8% 5% 5% 5% 3 4 6% 23%	2200
November	1st to 10th	Day	14% 14% 4 8% 5% 4 5% 3 5% 6% 32%	1926
	11th to 20th	Day	18% 22% 4 9% 4 4 4 3 5% 6% 23%	1685
	21st to end	Day	18% 28% 6% 10% 4 4 3 3 4 5% 14%	1680
December	1st to 10th	Day	18% 29% 15% 14% 4 3 3 2 2 2 8%	2680
	11th to 20th	Day	21% 25% 9% 12% 6% 3 3 2 4 4 11%	3070
	21st to end	Day	25% 25% 8% 14% 5% 4 3 2 2 3 9%	4048
January	1st to 10th	Night	15% 21% 10% 31% 6% 3 2 2 2 2 7%	7891
	11th to 20th	Night	11% 20% 12% 24% 9% 6% 4 2 3 3 8%	7680
	21st to end	Night	17% 18% 8% 15% 8% 5% 6% 3 3 4 14%	8401
February	1st to 10th	Night	18% 19% 6% 13% 8% 5% 4 3 3 4 18%	2730
	11th to 20th	Night	15% 17% 7% 16% 7% 6% 4 3 4 4 16%	2663
	21st to end	Night	18% 16% 5% 17% 6% 4 4 2 4 4 21%	2111
March	1st to 10th	Night	19% 17% 6% 9% 7% 3 3 3 4 6% 26%	5028
	11th to 20th	Night	14% 14% 6% 8% 4 3 3 3 4 6% 34%	5090
	21st to end	Night	18% 14% 6% 10% 5% 4 3 3 3 4 30%	5588
April	1st to 10th	Night	19% 16% 4 7% 4 4 4 4 5% 5% 27%	2188
	11th to 20th	Night	18% 11% 3 7% 5% 5% 4 4 5% 6% 32%	2018
	21st to end	Night	14% 12% 3 7% 4 5% 4 3 5% 7% 34%	1890
May	1st to 10th	Night	26% 11% 3 7% 4 4 4 3 4 5% 29%	4123
	11th to 20th	Night	16% 10% 4 7% 5% 4 4 3 5% 7% 35%	4005
	21st to end	Night	22% 7% 2 4 4 4 3 3 5% 9% 37%	3920
June	1st to 10th	Night	11% 6% 2 8% 4 5% 4 5% 7% 9% 40%	1600
	11th to 20th	Night	12% 7% 3 7% 5% 4 5% 4 5% 8% 41%	1600
	21st to end	Night	12% 6% 3 6% 4 5% 5% 4 7% 8% 41%	1600
July	1st to 10th	Night	13% 7% 3 6% 4 3 4 3 6% 10% 41%	3598
	11th to 20th	Night	13% 6% 2 6% 4 5% 4 3 7% 9% 41%	3997
	21st to end	Night	9% 5% 2 6% 4 4 5% 4 6% 9% 47%	4427
August	1st to 10th	Night	12% 8% 3 7% 5% 4 5% 4 6% 7% 39%	2915
	11th to 20th	Night	16% 8% 2 6% 4 4 5% 4 5% 7% 39%	2875
	21st to end	Night	9% 6% 2 7% 5% 5% 5% 4 7% 9% 42%	3376
September	1st to 10th	Night	13% 6% 2 5% 4 4 4 3 6% 8% 44%	2000
	11th to 20th	Night	12% 5% 2 5% 4 4 4 3 5% 7% 50%	2180
	21st to end	Night	18% 9% 3 7% 5% 4 4 3 6% 7% 35%	2205
October	1st to 10th	Night	17% 7% 2 6% 4 3 4 3 5% 6% 43%	2400
	11th to 20th	Night	20% 10% 3 7% 5% 5% 4 4 6% 6% 32%	2400
	21st to end	Night	25% 13% 3 6% 4 4 4 3 4 5% 28%	2640
November	1st to 10th	Night	15% 13% 3 8% 4 4 4 3 6% 7% 33%	2591
	11th to 20th	Night	19% 20% 4 8% 5% 5% 5% 3 5% 5% 21%	2932
	21st to end	Night	20% 22% 6% 9% 5% 4 4 3 4 5% 19%	2940
December	1st to 10th	Night	16% 23% 12% 13% 5% 4 4 3 3 4 13%	4720
	11th to 20th	Night	19% 24% 10% 12% 5% 4 3 2 3 4 14%	5345
	21st to end	Night	22% 23% 8% 12% 6% 4 4 3 4 4 12%	7150

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	86% 3 5% 4 2	4999
	11th to 20th	Day	85% 2 5% 6% 2	5280
	21st to end	Day	80% 3 6% 8% 4	5780
February	1st to 10th	Day	75% 3 6% 10% 5%	1890
	11th to 20th	Day	74% 4 6% 11% 5%	1852
	21st to end	Day	67% 4 9% 14% 4 2	1691
March	1st to 10th	Day	60% 4 12% 19% 5%	5045
	11th to 20th	Day	56% 5% 15% 18% 6% 1	5090
	21st to end	Day	62% 4 5% 18% 9% 1	5586
April	1st to 10th	Day	56% 5% 12% 15% 11% 1	2431
	11th to 20th	Day	53% 5% 9% 19% 11% 2	2569
	21st to end	Day	53% 4 10% 20% 10% 3	2730
May	1st to 10th	Day	55% 6% 8% 20% 8% 4	6027
	11th to 20th	Day	52% 7% 10% 20% 10% 1	6020
	21st to end	Day	48% 7% 12% 18% 12% 3	6949
June	1st to 10th	Day	45% 6% 12% 23% 12% 1	2800
	11th to 20th	Day	46% 7% 13% 21% 12% 1	2800
	21st to end	Day	42% 7% 12% 24% 14% 1	2800
July	1st to 10th	Day	38% 7% 13% 25% 17% 1	6290
	11th to 20th	Day	41% 7% 13% 23% 15% 1	5885
	21st to end	Day	39% 6% 11% 24% 19% 1	6240
August	1st to 10th	Day	46% 7% 14% 23% 10%	4221
	11th to 20th	Day	46% 9% 13% 24% 6% 1	4213
	21st to end	Day	39% 8% 12% 27% 12% 1	4487
September	1st to 10th	Day	43% 7% 11% 22% 14% 2	2400
	11th to 20th	Day	39% 7% 12% 27% 15% 1	2216
	21st to end	Day	51% 6% 11% 21% 11% 1	2195
October	1st to 10th	Day	39% 6% 14% 23% 17% 1	3260
	11th to 20th	Day	47% 6% 13% 18% 12% 4	3240
	21st to end	Day	48% 5% 14% 23% 9% 1	3538
November	1st to 10th	Day	52% 7% 10% 18% 11% 2	1926
	11th to 20th	Day	64% 5% 9% 14% 6% 2	1685
	21st to end	Day	73% 5% 8% 9% 4 1	1680
December	1st to 10th	Day	85% 4 5% 5% 1	2680
	11th to 20th	Day	77% 4 6% 11% 1	3066
	21st to end	Day	83% 3 7% 6% 1	4048
January	1st to 10th	Night	87% 3 4 5% 1	7892
	11th to 20th	Night	84% 3 6% 5% 2	7680
	21st to end	Night	75% 4 8% 10% 3	8401
February	1st to 10th	Night	72% 5% 7% 11% 5%	2730
	11th to 20th	Night	73% 4 8% 11% 4	2665
	21st to end	Night	69% 3 9% 12% 7% 1	2111
March	1st to 10th	Night	61% 4 12% 17% 6%	5028
	11th to 20th	Night	52% 4 12% 21% 9% 1	5090
	21st to end	Night	58% 5% 7% 15% 13% 1	5588
April	1st to 10th	Night	59% 5% 10% 15% 11% 1	2188
	11th to 20th	Night	52% 6% 10% 18% 12% 2	2018
	21st to end	Night	49% 6% 11% 20% 10% 3	1890
May	1st to 10th	Night	57% 5% 8% 15% 11% 5%	4150
	11th to 20th	Night	49% 7% 10% 22% 11% 1	4006
	21st to end	Night	44% 6% 11% 24% 13% 2	3927
June	1st to 10th	Night	40% 8% 11% 24% 15% 3	1600
	11th to 20th	Night	41% 7% 13% 22% 16% 2	1600
	21st to end	Night	39% 8% 13% 23% 17% 1	1600
July	1st to 10th	Night	40% 5% 11% 26% 17% 2	3560
	11th to 20th	Night	38% 7% 13% 26% 14% 1	3948
	21st to end	Night	34% 8% 12% 21% 25% 1	4297
August	1st to 10th	Night	42% 7% 12% 26% 12% 1	2904
	11th to 20th	Night	43% 8% 13% 28% 8% 1	2902
	21st to end	Night	35% 9% 15% 28% 11% 1	3376
September	1st to 10th	Night	37% 7% 13% 27% 14% 2	2000
	11th to 20th	Night	34% 6% 13% 27% 20% 1	2180
	21st to end	Night	48% 6% 12% 20% 13% 1	2205
October	1st to 10th	Night	38% 7% 10% 22% 22% 1	3920
	11th to 20th	Night	46% 6% 12% 15% 16% 4	3899
	21st to end	Night	43% 4 15% 23% 14% 1	4252
November	1st to 10th	Night	50% 6% 11% 21% 9% 2	2591
	11th to 20th	Night	64% 5% 9% 13% 6% 3	2932
	21st to end	Night	69% 5% 9% 13% 3 1	2940
December	1st to 10th	Night	77% 4 9% 8% 2	4720
	11th to 20th	Night	76% 3 8% 10% 2	5348
	21st to end	Night	77% 5% 8% 10% 1	7150

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	34% 28% 27% 7% 2	281
	11th to 20th	Day	36% 33% 23% 5% 2 1	330
	21st to end	Day	35% 29% 27% 7% 2	363
February	1st to 10th	Day	35% 24% 33% 7% 1	330
	11th to 20th	Day	41% 26% 27% 6% 1	330
	21st to end	Day	41% 26% 28% 5%	286
March	1st to 10th	Day	32% 24% 32% 9% 2	540
	11th to 20th	Day	35% 21% 31% 9% 2 1	540
	21st to end	Day	37% 22% 26% 12% 2	594
April	1st to 10th	Day	32% 24% 28% 11% 4 1	540
	11th to 20th	Day	34% 21% 24% 13% 6% 2	532
	21st to end	Day	30% 20% 30% 14% 5% 1	540
May	1st to 10th	Day	29% 20% 31% 15% 4 2	543
	11th to 20th	Day	26% 17% 26% 20% 9% 1	551
	21st to end	Day	20% 12% 31% 23% 12% 3	573
June	1st to 10th	Day	19% 16% 31% 23% 7% 3	530
	11th to 20th	Day	22% 12% 27% 26% 12% 2	530
	21st to end	Day	17% 13% 32% 25% 10% 2	530
July	1st to 10th	Day	11% 10% 26% 30% 17% 6%	525
	11th to 20th	Day	15% 10% 30% 27% 14% 5%	524
	21st to end	Day	11% 9% 35% 27% 15% 4	576
August	1st to 10th	Day	13% 8% 28% 28% 18% 6%	526
	11th to 20th	Day	17% 6% 27% 29% 13% 8%	519
	21st to end	Day	13% 9% 28% 28% 16% 5%	583
September	1st to 10th	Day	19% 10% 24% 24% 18% 5%	530
	11th to 20th	Day	11% 5% 25% 30% 22% 7%	530
	21st to end	Day	24% 14% 28% 19% 11% 4	530
October	1st to 10th	Day	20% 15% 25% 22% 14% 3	530
	11th to 20th	Day	27% 18% 34% 14% 5% 2	530
	21st to end	Day	35% 19% 30% 14% 2	583
November	1st to 10th	Day	31% 21% 28% 15% 4 1	404
	11th to 20th	Day	37% 24% 26% 9% 4	223
	21st to end	Day	36% 23% 34% 7%	220
December	1st to 10th	Day	40% 28% 25% 3 3	220
	11th to 20th	Day	43% 22% 27% 7% 1	220
	21st to end	Day	38% 23% 31% 6% 1	242
January	1st to 10th	Night	34% 26% 29% 9% 3	840
	11th to 20th	Night	36% 23% 27% 12% 1	840
	21st to end	Night	27% 28% 30% 13% 2	924
February	1st to 10th	Night	33% 21% 30% 15% 1	840
	11th to 20th	Night	30% 25% 31% 12% 1	823
	21st to end	Night	31% 23% 32% 12% 2	692
March	1st to 10th	Night	28% 20% 33% 15% 4	840
	11th to 20th	Night	30% 19% 30% 17% 4	840
	21st to end	Night	28% 18% 31% 18% 4 1	924
April	1st to 10th	Night	30% 18% 30% 18% 5%	840
	11th to 20th	Night	28% 14% 31% 20% 6% 1	774
	21st to end	Night	23% 14% 32% 27% 5%	630
May	1st to 10th	Night	25% 10% 28% 27% 8% 1	630
	11th to 20th	Night	20% 9% 31% 31% 8% 1	580
	21st to end	Night	15% 8% 28% 29% 18% 2	433
June	1st to 10th	Night	14% 7% 33% 33% 14%	400
	11th to 20th	Night	13% 10% 27% 39% 12%	400
	21st to end	Night	15% 6% 28% 33% 18% 1	400
July	1st to 10th	Night	11% 4 20% 42% 19% 4	390
	11th to 20th	Night	12% 4 21% 35% 25% 3	550
	21st to end	Night	13% 3 19% 38% 26% 1	657
August	1st to 10th	Night	11% 5% 19% 38% 25% 2	597
	11th to 20th	Night	13% 4 17% 34% 28% 3	594
	21st to end	Night	14% 5% 22% 31% 25% 3	748
September	1st to 10th	Night	17% 8% 19% 32% 22% 3	800
	11th to 20th	Night	13% 5% 19% 33% 26% 4	800
	21st to end	Night	23% 11% 21% 28% 15% 3	800
October	1st to 10th	Night	20% 8% 27% 26% 17% 2	800
	11th to 20th	Night	28% 14% 28% 22% 8% 1	800
	21st to end	Night	35% 14% 29% 18% 3	880
November	1st to 10th	Night	26% 16% 28% 23% 7%	820
	11th to 20th	Night	33% 20% 27% 13% 5% 1	840
	21st to end	Night	34% 20% 29% 12% 4	840
December	1st to 10th	Night	36% 23% 27% 11% 3	840
	11th to 20th	Night	32% 29% 25% 12% 2	840
	21st to end	Night	36% 20% 30% 13% 1	924

Page generated at 2026-05-14 01:58:55. Version ac07

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