Grant O. Gale Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Grant O. Gale Observatory 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	12% 8% 3 8% 5% 5% 5% 4 5% 7% 39%	131761
	Night	12% 9% 3 8% 4 4 4 3 5% 6% 41%	130357
Transparency	Day	44% 4 12% 25% 14% 1	135359
	Night	43% 3 12% 25% 15% 2	134836
Seeing	Day	17% 10% 31% 26% 13% 3	17682
	Night	16% 9% 33% 30% 10% 1	27698

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	8% 12% 5% 20% 7% 6% 5% 4 4 5% 24%	4691
	11th to 20th	Day	5% 10% 4 12% 6% 8% 6% 5% 6% 7% 32%	5112
	21st to end	Day	8% 12% 5% 13% 6% 7% 6% 4 4 5% 30%	5812
February	1st to 10th	Day	11% 13% 4 10% 6% 5% 5% 4 6% 7% 29%	1890
	11th to 20th	Day	12% 12% 4 8% 5% 5% 4 4 5% 7% 34%	1844
	21st to end	Day	12% 11% 4 10% 5% 4 4 3 4 6% 37%	1702
March	1st to 10th	Day	18% 10% 3 6% 3 3 3 2 3 5% 44%	4954
	11th to 20th	Day	10% 6% 3 7% 4 4 4 3 5% 7% 47%	5090
	21st to end	Day	19% 13% 4 9% 5% 4 4 4 5% 7% 26%	5665
April	1st to 10th	Day	17% 11% 4 7% 4 4 4 4 5% 7% 33%	2412
	11th to 20th	Day	17% 9% 3 8% 5% 4 5% 4 5% 6% 35%	2374
	21st to end	Day	22% 8% 3 7% 5% 4 4 4 6% 6% 32%	2478
May	1st to 10th	Day	16% 7% 2 7% 5% 4 5% 4 6% 7% 37%	5903
	11th to 20th	Day	15% 6% 2 5% 3 4 4 4 5% 7% 46%	6319
	21st to end	Day	15% 7% 2 7% 5% 4 5% 4 6% 7% 38%	6935
June	1st to 10th	Day	10% 6% 2 6% 5% 4 5% 4 7% 9% 44%	2800
	11th to 20th	Day	9% 5% 2 6% 5% 5% 5% 4 6% 9% 45%	2800
	21st to end	Day	10% 6% 2 6% 6% 4 6% 5% 7% 9% 39%	2800
July	1st to 10th	Day	8% 6% 3 7% 5% 5% 5% 4 7% 9% 40%	6360
	11th to 20th	Day	9% 5% 2 7% 4 4 4 4 6% 8% 47%	6360
	21st to end	Day	9% 5% 2 6% 5% 5% 5% 5% 7% 10% 40%	6523
August	1st to 10th	Day	7% 5% 2 6% 4 5% 6% 5% 7% 10% 44%	4234
	11th to 20th	Day	11% 6% 2 7% 5% 4 5% 4 6% 9% 40%	4171
	21st to end	Day	8% 5% 2 6% 4 5% 5% 3 5% 9% 49%	4320
September	1st to 10th	Day	12% 5% 2 5% 3 3 3 4 5% 7% 52%	2400
	11th to 20th	Day	10% 5% 2 5% 4 4 4 4 5% 8% 49%	2400
	21st to end	Day	11% 5% 2 5% 4 4 4 3 5% 8% 49%	2085
October	1st to 10th	Day	14% 6% 2 6% 4 3 4 3 4 5% 50%	2000
	11th to 20th	Day	15% 8% 3 7% 4 4 4 2 4 6% 44%	2000
	21st to end	Day	15% 10% 3 6% 4 4 4 3 4 6% 42%	2200
November	1st to 10th	Day	12% 6% 2 5% 4 5% 5% 4 6% 7% 45%	1890
	11th to 20th	Day	13% 10% 4 7% 5% 4 3 3 5% 7% 38%	1709
	21st to end	Day	15% 11% 4 7% 4 3 3 3 5% 7% 37%	1680
December	1st to 10th	Day	7% 9% 3 7% 5% 4 3 3 5% 7% 47%	2690
	11th to 20th	Day	13% 11% 4 16% 6% 5% 4 3 5% 5% 26%	3088
	21st to end	Day	23% 19% 9% 12% 4 3 3 3 4 4 16%	4070
January	1st to 10th	Night	10% 13% 5% 16% 6% 4 4 3 5% 6% 30%	8232
	11th to 20th	Night	7% 11% 4 10% 7% 6% 6% 4 7% 6% 31%	7860
	21st to end	Night	10% 12% 5% 11% 6% 4 4 3 4 6% 35%	8402
February	1st to 10th	Night	10% 13% 4 9% 6% 5% 4 3 5% 6% 34%	2730
	11th to 20th	Night	11% 10% 3 8% 4 4 4 3 6% 7% 40%	2663
	21st to end	Night	13% 10% 3 8% 4 5% 4 3 4 5% 42%	2102
March	1st to 10th	Night	17% 8% 3 5% 2 3 3 3 4 5% 48%	5152
	11th to 20th	Night	9% 7% 3 5% 4 3 3 3 6% 7% 49%	5090
	21st to end	Night	17% 11% 5% 11% 5% 4 4 4 5% 5% 28%	5506
April	1st to 10th	Night	15% 9% 3 7% 4 3 4 3 5% 7% 41%	2010
	11th to 20th	Night	14% 9% 3 7% 5% 4 3 4 4 6% 40%	1975
	21st to end	Night	18% 7% 2 6% 4 4 4 3 5% 6% 40%	1919
May	1st to 10th	Night	16% 7% 2 5% 4 4 4 3 6% 7% 43%	4009
	11th to 20th	Night	12% 6% 2 6% 4 4 3 3 5% 7% 48%	3596
	21st to end	Night	16% 7% 3 6% 4 4 4 3 5% 6% 42%	3942
June	1st to 10th	Night	8% 5% 2 5% 4 5% 4 4 6% 8% 50%	1600
	11th to 20th	Night	7% 5% 2 4 4 4 4 4 7% 11% 48%	1600
	21st to end	Night	11% 5% 2 5% 3 5% 4 5% 5% 8% 48%	1600
July	1st to 10th	Night	8% 4 2 6% 4 4 5% 4 7% 10% 46%	3618
	11th to 20th	Night	6% 4 2 6% 4 3 4 3 5% 8% 55%	3617
	21st to end	Night	9% 5% 2 6% 4 4 5% 4 6% 9% 46%	4330
August	1st to 10th	Night	5% 4 2 5% 4 5% 4 3 6% 9% 53%	2926
	11th to 20th	Night	9% 4 2 5% 4 3 3 3 5% 7% 56%	2888
	21st to end	Night	8% 4 2 5% 4 4 4 3 5% 8% 54%	3576
September	1st to 10th	Night	9% 5% 2 5% 3 3 4 2 4 6% 57%	2000
	11th to 20th	Night	11% 5% 1 5% 3 4 4 3 5% 7% 52%	2000
	21st to end	Night	9% 4 2 4 3 3 3 3 4 7% 57%	2290
October	1st to 10th	Night	13% 6% 2 5% 3 3 3 3 4 6% 53%	2400
	11th to 20th	Night	14% 6% 3 5% 3 4 3 3 5% 6% 49%	2400
	21st to end	Night	16% 8% 3 6% 4 4 3 3 4 6% 44%	2640
November	1st to 10th	Night	11% 5% 2 5% 3 3 3 3 4 6% 53%	2626
	11th to 20th	Night	13% 9% 3 7% 5% 3 4 3 5% 6% 42%	2909
	21st to end	Night	16% 10% 3 7% 4 3 4 3 5% 6% 40%	2940
December	1st to 10th	Night	6% 9% 4 8% 4 4 4 4 5% 7% 45%	4720
	11th to 20th	Night	13% 10% 4 13% 5% 4 4 3 4 6% 34%	5339
	21st to end	Night	20% 17% 8% 13% 5% 4 4 3 4 4 18%	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	63% 3 12% 13% 9% 1	4692
	11th to 20th	Day	53% 2 8% 21% 15% 1	5112
	21st to end	Day	58% 2 11% 22% 7%	5812
February	1st to 10th	Day	54% 3 15% 19% 8% 1	1890
	11th to 20th	Day	49% 5% 11% 23% 10% 2	1853
	21st to end	Day	50% 3 12% 22% 10% 3	1702
March	1st to 10th	Day	46% 2 9% 25% 16% 2	4954
	11th to 20th	Day	37% 4 9% 31% 17% 2	5090
	21st to end	Day	59% 4 16% 16% 5% 1	5667
April	1st to 10th	Day	51% 4 11% 24% 9%	2412
	11th to 20th	Day	50% 5% 11% 21% 12% 1	2374
	21st to end	Day	51% 5% 11% 21% 12% 1	2478
May	1st to 10th	Day	45% 5% 12% 20% 17% 1	5941
	11th to 20th	Day	38% 4 11% 28% 16% 3	6323
	21st to end	Day	45% 4 10% 26% 13% 1	6949
June	1st to 10th	Day	36% 6% 16% 25% 15% 2	2800
	11th to 20th	Day	36% 5% 13% 24% 20% 3	2800
	21st to end	Day	39% 6% 15% 23% 14% 2	2800
July	1st to 10th	Day	40% 3 13% 31% 11% 1	6291
	11th to 20th	Day	37% 3 13% 30% 16% 1	6285
	21st to end	Day	40% 4 19% 23% 13% 1	6338
August	1st to 10th	Day	34% 5% 16% 29% 15% 1	4220
	11th to 20th	Day	41% 4 13% 27% 14% 1	4210
	21st to end	Day	34% 3 16% 29% 16% 2	4319
September	1st to 10th	Day	32% 4 13% 25% 23% 4	2400
	11th to 20th	Day	34% 4 13% 30% 18% 1	2400
	21st to end	Day	34% 3 10% 33% 19% 1	2085
October	1st to 10th	Day	23% 2 6% 28% 39% 2	3260
	11th to 20th	Day	31% 2 8% 30% 25% 4	3240
	21st to end	Day	41% 2 9% 29% 16% 3	3538
November	1st to 10th	Day	37% 4 15% 25% 16% 2	1890
	11th to 20th	Day	45% 4 12% 23% 15% 1	1709
	21st to end	Day	47% 4 14% 24% 10% 1	1680
December	1st to 10th	Day	38% 2 13% 38% 7% 1	2690
	11th to 20th	Day	61% 3 11% 15% 10% 1	3085
	21st to end	Day	73% 3 8% 12% 4	4070
January	1st to 10th	Night	57% 2 15% 18% 8% 1	8233
	11th to 20th	Night	53% 2 11% 17% 16% 1	7860
	21st to end	Night	52% 2 13% 21% 12% 1	8402
February	1st to 10th	Night	52% 3 12% 22% 11% 1	2730
	11th to 20th	Night	44% 4 11% 27% 12% 2	2664
	21st to end	Night	45% 3 11% 22% 16% 3	2102
March	1st to 10th	Night	40% 2 7% 30% 19% 2	5152
	11th to 20th	Night	34% 3 8% 32% 18% 4	5090
	21st to end	Night	57% 5% 15% 15% 7% 2	5506
April	1st to 10th	Night	43% 4 10% 27% 14% 2	2010
	11th to 20th	Night	45% 3 11% 22% 17% 3	1975
	21st to end	Night	44% 4 13% 23% 13% 3	1919
May	1st to 10th	Night	41% 4 9% 25% 20% 1	4036
	11th to 20th	Night	37% 4 11% 27% 17% 4	3597
	21st to end	Night	43% 5% 10% 25% 15% 2	3950
June	1st to 10th	Night	31% 6% 15% 28% 16% 3	1600
	11th to 20th	Night	30% 5% 15% 23% 22% 5%	1600
	21st to end	Night	33% 6% 12% 27% 18% 4	1600
July	1st to 10th	Night	34% 3 13% 34% 15% 2	3580
	11th to 20th	Night	29% 3 16% 31% 19% 2	3572
	21st to end	Night	36% 4 18% 26% 15% 1	4204
August	1st to 10th	Night	28% 4 17% 34% 16% 2	2916
	11th to 20th	Night	30% 3 16% 35% 15% 1	2915
	21st to end	Night	31% 3 14% 28% 18% 6%	3576
September	1st to 10th	Night	29% 4 9% 29% 26% 3	2000
	11th to 20th	Night	33% 2 10% 33% 20% 2	2000
	21st to end	Night	29% 3 10% 31% 25% 3	2290
October	1st to 10th	Night	24% 2 5% 26% 40% 3	3920
	11th to 20th	Night	28% 2 9% 25% 28% 8%	3899
	21st to end	Night	44% 2 9% 24% 18% 3	4252
November	1st to 10th	Night	33% 3 14% 28% 18% 4	2626
	11th to 20th	Night	44% 3 11% 26% 13% 2	2909
	21st to end	Night	47% 3 11% 27% 12% 1	2940
December	1st to 10th	Night	40% 3 11% 37% 9% 1	4720
	11th to 20th	Night	53% 3 12% 24% 7% 1	5341
	21st to end	Night	70% 3 9% 12% 5%	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	23% 13% 35% 20% 8% 1	330
	11th to 20th	Day	22% 14% 45% 15% 4	330
	21st to end	Day	19% 14% 40% 24% 2 1	363
February	1st to 10th	Day	21% 18% 40% 15% 6%	330
	11th to 20th	Day	23% 15% 37% 18% 5% 1	330
	21st to end	Day	26% 15% 43% 13% 3 1	273
March	1st to 10th	Day	20% 20% 41% 15% 4	330
	11th to 20th	Day	28% 15% 31% 20% 6%	330
	21st to end	Day	34% 16% 31% 16% 3 1	394
April	1st to 10th	Day	28% 16% 35% 17% 5%	504
	11th to 20th	Day	25% 16% 30% 21% 7% 1	492
	21st to end	Day	26% 12% 37% 17% 7% 2	578
May	1st to 10th	Day	22% 11% 35% 21% 9% 1	703
	11th to 20th	Day	23% 10% 29% 24% 12% 2	725
	21st to end	Day	17% 8% 30% 27% 13% 4	789
June	1st to 10th	Day	10% 5% 28% 30% 21% 5%	730
	11th to 20th	Day	12% 5% 30% 34% 17% 2	730
	21st to end	Day	12% 8% 32% 31% 14% 3	730
July	1st to 10th	Day	9% 2 21% 36% 20% 12%	721
	11th to 20th	Day	9% 4 26% 35% 21% 6%	718
	21st to end	Day	7% 2 24% 38% 24% 4	794
August	1st to 10th	Day	9% 3 23% 36% 24% 5%	726
	11th to 20th	Day	11% 4 21% 34% 25% 5%	715
	21st to end	Day	10% 2 21% 38% 24% 6%	591
September	1st to 10th	Day	16% 6% 21% 30% 22% 6%	530
	11th to 20th	Day	10% 2 25% 35% 22% 5%	530
	21st to end	Day	13% 12% 27% 26% 18% 5%	330
October	1st to 10th	Day	18% 7% 30% 28% 12% 4	330
	11th to 20th	Day	19% 21% 37% 16% 7%	330
	21st to end	Day	23% 14% 35% 20% 6% 2	363
November	1st to 10th	Day	18% 13% 42% 19% 6% 2	330
	11th to 20th	Day	22% 19% 36% 16% 6% 1	330
	21st to end	Day	22% 15% 40% 19% 4	330
December	1st to 10th	Day	14% 13% 46% 18% 7% 2	330
	11th to 20th	Day	19% 17% 35% 20% 8% 2	330
	21st to end	Day	31% 14% 40% 13% 3	363
January	1st to 10th	Night	14% 13% 40% 25% 7%	1050
	11th to 20th	Night	17% 14% 38% 24% 5% 1	1050
	21st to end	Night	18% 12% 38% 25% 5%	1155
February	1st to 10th	Night	17% 13% 40% 25% 5%	1050
	11th to 20th	Night	17% 14% 39% 25% 4	1029
	21st to end	Night	19% 14% 40% 24% 3 1	718
March	1st to 10th	Night	19% 13% 39% 21% 7%	692
	11th to 20th	Night	19% 11% 36% 24% 9%	630
	21st to end	Night	23% 11% 39% 22% 5% 1	693
April	1st to 10th	Night	19% 16% 38% 24% 3	603
	11th to 20th	Night	20% 13% 37% 25% 6%	593
	21st to end	Night	22% 9% 36% 25% 8% 1	600
May	1st to 10th	Night	21% 6% 34% 32% 7% 2	600
	11th to 20th	Night	18% 6% 33% 31% 11%	591
	21st to end	Night	18% 4 30% 34% 14%	650
June	1st to 10th	Night	10% 5% 24% 43% 17% 1	600
	11th to 20th	Night	9% 4 29% 44% 14% 1	600
	21st to end	Night	13% 4 29% 40% 14% 1	600
July	1st to 10th	Night	7% 1 14% 46% 28% 4	585
	11th to 20th	Night	6% 1 23% 42% 25% 3	582
	21st to end	Night	7% 2 17% 49% 23% 2	657
August	1st to 10th	Night	7% 2 17% 47% 25% 3	597
	11th to 20th	Night	9% 2 16% 43% 28% 3	594
	21st to end	Night	11% 1 17% 43% 26% 2	660
September	1st to 10th	Night	12% 4 20% 41% 20% 3	600
	11th to 20th	Night	14% 2 21% 40% 21% 2	600
	21st to end	Night	11% 6% 29% 36% 17% 1	695
October	1st to 10th	Night	15% 6% 32% 33% 14% 1	800
	11th to 20th	Night	17% 10% 36% 29% 7%	800
	21st to end	Night	21% 11% 34% 26% 8%	880
November	1st to 10th	Night	16% 10% 37% 28% 8%	820
	11th to 20th	Night	18% 14% 40% 21% 6% 1	1019
	21st to end	Night	21% 12% 38% 21% 8%	1050
December	1st to 10th	Night	17% 13% 39% 27% 4 1	1050
	11th to 20th	Night	21% 12% 36% 26% 5% 1	1050
	21st to end	Night	26% 11% 37% 21% 5%	1155

Page generated at 2026-03-30 01:18:44. Version ac07

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