Scopehill Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Scopehill 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% 7% 2 6% 4 4 4 3 5% 7% 46%	131077
	Night	11% 7% 2 5% 4 3 4 3 5% 6% 50%	131027
Transparency	Day	38% 3 11% 27% 18% 3	134688
	Night	36% 2 10% 28% 20% 4	135493
Seeing	Day	15% 8% 26% 28% 18% 5%	17407
	Night	15% 7% 30% 34% 14% 1	27400

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% 9% 3 7% 5% 3 3 3 4 6% 41%	5289
	11th to 20th	Day	7% 9% 3 8% 5% 4 4 3 5% 6% 46%	5310
	21st to end	Day	15% 11% 3 6% 3 3 3 2 3 4 46%	5812
February	1st to 10th	Day	16% 10% 3 8% 4 4 3 3 3 6% 41%	1890
	11th to 20th	Day	13% 10% 3 7% 5% 4 3 2 3 5% 44%	1846
	21st to end	Day	12% 9% 3 6% 4 5% 4 4 5% 6% 42%	1829
March	1st to 10th	Day	13% 7% 3 6% 4 3 3 3 4 5% 50%	5046
	11th to 20th	Day	11% 6% 2 4 3 3 4 3 4 6% 55%	5090
	21st to end	Day	13% 10% 3 7% 5% 5% 5% 4 6% 7% 37%	5583
April	1st to 10th	Day	13% 8% 2 5% 5% 4 4 4 5% 6% 44%	2311
	11th to 20th	Day	15% 7% 2 6% 4 4 5% 3 5% 6% 42%	2374
	21st to end	Day	21% 7% 3 7% 5% 4 4 4 5% 5% 36%	2500
May	1st to 10th	Day	19% 8% 2 6% 4 4 4 3 5% 7% 39%	5862
	11th to 20th	Day	13% 6% 2 6% 4 4 4 3 6% 8% 44%	5862
	21st to end	Day	20% 6% 2 6% 4 4 4 3 5% 8% 39%	6434
June	1st to 10th	Day	7% 5% 2 5% 4 4 5% 5% 7% 10% 47%	2685
	11th to 20th	Day	8% 5% 2 5% 4 4 4 4 7% 9% 48%	2793
	21st to end	Day	5% 3 1 4 3 4 5% 4 6% 9% 56%	2600
July	1st to 10th	Day	9% 5% 2 7% 4 4 5% 4 6% 9% 44%	5900
	11th to 20th	Day	7% 5% 2 6% 4 5% 5% 4 6% 9% 47%	5900
	21st to end	Day	5% 4 2 5% 5% 5% 6% 5% 9% 11% 43%	6422
August	1st to 10th	Day	5% 3 2 5% 4 4 5% 4 6% 10% 54%	4234
	11th to 20th	Day	8% 5% 2 6% 5% 4 5% 4 6% 8% 47%	4171
	21st to end	Day	6% 4 2 5% 3 4 4 4 6% 8% 55%	4359
September	1st to 10th	Day	12% 5% 2 5% 3 4 4 3 4 7% 52%	2400
	11th to 20th	Day	10% 4 2 7% 4 4 4 3 6% 9% 48%	2209
	21st to end	Day	9% 4 1 5% 4 4 4 3 5% 6% 55%	2190
October	1st to 10th	Day	11% 5% 2 5% 3 4 3 3 4 6% 55%	2000
	11th to 20th	Day	12% 6% 2 5% 3 2 2 2 3 6% 57%	2000
	21st to end	Day	20% 6% 3 5% 2 3 3 3 4 4 48%	2200
November	1st to 10th	Day	11% 5% 2 5% 3 3 3 3 4 5% 56%	2058
	11th to 20th	Day	15% 6% 2 4 3 3 3 3 4 6% 52%	1938
	21st to end	Day	16% 7% 3 6% 3 2 2 2 4 5% 50%	1685
December	1st to 10th	Day	10% 8% 2 5% 3 2 3 2 4 5% 55%	2690
	11th to 20th	Day	12% 7% 3 5% 3 3 3 2 4 6% 53%	3088
	21st to end	Day	21% 19% 5% 7% 4 3 3 3 3 5% 28%	4517
January	1st to 10th	Night	14% 8% 3 8% 5% 4 4 3 4 5% 43%	7634
	11th to 20th	Night	8% 6% 2 7% 5% 4 5% 3 4 5% 50%	7680
	21st to end	Night	15% 9% 3 6% 4 4 4 2 4 5% 44%	8402
February	1st to 10th	Night	13% 11% 3 6% 4 3 4 3 5% 5% 42%	2730
	11th to 20th	Night	14% 8% 3 6% 3 4 3 3 5% 6% 44%	2663
	21st to end	Night	14% 8% 2 5% 4 3 3 3 5% 6% 47%	1992
March	1st to 10th	Night	15% 8% 2 5% 3 3 3 3 4 5% 49%	5029
	11th to 20th	Night	8% 6% 2 4 3 3 3 3 4 6% 57%	5090
	21st to end	Night	12% 6% 2 6% 4 4 5% 4 6% 7% 44%	5588
April	1st to 10th	Night	12% 6% 2 6% 4 3 4 3 4 7% 49%	2109
	11th to 20th	Night	14% 7% 2 6% 4 3 3 3 4 5% 49%	1975
	21st to end	Night	17% 7% 2 6% 5% 3 4 3 5% 6% 43%	1895
May	1st to 10th	Night	17% 8% 2 5% 4 3 3 3 4 6% 45%	4053
	11th to 20th	Night	13% 7% 3 5% 3 3 3 3 5% 7% 47%	4053
	21st to end	Night	17% 6% 2 4 3 3 3 2 4 7% 49%	4443
June	1st to 10th	Night	6% 3 2 4 4 4 4 4 7% 10% 52%	1700
	11th to 20th	Night	5% 4 1 4 4 4 5% 5% 8% 11% 51%	1600
	21st to end	Night	4 3 1 4 3 4 3 4 6% 8% 60%	1791
July	1st to 10th	Night	8% 6% 2 6% 4 4 4 4 5% 9% 49%	4078
	11th to 20th	Night	5% 3 2 5% 4 4 4 4 6% 9% 55%	4077
	21st to end	Night	5% 3 2 4 4 4 5% 4 8% 12% 49%	4449
August	1st to 10th	Night	3 2 1 4 3 3 4 3 6% 10% 62%	2926
	11th to 20th	Night	9% 5% 2 5% 4 4 4 3 6% 8% 51%	2888
	21st to end	Night	3 3 1 4 3 3 3 3 5% 9% 63%	3539
September	1st to 10th	Night	10% 5% 2 4 3 3 3 3 4 7% 56%	2000
	11th to 20th	Night	10% 4 1 5% 4 4 4 4 6% 7% 52%	2188
	21st to end	Night	7% 4 2 5% 3 3 4 3 5% 7% 58%	2200
October	1st to 10th	Night	8% 4 1 4 3 3 3 2 5% 6% 61%	2390
	11th to 20th	Night	13% 4 2 5% 2 3 3 2 4 5% 57%	2400
	21st to end	Night	16% 6% 2 5% 3 3 3 2 3 5% 53%	2640
November	1st to 10th	Night	11% 6% 1 3 3 2 2 2 3 5% 62%	2460
	11th to 20th	Night	11% 6% 2 5% 3 3 3 3 4 6% 54%	2678
	21st to end	Night	16% 7% 2 4 3 2 2 2 4 5% 52%	2930
December	1st to 10th	Night	8% 8% 2 4 3 3 3 3 5% 6% 56%	4720
	11th to 20th	Night	14% 7% 2 6% 3 3 3 3 4 5% 50%	5339
	21st to end	Night	20% 15% 5% 8% 4 4 3 3 3 4 32%	6698

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	47% 1 9% 28% 13% 2	5290
	11th to 20th	Day	41% 1 9% 27% 17% 4	5310
	21st to end	Day	45% 1 9% 21% 19% 6%	5812
February	1st to 10th	Day	48% 3 9% 27% 11% 2	1890
	11th to 20th	Day	45% 2 12% 22% 16% 3	1855
	21st to end	Day	43% 2 7% 25% 17% 6%	1829
March	1st to 10th	Day	39% 1 6% 23% 22% 8%	5046
	11th to 20th	Day	32% 1 8% 24% 25% 9%	5090
	21st to end	Day	48% 2 16% 26% 8% 1	5585
April	1st to 10th	Day	41% 3 12% 24% 17% 3	2311
	11th to 20th	Day	43% 3 11% 25% 16% 2	2374
	21st to end	Day	50% 2 11% 25% 11% 1	2500
May	1st to 10th	Day	47% 3 10% 21% 17% 1	5900
	11th to 20th	Day	39% 3 12% 25% 17% 3	5866
	21st to end	Day	46% 3 12% 25% 14% 1	6447
June	1st to 10th	Day	32% 4 17% 28% 18% 2	2685
	11th to 20th	Day	32% 4 14% 32% 17% 2	2793
	21st to end	Day	24% 5% 16% 34% 19% 4	2600
July	1st to 10th	Day	36% 4 11% 25% 21% 3	5836
	11th to 20th	Day	33% 4 12% 34% 15% 1	5831
	21st to end	Day	31% 6% 19% 33% 11%	6240
August	1st to 10th	Day	26% 4 17% 26% 24% 2	4220
	11th to 20th	Day	35% 5% 15% 28% 15% 1	4210
	21st to end	Day	27% 4 12% 39% 14% 4	4358
September	1st to 10th	Day	33% 3 11% 28% 17% 7%	2400
	11th to 20th	Day	33% 4 10% 27% 24% 2	2209
	21st to end	Day	31% 3 8% 27% 26% 5%	2190
October	1st to 10th	Day	20% 1 6% 20% 41% 12%	3260
	11th to 20th	Day	20% 1 4 29% 37% 10%	3240
	21st to end	Day	33% 3 7% 27% 26% 4	3538
November	1st to 10th	Day	31% 2 9% 31% 22% 4	2058
	11th to 20th	Day	35% 2 7% 27% 24% 5%	1938
	21st to end	Day	40% 2 9% 29% 19% 1	1685
December	1st to 10th	Day	33% 1 9% 39% 17%	2690
	11th to 20th	Day	37% 1 8% 32% 19% 2	3085
	21st to end	Day	63% 1 8% 20% 6% 1	4517
January	1st to 10th	Night	46% 2 11% 27% 13% 1	7635
	11th to 20th	Night	39% 1 7% 29% 21% 4	7680
	21st to end	Night	45% 1 9% 22% 18% 5%	8402
February	1st to 10th	Night	45% 2 10% 26% 15% 2	2730
	11th to 20th	Night	42% 2 10% 26% 17% 3	2664
	21st to end	Night	39% 2 7% 24% 21% 7%	1992
March	1st to 10th	Night	40% 1 7% 22% 24% 6%	5029
	11th to 20th	Night	30% 2 6% 23% 32% 8%	5090
	21st to end	Night	40% 3 14% 27% 13% 2	5588
April	1st to 10th	Night	37% 3 10% 23% 23% 5%	2109
	11th to 20th	Night	39% 3 8% 26% 20% 5%	1975
	21st to end	Night	44% 3 11% 25% 16% 2	1895
May	1st to 10th	Night	43% 3 12% 26% 14% 3	4080
	11th to 20th	Night	38% 2 12% 26% 15% 7%	4054
	21st to end	Night	38% 2 10% 28% 19% 3	4452
June	1st to 10th	Night	26% 3 19% 30% 18% 3	1700
	11th to 20th	Night	26% 4 16% 30% 21% 2	1600
	21st to end	Night	22% 3 14% 34% 22% 4	1791
July	1st to 10th	Night	32% 4 13% 26% 22% 3	4035
	11th to 20th	Night	26% 4 11% 40% 17% 2	4026
	21st to end	Night	27% 5% 17% 36% 13%	4320
August	1st to 10th	Night	18% 4 17% 34% 20% 7%	2916
	11th to 20th	Night	32% 4 13% 30% 19% 1	2915
	21st to end	Night	21% 4 13% 40% 18% 4	3539
September	1st to 10th	Night	29% 3 11% 27% 21% 8%	2000
	11th to 20th	Night	30% 4 10% 26% 25% 5%	2188
	21st to end	Night	28% 3 7% 27% 30% 4	2200
October	1st to 10th	Night	16% 2 6% 20% 43% 14%	3910
	11th to 20th	Night	22% 2 5% 23% 33% 15%	3899
	21st to end	Night	32% 2 6% 30% 26% 5%	4252
November	1st to 10th	Night	28% 2 10% 30% 24% 6%	2460
	11th to 20th	Night	34% 2 7% 27% 26% 5%	2678
	21st to end	Night	38% 1 8% 27% 22% 3	2930
December	1st to 10th	Night	30% 3 12% 37% 17% 1	4720
	11th to 20th	Night	39% 2 9% 28% 20% 2	5341
	21st to end	Night	59% 2 8% 22% 8% 2	6698

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	20% 11% 36% 25% 8% 1	330
	11th to 20th	Day	22% 17% 36% 22% 2	330
	21st to end	Day	17% 11% 45% 21% 6%	363
February	1st to 10th	Day	22% 16% 35% 21% 5%	330
	11th to 20th	Day	20% 16% 42% 18% 4 1	330
	21st to end	Day	20% 19% 41% 15% 4	273
March	1st to 10th	Day	24% 14% 36% 23% 3	330
	11th to 20th	Day	26% 11% 36% 21% 6%	330
	21st to end	Day	26% 13% 40% 17% 2 2	363
April	1st to 10th	Day	18% 18% 35% 21% 6%	403
	11th to 20th	Day	20% 14% 37% 23% 7%	492
	21st to end	Day	24% 12% 32% 22% 10%	600
May	1st to 10th	Day	20% 9% 33% 26% 10% 1	703
	11th to 20th	Day	18% 8% 29% 29% 13% 4	725
	21st to end	Day	17% 5% 27% 27% 17% 6%	789
June	1st to 10th	Day	8% 3 16% 36% 27% 10%	730
	11th to 20th	Day	8% 3 21% 38% 23% 7%	730
	21st to end	Day	7% 4 27% 36% 22% 4	730
July	1st to 10th	Day	10% 3 13% 29% 31% 13%	721
	11th to 20th	Day	5% 1 14% 36% 35% 8%	718
	21st to end	Day	9% 1 10% 32% 35% 13%	794
August	1st to 10th	Day	8% 2 13% 38% 32% 7%	726
	11th to 20th	Day	7% 2 13% 32% 37% 8%	715
	21st to end	Day	9% 2 9% 28% 35% 16%	617
September	1st to 10th	Day	13% 3 12% 31% 33% 9%	530
	11th to 20th	Day	5% 2 15% 37% 32% 10%	339
	21st to end	Day	13% 2 18% 35% 23% 8%	330
October	1st to 10th	Day	13% 5% 25% 29% 19% 9%	330
	11th to 20th	Day	12% 10% 36% 28% 11% 2	330
	21st to end	Day	28% 10% 32% 23% 7% 1	363
November	1st to 10th	Day	15% 9% 36% 26% 10% 4	330
	11th to 20th	Day	16% 10% 36% 24% 12% 1	330
	21st to end	Day	22% 11% 34% 25% 7% 2	330
December	1st to 10th	Day	13% 13% 42% 22% 8% 2	330
	11th to 20th	Day	16% 12% 38% 27% 7% 1	330
	21st to end	Day	21% 17% 40% 21% 1	363
January	1st to 10th	Night	15% 11% 39% 29% 6%	1050
	11th to 20th	Night	15% 13% 41% 25% 5%	1050
	21st to end	Night	15% 11% 39% 30% 4	1155
February	1st to 10th	Night	18% 11% 40% 27% 4	1050
	11th to 20th	Night	18% 12% 39% 28% 3	1029
	21st to end	Night	19% 12% 39% 26% 3	608
March	1st to 10th	Night	19% 10% 37% 30% 4	630
	11th to 20th	Night	21% 9% 35% 30% 5%	630
	21st to end	Night	21% 11% 38% 27% 3	693
April	1st to 10th	Night	14% 10% 38% 30% 6%	603
	11th to 20th	Night	18% 12% 34% 30% 6%	593
	21st to end	Night	21% 6% 37% 28% 8%	600
May	1st to 10th	Night	17% 6% 32% 37% 9%	600
	11th to 20th	Night	17% 3 27% 36% 16% 1	591
	21st to end	Night	14% 5% 24% 35% 18% 3	650
June	1st to 10th	Night	7% 4 15% 42% 31% 1	600
	11th to 20th	Night	6% 1 21% 49% 22% 1	600
	21st to end	Night	6% 1 25% 49% 18% 1	600
July	1st to 10th	Night	8% 2 10% 42% 34% 4	585
	11th to 20th	Night	3 1 11% 47% 34% 2	582
	21st to end	Night	8% 1 9% 40% 40% 2	657
August	1st to 10th	Night	6% 2 11% 46% 34% 2	597
	11th to 20th	Night	9% 1 9% 45% 33% 3	594
	21st to end	Night	6% 1 8% 39% 41% 4	660
September	1st to 10th	Night	12% 2 11% 44% 28% 4	600
	11th to 20th	Night	11% 2 14% 40% 30% 5%	600
	21st to end	Night	9% 1 20% 43% 24% 3	600
October	1st to 10th	Night	11% 2 24% 39% 23% 2	790
	11th to 20th	Night	14% 6% 35% 35% 10%	800
	21st to end	Night	19% 9% 33% 30% 9% 1	880
November	1st to 10th	Night	13% 6% 37% 32% 11% 1	820
	11th to 20th	Night	15% 9% 36% 30% 9%	998
	21st to end	Night	20% 10% 35% 28% 8%	1050
December	1st to 10th	Night	17% 8% 39% 30% 6%	1050
	11th to 20th	Night	21% 8% 34% 31% 5%	1050
	21st to end	Night	20% 12% 35% 28% 4	1155

Page generated at 2026-03-30 01:18:40. Version ac07

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