Scott's Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Scott's Observatory clear sky chart from 2005-11-06 to 2026-07-31.

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	7% 5% 2 6% 5% 5% 6% 5% 7% 10% 42%	138195
	Night	8% 5% 2 7% 5% 5% 5% 4 7% 9% 43%	134809
Transparency	Day	34% 10% 18% 20% 14% 5%	141955
	Night	35% 9% 17% 20% 14% 5%	139121
Seeing	Day	7% 4 17% 27% 27% 18%	17827
	Night	8% 3 18% 34% 28% 9%	27846

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	6% 5% 2 5% 3 3 4 4 6% 8% 53%	5258
	11th to 20th	Day	18% 8% 3 8% 5% 5% 5% 4 5% 6% 34%	5280
	21st to end	Day	12% 5% 2 6% 4 3 4 3 6% 8% 47%	6352
February	1st to 10th	Day	11% 6% 2 5% 3 4 4 4 6% 7% 49%	2100
	11th to 20th	Day	6% 4 2 5% 3 3 4 5% 5% 8% 55%	2048
	21st to end	Day	8% 4 1 6% 4 4 4 3 5% 9% 53%	1788
March	1st to 10th	Day	8% 4 1 6% 4 5% 5% 4 5% 8% 48%	5045
	11th to 20th	Day	5% 3 2 4 3 3 3 3 5% 9% 61%	5090
	21st to end	Day	7% 4 2 5% 4 4 4 4 6% 9% 52%	5586
April	1st to 10th	Day	6% 4 1 5% 4 5% 5% 5% 7% 8% 51%	2310
	11th to 20th	Day	7% 4 1 6% 4 4 4 4 6% 8% 50%	2286
	21st to end	Day	4 3 1 4 4 4 5% 4 7% 9% 55%	2483
May	1st to 10th	Day	3 2 1 4 3 4 5% 4 8% 10% 54%	7616
	11th to 20th	Day	8% 4 2 5% 4 4 5% 4 7% 10% 49%	7626
	21st to end	Day	3 3 1 4 4 4 6% 5% 9% 15% 45%	8391
June	1st to 10th	Day	11% 7% 3 9% 6% 6% 7% 6% 7% 10% 28%	2730
	11th to 20th	Day	6% 6% 2 7% 8% 6% 8% 7% 9% 12% 29%	2730
	21st to end	Day	6% 6% 2 8% 7% 8% 9% 7% 9% 10% 28%	2730
July	1st to 10th	Day	5% 5% 3 8% 7% 9% 9% 7% 10% 14% 23%	6030
	11th to 20th	Day	3 4 2 9% 7% 9% 10% 8% 11% 13% 24%	6030
	21st to end	Day	3 4 2 7% 7% 7% 9% 7% 11% 13% 28%	6554
August	1st to 10th	Day	6% 5% 3 9% 7% 7% 8% 7% 9% 13% 25%	4152
	11th to 20th	Day	4 4 2 7% 6% 7% 8% 6% 9% 11% 34%	3827
	21st to end	Day	7% 5% 3 8% 7% 7% 9% 7% 11% 13% 23%	3949
September	1st to 10th	Day	8% 6% 2 9% 8% 8% 9% 6% 10% 12% 23%	2200
	11th to 20th	Day	9% 7% 4 9% 8% 7% 7% 5% 9% 10% 26%	2200
	21st to end	Day	7% 6% 3 9% 6% 7% 6% 6% 7% 11% 33%	2200
October	1st to 10th	Day	9% 6% 3 9% 6% 5% 6% 5% 6% 9% 35%	2200
	11th to 20th	Day	6% 4 2 5% 5% 4 5% 4 6% 8% 50%	2053
	21st to end	Day	7% 5% 2 6% 3 4 5% 3 6% 8% 50%	2200
November	1st to 10th	Day	6% 4 1 5% 4 5% 5% 4 7% 9% 50%	2057
	11th to 20th	Day	8% 6% 2 5% 5% 4 5% 4 5% 7% 50%	2100
	21st to end	Day	8% 5% 2 5% 4 5% 5% 4 6% 8% 49%	1946
December	1st to 10th	Day	4 3 1 4 3 4 4 3 6% 8% 60%	3020
	11th to 20th	Day	8% 6% 3 7% 4 4 4 4 5% 8% 48%	3463
	21st to end	Day	10% 7% 3 8% 6% 5% 5% 4 5% 7% 39%	4565
January	1st to 10th	Night	7% 4 2 6% 4 4 4 3 5% 9% 52%	7635
	11th to 20th	Night	17% 10% 4 8% 5% 4 4 3 5% 7% 33%	7680
	21st to end	Night	12% 5% 2 6% 4 4 4 3 5% 7% 46%	7825
February	1st to 10th	Night	10% 6% 2 6% 4 5% 3 3 5% 8% 47%	2520
	11th to 20th	Night	9% 5% 2 4 4 4 4 3 6% 7% 52%	2458
	21st to end	Night	7% 4 2 5% 4 3 4 4 7% 7% 52%	2015
March	1st to 10th	Night	9% 5% 2 7% 4 4 4 4 5% 8% 48%	5028
	11th to 20th	Night	5% 3 1 4 3 3 4 4 7% 9% 57%	5090
	21st to end	Night	8% 5% 3 6% 4 4 4 3 5% 6% 52%	5588
April	1st to 10th	Night	6% 4 2 4 4 4 4 4 7% 9% 53%	2310
	11th to 20th	Night	5% 4 1 5% 4 4 4 4 6% 9% 53%	2283
	21st to end	Night	4 4 1 4 4 4 4 3 5% 9% 56%	2151
May	1st to 10th	Night	4 4 1 5% 3 4 5% 4 7% 11% 51%	5237
	11th to 20th	Night	8% 5% 2 6% 4 4 5% 4 6% 9% 50%	5234
	21st to end	Night	4 4 2 6% 4 5% 5% 4 7% 11% 47%	5761
June	1st to 10th	Night	10% 6% 2 6% 4 5% 6% 5% 7% 10% 38%	1890
	11th to 20th	Night	6% 5% 2 5% 6% 6% 6% 6% 8% 13% 37%	1890
	21st to end	Night	5% 6% 3 7% 6% 6% 6% 5% 9% 12% 34%	1890
July	1st to 10th	Night	5% 6% 3 10% 8% 7% 7% 7% 10% 13% 24%	4148
	11th to 20th	Night	4 5% 3 8% 7% 7% 8% 7% 11% 15% 26%	4147
	21st to end	Night	4 6% 3 11% 8% 7% 9% 7% 10% 12% 23%	4517
August	1st to 10th	Night	5% 4 3 7% 5% 6% 7% 5% 10% 13% 34%	2977
	11th to 20th	Night	4 4 2 6% 5% 5% 7% 6% 10% 16% 35%	3200
	21st to end	Night	6% 5% 3 8% 6% 8% 8% 7% 10% 12% 26%	3934
September	1st to 10th	Night	5% 6% 2 9% 7% 7% 7% 6% 9% 12% 29%	2200
	11th to 20th	Night	8% 7% 3 9% 7% 7% 7% 5% 8% 9% 30%	2200
	21st to end	Night	8% 7% 3 8% 6% 6% 6% 5% 8% 10% 33%	2200
October	1st to 10th	Night	8% 6% 3 7% 5% 5% 6% 5% 8% 10% 38%	2200
	11th to 20th	Night	6% 5% 2 5% 4 4 4 4 6% 8% 51%	2345
	21st to end	Night	8% 5% 2 5% 4 4 4 3 6% 8% 50%	2640
November	1st to 10th	Night	7% 5% 2 5% 4 4 4 4 6% 8% 51%	2461
	11th to 20th	Night	8% 5% 2 6% 5% 4 4 4 5% 7% 51%	2520
	21st to end	Night	7% 5% 2 6% 5% 4 4 4 6% 8% 48%	2670
December	1st to 10th	Night	5% 3 1 4 3 3 3 3 6% 8% 62%	4380
	11th to 20th	Night	11% 6% 2 5% 4 4 4 3 5% 7% 50%	4952
	21st to end	Night	13% 8% 3 7% 5% 5% 5% 4 6% 8% 36%	6633

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	27% 5% 11% 23% 26% 8%	5259
	11th to 20th	Day	50% 4 8% 16% 14% 9%	5280
	21st to end	Day	34% 4 9% 28% 15% 10%	6352
February	1st to 10th	Day	33% 6% 10% 19% 19% 12%	2100
	11th to 20th	Day	26% 6% 12% 23% 24% 10%	2058
	21st to end	Day	29% 5% 12% 18% 25% 10%	1788
March	1st to 10th	Day	33% 7% 12% 20% 21% 6%	5045
	11th to 20th	Day	21% 5% 11% 24% 22% 17%	5090
	21st to end	Day	28% 5% 10% 28% 22% 7%	5586
April	1st to 10th	Day	27% 8% 18% 23% 18% 7%	2310
	11th to 20th	Day	30% 8% 13% 17% 23% 10%	2286
	21st to end	Day	23% 7% 13% 25% 25% 6%	2483
May	1st to 10th	Day	20% 8% 24% 30% 15% 3	7651
	11th to 20th	Day	30% 8% 20% 26% 15% 2	7630
	21st to end	Day	24% 12% 26% 26% 11% 1	8405
June	1st to 10th	Day	46% 12% 19% 17% 5%	2730
	11th to 20th	Day	41% 16% 23% 17% 3	2730
	21st to end	Day	43% 17% 25% 13% 2	2730
July	1st to 10th	Day	45% 17% 31% 6% 1	5965
	11th to 20th	Day	43% 17% 31% 8%	5961
	21st to end	Day	39% 18% 27% 16% 1	6370
August	1st to 10th	Day	44% 15% 29% 10% 2	4139
	11th to 20th	Day	37% 15% 26% 17% 5%	3863
	21st to end	Day	45% 17% 26% 11% 1	3949
September	1st to 10th	Day	49% 18% 21% 12% 1	2200
	11th to 20th	Day	48% 15% 20% 13% 3	2200
	21st to end	Day	42% 13% 16% 17% 10% 2	2200
October	1st to 10th	Day	53% 11% 15% 9% 7% 4	3590
	11th to 20th	Day	26% 6% 10% 21% 33% 4	3319
	21st to end	Day	29% 5% 8% 23% 27% 8%	3538
November	1st to 10th	Day	29% 7% 12% 22% 20% 11%	2057
	11th to 20th	Day	32% 7% 9% 21% 22% 10%	2100
	21st to end	Day	31% 6% 12% 21% 20% 10%	1946
December	1st to 10th	Day	22% 5% 16% 28% 20% 8%	3020
	11th to 20th	Day	34% 5% 11% 19% 21% 10%	3460
	21st to end	Day	42% 6% 12% 21% 16% 4	4565
January	1st to 10th	Night	29% 5% 10% 25% 25% 6%	7636
	11th to 20th	Night	51% 5% 9% 15% 13% 7%	7680
	21st to end	Night	37% 5% 12% 22% 15% 9%	7825
February	1st to 10th	Night	34% 6% 12% 18% 21% 10%	2520
	11th to 20th	Night	31% 5% 12% 20% 23% 9%	2459
	21st to end	Night	27% 7% 13% 20% 26% 7%	2015
March	1st to 10th	Night	34% 6% 13% 21% 18% 8%	5028
	11th to 20th	Night	21% 7% 17% 21% 18% 16%	5090
	21st to end	Night	32% 5% 10% 27% 22% 5%	5588
April	1st to 10th	Night	25% 7% 19% 26% 16% 7%	2310
	11th to 20th	Night	25% 8% 15% 21% 23% 7%	2283
	21st to end	Night	24% 6% 14% 27% 24% 5%	2151
May	1st to 10th	Night	23% 10% 23% 29% 12% 3	5264
	11th to 20th	Night	30% 7% 20% 27% 14% 1	5235
	21st to end	Night	29% 10% 22% 26% 12%	5771
June	1st to 10th	Night	38% 12% 24% 21% 6%	1890
	11th to 20th	Night	35% 13% 28% 20% 4	1890
	21st to end	Night	37% 17% 26% 15% 6%	1890
July	1st to 10th	Night	47% 16% 30% 6% 1	4105
	11th to 20th	Night	42% 18% 33% 6%	4096
	21st to end	Night	48% 15% 25% 12%	4387
August	1st to 10th	Night	37% 16% 30% 11% 6%	2966
	11th to 20th	Night	31% 16% 35% 16% 1	3230
	21st to end	Night	44% 18% 28% 9% 1	3934
September	1st to 10th	Night	43% 19% 24% 13% 1	2200
	11th to 20th	Night	46% 14% 23% 14% 2	2200
	21st to end	Night	42% 13% 20% 16% 8% 1	2200
October	1st to 10th	Night	54% 11% 13% 11% 7% 4	3590
	11th to 20th	Night	27% 5% 10% 28% 24% 5%	3818
	21st to end	Night	30% 4 9% 26% 26% 6%	4252
November	1st to 10th	Night	29% 7% 13% 20% 20% 11%	2461
	11th to 20th	Night	31% 7% 11% 21% 21% 9%	2520
	21st to end	Night	32% 6% 11% 23% 19% 8%	2670
December	1st to 10th	Night	21% 5% 18% 29% 15% 12%	4380
	11th to 20th	Night	33% 5% 12% 21% 20% 9%	4954
	21st to end	Night	44% 7% 14% 17% 15% 3	6633

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	9% 8% 35% 33% 12% 2	330
	11th to 20th	Day	9% 14% 33% 28% 13% 3	330
	21st to end	Day	12% 13% 43% 26% 6%	363
February	1st to 10th	Day	8% 10% 30% 33% 14% 5%	330
	11th to 20th	Day	5% 9% 32% 35% 16% 3	330
	21st to end	Day	7% 7% 27% 36% 19% 3	331
March	1st to 10th	Day	5% 7% 36% 37% 14% 1	540
	11th to 20th	Day	7% 8% 31% 28% 24% 1	540
	21st to end	Day	5% 6% 29% 36% 20% 5%	594
April	1st to 10th	Day	6% 7% 30% 35% 20% 3	540
	11th to 20th	Day	6% 7% 25% 36% 20% 5%	532
	21st to end	Day	4 2 21% 34% 31% 7%	540
May	1st to 10th	Day	4 3 18% 37% 29% 10%	543
	11th to 20th	Day	9% 4 15% 30% 29% 13%	567
	21st to end	Day	5% 2 9% 25% 35% 24%	617
June	1st to 10th	Day	11% 2 9% 26% 32% 20%	570
	11th to 20th	Day	7% 2 6% 23% 32% 30%	570
	21st to end	Day	5% 1 4 13% 31% 47%	570
July	1st to 10th	Day	5% 2 2 12% 27% 52%	565
	11th to 20th	Day	5% 1 4 12% 32% 46%	564
	21st to end	Day	7% 1 5% 13% 31% 43%	616
August	1st to 10th	Day	8% 1 5% 10% 32% 44%	526
	11th to 20th	Day	4 2 6% 11% 29% 48%	519
	21st to end	Day	8% 2 7% 16% 32% 36%	583
September	1st to 10th	Day	6% 2 5% 15% 32% 40%	530
	11th to 20th	Day	8% 1 5% 24% 35% 27%	530
	21st to end	Day	6% 2 5% 22% 35% 30%	530
October	1st to 10th	Day	11% 3 12% 31% 34% 9%	530
	11th to 20th	Day	6% 2 16% 31% 33% 11%	530
	21st to end	Day	8% 3 17% 35% 31% 6%	583
November	1st to 10th	Day	8% 4 20% 33% 29% 6%	535
	11th to 20th	Day	9% 6% 20% 38% 23% 4	540
	21st to end	Day	6% 6% 27% 33% 24% 5%	386
December	1st to 10th	Day	4 7% 33% 33% 17% 6%	330
	11th to 20th	Day	8% 6% 32% 33% 18% 2	330
	21st to end	Day	6% 7% 28% 34% 20% 5%	363
January	1st to 10th	Night	9% 6% 28% 39% 16% 1	840
	11th to 20th	Night	11% 7% 27% 40% 13% 1	840
	21st to end	Night	12% 9% 31% 36% 11%	924
February	1st to 10th	Night	12% 6% 29% 38% 15%	840
	11th to 20th	Night	10% 6% 26% 41% 17% 1	823
	21st to end	Night	7% 5% 25% 48% 15%	692
March	1st to 10th	Night	5% 6% 33% 44% 11% 1	840
	11th to 20th	Night	6% 6% 25% 44% 18% 1	840
	21st to end	Night	6% 4 27% 44% 18% 1	924
April	1st to 10th	Night	6% 3 27% 45% 19%	840
	11th to 20th	Night	6% 4 26% 45% 18% 1	831
	21st to end	Night	5% 2 24% 49% 19% 1	834
May	1st to 10th	Night	3 1 13% 48% 33% 3	630
	11th to 20th	Night	6% 2 13% 42% 33% 4	621
	21st to end	Night	4 2 10% 31% 45% 8%	683
June	1st to 10th	Night	10% 1 5% 27% 47% 11%	630
	11th to 20th	Night	5% 2 4 16% 49% 24%	630
	21st to end	Night	6% 2 3 12% 46% 32%	630
July	1st to 10th	Night	6% 3 9% 44% 38%	615
	11th to 20th	Night	5% 1 3 9% 42% 40%	612
	21st to end	Night	5% 1 4 10% 47% 34%	687
August	1st to 10th	Night	6% 1 2 9% 47% 36%	597
	11th to 20th	Night	6% 1 2 9% 39% 43%	594
	21st to end	Night	10% 1 7% 18% 42% 23%	865
September	1st to 10th	Night	8% 1 7% 17% 46% 22%	800
	11th to 20th	Night	9% 2 6% 26% 42% 15%	800
	21st to end	Night	12% 2 6% 29% 39% 13%	800
October	1st to 10th	Night	11% 2 13% 39% 31% 4	800
	11th to 20th	Night	7% 3 15% 37% 36% 3	800
	21st to end	Night	8% 3 20% 40% 26% 3	880
November	1st to 10th	Night	8% 2 23% 42% 22% 2	820
	11th to 20th	Night	8% 4 21% 44% 21% 2	840
	21st to end	Night	7% 4 25% 45% 19% 1	840
December	1st to 10th	Night	9% 3 24% 40% 23% 1	840
	11th to 20th	Night	9% 5% 23% 43% 19% 2	840
	21st to end	Night	8% 5% 27% 39% 20% 1	924

Page generated at 2026-07-28 03:25:23. Version ac07

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