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 2025-03-27.

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	8% 5% 2 6% 5% 5% 6% 5% 7% 10% 41%	109386
	Night	8% 5% 2 6% 5% 5% 5% 4 7% 9% 43%	109949
Transparency	Day	36% 10% 17% 19% 14% 4	113190
	Night	35% 9% 17% 20% 14% 4	114284
Seeing	Day	7% 4 17% 26% 26% 19%	15958
	Night	8% 3 18% 34% 28% 9%	26090

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% 6% 2 6% 4 4 5% 4 5% 8% 49%	4056
	11th to 20th	Day	19% 8% 3 7% 5% 4 5% 4 5% 7% 32%	4060
	21st to end	Day	13% 5% 2 6% 4 3 4 3 6% 9% 45%	4902
February	1st to 10th	Day	11% 6% 2 6% 4 4 4 4 6% 7% 47%	2000
	11th to 20th	Day	6% 4 2 4 4 3 4 5% 5% 8% 55%	1948
	21st to end	Day	8% 4 1 6% 4 4 4 4 5% 9% 52%	1705
March	1st to 10th	Day	7% 5% 2 8% 5% 7% 6% 4 6% 9% 41%	3545
	11th to 20th	Day	6% 3 2 5% 3 4 4 4 7% 11% 53%	3590
	21st to end	Day	9% 4 1 4 4 3 4 4 6% 9% 52%	3881
April	1st to 10th	Day	6% 4 1 4 4 5% 5% 5% 8% 9% 50%	2090
	11th to 20th	Day	8% 5% 2 6% 4 5% 4 5% 7% 8% 47%	2066
	21st to end	Day	4 3 2 4 4 4 5% 4 8% 9% 54%	2247
May	1st to 10th	Day	4 2 1 3 3 4 4 4 6% 9% 62%	4082
	11th to 20th	Day	9% 4 1 5% 4 5% 5% 4 8% 10% 43%	4081
	21st to end	Day	4 3 1 4 4 4 4 4 6% 9% 57%	4476
June	1st to 10th	Day	11% 7% 3 8% 6% 6% 7% 6% 8% 10% 29%	2470
	11th to 20th	Day	6% 6% 2 7% 8% 6% 8% 7% 9% 11% 30%	2470
	21st to end	Day	7% 7% 2 8% 7% 8% 9% 7% 9% 10% 27%	2470
July	1st to 10th	Day	6% 5% 2 7% 8% 9% 9% 7% 9% 13% 25%	4120
	11th to 20th	Day	4 4 2 9% 7% 9% 10% 8% 11% 13% 22%	4120
	21st to end	Day	5% 5% 2 8% 9% 7% 9% 8% 11% 13% 24%	4466
August	1st to 10th	Day	6% 5% 3 9% 7% 7% 8% 7% 9% 13% 26%	4024
	11th to 20th	Day	4 4 2 7% 6% 7% 8% 6% 10% 11% 34%	3708
	21st to end	Day	7% 5% 3 8% 7% 7% 9% 7% 11% 13% 23%	3828
September	1st to 10th	Day	8% 6% 2 8% 8% 9% 9% 6% 9% 12% 23%	2090
	11th to 20th	Day	9% 7% 4 9% 8% 8% 8% 5% 9% 10% 25%	2090
	21st to end	Day	7% 6% 3 9% 7% 7% 6% 5% 7% 11% 32%	2090
October	1st to 10th	Day	9% 6% 3 9% 6% 5% 6% 5% 6% 9% 35%	2090
	11th to 20th	Day	6% 4 2 5% 5% 5% 5% 5% 6% 8% 49%	1951
	21st to end	Day	7% 5% 2 6% 3 4 5% 3 6% 8% 49%	2090
November	1st to 10th	Day	6% 5% 1 5% 4 5% 5% 4 7% 9% 50%	1957
	11th to 20th	Day	9% 6% 2 6% 5% 4 5% 4 5% 7% 48%	2000
	21st to end	Day	8% 5% 2 5% 4 5% 5% 4 6% 8% 48%	1854
December	1st to 10th	Day	4 3 1 4 3 4 4 3 6% 8% 61%	2930
	11th to 20th	Day	9% 6% 3 7% 4 4 4 4 5% 7% 47%	3373
	21st to end	Day	11% 7% 3 8% 6% 5% 5% 4 5% 7% 38%	4466
January	1st to 10th	Night	8% 5% 3 6% 4 4 4 4 6% 9% 47%	5889
	11th to 20th	Night	19% 11% 4 8% 5% 5% 4 3 6% 7% 29%	5900
	21st to end	Night	14% 6% 2 7% 4 5% 5% 3 6% 7% 42%	6050
February	1st to 10th	Night	10% 6% 2 7% 4 5% 3 3 5% 8% 45%	2400
	11th to 20th	Night	10% 5% 2 4 4 4 4 3 6% 7% 51%	2338
	21st to end	Night	7% 4 2 5% 4 3 5% 4 7% 8% 51%	1922
March	1st to 10th	Night	12% 7% 2 8% 5% 5% 5% 4 5% 7% 40%	3528
	11th to 20th	Night	4 3 1 6% 4 4 4 5% 9% 12% 49%	3590
	21st to end	Night	9% 4 2 5% 4 3 4 3 5% 6% 54%	3890
April	1st to 10th	Night	5% 3 1 4 4 4 4 4 7% 10% 54%	2090
	11th to 20th	Night	6% 5% 1 6% 4 4 4 5% 6% 10% 50%	2063
	21st to end	Night	4 4 1 4 4 4 4 3 5% 10% 57%	1947
May	1st to 10th	Night	4 3 1 4 3 4 4 3 6% 9% 60%	2813
	11th to 20th	Night	9% 4 2 5% 4 4 5% 4 6% 9% 49%	2812
	21st to end	Night	3 3 1 4 3 3 4 3 6% 7% 63%	3079
June	1st to 10th	Night	10% 6% 2 6% 4 5% 5% 5% 7% 9% 40%	1710
	11th to 20th	Night	6% 4 1 5% 6% 5% 5% 6% 8% 13% 39%	1710
	21st to end	Night	5% 6% 3 7% 6% 6% 6% 5% 9% 12% 35%	1710
July	1st to 10th	Night	6% 6% 3 10% 7% 7% 6% 6% 9% 12% 27%	2840
	11th to 20th	Night	3 5% 2 8% 7% 8% 8% 7% 11% 14% 28%	2840
	21st to end	Night	5% 7% 4 13% 9% 8% 9% 6% 9% 11% 19%	3087
August	1st to 10th	Night	5% 4 3 7% 5% 6% 6% 5% 11% 13% 35%	2885
	11th to 20th	Night	4 4 2 5% 5% 5% 7% 6% 10% 16% 35%	3100
	21st to end	Night	6% 5% 2 8% 6% 8% 8% 7% 11% 12% 26%	3814
September	1st to 10th	Night	5% 6% 2 9% 7% 7% 7% 6% 9% 12% 30%	2090
	11th to 20th	Night	8% 7% 3 9% 7% 7% 7% 5% 8% 9% 30%	2090
	21st to end	Night	8% 7% 3 9% 6% 6% 6% 5% 8% 10% 32%	2090
October	1st to 10th	Night	8% 6% 3 7% 5% 5% 5% 5% 8% 10% 39%	2090
	11th to 20th	Night	6% 5% 2 6% 4 4 3 4 6% 9% 50%	2228
	21st to end	Night	8% 5% 2 5% 4 4 4 3 6% 8% 50%	2508
November	1st to 10th	Night	7% 4 2 6% 4 4 4 4 6% 8% 51%	2341
	11th to 20th	Night	8% 5% 2 7% 5% 4 4 4 5% 7% 49%	2400
	21st to end	Night	8% 5% 2 6% 5% 4 4 4 6% 8% 48%	2543
December	1st to 10th	Night	4 3 1 4 3 3 3 3 6% 8% 63%	4250
	11th to 20th	Night	11% 6% 2 5% 4 4 4 3 5% 7% 50%	4822
	21st to end	Night	13% 8% 3 7% 5% 5% 5% 4 6% 8% 36%	6490

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	32% 5% 10% 18% 26% 10%	4057
	11th to 20th	Day	51% 4 7% 18% 15% 5%	4060
	21st to end	Day	35% 5% 8% 28% 19% 5%	4902
February	1st to 10th	Day	35% 7% 11% 19% 18% 11%	2000
	11th to 20th	Day	26% 6% 11% 22% 24% 10%	1958
	21st to end	Day	29% 6% 12% 18% 25% 10%	1705
March	1st to 10th	Day	37% 10% 15% 18% 15% 6%	3545
	11th to 20th	Day	25% 7% 15% 27% 19% 7%	3590
	21st to end	Day	28% 5% 10% 30% 18% 8%	3881
April	1st to 10th	Day	27% 8% 18% 22% 18% 7%	2090
	11th to 20th	Day	32% 8% 14% 18% 22% 6%	2066
	21st to end	Day	24% 7% 13% 24% 25% 6%	2247
May	1st to 10th	Day	14% 10% 17% 32% 22% 5%	4120
	11th to 20th	Day	32% 10% 20% 22% 14% 1	4085
	21st to end	Day	22% 10% 14% 33% 20% 1	4489
June	1st to 10th	Day	46% 12% 20% 17% 4	2470
	11th to 20th	Day	41% 16% 22% 18% 3	2470
	21st to end	Day	45% 17% 24% 12% 2	2470
July	1st to 10th	Day	44% 17% 30% 8% 1	4055
	11th to 20th	Day	43% 19% 32% 6%	4051
	21st to end	Day	44% 20% 26% 10%	4324
August	1st to 10th	Day	44% 15% 28% 10% 2	4011
	11th to 20th	Day	37% 15% 26% 17% 5%	3744
	21st to end	Day	45% 17% 27% 11% 1	3828
September	1st to 10th	Day	48% 17% 21% 12% 1	2090
	11th to 20th	Day	49% 16% 20% 13% 3	2090
	21st to end	Day	43% 13% 16% 17% 9% 2	2090
October	1st to 10th	Day	54% 11% 15% 9% 7% 4	3480
	11th to 20th	Day	27% 6% 10% 21% 33% 4	3217
	21st to end	Day	29% 5% 8% 23% 27% 8%	3428
November	1st to 10th	Day	29% 7% 12% 22% 19% 11%	1957
	11th to 20th	Day	34% 7% 9% 21% 20% 9%	2000
	21st to end	Day	31% 7% 13% 19% 20% 10%	1854
December	1st to 10th	Day	22% 5% 15% 29% 21% 9%	2930
	11th to 20th	Day	34% 5% 11% 19% 21% 10%	3370
	21st to end	Day	43% 6% 12% 20% 15% 4	4466
January	1st to 10th	Night	32% 6% 9% 19% 26% 8%	5889
	11th to 20th	Night	54% 5% 10% 14% 13% 3	5900
	21st to end	Night	40% 6% 10% 21% 17% 6%	6050
February	1st to 10th	Night	35% 6% 12% 18% 20% 8%	2400
	11th to 20th	Night	31% 5% 12% 19% 23% 9%	2339
	21st to end	Night	28% 7% 13% 20% 25% 7%	1922
March	1st to 10th	Night	42% 8% 13% 18% 14% 5%	3528
	11th to 20th	Night	24% 9% 22% 24% 16% 5%	3590
	21st to end	Night	30% 5% 10% 27% 21% 6%	3890
April	1st to 10th	Night	24% 7% 20% 26% 16% 7%	2090
	11th to 20th	Night	27% 9% 16% 21% 22% 5%	2063
	21st to end	Night	23% 6% 15% 26% 25% 5%	1947
May	1st to 10th	Night	17% 12% 18% 31% 17% 5%	2840
	11th to 20th	Night	28% 11% 22% 27% 12%	2813
	21st to end	Night	18% 8% 16% 37% 20% 1	3087
June	1st to 10th	Night	36% 13% 23% 22% 6%	1710
	11th to 20th	Night	33% 13% 28% 22% 4	1710
	21st to end	Night	37% 17% 25% 15% 6%	1710
July	1st to 10th	Night	46% 16% 29% 8% 1	2795
	11th to 20th	Night	41% 19% 33% 6%	2786
	21st to end	Night	55% 16% 21% 7%	2988
August	1st to 10th	Night	37% 16% 29% 12% 6%	2874
	11th to 20th	Night	30% 16% 36% 17% 2	3130
	21st to end	Night	44% 18% 28% 8% 1	3814
September	1st to 10th	Night	41% 19% 25% 13% 2	2090
	11th to 20th	Night	46% 15% 23% 14% 2	2090
	21st to end	Night	42% 13% 20% 16% 8% 1	2090
October	1st to 10th	Night	54% 11% 13% 11% 8% 4	3480
	11th to 20th	Night	27% 5% 10% 28% 24% 5%	3701
	21st to end	Night	30% 4 9% 26% 25% 5%	4120
November	1st to 10th	Night	29% 7% 13% 20% 20% 11%	2341
	11th to 20th	Night	32% 7% 12% 21% 19% 8%	2400
	21st to end	Night	32% 7% 12% 22% 19% 8%	2543
December	1st to 10th	Night	20% 5% 18% 30% 16% 12%	4250
	11th to 20th	Night	33% 5% 13% 21% 20% 9%	4824
	21st to end	Night	44% 7% 14% 17% 14% 3	6490

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	10% 9% 38% 31% 11% 2	300
	11th to 20th	Day	9% 14% 32% 28% 13% 4	300
	21st to end	Day	12% 13% 42% 27% 6%	330
February	1st to 10th	Day	8% 9% 30% 34% 15% 5%	300
	11th to 20th	Day	6% 9% 32% 33% 17% 3	300
	21st to end	Day	7% 6% 27% 37% 20% 3	304
March	1st to 10th	Day	5% 7% 39% 36% 12% 1	500
	11th to 20th	Day	7% 8% 29% 29% 25% 2	500
	21st to end	Day	5% 6% 29% 36% 19% 4	530
April	1st to 10th	Day	5% 6% 31% 35% 20% 2	460
	11th to 20th	Day	7% 8% 25% 37% 20% 4	452
	21st to end	Day	5% 2 22% 35% 28% 8%	460
May	1st to 10th	Day	5% 2 15% 37% 30% 11%	463
	11th to 20th	Day	9% 4 13% 30% 30% 14%	487
	21st to end	Day	5% 2 8% 23% 35% 26%	529
June	1st to 10th	Day	11% 2 8% 26% 31% 22%	490
	11th to 20th	Day	7% 3 7% 22% 31% 30%	490
	21st to end	Day	6% 1 3 12% 31% 47%	490
July	1st to 10th	Day	6% 2 1 10% 28% 53%	485
	11th to 20th	Day	5% 1 3 9% 33% 49%	484
	21st to end	Day	8% 1 5% 11% 30% 45%	532
August	1st to 10th	Day	8% 5% 10% 32% 45%	486
	11th to 20th	Day	5% 2 6% 10% 29% 49%	479
	21st to end	Day	8% 2 6% 16% 31% 37%	539
September	1st to 10th	Day	6% 2 5% 14% 31% 42%	490
	11th to 20th	Day	8% 1 5% 23% 34% 29%	490
	21st to end	Day	6% 2 5% 21% 35% 32%	490
October	1st to 10th	Day	10% 3 13% 31% 33% 10%	490
	11th to 20th	Day	7% 2 17% 30% 32% 12%	490
	21st to end	Day	8% 3 15% 35% 32% 6%	539
November	1st to 10th	Day	8% 3 19% 33% 30% 6%	495
	11th to 20th	Day	10% 6% 20% 38% 22% 4	500
	21st to end	Day	6% 6% 26% 33% 24% 4	354
December	1st to 10th	Day	4 7% 31% 33% 18% 7%	300
	11th to 20th	Day	8% 5% 32% 33% 19% 3	300
	21st to end	Day	6% 7% 29% 35% 19% 4	330
January	1st to 10th	Night	10% 7% 29% 39% 15% 1	800
	11th to 20th	Night	12% 7% 26% 40% 14% 1	800
	21st to end	Night	13% 9% 31% 36% 12%	880
February	1st to 10th	Night	12% 6% 28% 38% 16%	800
	11th to 20th	Night	10% 6% 27% 39% 17% 1	783
	21st to end	Night	7% 5% 25% 48% 15%	660
March	1st to 10th	Night	5% 6% 34% 44% 10% 1	800
	11th to 20th	Night	6% 6% 25% 45% 18% 1	800
	21st to end	Night	6% 4 26% 45% 17% 1	863
April	1st to 10th	Night	5% 3 28% 44% 20%	760
	11th to 20th	Night	7% 4 26% 45% 17% 1	751
	21st to end	Night	5% 2 24% 48% 20% 1	755
May	1st to 10th	Night	4 12% 47% 34% 3	570
	11th to 20th	Night	7% 2 14% 40% 33% 4	561
	21st to end	Night	4 1 10% 32% 45% 8%	617
June	1st to 10th	Night	10% 1 4 26% 47% 12%	570
	11th to 20th	Night	5% 2 4 17% 47% 25%	570
	21st to end	Night	5% 2 3 13% 46% 31%	570
July	1st to 10th	Night	6% 3 9% 44% 38%	555
	11th to 20th	Night	4 1 3 9% 42% 42%	552
	21st to end	Night	4 1 3 10% 46% 36%	624
August	1st to 10th	Night	6% 1 2 9% 47% 35%	567
	11th to 20th	Night	6% 1 2 9% 38% 44%	564
	21st to end	Night	10% 1 7% 18% 41% 24%	822
September	1st to 10th	Night	8% 1 6% 16% 47% 22%	760
	11th to 20th	Night	9% 2 5% 24% 43% 16%	760
	21st to end	Night	12% 1 6% 29% 39% 13%	760
October	1st to 10th	Night	11% 2 14% 38% 31% 4	760
	11th to 20th	Night	7% 3 15% 36% 35% 3	760
	21st to end	Night	8% 3 20% 40% 27% 3	836
November	1st to 10th	Night	7% 2 23% 43% 22% 2	780
	11th to 20th	Night	8% 4 22% 44% 20% 2	800
	21st to end	Night	7% 4 25% 46% 18% 1	800
December	1st to 10th	Night	8% 3 24% 40% 24% 1	800
	11th to 20th	Night	9% 5% 23% 42% 20% 2	800
	21st to end	Night	8% 5% 27% 40% 19% 1	880

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