Clayton
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Clayton clear sky chart from 2005-11-06 to 2025-05-11.

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	10% 7% 3 7% 4 4 4 4 5% 7% 44%	117348
	Night	9% 7% 3 7% 4 4 4 4 5% 7% 46%	120319
Transparency	Day	39% 5% 14% 25% 14% 2	117099
	Night	37% 5% 14% 26% 16% 2	120158
Seeing	Day	15% 9% 30% 30% 14% 2	16557
	Night	13% 8% 34% 35% 10% 1	26594

By Month

Percentage Cloud Cover SW (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	9% 12% 4 14% 9% 7% 7% 5% 5% 5% 23%	3638
	11th to 20th	Day	5% 10% 5% 11% 8% 5% 6% 3 8% 7% 31%	3640
	21st to end	Day	4 9% 6% 12% 6% 6% 5% 3 5% 5% 38%	4427
February	1st to 10th	Day	9% 12% 4 9% 6% 6% 6% 4 6% 5% 31%	1800
	11th to 20th	Day	8% 10% 4 9% 7% 6% 5% 4 4 7% 36%	1830
	21st to end	Day	9% 9% 3 10% 5% 6% 4 3 6% 7% 37%	1650
March	1st to 10th	Day	10% 8% 3 9% 4 3 3 3 4 6% 48%	4666
	11th to 20th	Day	7% 6% 3 6% 3 3 4 3 5% 8% 51%	4980
	21st to end	Day	21% 11% 4 10% 5% 4 5% 4 5% 6% 25%	5464
April	1st to 10th	Day	16% 7% 2 7% 4 4 4 4 5% 6% 40%	2280
	11th to 20th	Day	17% 8% 3 6% 4 4 4 3 5% 6% 39%	2569
	21st to end	Day	18% 8% 3 6% 5% 3 3 3 5% 7% 39%	2600
May	1st to 10th	Day	15% 7% 2 6% 4 5% 5% 4 6% 7% 40%	4250
	11th to 20th	Day	11% 6% 2 5% 4 4 4 3 5% 5% 50%	4083
	21st to end	Day	17% 6% 2 6% 4 4 4 3 6% 7% 40%	4667
June	1st to 10th	Day	11% 4 2 5% 3 3 4 4 5% 9% 50%	2844
	11th to 20th	Day	12% 4 2 6% 4 4 4 4 7% 7% 46%	2850
	21st to end	Day	8% 5% 2 4 3 3 5% 4 6% 9% 51%	2850
July	1st to 10th	Day	11% 5% 2 5% 4 4 5% 4 6% 9% 45%	4564
	11th to 20th	Day	6% 3 1 4 3 3 4 3 5% 7% 59%	4430
	21st to end	Day	4 3 1 4 4 4 5% 4 7% 9% 55%	4532
August	1st to 10th	Day	7% 5% 2 6% 4 4 5% 4 6% 8% 48%	4103
	11th to 20th	Day	18% 8% 2 6% 3 4 4 4 5% 7% 38%	4041
	21st to end	Day	7% 5% 2 5% 3 4 4 4 5% 7% 55%	4532
September	1st to 10th	Day	12% 5% 2 4 3 3 3 2 4 7% 54%	2188
	11th to 20th	Day	8% 4 2 4 3 3 3 3 5% 7% 58%	2090
	21st to end	Day	11% 4 2 5% 3 3 4 3 4 6% 56%	2090
October	1st to 10th	Day	8% 3 1 4 2 2 3 2 4 6% 64%	3324
	11th to 20th	Day	8% 5% 2 4 4 4 3 3 5% 7% 56%	3140
	21st to end	Day	11% 7% 2 6% 4 4 5% 4 5% 7% 45%	3201
November	1st to 10th	Day	8% 7% 3 7% 4 4 4 4 6% 7% 46%	1763
	11th to 20th	Day	7% 8% 2 9% 6% 5% 4 3 5% 7% 45%	1740
	21st to end	Day	9% 9% 3 7% 4 3 4 3 6% 8% 44%	1600
December	1st to 10th	Day	4 9% 4 9% 7% 7% 6% 5% 5% 6% 38%	2610
	11th to 20th	Day	9% 11% 5% 9% 5% 5% 6% 4 6% 7% 34%	2754
	21st to end	Day	13% 9% 3 14% 6% 4 4 4 6% 6% 30%	3558
January	1st to 10th	Night	7% 12% 5% 13% 10% 7% 6% 4 5% 6% 24%	6327
	11th to 20th	Night	3 10% 4 10% 6% 5% 7% 4 8% 8% 35%	6340
	21st to end	Night	3 8% 5% 10% 5% 5% 5% 4 6% 6% 43%	6535
February	1st to 10th	Night	9% 10% 4 9% 6% 4 4 4 6% 7% 37%	2600
	11th to 20th	Night	7% 12% 4 8% 5% 5% 4 4 6% 7% 41%	2473
	21st to end	Night	8% 8% 3 6% 5% 4 4 5% 5% 7% 45%	1980
March	1st to 10th	Night	10% 7% 2 5% 3 3 4 3 5% 6% 51%	5199
	11th to 20th	Night	9% 6% 2 5% 3 3 3 3 5% 9% 52%	4980
	21st to end	Night	16% 11% 3 8% 5% 4 4 3 5% 6% 35%	5467
April	1st to 10th	Night	14% 8% 2 5% 5% 3 4 4 7% 7% 40%	2113
	11th to 20th	Night	14% 8% 3 7% 4 4 4 2 4 7% 43%	1779
	21st to end	Night	16% 8% 2 6% 3 4 4 4 5% 6% 42%	1800
May	1st to 10th	Night	15% 5% 2 5% 4 3 4 3 6% 7% 46%	2940
	11th to 20th	Night	10% 4 2 5% 4 4 4 4 5% 8% 52%	2828
	21st to end	Night	15% 6% 2 5% 4 3 4 4 5% 8% 44%	2916
June	1st to 10th	Night	7% 5% 2 4 3 3 4 3 5% 7% 57%	1331
	11th to 20th	Night	8% 4 1 5% 4 3 4 3 6% 10% 52%	1330
	21st to end	Night	6% 3 1 3 3 4 3 3 6% 9% 58%	1330
July	1st to 10th	Night	9% 5% 2 4 3 4 4 4 6% 8% 51%	2401
	11th to 20th	Night	4 3 1 3 3 3 3 3 5% 7% 64%	2540
	21st to end	Night	2 2 1 4 3 3 4 3 6% 10% 62%	3026
August	1st to 10th	Night	7% 4 2 5% 4 4 4 4 7% 10% 49%	2836
	11th to 20th	Night	10% 5% 3 5% 4 4 5% 4 5% 7% 49%	2798
	21st to end	Night	7% 3 1 5% 3 3 4 4 5% 9% 56%	3135
September	1st to 10th	Night	8% 4 1 4 3 3 4 2 4 6% 59%	1988
	11th to 20th	Night	6% 4 1 4 3 4 4 3 5% 6% 60%	2090
	21st to end	Night	7% 5% 1 4 3 3 4 2 4 6% 61%	2090
October	1st to 10th	Night	9% 3 1 2 2 1 2 2 3 4 70%	3603
	11th to 20th	Night	8% 4 1 3 2 2 2 2 5% 8% 60%	3779
	21st to end	Night	11% 6% 2 6% 3 4 4 4 6% 8% 47%	4340
November	1st to 10th	Night	8% 5% 2 5% 4 3 3 3 5% 6% 53%	2535
	11th to 20th	Night	7% 7% 2 7% 5% 4 4 3 5% 7% 47%	2658
	21st to end	Night	8% 7% 2 6% 4 4 4 4 5% 8% 48%	2800
December	1st to 10th	Night	5% 6% 4 9% 6% 5% 4 4 5% 6% 47%	4580
	11th to 20th	Night	8% 9% 4 8% 6% 6% 5% 4 5% 7% 37%	5436
	21st to end	Night	15% 11% 4 12% 5% 4 4 3 4 5% 32%	7416

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	61% 6% 14% 14% 5%	3639
	11th to 20th	Day	51% 4 14% 16% 15%	3640
	21st to end	Day	45% 6% 17% 21% 10% 1	4427
February	1st to 10th	Day	51% 8% 14% 17% 9%	1800
	11th to 20th	Day	47% 6% 13% 20% 12% 1	1839
	21st to end	Day	45% 6% 15% 21% 12% 2	1650
March	1st to 10th	Day	38% 5% 11% 31% 14% 2	4666
	11th to 20th	Day	31% 4 10% 31% 22% 3	4980
	21st to end	Day	59% 5% 15% 16% 3 2	5464
April	1st to 10th	Day	43% 5% 14% 25% 12% 1	2280
	11th to 20th	Day	45% 6% 11% 22% 14% 2	2572
	21st to end	Day	46% 5% 13% 25% 12%	2600
May	1st to 10th	Day	39% 8% 12% 24% 17%	4250
	11th to 20th	Day	35% 5% 12% 28% 16% 3	4087
	21st to end	Day	41% 6% 14% 25% 13% 1	4667
June	1st to 10th	Day	32% 6% 18% 30% 12% 2	2844
	11th to 20th	Day	35% 7% 15% 24% 16% 3	2850
	21st to end	Day	29% 7% 17% 30% 15% 3	2850
July	1st to 10th	Day	34% 7% 19% 27% 11% 2	4495
	11th to 20th	Day	24% 5% 13% 34% 20% 4	4355
	21st to end	Day	23% 7% 16% 29% 22% 3	4388
August	1st to 10th	Day	32% 6% 16% 29% 15% 3	4090
	11th to 20th	Day	44% 6% 14% 25% 10% 1	4080
	21st to end	Day	28% 6% 18% 27% 18% 3	4531
September	1st to 10th	Day	30% 4 12% 30% 20% 3	2188
	11th to 20th	Day	27% 5% 16% 33% 18% 1	2090
	21st to end	Day	29% 5% 10% 32% 21% 2	2090
October	1st to 10th	Day	24% 2 9% 25% 36% 4	3324
	11th to 20th	Day	29% 4 13% 30% 20% 4	3140
	21st to end	Day	38% 3 12% 25% 19% 2	3201
November	1st to 10th	Day	36% 6% 17% 25% 13% 3	1763
	11th to 20th	Day	38% 5% 15% 27% 13% 2	1740
	21st to end	Day	39% 4 17% 26% 13% 1	1600
December	1st to 10th	Day	45% 6% 17% 24% 8% 1	2610
	11th to 20th	Day	49% 4 19% 21% 7%	2751
	21st to end	Day	53% 4 14% 23% 5%	3558
January	1st to 10th	Night	60% 6% 11% 15% 8%	6327
	11th to 20th	Night	44% 5% 16% 18% 16% 1	6340
	21st to end	Night	40% 6% 15% 26% 13%	6535
February	1st to 10th	Night	46% 5% 16% 20% 11% 1	2600
	11th to 20th	Night	43% 4 15% 21% 13% 3	2474
	21st to end	Night	38% 5% 14% 26% 13% 3	1980
March	1st to 10th	Night	34% 4 11% 30% 20% 2	5199
	11th to 20th	Night	30% 3 10% 30% 25% 2	4980
	21st to end	Night	51% 4 14% 23% 7% 1	5467
April	1st to 10th	Night	39% 7% 15% 24% 14% 1	2113
	11th to 20th	Night	40% 5% 10% 24% 19% 2	1779
	21st to end	Night	41% 6% 12% 26% 13% 2	1800
May	1st to 10th	Night	35% 7% 15% 23% 17% 3	2940
	11th to 20th	Night	30% 6% 13% 25% 24% 3	2829
	21st to end	Night	38% 5% 14% 26% 16% 1	2917
June	1st to 10th	Night	27% 6% 15% 29% 19% 4	1331
	11th to 20th	Night	28% 5% 17% 24% 21% 5%	1330
	21st to end	Night	22% 5% 16% 33% 20% 3	1330
July	1st to 10th	Night	31% 5% 22% 25% 12% 4	2361
	11th to 20th	Night	19% 5% 13% 37% 21% 5%	2492
	21st to end	Night	18% 7% 18% 32% 22% 4	2931
August	1st to 10th	Night	30% 6% 16% 30% 15% 2	2826
	11th to 20th	Night	34% 7% 15% 28% 14% 2	2825
	21st to end	Night	26% 6% 18% 25% 21% 4	3135
September	1st to 10th	Night	27% 4 14% 27% 24% 5%	1988
	11th to 20th	Night	23% 6% 14% 34% 20% 2	2090
	21st to end	Night	26% 4 9% 31% 26% 4	2090
October	1st to 10th	Night	20% 3 7% 20% 46% 6%	3603
	11th to 20th	Night	24% 3 11% 32% 25% 5%	3779
	21st to end	Night	35% 4 13% 27% 19% 1	4340
November	1st to 10th	Night	31% 4 14% 26% 20% 4	2535
	11th to 20th	Night	36% 5% 14% 24% 18% 3	2658
	21st to end	Night	35% 4 17% 28% 16% 1	2800
December	1st to 10th	Night	39% 3 16% 31% 10% 1	4580
	11th to 20th	Night	46% 5% 18% 24% 7%	5438
	21st to end	Night	54% 5% 12% 23% 7%	7416

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	18% 11% 30% 29% 11% 1	300
	11th to 20th	Day	14% 14% 35% 29% 8% 1	300
	21st to end	Day	14% 11% 35% 29% 9% 2	330
February	1st to 10th	Day	13% 19% 39% 24% 5%	300
	11th to 20th	Day	14% 13% 43% 21% 8% 1	300
	21st to end	Day	16% 13% 38% 29% 4	249
March	1st to 10th	Day	14% 13% 40% 27% 5% 1	300
	11th to 20th	Day	18% 10% 37% 23% 10% 2	300
	21st to end	Day	27% 10% 34% 22% 6% 1	330
April	1st to 10th	Day	25% 15% 37% 19% 4	380
	11th to 20th	Day	23% 14% 31% 25% 6%	690
	21st to end	Day	25% 13% 29% 23% 8% 2	700
May	1st to 10th	Day	18% 13% 32% 25% 11% 2	703
	11th to 20th	Day	20% 8% 29% 25% 17% 2	676
	21st to end	Day	19% 9% 28% 30% 12% 2	734
June	1st to 10th	Day	10% 6% 24% 33% 22% 5%	680
	11th to 20th	Day	13% 7% 34% 28% 15% 2	680
	21st to end	Day	11% 6% 28% 35% 17% 2	680
July	1st to 10th	Day	7% 3 20% 38% 26% 6%	671
	11th to 20th	Day	6% 4 26% 41% 20% 3	668
	21st to end	Day	5% 3 25% 43% 21% 4	739
August	1st to 10th	Day	9% 3 21% 41% 22% 4	676
	11th to 20th	Day	11% 4 24% 34% 23% 4	665
	21st to end	Day	7% 4 26% 37% 21% 4	748
September	1st to 10th	Day	14% 6% 30% 31% 16% 4	398
	11th to 20th	Day	8% 6% 36% 29% 17% 4	300
	21st to end	Day	14% 11% 28% 31% 12% 4	300
October	1st to 10th	Day	17% 7% 26% 31% 15% 3	300
	11th to 20th	Day	17% 14% 32% 27% 8% 2	300
	21st to end	Day	20% 16% 31% 23% 8% 1	330
November	1st to 10th	Day	19% 11% 35% 25% 7% 3	300
	11th to 20th	Day	14% 16% 38% 25% 7%	300
	21st to end	Day	19% 16% 30% 26% 8% 2	300
December	1st to 10th	Day	14% 8% 35% 30% 11% 1	300
	11th to 20th	Day	17% 13% 37% 27% 6%	300
	21st to end	Day	19% 13% 36% 27% 4 1	330
January	1st to 10th	Night	12% 10% 36% 33% 9% 1	1000
	11th to 20th	Night	11% 13% 36% 32% 8% 1	1000
	21st to end	Night	13% 10% 40% 30% 7% 1	1100
February	1st to 10th	Night	14% 12% 40% 29% 5% 1	1000
	11th to 20th	Night	12% 12% 37% 32% 7% 1	919
	21st to end	Night	14% 9% 37% 31% 8% 1	660
March	1st to 10th	Night	13% 9% 39% 33% 5% 1	729
	11th to 20th	Night	16% 10% 36% 33% 5%	600
	21st to end	Night	19% 11% 36% 28% 7%	660
April	1st to 10th	Night	18% 14% 40% 25% 4	600
	11th to 20th	Night	19% 12% 36% 25% 8%	593
	21st to end	Night	20% 9% 36% 28% 6% 1	600
May	1st to 10th	Night	16% 6% 35% 30% 12% 1	600
	11th to 20th	Night	15% 7% 32% 34% 12%	563
	21st to end	Night	17% 5% 35% 33% 9% 1	617
June	1st to 10th	Night	9% 5% 29% 40% 17% 1	570
	11th to 20th	Night	11% 3 31% 43% 10% 1	570
	21st to end	Night	7% 4 27% 48% 14% 1	570
July	1st to 10th	Night	6% 3 22% 44% 25% 1	555
	11th to 20th	Night	5% 1 24% 52% 17% 1	552
	21st to end	Night	3 2 26% 51% 16% 1	624
August	1st to 10th	Night	6% 4 22% 51% 16% 1	567
	11th to 20th	Night	7% 4 25% 44% 20% 1	564
	21st to end	Night	6% 4 30% 42% 18% 1	627
September	1st to 10th	Night	10% 4 27% 42% 14% 2	570
	11th to 20th	Night	7% 5% 34% 39% 12% 2	570
	21st to end	Night	10% 5% 36% 34% 14% 1	570
October	1st to 10th	Night	17% 8% 31% 33% 11% 1	641
	11th to 20th	Night	15% 10% 36% 29% 9%	760
	21st to end	Night	16% 10% 34% 32% 8%	968
November	1st to 10th	Night	11% 9% 36% 34% 9% 1	975
	11th to 20th	Night	12% 11% 38% 31% 8% 1	1000
	21st to end	Night	12% 9% 37% 35% 8%	1000
December	1st to 10th	Night	13% 11% 38% 32% 7% 1	1000
	11th to 20th	Night	14% 9% 35% 34% 8%	1000
	21st to end	Night	20% 11% 33% 28% 8%	1100

Page generated at 2025-05-08 08:34:01. Version ac07

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