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 2026-09-14.

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% 2 7% 4 4 4 4 5% 7% 45%	157585
	Night	9% 7% 3 7% 4 4 4 4 5% 7% 46%	150896
Transparency	Day	38% 5% 14% 26% 15% 2	152264
	Night	37% 5% 14% 26% 16% 2	147229
Seeing	Day	15% 9% 30% 30% 13% 2	18556
	Night	13% 8% 34% 35% 10% 1	28332

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% 13% 4 14% 8% 6% 6% 4 5% 5% 26%	4720
	11th to 20th	Day	4 10% 5% 12% 8% 7% 6% 4 7% 7% 31%	4740
	21st to end	Day	4 9% 5% 11% 7% 6% 6% 4 5% 6% 36%	5738
February	1st to 10th	Day	9% 13% 4 10% 6% 6% 6% 4 6% 6% 31%	1890
	11th to 20th	Day	8% 10% 4 9% 7% 5% 5% 4 4 7% 37%	1924
	21st to end	Day	9% 9% 3 10% 5% 6% 4 3 6% 7% 38%	1730
March	1st to 10th	Day	10% 8% 3 8% 4 3 3 3 4 6% 48%	4770
	11th to 20th	Day	7% 6% 3 6% 3 3 4 3 5% 8% 51%	5090
	21st to end	Day	20% 11% 4 10% 5% 4 5% 3 5% 6% 25%	5585
April	1st to 10th	Day	17% 8% 2 7% 4 4 4 4 5% 6% 40%	2395
	11th to 20th	Day	17% 8% 3 7% 4 4 4 3 5% 6% 39%	2699
	21st to end	Day	19% 8% 3 6% 4 3 3 3 5% 6% 39%	2730
May	1st to 10th	Day	8% 4 1 4 4 4 5% 4 6% 8% 52%	7655
	11th to 20th	Day	15% 7% 2 5% 3 3 4 3 4 5% 48%	7627
	21st to end	Day	19% 7% 2 6% 4 4 4 3 5% 7% 38%	8722
June	1st to 10th	Day	11% 4 2 6% 3 3 4 4 6% 8% 49%	3144
	11th to 20th	Day	12% 4 2 6% 4 4 4 4 7% 7% 46%	3150
	21st to end	Day	9% 5% 2 5% 4 3 5% 4 6% 8% 50%	3150
July	1st to 10th	Day	8% 4 2 5% 4 4 5% 4 7% 9% 47%	8495
	11th to 20th	Day	7% 3 1 4 3 3 3 3 5% 7% 61%	8250
	21st to end	Day	8% 5% 2 6% 4 4 5% 4 7% 9% 46%	8456
August	1st to 10th	Day	8% 5% 2 6% 4 4 5% 4 6% 8% 47%	7581
	11th to 20th	Day	14% 7% 2 6% 4 4 4 4 6% 8% 40%	7598
	21st to end	Day	7% 4 2 5% 3 3 4 3 4 7% 56%	8408
September	1st to 10th	Day	12% 5% 2 5% 3 3 3 3 5% 7% 53%	2418
	11th to 20th	Day	9% 5% 2 4 3 3 3 3 5% 7% 56%	2233
	21st to end	Day	11% 4 2 4 3 2 4 3 4 6% 57%	2200
October	1st to 10th	Day	8% 3 1 4 2 2 3 2 4 6% 65%	3429
	11th to 20th	Day	9% 5% 2 4 4 4 3 3 5% 7% 55%	3240
	21st to end	Day	11% 7% 2 6% 4 4 5% 4 6% 7% 45%	3304
November	1st to 10th	Day	8% 7% 3 7% 4 4 4 4 6% 7% 46%	1853
	11th to 20th	Day	7% 8% 2 9% 6% 5% 4 3 5% 7% 45%	1827
	21st to end	Day	10% 9% 3 7% 4 3 4 3 6% 8% 43%	1680
December	1st to 10th	Day	4 9% 4 10% 7% 7% 6% 5% 5% 7% 37%	2690
	11th to 20th	Day	9% 11% 5% 9% 5% 5% 5% 4 6% 8% 34%	2828
	21st to end	Day	13% 9% 3 14% 6% 4 4 4 6% 6% 30%	3636
January	1st to 10th	Night	6% 11% 5% 13% 9% 7% 6% 4 5% 6% 28%	8201
	11th to 20th	Night	2 9% 3 10% 7% 6% 7% 4 8% 8% 35%	8250
	21st to end	Night	3 8% 4 10% 5% 5% 5% 4 6% 7% 43%	8460
February	1st to 10th	Night	9% 11% 4 9% 6% 4 4 5% 6% 7% 36%	2730
	11th to 20th	Night	7% 11% 4 8% 5% 5% 4 4 6% 6% 41%	2600
	21st to end	Night	8% 8% 3 6% 5% 5% 4 5% 5% 7% 45%	2076
March	1st to 10th	Night	10% 7% 2 5% 3 3 4 3 5% 7% 51%	5315
	11th to 20th	Night	9% 6% 2 5% 3 3 3 3 5% 9% 52%	5090
	21st to end	Night	16% 11% 3 8% 5% 4 4 3 5% 6% 36%	5588
April	1st to 10th	Night	14% 8% 2 6% 5% 3 4 4 7% 7% 40%	2218
	11th to 20th	Night	13% 8% 3 6% 4 4 4 2 4 7% 44%	1869
	21st to end	Night	16% 8% 2 6% 3 4 4 4 5% 6% 42%	1890
May	1st to 10th	Night	10% 4 1 4 4 3 4 3 7% 7% 53%	5292
	11th to 20th	Night	12% 7% 2 4 3 3 3 3 4 7% 51%	5265
	21st to end	Night	17% 7% 2 6% 4 4 4 4 5% 7% 40%	5467
June	1st to 10th	Night	7% 5% 2 4 4 3 4 3 5% 7% 56%	1471
	11th to 20th	Night	8% 4 1 5% 4 4 4 3 6% 9% 52%	1470
	21st to end	Night	8% 3 1 4 3 4 3 3 7% 9% 55%	1470
July	1st to 10th	Night	7% 4 2 5% 4 5% 5% 5% 8% 10% 47%	4475
	11th to 20th	Night	6% 3 1 3 2 3 3 3 4 7% 64%	4740
	21st to end	Night	7% 4 2 5% 3 4 4 3 6% 9% 52%	5648
August	1st to 10th	Night	6% 4 2 5% 3 4 5% 4 7% 10% 50%	5241
	11th to 20th	Night	8% 5% 2 6% 4 4 5% 5% 7% 9% 46%	5258
	21st to end	Night	6% 3 2 5% 3 4 4 4 6% 9% 54%	5808
September	1st to 10th	Night	8% 4 1 5% 3 3 4 2 4 6% 59%	2198
	11th to 20th	Night	6% 4 1 4 3 4 4 3 5% 7% 58%	2233
	21st to end	Night	7% 5% 1 5% 3 3 4 2 4 6% 61%	2200
October	1st to 10th	Night	9% 3 1 2 2 1 2 2 3 4 71%	3717
	11th to 20th	Night	9% 4 1 4 2 2 2 2 5% 8% 59%	3899
	21st to end	Night	11% 6% 2 6% 3 4 4 4 6% 7% 47%	4479
November	1st to 10th	Night	8% 5% 2 6% 4 3 3 4 5% 6% 53%	2665
	11th to 20th	Night	7% 7% 2 7% 5% 4 4 4 6% 7% 47%	2791
	21st to end	Night	8% 7% 2 6% 4 4 4 4 5% 8% 47%	2940
December	1st to 10th	Night	5% 6% 4 10% 6% 5% 4 4 5% 6% 46%	4720
	11th to 20th	Night	8% 10% 4 8% 6% 6% 5% 4 5% 7% 37%	5582
	21st to end	Night	14% 11% 4 11% 5% 4 4 3 4 5% 33%	7580

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	59% 5% 14% 15% 6%	4723
	11th to 20th	Day	51% 4 13% 18% 14%	4740
	21st to end	Day	46% 6% 17% 21% 10% 1	5738
February	1st to 10th	Day	51% 7% 15% 17% 9%	1890
	11th to 20th	Day	46% 6% 13% 21% 12% 1	1933
	21st to end	Day	44% 6% 14% 22% 12% 2	1730
March	1st to 10th	Day	38% 5% 11% 31% 13% 2	4770
	11th to 20th	Day	31% 4 10% 30% 22% 3	5090
	21st to end	Day	59% 5% 14% 16% 4 2	5585
April	1st to 10th	Day	45% 4 14% 25% 11% 1	2395
	11th to 20th	Day	45% 6% 11% 22% 14% 2	2702
	21st to end	Day	46% 4 12% 25% 12%	2730
May	1st to 10th	Day	29% 5% 9% 29% 27% 1	7655
	11th to 20th	Day	39% 4 10% 22% 19% 8%	7631
	21st to end	Day	45% 5% 11% 26% 13% 1	8723
June	1st to 10th	Day	32% 6% 17% 30% 12% 2	3144
	11th to 20th	Day	34% 7% 16% 25% 16% 3	3150
	21st to end	Day	30% 7% 16% 29% 15% 3	3150
July	1st to 10th	Day	32% 6% 19% 30% 12% 1	8427
	11th to 20th	Day	24% 4 12% 29% 23% 8%	8175
	21st to end	Day	33% 6% 18% 27% 14% 1	8269
August	1st to 10th	Day	30% 5% 14% 32% 15% 3	6000
	11th to 20th	Day	48% 7% 16% 22% 7%	5987
	21st to end	Day	27% 6% 21% 28% 16% 2	6592
September	1st to 10th	Day	31% 4 13% 30% 19% 3	2418
	11th to 20th	Day	29% 5% 16% 33% 16% 1	2233
	21st to end	Day	29% 5% 10% 32% 22% 2	2200
October	1st to 10th	Day	24% 2 8% 26% 35% 4	3429
	11th to 20th	Day	30% 4 13% 30% 20% 4	3240
	21st to end	Day	38% 3 12% 25% 19% 2	3304
November	1st to 10th	Day	35% 6% 17% 27% 13% 2	1853
	11th to 20th	Day	38% 5% 16% 26% 13% 2	1827
	21st to end	Day	40% 4 16% 26% 13% 1	1680
December	1st to 10th	Day	46% 6% 17% 24% 8% 1	2690
	11th to 20th	Day	48% 4 19% 21% 7%	2825
	21st to end	Day	53% 4 15% 23% 5%	3636
January	1st to 10th	Night	55% 5% 14% 17% 8% 1	8202
	11th to 20th	Night	44% 5% 15% 20% 15% 1	8250
	21st to end	Night	39% 5% 17% 26% 14%	8460
February	1st to 10th	Night	46% 6% 16% 20% 11% 1	2730
	11th to 20th	Night	43% 4 14% 22% 14% 3	2601
	21st to end	Night	38% 5% 14% 26% 14% 3	2076
March	1st to 10th	Night	34% 4 11% 30% 20% 2	5315
	11th to 20th	Night	31% 3 10% 29% 25% 3	5090
	21st to end	Night	50% 4 14% 23% 8% 1	5588
April	1st to 10th	Night	40% 6% 15% 24% 13% 1	2218
	11th to 20th	Night	40% 5% 10% 24% 19% 2	1869
	21st to end	Night	41% 5% 12% 25% 13% 2	1890
May	1st to 10th	Night	28% 5% 11% 29% 25% 2	5293
	11th to 20th	Night	34% 4 10% 19% 26% 8%	5266
	21st to end	Night	44% 4 12% 25% 14% 1	5469
June	1st to 10th	Night	27% 6% 16% 29% 19% 3	1471
	11th to 20th	Night	28% 5% 16% 23% 22% 5%	1470
	21st to end	Night	25% 5% 17% 32% 19% 3	1470
July	1st to 10th	Night	31% 5% 20% 30% 11% 2	4435
	11th to 20th	Night	21% 5% 10% 32% 26% 6%	4692
	21st to end	Night	28% 6% 22% 28% 14% 2	5522
August	1st to 10th	Night	27% 5% 16% 32% 18% 2	4146
	11th to 20th	Night	35% 9% 22% 23% 10% 1	4145
	21st to end	Night	26% 6% 18% 28% 19% 3	4555
September	1st to 10th	Night	27% 4 15% 26% 23% 4	2198
	11th to 20th	Night	24% 6% 15% 33% 19% 2	2233
	21st to end	Night	26% 3 9% 32% 26% 4	2200
October	1st to 10th	Night	19% 3 7% 21% 45% 5%	3717
	11th to 20th	Night	25% 3 11% 32% 24% 5%	3899
	21st to end	Night	35% 4 13% 27% 19% 1	4479
November	1st to 10th	Night	31% 4 15% 26% 20% 4	2665
	11th to 20th	Night	35% 5% 15% 24% 18% 3	2791
	21st to end	Night	36% 4 17% 28% 15% 1	2940
December	1st to 10th	Night	39% 3 15% 31% 10% 1	4720
	11th to 20th	Night	46% 5% 17% 24% 7%	5584
	21st to end	Night	54% 5% 12% 23% 7%	7580

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	19% 11% 30% 28% 11% 1	330
	11th to 20th	Day	15% 15% 35% 27% 7% 1	330
	21st to end	Day	13% 11% 37% 28% 9% 2	363
February	1st to 10th	Day	12% 19% 40% 23% 5%	330
	11th to 20th	Day	14% 13% 41% 22% 8% 1	330
	21st to end	Day	16% 15% 37% 28% 4	273
March	1st to 10th	Day	15% 12% 41% 26% 5% 1	330
	11th to 20th	Day	18% 10% 38% 23% 10% 2	330
	21st to end	Day	25% 13% 34% 22% 6% 1	363
April	1st to 10th	Day	26% 15% 36% 19% 4	415
	11th to 20th	Day	22% 15% 31% 25% 5%	740
	21st to end	Day	26% 12% 29% 23% 8% 2	750
May	1st to 10th	Day	16% 13% 33% 25% 11% 2	753
	11th to 20th	Day	21% 9% 28% 25% 16% 1	775
	21st to end	Day	19% 9% 28% 30% 12% 3	844
June	1st to 10th	Day	10% 6% 25% 32% 21% 5%	780
	11th to 20th	Day	14% 7% 35% 28% 14% 2	780
	21st to end	Day	11% 7% 29% 34% 16% 2	780
July	1st to 10th	Day	7% 3 21% 38% 26% 6%	771
	11th to 20th	Day	7% 4 25% 41% 20% 3	768
	21st to end	Day	6% 3 26% 42% 19% 3	849
August	1st to 10th	Day	9% 4 23% 40% 21% 3	776
	11th to 20th	Day	12% 4 25% 34% 22% 4	765
	21st to end	Day	7% 4 26% 39% 20% 4	858
September	1st to 10th	Day	13% 7% 31% 30% 16% 3	468
	11th to 20th	Day	9% 6% 36% 29% 16% 3	339
	21st to end	Day	14% 10% 26% 31% 15% 4	330
October	1st to 10th	Day	16% 7% 28% 31% 15% 4	330
	11th to 20th	Day	18% 15% 32% 25% 8% 2	330
	21st to end	Day	21% 16% 31% 22% 9% 1	363
November	1st to 10th	Day	18% 11% 36% 25% 7% 3	330
	11th to 20th	Day	13% 15% 39% 24% 7%	330
	21st to end	Day	20% 15% 30% 24% 8% 2	330
December	1st to 10th	Day	13% 9% 36% 31% 11% 1	330
	11th to 20th	Day	17% 15% 37% 25% 5%	330
	21st to end	Day	18% 13% 37% 28% 4 1	363
January	1st to 10th	Night	12% 10% 36% 33% 9% 1	1050
	11th to 20th	Night	10% 14% 36% 31% 7% 1	1050
	21st to end	Night	12% 10% 40% 30% 7% 1	1155
February	1st to 10th	Night	14% 12% 40% 29% 5% 1	1050
	11th to 20th	Night	12% 12% 37% 32% 6% 1	966
	21st to end	Night	13% 9% 38% 30% 8% 1	692
March	1st to 10th	Night	13% 9% 39% 33% 5% 1	765
	11th to 20th	Night	17% 10% 35% 33% 5%	630
	21st to end	Night	18% 11% 37% 27% 6%	693
April	1st to 10th	Night	18% 13% 41% 24% 3	630
	11th to 20th	Night	19% 12% 37% 25% 7%	623
	21st to end	Night	20% 10% 35% 28% 6% 1	630
May	1st to 10th	Night	16% 6% 36% 32% 11%	630
	11th to 20th	Night	16% 8% 31% 33% 11%	621
	21st to end	Night	17% 6% 34% 33% 9% 1	683
June	1st to 10th	Night	10% 5% 29% 40% 17%	630
	11th to 20th	Night	10% 5% 32% 43% 10% 1	630
	21st to end	Night	9% 5% 28% 46% 12%	630
July	1st to 10th	Night	6% 2 22% 46% 24% 1	615
	11th to 20th	Night	6% 2 24% 52% 16% 1	612
	21st to end	Night	5% 3 27% 49% 15% 1	690
August	1st to 10th	Night	7% 4 23% 49% 15% 1	627
	11th to 20th	Night	7% 4 25% 45% 20%	624
	21st to end	Night	6% 4 29% 44% 17% 1	693
September	1st to 10th	Night	10% 5% 26% 43% 14% 2	630
	11th to 20th	Night	7% 5% 34% 40% 12% 2	609
	21st to end	Night	9% 5% 35% 35% 15% 1	600
October	1st to 10th	Night	17% 8% 31% 33% 11% 1	675
	11th to 20th	Night	16% 11% 37% 28% 9%	800
	21st to end	Night	16% 10% 34% 32% 7%	1019
November	1st to 10th	Night	11% 10% 37% 33% 8% 1	1025
	11th to 20th	Night	12% 10% 38% 31% 8% 1	1050
	21st to end	Night	12% 9% 36% 35% 8%	1050
December	1st to 10th	Night	13% 10% 38% 32% 6%	1050
	11th to 20th	Night	15% 9% 35% 33% 8%	1050
	21st to end	Night	19% 11% 34% 29% 7%	1155

Page generated at 2026-09-11 03:11:51. Version ac07

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