Astronomers by Necessity
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Astronomers by Necessity 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	8% 4 2 4 3 3 4 3 5% 7% 57%	148694
	Night	7% 4 1 4 3 3 3 3 5% 7% 60%	142839
Transparency	Day	27% 4 9% 24% 26% 11%	152474
	Night	25% 4 8% 23% 27% 14%	147125
Seeing	Day	10% 8% 25% 31% 21% 6%	17634
	Night	10% 8% 33% 36% 13% 1	27438

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% 5% 3 5% 3 3 3 2 4 5% 59%	5289
	11th to 20th	Day	4 4 2 3 2 2 3 3 4 6% 67%	5310
	21st to end	Day	17% 8% 2 5% 3 3 2 2 4 5% 49%	6089
February	1st to 10th	Day	12% 4 2 4 3 3 3 4 4 5% 54%	2100
	11th to 20th	Day	11% 8% 3 5% 3 3 4 3 4 5% 52%	2049
	21st to end	Day	13% 6% 2 3 3 2 2 2 3 4 59%	1730
March	1st to 10th	Day	11% 5% 2 4 3 3 3 2 5% 6% 55%	4841
	11th to 20th	Day	10% 4 2 4 2 3 3 3 4 5% 60%	5090
	21st to end	Day	5% 4 1 5% 4 3 4 4 6% 7% 56%	5583
April	1st to 10th	Day	8% 5% 2 5% 3 4 4 4 4 7% 54%	2310
	11th to 20th	Day	11% 6% 2 5% 4 3 3 3 4 5% 54%	2437
	21st to end	Day	9% 5% 1 5% 3 3 4 3 5% 6% 55%	2693
May	1st to 10th	Day	11% 6% 2 7% 5% 4 4 3 5% 6% 47%	7617
	11th to 20th	Day	11% 6% 2 6% 4 4 4 4 4 5% 49%	7627
	21st to end	Day	9% 6% 3 7% 5% 5% 5% 4 6% 8% 42%	8392
June	1st to 10th	Day	6% 4 1 5% 3 3 4 4 5% 9% 57%	2730
	11th to 20th	Day	4 3 1 3 3 4 4 4 6% 9% 58%	2730
	21st to end	Day	4 4 1 3 4 4 4 4 5% 7% 60%	2730
July	1st to 10th	Day	8% 5% 2 6% 4 4 5% 3 5% 8% 49%	7680
	11th to 20th	Day	4 3 2 5% 4 4 4 4 6% 8% 56%	7680
	21st to end	Day	2 1 1 3 2 3 3 3 5% 8% 69%	8337
August	1st to 10th	Day	1 1 1 2 2 2 3 3 4 6% 75%	6014
	11th to 20th	Day	2 2 1 2 2 2 2 2 5% 8% 72%	5560
	21st to end	Day	2 2 1 3 3 3 4 3 5% 8% 66%	6013
September	1st to 10th	Day	10% 3 2 5% 3 5% 4 3 4 5% 56%	2310
	11th to 20th	Day	10% 5% 2 6% 4 4 4 3 5% 7% 51%	2233
	21st to end	Day	7% 3 2 4 3 3 4 3 5% 7% 60%	2200
October	1st to 10th	Day	4 3 2 4 3 2 3 3 4 7% 66%	2200
	11th to 20th	Day	8% 3 1 4 2 3 2 3 4 4 67%	2074
	21st to end	Day	10% 5% 2 3 2 2 2 1 3 4 66%	2200
November	1st to 10th	Day	8% 4 2 4 2 3 3 3 4 6% 62%	1968
	11th to 20th	Day	4 3 1 2 3 3 3 4 5% 6% 65%	1890
	21st to end	Day	11% 5% 2 4 3 3 3 3 4 6% 56%	1890
December	1st to 10th	Day	12% 6% 2 5% 3 3 4 3 5% 6% 52%	3030
	11th to 20th	Day	9% 5% 1 5% 3 4 5% 3 4 5% 55%	3481
	21st to end	Day	10% 7% 2 5% 3 3 3 2 3 4 60%	4587
January	1st to 10th	Night	9% 5% 2 4 3 3 3 3 4 6% 60%	7634
	11th to 20th	Night	5% 2 1 2 2 2 3 3 4 5% 71%	7680
	21st to end	Night	15% 8% 2 5% 3 4 4 3 4 6% 46%	8181
February	1st to 10th	Night	10% 6% 2 5% 3 3 3 2 5% 6% 57%	2520
	11th to 20th	Night	12% 6% 2 5% 4 3 3 3 3 6% 53%	2458
	21st to end	Night	10% 5% 2 4 3 3 3 3 4 5% 59%	2076
March	1st to 10th	Night	8% 8% 2 4 3 3 3 2 5% 6% 56%	5241
	11th to 20th	Night	9% 4 1 4 2 2 3 3 5% 7% 59%	5090
	21st to end	Night	5% 5% 2 6% 4 3 3 3 4 6% 58%	5588
April	1st to 10th	Night	7% 4 2 5% 4 3 3 4 6% 7% 56%	2310
	11th to 20th	Night	9% 5% 2 5% 4 4 3 3 5% 6% 55%	2128
	21st to end	Night	8% 4 1 4 3 3 4 3 4 7% 59%	1925
May	1st to 10th	Night	10% 6% 2 6% 4 4 4 3 5% 6% 48%	5266
	11th to 20th	Night	7% 5% 2 5% 4 4 4 3 5% 7% 53%	5265
	21st to end	Night	9% 6% 2 6% 4 4 4 4 7% 9% 45%	5793
June	1st to 10th	Night	3 2 1 3 3 3 4 3 8% 11% 59%	1890
	11th to 20th	Night	2 2 1 3 3 3 3 4 7% 9% 62%	1890
	21st to end	Night	3 3 1 4 2 3 2 3 4 8% 68%	1890
July	1st to 10th	Night	6% 4 1 5% 4 3 4 3 6% 11% 52%	5308
	11th to 20th	Night	2 2 1 4 3 3 4 3 5% 10% 64%	5307
	21st to end	Night	1 2 1 2 2 2 3 3 4 7% 74%	5770
August	1st to 10th	Night	1 1 1 1 1 2 2 4 7% 80%	4156
	11th to 20th	Night	2 1 1 2 2 1 2 2 4 8% 76%	4489
	21st to end	Night	2 2 1 3 2 3 3 3 5% 9% 67%	5131
September	1st to 10th	Night	7% 4 1 4 3 4 4 3 5% 6% 59%	2310
	11th to 20th	Night	9% 5% 2 6% 4 4 3 3 5% 8% 52%	2233
	21st to end	Night	7% 2 1 3 3 3 3 3 5% 7% 64%	2200
October	1st to 10th	Night	4 2 1 3 2 3 2 2 5% 7% 69%	2200
	11th to 20th	Night	8% 3 1 4 3 2 2 1 3 4 69%	2305
	21st to end	Night	9% 5% 1 3 2 3 2 2 3 4 64%	2640
November	1st to 10th	Night	8% 4 1 4 2 3 2 3 3 5% 65%	2546
	11th to 20th	Night	3 3 2 5% 3 3 3 3 4 6% 67%	2730
	21st to end	Night	13% 5% 2 4 3 2 3 3 5% 7% 55%	2730
December	1st to 10th	Night	12% 6% 1 4 3 3 3 2 4 5% 56%	4380
	11th to 20th	Night	8% 5% 2 5% 4 3 4 3 4 6% 56%	4946
	21st to end	Night	9% 6% 2 5% 3 3 3 2 3 5% 60%	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	28% 2 6% 23% 30% 10%	5290
	11th to 20th	Day	20% 2 5% 24% 37% 13%	5310
	21st to end	Day	39% 3 4 18% 23% 14%	6089
February	1st to 10th	Day	32% 3 5% 17% 26% 16%	2100
	11th to 20th	Day	36% 2 6% 20% 23% 12%	2059
	21st to end	Day	32% 2 4 14% 27% 21%	1730
March	1st to 10th	Day	30% 3 5% 13% 24% 25%	4841
	11th to 20th	Day	28% 2 10% 12% 22% 26%	5090
	21st to end	Day	27% 4 10% 23% 25% 12%	5585
April	1st to 10th	Day	31% 5% 9% 20% 21% 14%	2310
	11th to 20th	Day	33% 3 8% 18% 22% 15%	2437
	21st to end	Day	32% 4 8% 19% 29% 8%	2693
May	1st to 10th	Day	37% 5% 10% 22% 19% 7%	7655
	11th to 20th	Day	36% 4 7% 15% 30% 7%	7631
	21st to end	Day	38% 7% 13% 23% 11% 7%	8405
June	1st to 10th	Day	26% 4 14% 32% 20% 4	2730
	11th to 20th	Day	22% 5% 13% 31% 23% 6%	2730
	21st to end	Day	23% 4 12% 28% 25% 8%	2730
July	1st to 10th	Day	32% 6% 10% 32% 19% 1	7616
	11th to 20th	Day	25% 6% 18% 28% 20% 3	7611
	21st to end	Day	13% 5% 12% 40% 29% 2	8155
August	1st to 10th	Day	11% 3 9% 31% 35% 11%	6000
	11th to 20th	Day	13% 3 11% 35% 34% 3	5597
	21st to end	Day	16% 5% 14% 29% 32% 4	6012
September	1st to 10th	Day	30% 4 9% 20% 26% 10%	2310
	11th to 20th	Day	33% 5% 9% 22% 24% 7%	2233
	21st to end	Day	24% 4 8% 20% 29% 15%	2200
October	1st to 10th	Day	13% 2 6% 16% 37% 26%	3590
	11th to 20th	Day	17% 3 8% 22% 31% 20%	3353
	21st to end	Day	19% 2 3 26% 35% 14%	3538
November	1st to 10th	Day	25% 4 6% 23% 27% 15%	1968
	11th to 20th	Day	20% 3 8% 23% 30% 16%	1890
	21st to end	Day	32% 3 4 22% 26% 14%	1890
December	1st to 10th	Day	35% 3 5% 21% 27% 9%	3030
	11th to 20th	Day	32% 4 7% 14% 28% 16%	3479
	21st to end	Day	30% 3 4 24% 25% 15%	4587
January	1st to 10th	Night	27% 3 5% 22% 34% 10%	7635
	11th to 20th	Night	17% 2 5% 19% 37% 21%	7680
	21st to end	Night	39% 3 6% 15% 23% 14%	8181
February	1st to 10th	Night	29% 3 6% 17% 26% 18%	2520
	11th to 20th	Night	34% 3 6% 15% 25% 16%	2458
	21st to end	Night	29% 3 5% 16% 24% 24%	2076
March	1st to 10th	Night	30% 3 4 15% 23% 24%	5241
	11th to 20th	Night	26% 2 8% 18% 16% 30%	5090
	21st to end	Night	28% 4 7% 20% 29% 12%	5588
April	1st to 10th	Night	28% 4 11% 21% 20% 18%	2310
	11th to 20th	Night	31% 4 10% 17% 22% 16%	2128
	21st to end	Night	26% 4 12% 22% 28% 8%	1925
May	1st to 10th	Night	36% 5% 10% 23% 18% 7%	5293
	11th to 20th	Night	29% 5% 10% 19% 24% 12%	5266
	21st to end	Night	34% 6% 15% 23% 12% 8%	5805
June	1st to 10th	Night	19% 6% 18% 33% 20% 4	1890
	11th to 20th	Night	18% 5% 13% 31% 24% 8%	1890
	21st to end	Night	18% 4 11% 31% 27% 9%	1890
July	1st to 10th	Night	26% 6% 11% 34% 21% 2	5265
	11th to 20th	Night	17% 6% 15% 33% 24% 5%	5256
	21st to end	Night	11% 4 12% 37% 34% 3	5641
August	1st to 10th	Night	6% 2 8% 31% 39% 13%	4146
	11th to 20th	Night	11% 2 12% 35% 34% 6%	4517
	21st to end	Night	15% 5% 16% 28% 31% 5%	5131
September	1st to 10th	Night	26% 5% 9% 23% 28% 10%	2310
	11th to 20th	Night	30% 5% 11% 23% 22% 10%	2233
	21st to end	Night	21% 3 8% 23% 27% 18%	2200
October	1st to 10th	Night	11% 2 6% 16% 32% 34%	3590
	11th to 20th	Night	15% 2 8% 19% 33% 23%	3752
	21st to end	Night	20% 3 4 21% 38% 14%	4252
November	1st to 10th	Night	24% 3 7% 21% 26% 19%	2546
	11th to 20th	Night	21% 3 5% 23% 29% 18%	2730
	21st to end	Night	31% 3 6% 17% 28% 15%	2730
December	1st to 10th	Night	31% 3 5% 21% 27% 12%	4380
	11th to 20th	Night	30% 3 6% 15% 29% 17%	4947
	21st to end	Night	29% 3 5% 21% 26% 16%	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	12% 21% 38% 26% 4	330
	11th to 20th	Day	15% 18% 35% 25% 5% 1	330
	21st to end	Day	15% 17% 39% 22% 6%	363
February	1st to 10th	Day	16% 16% 39% 25% 3 1	330
	11th to 20th	Day	19% 18% 43% 17% 3	330
	21st to end	Day	20% 17% 41% 19% 3	273
March	1st to 10th	Day	19% 18% 36% 22% 4	330
	11th to 20th	Day	19% 16% 41% 19% 4 1	330
	21st to end	Day	12% 14% 45% 25% 4	363
April	1st to 10th	Day	14% 19% 41% 19% 6%	330
	11th to 20th	Day	15% 20% 45% 16% 4	475
	21st to end	Day	13% 16% 42% 22% 6%	713
May	1st to 10th	Day	11% 13% 40% 28% 6% 1	753
	11th to 20th	Day	14% 10% 33% 28% 13% 3	775
	21st to end	Day	10% 6% 34% 33% 14% 3	844
June	1st to 10th	Day	6% 3 20% 42% 23% 5%	780
	11th to 20th	Day	5% 2 21% 49% 20% 4	780
	21st to end	Day	3 2 14% 40% 32% 8%	780
July	1st to 10th	Day	5% 2 8% 39% 35% 12%	771
	11th to 20th	Day	3 2 7% 38% 39% 12%	768
	21st to end	Day	2 1 6% 36% 40% 15%	849
August	1st to 10th	Day	3 6% 34% 46% 12%	776
	11th to 20th	Day	5% 2 5% 33% 43% 12%	596
	21st to end	Day	4 1 7% 31% 38% 18%	600
September	1st to 10th	Day	11% 1 8% 19% 43% 18%	360
	11th to 20th	Day	3 2 7% 29% 44% 15%	339
	21st to end	Day	10% 2 8% 35% 32% 13%	330
October	1st to 10th	Day	4 2 13% 38% 33% 11%	330
	11th to 20th	Day	8% 5% 32% 34% 19% 2	330
	21st to end	Day	18% 8% 28% 28% 16% 2	363
November	1st to 10th	Day	14% 7% 33% 30% 12% 3	330
	11th to 20th	Day	8% 12% 33% 31% 12% 4	330
	21st to end	Day	15% 19% 36% 20% 9% 2	330
December	1st to 10th	Day	15% 15% 35% 26% 8% 1	330
	11th to 20th	Day	11% 13% 44% 24% 7% 1	330
	21st to end	Day	17% 13% 35% 28% 6%	363
January	1st to 10th	Night	12% 10% 39% 31% 7%	1050
	11th to 20th	Night	13% 14% 34% 32% 7%	1050
	21st to end	Night	13% 13% 39% 27% 6%	1076
February	1st to 10th	Night	13% 11% 43% 29% 4	840
	11th to 20th	Night	16% 16% 40% 25% 3	824
	21st to end	Night	13% 16% 47% 21% 2	692
March	1st to 10th	Night	14% 15% 43% 25% 3	730
	11th to 20th	Night	15% 13% 44% 23% 6%	630
	21st to end	Night	8% 14% 47% 28% 4	693
April	1st to 10th	Night	9% 17% 47% 25% 3	630
	11th to 20th	Night	13% 13% 49% 23% 2	623
	21st to end	Night	11% 14% 46% 22% 6%	630
May	1st to 10th	Night	12% 11% 45% 27% 4	630
	11th to 20th	Night	10% 7% 42% 29% 11% 1	621
	21st to end	Night	8% 5% 43% 32% 12% 1	683
June	1st to 10th	Night	3 3 33% 42% 18% 1	630
	11th to 20th	Night	3 2 33% 53% 9%	630
	21st to end	Night	5% 1 21% 51% 20% 1	630
July	1st to 10th	Night	5% 2 13% 51% 28% 1	615
	11th to 20th	Night	3 11% 57% 28% 1	612
	21st to end	Night	2 1 9% 57% 31% 1	690
August	1st to 10th	Night	1 8% 59% 30% 1	627
	11th to 20th	Night	7% 1 7% 52% 30% 3	624
	21st to end	Night	4 2 5% 49% 35% 5%	693
September	1st to 10th	Night	7% 2 10% 47% 30% 3	630
	11th to 20th	Night	11% 1 12% 46% 28% 2	609
	21st to end	Night	8% 1 17% 45% 27% 2	600
October	1st to 10th	Night	6% 2 24% 45% 22% 2	600
	11th to 20th	Night	8% 5% 33% 40% 13% 1	705
	21st to end	Night	13% 10% 31% 33% 13% 1	880
November	1st to 10th	Night	10% 6% 33% 38% 12% 1	906
	11th to 20th	Night	5% 9% 38% 36% 11% 1	1050
	21st to end	Night	15% 11% 38% 26% 9% 1	1050
December	1st to 10th	Night	11% 10% 38% 33% 7%	1050
	11th to 20th	Night	12% 11% 38% 32% 6%	1050
	21st to end	Night	13% 12% 39% 29% 7%	1155

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

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