Astro-Physics
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Astro-Physics 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	12% 9% 3 8% 5% 4 5% 4 6% 7% 38%	149388
	Night	12% 10% 4 8% 5% 4 4 3 5% 6% 38%	142036
Transparency	Day	43% 6% 12% 24% 13% 1	152882
	Night	45% 5% 11% 23% 14% 2	146619
Seeing	Day	16% 11% 29% 25% 13% 5%	18460
	Night	17% 9% 31% 30% 11% 1	28206

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	10% 13% 7% 15% 6% 6% 5% 4 5% 5% 24%	4691
	11th to 20th	Day	7% 11% 7% 13% 4 4 5% 6% 6% 6% 31%	4710
	21st to end	Day	10% 13% 7% 9% 5% 5% 4 3 4 6% 33%	5430
February	1st to 10th	Day	11% 15% 5% 9% 6% 6% 6% 3 5% 7% 27%	1890
	11th to 20th	Day	13% 11% 3 8% 6% 6% 4 3 4 5% 37%	2016
	21st to end	Day	13% 15% 4 11% 5% 4 4 3 5% 6% 32%	1730
March	1st to 10th	Day	15% 9% 3 7% 4 3 4 3 4 5% 44%	4580
	11th to 20th	Day	11% 7% 3 8% 4 4 4 3 6% 8% 42%	4855
	21st to end	Day	20% 12% 5% 11% 6% 5% 5% 3 5% 5% 23%	6069
April	1st to 10th	Day	18% 13% 3 8% 5% 4 4 4 5% 5% 31%	2520
	11th to 20th	Day	16% 10% 4 7% 5% 4 5% 4 6% 6% 33%	2494
	21st to end	Day	19% 9% 3 8% 5% 4 4 3 5% 6% 33%	2593
May	1st to 10th	Day	12% 8% 3 7% 5% 5% 5% 4 6% 7% 37%	7711
	11th to 20th	Day	13% 6% 2 6% 4 5% 5% 4 6% 8% 41%	8222
	21st to end	Day	13% 7% 3 7% 5% 5% 5% 4 6% 9% 37%	9047
June	1st to 10th	Day	11% 5% 2 6% 5% 5% 4 4 6% 8% 43%	2940
	11th to 20th	Day	10% 5% 2 6% 4 4 5% 4 7% 8% 44%	2940
	21st to end	Day	11% 7% 2 6% 5% 5% 5% 4 7% 10% 38%	2940
July	1st to 10th	Day	9% 7% 2 8% 5% 5% 5% 4 7% 9% 39%	8280
	11th to 20th	Day	7% 5% 2 6% 4 4 5% 4 6% 9% 49%	8280
	21st to end	Day	9% 6% 3 7% 4 5% 5% 4 6% 9% 42%	8990
August	1st to 10th	Day	13% 6% 2 6% 5% 4 5% 4 7% 9% 39%	6393
	11th to 20th	Day	15% 7% 3 7% 4 4 5% 4 6% 8% 37%	5518
	21st to end	Day	8% 6% 2 5% 4 4 4 4 6% 9% 48%	6080
September	1st to 10th	Day	11% 5% 2 6% 4 4 4 3 6% 8% 47%	2520
	11th to 20th	Day	12% 4 1 4 3 3 4 4 5% 9% 52%	2351
	21st to end	Day	12% 6% 2 7% 4 4 4 4 5% 7% 45%	2200
October	1st to 10th	Day	13% 7% 2 7% 3 4 3 3 5% 6% 49%	2075
	11th to 20th	Day	19% 10% 3 5% 4 4 4 3 5% 7% 36%	2000
	21st to end	Day	19% 10% 3 7% 4 4 3 3 4 5% 38%	2006
November	1st to 10th	Day	13% 8% 4 6% 4 3 4 4 4 7% 44%	1852
	11th to 20th	Day	14% 13% 3 10% 6% 4 4 3 4 6% 32%	1890
	21st to end	Day	16% 15% 4 9% 5% 3 2 2 4 5% 34%	1727
December	1st to 10th	Day	8% 10% 4 9% 5% 4 4 4 5% 6% 40%	2690
	11th to 20th	Day	12% 15% 5% 12% 6% 5% 5% 4 5% 7% 24%	3088
	21st to end	Day	20% 22% 9% 12% 5% 4 4 2 4 5% 13%	4070
January	1st to 10th	Night	10% 13% 7% 14% 6% 5% 3 3 4 4 31%	8232
	11th to 20th	Night	9% 14% 6% 15% 7% 7% 5% 5% 6% 5% 22%	8280
	21st to end	Night	11% 14% 6% 9% 5% 5% 4 3 5% 6% 32%	8782
February	1st to 10th	Night	13% 14% 4 9% 6% 5% 3 3 4 5% 33%	2730
	11th to 20th	Night	12% 11% 4 7% 5% 5% 4 3 5% 6% 38%	2489
	21st to end	Night	16% 13% 3 9% 3 4 3 3 5% 5% 36%	2076
March	1st to 10th	Night	16% 9% 3 7% 4 4 3 3 4 5% 42%	5493
	11th to 20th	Night	9% 7% 2 6% 5% 4 4 3 5% 8% 47%	5357
	21st to end	Night	21% 13% 5% 9% 5% 4 4 3 4 5% 28%	5101
April	1st to 10th	Night	18% 9% 4 6% 4 4 3 4 4 5% 37%	2100
	11th to 20th	Night	19% 9% 3 7% 4 4 3 3 4 6% 39%	2075
	21st to end	Night	17% 8% 2 7% 4 4 4 4 5% 6% 37%	2026
May	1st to 10th	Night	14% 6% 2 6% 5% 4 5% 4 6% 7% 40%	5132
	11th to 20th	Night	10% 5% 2 6% 4 4 5% 4 6% 8% 44%	4670
	21st to end	Night	13% 8% 2 7% 5% 5% 5% 4 6% 7% 39%	5139
June	1st to 10th	Night	9% 4 1 5% 4 5% 6% 5% 6% 9% 47%	1680
	11th to 20th	Night	10% 6% 1 6% 4 4 5% 4 7% 9% 46%	1680
	21st to end	Night	10% 6% 3 7% 5% 4 5% 4 5% 7% 45%	1680
July	1st to 10th	Night	11% 6% 2 8% 5% 4 4 3 6% 8% 43%	4708
	11th to 20th	Night	6% 5% 2 5% 3 4 4 3 5% 9% 54%	4707
	21st to end	Night	7% 6% 2 7% 4 4 5% 4 6% 7% 47%	5117
August	1st to 10th	Night	13% 6% 2 7% 4 4 4 3 5% 7% 46%	3773
	11th to 20th	Night	12% 8% 3 7% 4 4 5% 3 5% 7% 42%	4517
	21st to end	Night	7% 4 2 5% 4 4 4 4 6% 9% 52%	5046
September	1st to 10th	Night	8% 5% 3 6% 3 3 3 3 5% 6% 54%	2100
	11th to 20th	Night	10% 4 1 4 4 4 4 4 5% 6% 54%	2094
	21st to end	Night	11% 6% 2 5% 4 4 4 3 5% 7% 49%	2200
October	1st to 10th	Night	11% 6% 3 6% 3 3 4 3 4 6% 51%	2322
	11th to 20th	Night	17% 7% 2 6% 4 3 3 3 4 6% 44%	2400
	21st to end	Night	19% 9% 3 6% 4 3 4 3 5% 6% 38%	2833
November	1st to 10th	Night	12% 8% 3 6% 3 4 3 3 5% 6% 48%	2666
	11th to 20th	Night	14% 10% 3 9% 5% 4 4 4 5% 6% 36%	2730
	21st to end	Night	17% 12% 4 8% 5% 3 3 2 4 6% 36%	2892
December	1st to 10th	Night	10% 10% 3 8% 5% 5% 4 4 5% 7% 40%	4720
	11th to 20th	Night	16% 18% 4 9% 5% 4 4 3 4 5% 28%	5339
	21st to end	Night	20% 19% 7% 11% 5% 4 5% 3 4 5% 16%	7150

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	62% 5% 14% 14% 5%	4692
	11th to 20th	Day	51% 4 11% 16% 15% 2	4710
	21st to end	Day	52% 4 13% 21% 9%	5430
February	1st to 10th	Day	56% 5% 13% 15% 10% 1	1890
	11th to 20th	Day	50% 4 11% 24% 9% 2	2026
	21st to end	Day	54% 5% 9% 21% 10% 2	1730
March	1st to 10th	Day	43% 4 12% 27% 10% 4	4580
	11th to 20th	Day	39% 5% 13% 31% 11% 1	4855
	21st to end	Day	61% 7% 11% 13% 7% 1	6071
April	1st to 10th	Day	54% 6% 10% 21% 9%	2520
	11th to 20th	Day	49% 7% 10% 22% 10% 2	2494
	21st to end	Day	51% 6% 12% 18% 11% 1	2593
May	1st to 10th	Day	43% 6% 10% 23% 18% 1	7746
	11th to 20th	Day	40% 7% 11% 25% 17% 1	8226
	21st to end	Day	41% 7% 11% 22% 14% 5%	9061
June	1st to 10th	Day	37% 8% 13% 25% 17% 1	2940
	11th to 20th	Day	35% 7% 13% 24% 19% 2	2940
	21st to end	Day	39% 8% 15% 23% 13% 2	2940
July	1st to 10th	Day	40% 6% 15% 25% 13% 1	8211
	11th to 20th	Day	31% 5% 13% 31% 19% 1	8205
	21st to end	Day	37% 6% 14% 23% 18% 2	8794
August	1st to 10th	Day	39% 7% 14% 26% 13% 1	6378
	11th to 20th	Day	42% 6% 14% 28% 10%	5554
	21st to end	Day	33% 6% 13% 32% 16% 1	6079
September	1st to 10th	Day	35% 7% 11% 28% 17% 2	2520
	11th to 20th	Day	30% 5% 12% 32% 18% 2	2351
	21st to end	Day	38% 6% 9% 28% 18% 1	2200
October	1st to 10th	Day	24% 3 7% 38% 26% 1	3374
	11th to 20th	Day	34% 4 10% 27% 23% 1	3240
	21st to end	Day	43% 4 9% 28% 14% 3	3218
November	1st to 10th	Day	40% 5% 14% 25% 15% 1	1852
	11th to 20th	Day	54% 4 12% 20% 9% 1	1890
	21st to end	Day	52% 5% 13% 23% 7%	1727
December	1st to 10th	Day	44% 4 19% 24% 7% 1	2690
	11th to 20th	Day	59% 4 12% 17% 6% 1	3085
	21st to end	Day	76% 4 7% 9% 5%	4070
January	1st to 10th	Night	57% 4 11% 19% 9%	8233
	11th to 20th	Night	63% 5% 9% 10% 12% 1	8280
	21st to end	Night	54% 3 12% 21% 10%	8782
February	1st to 10th	Night	54% 4 11% 18% 11% 1	2730
	11th to 20th	Night	47% 5% 11% 24% 13% 1	2490
	21st to end	Night	49% 5% 12% 20% 11% 4	2076
March	1st to 10th	Night	45% 5% 12% 22% 10% 6%	5493
	11th to 20th	Night	35% 5% 11% 33% 12% 3	5357
	21st to end	Night	59% 5% 9% 17% 9% 1	5101
April	1st to 10th	Night	47% 6% 11% 21% 14% 1	2100
	11th to 20th	Night	48% 4 7% 21% 17% 3	2075
	21st to end	Night	45% 7% 12% 23% 11% 2	2026
May	1st to 10th	Night	40% 7% 11% 22% 19% 1	5159
	11th to 20th	Night	35% 8% 14% 23% 17% 3	4671
	21st to end	Night	42% 6% 10% 21% 17% 4	5149
June	1st to 10th	Night	31% 8% 15% 24% 20% 2	1680
	11th to 20th	Night	33% 7% 11% 23% 23% 3	1680
	21st to end	Night	37% 6% 14% 22% 18% 2	1680
July	1st to 10th	Night	39% 5% 16% 24% 14% 2	4670
	11th to 20th	Night	26% 5% 14% 31% 23% 1	4662
	21st to end	Night	33% 6% 13% 27% 19% 2	5002
August	1st to 10th	Night	38% 5% 13% 27% 17% 1	3764
	11th to 20th	Night	41% 6% 15% 26% 12%	4547
	21st to end	Night	28% 6% 14% 33% 16% 2	5046
September	1st to 10th	Night	31% 6% 12% 28% 22% 2	2100
	11th to 20th	Night	29% 6% 10% 33% 20% 3	2094
	21st to end	Night	35% 6% 10% 29% 19% 1	2200
October	1st to 10th	Night	25% 4 6% 35% 28% 1	3803
	11th to 20th	Night	35% 4 9% 26% 24% 3	3899
	21st to end	Night	42% 5% 9% 25% 16% 3	4571
November	1st to 10th	Night	38% 5% 10% 27% 18% 2	2666
	11th to 20th	Night	48% 5% 13% 21% 11% 1	2730
	21st to end	Night	52% 4 11% 22% 10% 1	2892
December	1st to 10th	Night	44% 5% 15% 26% 9% 1	4720
	11th to 20th	Night	58% 4 11% 16% 9% 1	5341
	21st to end	Night	70% 5% 9% 11% 5%	7150

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% 16% 33% 25% 8%	330
	11th to 20th	Day	19% 18% 38% 22% 2 1	330
	21st to end	Day	20% 15% 37% 23% 4 1	363
February	1st to 10th	Day	16% 21% 40% 18% 5% 1	330
	11th to 20th	Day	21% 19% 36% 20% 5%	330
	21st to end	Day	24% 20% 40% 15% 1	273
March	1st to 10th	Day	20% 21% 40% 14% 5%	330
	11th to 20th	Day	25% 19% 35% 14% 6% 1	330
	21st to end	Day	27% 17% 32% 18% 5% 1	577
April	1st to 10th	Day	26% 14% 34% 21% 4	540
	11th to 20th	Day	21% 22% 27% 19% 8% 2	532
	21st to end	Day	24% 13% 30% 22% 9% 1	540
May	1st to 10th	Day	17% 17% 31% 23% 8% 3	574
	11th to 20th	Day	21% 12% 30% 23% 12% 3	775
	21st to end	Day	16% 10% 29% 24% 15% 6%	844
June	1st to 10th	Day	10% 9% 32% 28% 17% 4	780
	11th to 20th	Day	12% 6% 35% 28% 14% 5%	780
	21st to end	Day	13% 9% 28% 33% 14% 4	780
July	1st to 10th	Day	10% 2 19% 32% 25% 12%	771
	11th to 20th	Day	10% 5% 25% 30% 22% 9%	768
	21st to end	Day	8% 4 23% 37% 20% 7%	849
August	1st to 10th	Day	9% 4 20% 32% 24% 11%	776
	11th to 20th	Day	13% 4 20% 26% 22% 15%	578
	21st to end	Day	11% 4 18% 31% 27% 9%	627
September	1st to 10th	Day	13% 7% 22% 28% 22% 8%	570
	11th to 20th	Day	11% 3 19% 31% 24% 12%	542
	21st to end	Day	14% 10% 25% 26% 18% 8%	530
October	1st to 10th	Day	14% 11% 28% 25% 19% 4	405
	11th to 20th	Day	22% 20% 30% 19% 8% 1	330
	21st to end	Day	27% 14% 31% 22% 5% 1	363
November	1st to 10th	Day	17% 18% 35% 19% 8% 2	330
	11th to 20th	Day	21% 19% 37% 15% 8%	330
	21st to end	Day	28% 11% 40% 16% 5%	330
December	1st to 10th	Day	16% 18% 41% 18% 5% 2	330
	11th to 20th	Day	18% 15% 40% 19% 8% 1	330
	21st to end	Day	27% 15% 39% 16% 2	363
January	1st to 10th	Night	19% 13% 37% 27% 5%	1050
	11th to 20th	Night	20% 14% 38% 22% 5% 1	1050
	21st to end	Night	19% 12% 36% 27% 6%	1155
February	1st to 10th	Night	22% 11% 39% 23% 5%	1050
	11th to 20th	Night	19% 14% 36% 27% 4	854
	21st to end	Night	22% 14% 40% 21% 3	692
March	1st to 10th	Night	21% 11% 38% 24% 5%	840
	11th to 20th	Night	18% 13% 34% 28% 7%	754
	21st to end	Night	23% 14% 30% 27% 5% 1	693
April	1st to 10th	Night	24% 14% 34% 24% 4	630
	11th to 20th	Night	23% 12% 31% 26% 7%	623
	21st to end	Night	21% 9% 33% 30% 7%	630
May	1st to 10th	Night	17% 10% 35% 30% 8%	630
	11th to 20th	Night	17% 7% 33% 32% 10%	621
	21st to end	Night	14% 6% 29% 36% 15%	683
June	1st to 10th	Night	9% 5% 29% 43% 13%	630
	11th to 20th	Night	12% 5% 32% 39% 11% 1	630
	21st to end	Night	10% 4 30% 39% 17% 1	630
July	1st to 10th	Night	8% 2 17% 41% 28% 3	615
	11th to 20th	Night	8% 3 20% 43% 25% 1	612
	21st to end	Night	8% 2 19% 47% 23% 1	690
August	1st to 10th	Night	6% 3 14% 47% 28% 3	627
	11th to 20th	Night	9% 4 13% 40% 29% 4	624
	21st to end	Night	9% 2 16% 43% 27% 3	693
September	1st to 10th	Night	8% 5% 21% 38% 24% 4	630
	11th to 20th	Night	11% 4 21% 35% 28% 3	673
	21st to end	Night	12% 8% 25% 32% 21% 2	800
October	1st to 10th	Night	13% 6% 32% 33% 16% 1	800
	11th to 20th	Night	21% 11% 31% 28% 9%	800
	21st to end	Night	23% 11% 33% 25% 8%	880
November	1st to 10th	Night	17% 10% 39% 23% 9% 1	820
	11th to 20th	Night	20% 15% 36% 24% 6%	840
	21st to end	Night	23% 13% 34% 24% 6%	1002
December	1st to 10th	Night	22% 12% 39% 22% 6%	1050
	11th to 20th	Night	22% 12% 35% 25% 5% 1	1050
	21st to end	Night	29% 12% 34% 23% 2	1155

Page generated at 2026-09-11 03:11:54. Version ac07

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