Aero Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Aero Observatory 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% 7% 3 7% 4 4 4 4 6% 8% 41%	149143
	Night	13% 8% 3 7% 4 4 4 3 5% 7% 43%	142305
Transparency	Day	40% 5% 13% 24% 16% 2	152709
	Night	41% 4 12% 24% 17% 2	146819
Seeing	Day	15% 11% 28% 26% 16% 5%	18261
	Night	16% 10% 30% 30% 13% 1	27971

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	11% 11% 6% 13% 5% 5% 4 3 5% 6% 31%	4691
	11th to 20th	Day	7% 9% 3 8% 7% 7% 5% 4 5% 6% 38%	5045
	21st to end	Day	14% 15% 6% 9% 5% 4 3 2 3 4 35%	5812
February	1st to 10th	Day	15% 9% 5% 9% 5% 5% 4 3 5% 7% 33%	1890
	11th to 20th	Day	14% 10% 3 7% 6% 5% 5% 3 5% 7% 35%	1996
	21st to end	Day	14% 10% 3 8% 5% 3 4 3 4 7% 39%	1730
March	1st to 10th	Day	16% 10% 2 6% 3 3 3 2 3 5% 48%	4669
	11th to 20th	Day	8% 7% 2 5% 3 4 4 4 6% 8% 50%	5090
	21st to end	Day	14% 9% 3 8% 6% 5% 5% 3 6% 7% 34%	5864
April	1st to 10th	Day	14% 8% 2 7% 5% 5% 5% 3 5% 7% 40%	2520
	11th to 20th	Day	14% 9% 3 7% 4 4 4 3 5% 6% 40%	2494
	21st to end	Day	17% 7% 3 7% 5% 4 4 3 6% 8% 36%	2520
May	1st to 10th	Day	15% 7% 3 8% 5% 5% 5% 4 6% 9% 33%	7289
	11th to 20th	Day	14% 7% 3 7% 4 4 4 3 5% 7% 41%	8222
	21st to end	Day	14% 8% 3 8% 5% 5% 5% 4 6% 7% 35%	9047
June	1st to 10th	Day	8% 4 2 5% 4 4 6% 4 7% 10% 46%	2940
	11th to 20th	Day	8% 4 2 5% 4 5% 5% 4 7% 9% 48%	2940
	21st to end	Day	7% 4 2 6% 5% 5% 6% 6% 6% 10% 45%	2940
July	1st to 10th	Day	9% 5% 2 5% 4 4 4 4 6% 11% 45%	8280
	11th to 20th	Day	8% 4 2 6% 5% 5% 6% 4 7% 9% 44%	8280
	21st to end	Day	10% 5% 2 6% 4 5% 5% 5% 7% 10% 40%	8615
August	1st to 10th	Day	9% 5% 2 5% 4 4 5% 4 7% 10% 45%	6014
	11th to 20th	Day	11% 5% 2 7% 5% 5% 5% 5% 7% 10% 39%	5534
	21st to end	Day	4 3 1 4 3 4 4 4 6% 12% 54%	6080
September	1st to 10th	Day	12% 4 2 4 3 3 4 4 5% 7% 53%	2520
	11th to 20th	Day	9% 5% 2 5% 3 4 4 4 6% 8% 51%	2420
	21st to end	Day	8% 5% 2 5% 4 3 4 4 6% 8% 52%	2175
October	1st to 10th	Day	10% 4 2 5% 3 4 3 3 4 6% 55%	2000
	11th to 20th	Day	14% 7% 2 6% 3 3 3 3 4 7% 50%	2000
	21st to end	Day	17% 7% 3 6% 4 4 3 2 4 5% 46%	2172
November	1st to 10th	Day	11% 5% 2 5% 3 3 4 3 4 6% 52%	1852
	11th to 20th	Day	14% 8% 3 7% 3 4 4 3 4 6% 45%	1890
	21st to end	Day	16% 10% 4 7% 5% 3 3 3 3 6% 39%	1764
December	1st to 10th	Day	7% 10% 3 6% 3 3 3 3 4 6% 50%	2690
	11th to 20th	Day	13% 9% 6% 9% 6% 4 5% 3 5% 6% 35%	3088
	21st to end	Day	23% 17% 5% 11% 5% 4 3 3 4 4 21%	4070
January	1st to 10th	Night	12% 11% 6% 11% 5% 4 4 3 4 5% 36%	8232
	11th to 20th	Night	8% 10% 4 12% 6% 5% 6% 4 4 5% 37%	7943
	21st to end	Night	15% 13% 5% 10% 5% 4 4 3 4 5% 33%	8402
February	1st to 10th	Night	14% 11% 5% 8% 5% 4 5% 3 5% 6% 34%	2730
	11th to 20th	Night	14% 9% 4 7% 5% 4 4 3 5% 5% 39%	2508
	21st to end	Night	15% 8% 3 7% 4 3 4 3 4 5% 44%	2076
March	1st to 10th	Night	20% 11% 3 6% 3 3 3 2 3 4 42%	5447
	11th to 20th	Night	12% 7% 2 5% 3 3 3 3 5% 7% 50%	5090
	21st to end	Night	17% 10% 3 8% 4 4 4 4 5% 6% 34%	5307
April	1st to 10th	Night	13% 7% 2 7% 5% 3 4 2 6% 7% 43%	2100
	11th to 20th	Night	13% 6% 2 5% 3 4 5% 3 5% 8% 45%	2075
	21st to end	Night	18% 7% 3 6% 4 4 4 3 4 7% 41%	2100
May	1st to 10th	Night	18% 7% 2 7% 5% 5% 4 4 5% 7% 35%	5563
	11th to 20th	Night	13% 5% 2 5% 4 4 4 4 6% 8% 45%	4670
	21st to end	Night	14% 8% 3 7% 4 4 5% 4 6% 8% 37%	5139
June	1st to 10th	Night	8% 4 2 5% 4 4 4 4 6% 9% 50%	1680
	11th to 20th	Night	6% 5% 2 6% 5% 4 4 4 6% 9% 49%	1680
	21st to end	Night	5% 5% 2 5% 4 5% 4 5% 7% 9% 50%	1680
July	1st to 10th	Night	9% 4 2 7% 5% 4 4 4 5% 9% 48%	4708
	11th to 20th	Night	6% 3 2 6% 4 5% 5% 4 6% 8% 51%	4707
	21st to end	Night	8% 5% 2 7% 5% 4 5% 4 7% 10% 44%	5488
August	1st to 10th	Night	9% 3 2 5% 4 3 3 3 5% 8% 55%	4156
	11th to 20th	Night	9% 5% 2 6% 4 4 4 4 7% 8% 47%	4512
	21st to end	Night	4 3 1 4 3 3 4 4 6% 9% 60%	5046
September	1st to 10th	Night	11% 4 2 5% 3 3 3 3 4 7% 56%	2100
	11th to 20th	Night	8% 5% 2 6% 3 4 4 4 6% 7% 52%	2030
	21st to end	Night	9% 5% 1 5% 3 4 4 3 5% 7% 55%	2215
October	1st to 10th	Night	8% 4 2 4 3 3 3 3 4 6% 58%	2400
	11th to 20th	Night	13% 5% 2 5% 3 3 3 2 4 7% 52%	2400
	21st to end	Night	18% 8% 2 5% 4 3 3 3 5% 5% 46%	2665
November	1st to 10th	Night	10% 6% 2 5% 3 4 3 3 4 5% 53%	2666
	11th to 20th	Night	12% 6% 3 6% 4 3 3 3 5% 6% 49%	2730
	21st to end	Night	17% 9% 4 6% 4 3 3 3 4 5% 41%	2851
December	1st to 10th	Night	11% 9% 3 7% 4 4 4 3 4 5% 46%	4720
	11th to 20th	Night	14% 10% 3 6% 4 4 4 3 5% 7% 41%	5339
	21st to end	Night	23% 18% 5% 10% 5% 4 3 3 3 4 21%	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	54% 5% 11% 20% 9% 1	4692
	11th to 20th	Day	46% 4 13% 19% 15% 3	5045
	21st to end	Day	55% 3 9% 19% 12% 3	5812
February	1st to 10th	Day	51% 5% 12% 21% 10% 1	1890
	11th to 20th	Day	50% 5% 11% 19% 14% 2	2006
	21st to end	Day	45% 4 11% 23% 14% 3	1730
March	1st to 10th	Day	42% 3 6% 23% 24% 2	4669
	11th to 20th	Day	31% 5% 10% 28% 22% 3	5090
	21st to end	Day	47% 7% 15% 24% 7% 1	5866
April	1st to 10th	Day	44% 6% 11% 23% 16% 1	2520
	11th to 20th	Day	45% 5% 10% 25% 13% 3	2494
	21st to end	Day	45% 6% 13% 22% 13% 1	2520
May	1st to 10th	Day	45% 7% 13% 22% 13%	7324
	11th to 20th	Day	43% 5% 10% 23% 17% 2	8226
	21st to end	Day	47% 6% 10% 21% 16% 1	9061
June	1st to 10th	Day	32% 7% 14% 27% 16% 2	2940
	11th to 20th	Day	31% 7% 13% 25% 21% 2	2940
	21st to end	Day	32% 9% 16% 23% 16% 3	2940
July	1st to 10th	Day	33% 5% 14% 29% 17% 1	8211
	11th to 20th	Day	35% 6% 16% 24% 18% 2	8205
	21st to end	Day	37% 5% 19% 31% 7% 1	8427
August	1st to 10th	Day	33% 6% 15% 30% 15% 1	6000
	11th to 20th	Day	39% 6% 18% 26% 11% 1	5570
	21st to end	Day	23% 4 15% 36% 18% 2	6079
September	1st to 10th	Day	31% 5% 10% 28% 21% 5%	2520
	11th to 20th	Day	30% 5% 11% 28% 22% 4	2420
	21st to end	Day	30% 6% 10% 28% 23% 3	2175
October	1st to 10th	Day	20% 3 6% 18% 47% 6%	3260
	11th to 20th	Day	24% 3 8% 21% 37% 7%	3240
	21st to end	Day	37% 3 7% 27% 25% 2	3486
November	1st to 10th	Day	33% 4 13% 26% 20% 4	1852
	11th to 20th	Day	41% 4 11% 23% 18% 4	1890
	21st to end	Day	48% 4 10% 25% 12%	1764
December	1st to 10th	Day	36% 4 17% 30% 13%	2690
	11th to 20th	Day	50% 4 12% 22% 11% 2	3085
	21st to end	Day	67% 5% 11% 12% 5% 1	4070
January	1st to 10th	Night	52% 4 14% 23% 8%	8233
	11th to 20th	Night	50% 4 7% 22% 15% 3	7943
	21st to end	Night	54% 3 10% 19% 11% 2	8402
February	1st to 10th	Night	52% 4 11% 21% 11% 2	2730
	11th to 20th	Night	47% 4 10% 22% 15% 2	2509
	21st to end	Night	42% 5% 11% 25% 14% 2	2076
March	1st to 10th	Night	47% 4 6% 22% 20% 2	5447
	11th to 20th	Night	35% 4 8% 26% 21% 6%	5090
	21st to end	Night	49% 5% 11% 24% 9% 1	5307
April	1st to 10th	Night	40% 5% 10% 25% 16% 4	2100
	11th to 20th	Night	37% 6% 12% 23% 18% 4	2075
	21st to end	Night	45% 5% 9% 22% 17% 2	2100
May	1st to 10th	Night	47% 6% 13% 23% 11% 1	5593
	11th to 20th	Night	37% 6% 13% 25% 16% 3	4671
	21st to end	Night	44% 6% 12% 20% 17% 1	5149
June	1st to 10th	Night	28% 7% 16% 27% 18% 4	1680
	11th to 20th	Night	30% 7% 11% 23% 25% 4	1680
	21st to end	Night	28% 7% 16% 25% 17% 6%	1680
July	1st to 10th	Night	34% 4 12% 29% 18% 1	4670
	11th to 20th	Night	31% 6% 15% 29% 19% 1	4662
	21st to end	Night	35% 4 22% 29% 9% 1	5365
August	1st to 10th	Night	29% 4 15% 34% 17% 1	4146
	11th to 20th	Night	34% 4 17% 28% 16% 1	4542
	21st to end	Night	21% 4 16% 38% 19% 2	5046
September	1st to 10th	Night	29% 5% 10% 25% 24% 7%	2100
	11th to 20th	Night	30% 5% 9% 26% 24% 6%	2030
	21st to end	Night	29% 5% 9% 25% 27% 4	2215
October	1st to 10th	Night	19% 3 6% 20% 47% 5%	3920
	11th to 20th	Night	24% 2 9% 22% 35% 7%	3899
	21st to end	Night	33% 3 9% 25% 25% 5%	4301
November	1st to 10th	Night	32% 5% 11% 26% 20% 5%	2666
	11th to 20th	Night	35% 4 10% 28% 19% 3	2730
	21st to end	Night	46% 4 11% 24% 14% 1	2851
December	1st to 10th	Night	41% 3 11% 31% 13% 1	4720
	11th to 20th	Night	44% 4 12% 24% 15%	5341
	21st to end	Night	68% 4 11% 12% 5% 1	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	21% 21% 33% 19% 5% 1	330
	11th to 20th	Day	25% 25% 35% 13% 2	330
	21st to end	Day	17% 24% 42% 14% 3	363
February	1st to 10th	Day	22% 18% 41% 16% 2	330
	11th to 20th	Day	23% 25% 34% 14% 4	330
	21st to end	Day	23% 17% 44% 15% 1	273
March	1st to 10th	Day	20% 22% 42% 15% 1	330
	11th to 20th	Day	29% 18% 33% 16% 3	330
	21st to end	Day	27% 17% 37% 15% 3	489
April	1st to 10th	Day	23% 14% 37% 20% 6% 1	540
	11th to 20th	Day	22% 18% 32% 18% 10%	532
	21st to end	Day	20% 18% 33% 22% 6% 1	540
May	1st to 10th	Day	17% 13% 36% 25% 7% 2	559
	11th to 20th	Day	17% 12% 30% 24% 14% 3	775
	21st to end	Day	16% 8% 31% 27% 12% 6%	844
June	1st to 10th	Day	9% 6% 26% 30% 23% 6%	780
	11th to 20th	Day	10% 4 26% 34% 21% 5%	780
	21st to end	Day	7% 7% 28% 34% 19% 4	780
July	1st to 10th	Day	10% 3 16% 31% 28% 13%	771
	11th to 20th	Day	7% 3 19% 36% 28% 8%	768
	21st to end	Day	10% 2 18% 33% 30% 7%	849
August	1st to 10th	Day	7% 3 17% 39% 25% 9%	776
	11th to 20th	Day	10% 4 16% 31% 29% 10%	582
	21st to end	Day	8% 2 15% 34% 31% 10%	627
September	1st to 10th	Day	11% 5% 17% 31% 27% 9%	570
	11th to 20th	Day	9% 3 13% 33% 31% 11%	542
	21st to end	Day	9% 10% 21% 28% 23% 9%	505
October	1st to 10th	Day	10% 8% 24% 35% 18% 5%	330
	11th to 20th	Day	16% 19% 30% 24% 9% 2	330
	21st to end	Day	26% 20% 30% 18% 6%	363
November	1st to 10th	Day	13% 13% 39% 23% 8% 3	330
	11th to 20th	Day	20% 18% 33% 19% 8% 1	330
	21st to end	Day	26% 16% 31% 22% 5% 1	330
December	1st to 10th	Day	13% 20% 39% 19% 6% 3	330
	11th to 20th	Day	21% 18% 37% 20% 4	330
	21st to end	Day	26% 22% 35% 15% 2	363
January	1st to 10th	Night	17% 15% 39% 23% 5%	1050
	11th to 20th	Night	20% 18% 37% 21% 4	1050
	21st to end	Night	19% 17% 39% 24% 2	1155
February	1st to 10th	Night	21% 14% 38% 24% 3	1050
	11th to 20th	Night	21% 16% 37% 23% 3	873
	21st to end	Night	21% 17% 39% 21% 3	692
March	1st to 10th	Night	21% 16% 36% 22% 3	820
	11th to 20th	Night	22% 13% 35% 26% 5%	630
	21st to end	Night	24% 12% 35% 25% 5%	693
April	1st to 10th	Night	17% 16% 37% 26% 4	630
	11th to 20th	Night	15% 15% 35% 26% 9%	623
	21st to end	Night	21% 11% 36% 27% 6%	630
May	1st to 10th	Night	17% 8% 34% 32% 9%	630
	11th to 20th	Night	16% 7% 28% 36% 13% 1	621
	21st to end	Night	12% 5% 26% 35% 19% 2	683
June	1st to 10th	Night	10% 3 24% 39% 24%	630
	11th to 20th	Night	9% 3 23% 46% 18% 2	630
	21st to end	Night	5% 3 26% 42% 22% 1	630
July	1st to 10th	Night	11% 2 11% 35% 36% 5%	615
	11th to 20th	Night	6% 2 14% 45% 30% 3	612
	21st to end	Night	7% 2 15% 42% 33% 2	690
August	1st to 10th	Night	9% 2 12% 43% 32% 2	627
	11th to 20th	Night	9% 2 14% 40% 30% 4	624
	21st to end	Night	5% 2 12% 41% 39% 2	693
September	1st to 10th	Night	12% 3 16% 38% 27% 4	630
	11th to 20th	Night	9% 2 13% 39% 33% 5%	609
	21st to end	Night	11% 6% 23% 32% 25% 4	795
October	1st to 10th	Night	10% 5% 25% 34% 23% 3	800
	11th to 20th	Night	17% 9% 33% 30% 11% 1	800
	21st to end	Night	23% 12% 32% 27% 6%	880
November	1st to 10th	Night	14% 9% 38% 30% 10%	820
	11th to 20th	Night	15% 14% 37% 29% 5%	840
	21st to end	Night	23% 13% 33% 24% 6%	961
December	1st to 10th	Night	20% 13% 38% 24% 4 1	1050
	11th to 20th	Night	22% 12% 32% 29% 5%	1050
	21st to end	Night	26% 15% 34% 21% 3	1155

Page generated at 2026-09-11 03:11:52. Version ac07

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