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 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	12% 7% 3 7% 4 4 4 4 5% 7% 42%	113870
	Night	13% 8% 3 7% 4 4 4 3 5% 6% 42%	115290
Transparency	Day	40% 6% 12% 24% 17% 2	117478
	Night	41% 5% 11% 24% 17% 3	119825
Seeing	Day	15% 11% 28% 26% 16% 5%	16288
	Night	16% 10% 30% 30% 13% 1	26250

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	12% 12% 6% 15% 5% 4 4 3 6% 5% 28%	3617
	11th to 20th	Day	9% 10% 4 9% 7% 7% 4 4 5% 5% 35%	3871
	21st to end	Day	17% 14% 6% 9% 5% 4 3 2 3 4 33%	4488
February	1st to 10th	Day	16% 9% 5% 9% 5% 5% 3 3 5% 7% 32%	1800
	11th to 20th	Day	14% 10% 3 7% 6% 5% 5% 3 5% 8% 34%	1898
	21st to end	Day	14% 11% 3 7% 4 3 4 3 4 7% 39%	1650
March	1st to 10th	Day	16% 10% 2 6% 3 3 3 2 3 5% 49%	4567
	11th to 20th	Day	8% 7% 2 5% 3 4 4 4 6% 8% 50%	4980
	21st to end	Day	14% 9% 3 8% 5% 5% 5% 3 6% 7% 34%	5737
April	1st to 10th	Day	14% 8% 3 7% 5% 5% 5% 3 5% 7% 40%	2400
	11th to 20th	Day	15% 9% 3 7% 4 4 4 3 5% 6% 40%	2374
	21st to end	Day	18% 7% 3 7% 5% 4 4 4 6% 8% 36%	2400
May	1st to 10th	Day	13% 8% 3 8% 5% 5% 5% 4 7% 10% 32%	4035
	11th to 20th	Day	17% 8% 2 7% 4 4 4 4 5% 7% 37%	4400
	21st to end	Day	9% 6% 2 6% 5% 4 4 4 6% 7% 47%	4824
June	1st to 10th	Day	7% 4 2 5% 4 4 6% 4 7% 10% 47%	2660
	11th to 20th	Day	8% 4 2 4 4 5% 5% 4 7% 9% 50%	2660
	21st to end	Day	6% 4 2 6% 5% 4 6% 5% 7% 9% 46%	2660
July	1st to 10th	Day	13% 6% 2 5% 4 5% 5% 4 6% 10% 39%	4440
	11th to 20th	Day	5% 4 2 5% 4 5% 5% 4 6% 10% 50%	4440
	21st to end	Day	7% 5% 2 7% 5% 5% 6% 5% 7% 11% 41%	4606
August	1st to 10th	Day	6% 4 2 5% 4 4 5% 4 7% 9% 50%	4104
	11th to 20th	Day	12% 5% 2 6% 4 5% 5% 4 7% 9% 42%	3762
	21st to end	Day	6% 4 1 5% 4 5% 5% 4 7% 11% 50%	4180
September	1st to 10th	Day	13% 4 2 4 3 3 4 4 5% 7% 52%	2280
	11th to 20th	Day	9% 5% 2 5% 3 4 4 4 6% 7% 51%	2265
	21st to end	Day	8% 5% 2 5% 4 3 4 4 6% 8% 51%	2066
October	1st to 10th	Day	11% 4 2 5% 3 4 3 3 4 7% 55%	1900
	11th to 20th	Day	15% 7% 2 6% 2 3 3 3 3 6% 50%	1900
	21st to end	Day	17% 7% 3 6% 4 4 3 2 4 5% 46%	2064
November	1st to 10th	Day	11% 5% 2 5% 3 3 4 3 5% 6% 51%	1762
	11th to 20th	Day	14% 8% 3 7% 3 4 3 3 5% 6% 45%	1800
	21st to end	Day	16% 10% 4 7% 5% 4 3 3 4 6% 39%	1680
December	1st to 10th	Day	7% 10% 3 6% 3 3 3 3 4 6% 51%	2610
	11th to 20th	Day	12% 9% 6% 9% 6% 4 5% 3 5% 6% 35%	3008
	21st to end	Day	23% 17% 5% 11% 5% 4 3 3 4 4 21%	3982
January	1st to 10th	Night	14% 11% 6% 12% 5% 4 4 3 4 5% 31%	6348
	11th to 20th	Night	10% 11% 5% 13% 6% 5% 6% 4 4 4 31%	6107
	21st to end	Night	18% 14% 4 9% 4 4 3 2 4 5% 32%	6490
February	1st to 10th	Night	15% 11% 6% 8% 5% 4 5% 3 5% 6% 33%	2600
	11th to 20th	Night	15% 9% 4 8% 5% 4 4 3 4 5% 39%	2386
	21st to end	Night	15% 8% 4 7% 4 3 4 3 4 5% 43%	1980
March	1st to 10th	Night	19% 12% 3 6% 3 3 3 2 3 4 42%	5328
	11th to 20th	Night	13% 7% 2 5% 3 3 3 3 5% 7% 51%	4980
	21st to end	Night	17% 10% 3 8% 4 4 4 4 5% 6% 34%	5192
April	1st to 10th	Night	13% 7% 2 7% 5% 3 4 2 6% 7% 44%	2000
	11th to 20th	Night	13% 6% 2 5% 3 4 5% 3 5% 8% 45%	1975
	21st to end	Night	19% 7% 3 6% 4 4 4 3 4 6% 40%	2000
May	1st to 10th	Night	16% 9% 2 7% 4 4 4 4 6% 8% 37%	3077
	11th to 20th	Night	15% 6% 3 6% 4 4 4 4 5% 7% 42%	2511
	21st to end	Night	8% 6% 2 6% 5% 4 5% 4 6% 9% 46%	2743
June	1st to 10th	Night	7% 4 2 5% 4 4 4 5% 6% 10% 51%	1520
	11th to 20th	Night	6% 5% 2 5% 5% 4 4 4 5% 9% 51%	1520
	21st to end	Night	5% 5% 2 5% 4 5% 4 5% 7% 10% 50%	1520
July	1st to 10th	Night	12% 5% 2 7% 4 4 4 4 6% 9% 43%	2530
	11th to 20th	Night	5% 2 1 4 3 4 3 3 6% 9% 60%	2530
	21st to end	Night	9% 6% 2 7% 5% 5% 5% 4 7% 9% 42%	2953
August	1st to 10th	Night	5% 3 2 4 4 3 3 3 5% 8% 59%	2836
	11th to 20th	Night	10% 5% 2 5% 4 4 4 3 6% 7% 50%	3066
	21st to end	Night	5% 3 1 4 3 4 4 4 6% 10% 56%	3476
September	1st to 10th	Night	11% 4 2 5% 3 3 3 3 4 7% 56%	1900
	11th to 20th	Night	8% 5% 2 6% 3 4 4 4 5% 7% 52%	1900
	21st to end	Night	9% 5% 1 5% 4 4 4 3 5% 7% 55%	2104
October	1st to 10th	Night	9% 4 2 4 3 3 3 3 4 6% 58%	2280
	11th to 20th	Night	13% 5% 2 5% 3 3 3 2 4 7% 52%	2280
	21st to end	Night	17% 8% 2 5% 4 4 3 3 5% 5% 46%	2532
November	1st to 10th	Night	10% 6% 2 5% 3 4 3 3 4 5% 54%	2536
	11th to 20th	Night	12% 6% 3 6% 4 3 3 3 5% 6% 49%	2600
	21st to end	Night	17% 9% 4 6% 4 3 3 3 4 5% 42%	2715
December	1st to 10th	Night	11% 9% 3 7% 4 4 4 3 4 5% 47%	4580
	11th to 20th	Night	15% 10% 3 6% 4 4 4 3 5% 7% 41%	5199
	21st to end	Night	24% 19% 5% 10% 5% 4 3 3 3 4 20%	6996

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	56% 5% 10% 17% 9% 1	3618
	11th to 20th	Day	49% 4 14% 16% 13% 4	3871
	21st to end	Day	57% 3 8% 16% 13% 3	4488
February	1st to 10th	Day	51% 5% 12% 21% 10% 1	1800
	11th to 20th	Day	49% 5% 12% 18% 14% 2	1908
	21st to end	Day	46% 4 11% 23% 14% 3	1650
March	1st to 10th	Day	41% 3 6% 24% 24% 2	4567
	11th to 20th	Day	31% 5% 10% 27% 22% 3	4980
	21st to end	Day	47% 7% 15% 24% 7% 1	5739
April	1st to 10th	Day	43% 6% 11% 24% 16% 1	2400
	11th to 20th	Day	45% 5% 10% 25% 13% 3	2374
	21st to end	Day	46% 6% 14% 21% 13% 1	2400
May	1st to 10th	Day	43% 9% 15% 22% 9%	4070
	11th to 20th	Day	45% 8% 10% 25% 11% 1	4404
	21st to end	Day	34% 8% 10% 24% 22% 1	4837
June	1st to 10th	Day	31% 7% 15% 28% 16% 3	2660
	11th to 20th	Day	30% 7% 13% 25% 22% 3	2660
	21st to end	Day	31% 9% 16% 23% 17% 3	2660
July	1st to 10th	Day	39% 7% 12% 28% 13% 2	4371
	11th to 20th	Day	29% 6% 15% 29% 19% 2	4365
	21st to end	Day	35% 7% 19% 27% 11% 1	4461
August	1st to 10th	Day	28% 7% 16% 35% 14% 1	4090
	11th to 20th	Day	38% 7% 13% 26% 15% 1	3798
	21st to end	Day	28% 6% 17% 32% 14% 3	4179
September	1st to 10th	Day	32% 6% 10% 26% 21% 5%	2280
	11th to 20th	Day	30% 5% 11% 28% 22% 4	2265
	21st to end	Day	30% 6% 10% 29% 22% 3	2066
October	1st to 10th	Day	20% 3 6% 18% 47% 6%	3160
	11th to 20th	Day	24% 3 7% 21% 37% 7%	3140
	21st to end	Day	36% 3 7% 27% 25% 2	3378
November	1st to 10th	Day	33% 4 13% 26% 20% 4	1762
	11th to 20th	Day	40% 4 11% 23% 18% 4	1800
	21st to end	Day	47% 4 10% 26% 12%	1680
December	1st to 10th	Day	35% 4 17% 30% 14%	2610
	11th to 20th	Day	50% 4 12% 21% 11% 2	3005
	21st to end	Day	67% 5% 11% 12% 5% 1	3982
January	1st to 10th	Night	55% 4 11% 22% 7%	6348
	11th to 20th	Night	56% 4 8% 18% 11% 2	6107
	21st to end	Night	55% 4 8% 19% 12% 2	6490
February	1st to 10th	Night	53% 5% 11% 19% 11% 2	2600
	11th to 20th	Night	47% 5% 10% 20% 15% 2	2387
	21st to end	Night	42% 5% 11% 24% 14% 2	1980
March	1st to 10th	Night	47% 4 6% 22% 20% 2	5328
	11th to 20th	Night	35% 4 8% 26% 22% 6%	4980
	21st to end	Night	50% 5% 11% 24% 9% 1	5192
April	1st to 10th	Night	39% 6% 11% 25% 16% 4	2000
	11th to 20th	Night	37% 6% 11% 23% 19% 4	1975
	21st to end	Night	45% 5% 9% 22% 17% 2	2000
May	1st to 10th	Night	43% 9% 16% 20% 12% 1	3107
	11th to 20th	Night	40% 8% 12% 27% 12% 1	2512
	21st to end	Night	32% 8% 15% 22% 21% 3	2751
June	1st to 10th	Night	27% 8% 15% 27% 19% 4	1520
	11th to 20th	Night	28% 7% 11% 23% 26% 4	1520
	21st to end	Night	27% 8% 16% 25% 18% 7%	1520
July	1st to 10th	Night	38% 5% 10% 31% 13% 2	2490
	11th to 20th	Night	21% 4 13% 38% 21% 2	2482
	21st to end	Night	37% 6% 18% 24% 14% 1	2859
August	1st to 10th	Night	24% 5% 16% 36% 17% 1	2826
	11th to 20th	Night	34% 4 12% 27% 22% 1	3096
	21st to end	Night	24% 5% 17% 33% 17% 4	3476
September	1st to 10th	Night	29% 5% 10% 24% 24% 8%	1900
	11th to 20th	Night	30% 6% 9% 25% 24% 6%	1900
	21st to end	Night	29% 6% 9% 26% 26% 4	2104
October	1st to 10th	Night	19% 3 6% 20% 46% 5%	3800
	11th to 20th	Night	24% 2 9% 22% 36% 7%	3779
	21st to end	Night	33% 4 9% 25% 25% 5%	4168
November	1st to 10th	Night	32% 5% 12% 26% 20% 6%	2536
	11th to 20th	Night	36% 4 10% 28% 19% 4	2600
	21st to end	Night	45% 4 11% 24% 15% 1	2715
December	1st to 10th	Night	41% 4 10% 32% 13% 1	4580
	11th to 20th	Night	44% 4 13% 23% 15%	5201
	21st to end	Night	69% 4 11% 11% 5% 1	6996

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% 6% 1	300
	11th to 20th	Day	25% 23% 35% 14% 2	300
	21st to end	Day	15% 24% 42% 15% 3	330
February	1st to 10th	Day	23% 18% 41% 15% 2	300
	11th to 20th	Day	23% 24% 34% 15% 4	300
	21st to end	Day	24% 16% 43% 16% 2	249
March	1st to 10th	Day	19% 21% 43% 16% 1	300
	11th to 20th	Day	30% 17% 32% 18% 4	300
	21st to end	Day	29% 17% 35% 15% 4	450
April	1st to 10th	Day	21% 13% 37% 22% 6% 1	500
	11th to 20th	Day	22% 17% 32% 19% 11%	492
	21st to end	Day	21% 17% 33% 22% 6% 1	500
May	1st to 10th	Day	17% 13% 36% 26% 7% 2	518
	11th to 20th	Day	17% 10% 30% 25% 14% 3	676
	21st to end	Day	14% 7% 31% 27% 13% 7%	734
June	1st to 10th	Day	7% 6% 26% 31% 24% 7%	680
	11th to 20th	Day	9% 3 25% 35% 22% 5%	680
	21st to end	Day	6% 7% 29% 34% 19% 4	680
July	1st to 10th	Day	11% 3 16% 29% 28% 13%	671
	11th to 20th	Day	6% 3 19% 36% 28% 7%	668
	21st to end	Day	10% 2 17% 34% 31% 7%	739
August	1st to 10th	Day	8% 2 16% 38% 26% 9%	676
	11th to 20th	Day	11% 4 17% 29% 29% 10%	500
	21st to end	Day	9% 2 14% 32% 32% 10%	539
September	1st to 10th	Day	12% 5% 16% 29% 28% 10%	490
	11th to 20th	Day	9% 2 13% 33% 31% 11%	490
	21st to end	Day	9% 10% 22% 29% 23% 7%	466
October	1st to 10th	Day	11% 8% 26% 33% 17% 5%	300
	11th to 20th	Day	17% 19% 31% 24% 8% 2	300
	21st to end	Day	25% 20% 29% 19% 6%	330
November	1st to 10th	Day	13% 12% 38% 24% 9% 3	300
	11th to 20th	Day	20% 19% 32% 19% 9% 1	300
	21st to end	Day	24% 16% 32% 22% 4 1	300
December	1st to 10th	Day	12% 21% 39% 19% 6% 3	300
	11th to 20th	Day	21% 18% 36% 21% 5%	300
	21st to end	Day	26% 22% 34% 15% 2	330
January	1st to 10th	Night	18% 15% 39% 23% 5%	1000
	11th to 20th	Night	20% 18% 37% 21% 5%	1000
	21st to end	Night	19% 15% 39% 25% 2	1100
February	1st to 10th	Night	21% 14% 38% 24% 3	1000
	11th to 20th	Night	21% 16% 37% 23% 3	831
	21st to end	Night	21% 16% 39% 21% 3	660
March	1st to 10th	Night	20% 16% 37% 23% 4	781
	11th to 20th	Night	21% 12% 35% 26% 6%	600
	21st to end	Night	24% 12% 34% 25% 5%	660
April	1st to 10th	Night	16% 16% 36% 27% 5%	600
	11th to 20th	Night	15% 15% 34% 27% 9% 1	593
	21st to end	Night	22% 10% 36% 27% 6%	600
May	1st to 10th	Night	17% 8% 33% 33% 9% 1	600
	11th to 20th	Night	16% 5% 27% 37% 13% 1	563
	21st to end	Night	11% 5% 27% 34% 20% 3	617
June	1st to 10th	Night	9% 3 24% 39% 25% 1	570
	11th to 20th	Night	9% 2 23% 46% 19% 2	570
	21st to end	Night	5% 3 26% 42% 23% 1	570
July	1st to 10th	Night	12% 2 11% 34% 35% 5%	555
	11th to 20th	Night	6% 3 14% 45% 29% 3	552
	21st to end	Night	7% 2 14% 42% 34% 2	624
August	1st to 10th	Night	8% 2 11% 44% 33% 2	567
	11th to 20th	Night	9% 2 14% 39% 31% 5%	564
	21st to end	Night	5% 1 12% 40% 40% 2	627
September	1st to 10th	Night	12% 3 16% 37% 28% 5%	570
	11th to 20th	Night	9% 2 13% 39% 31% 5%	570
	21st to end	Night	10% 6% 24% 32% 24% 3	755
October	1st to 10th	Night	10% 5% 26% 32% 23% 3	760
	11th to 20th	Night	16% 9% 34% 29% 11% 1	760
	21st to end	Night	22% 12% 32% 27% 7%	836
November	1st to 10th	Night	14% 9% 37% 30% 10%	780
	11th to 20th	Night	15% 14% 36% 29% 6%	800
	21st to end	Night	22% 13% 34% 24% 6%	915
December	1st to 10th	Night	20% 13% 38% 24% 5% 1	1000
	11th to 20th	Night	22% 11% 32% 29% 5%	1000
	21st to end	Night	27% 16% 34% 21% 3	1100

Page generated at 2025-05-08 08:33:59. Version ac07

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