AIG public observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the AIG public 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	15% 9% 3 7% 5% 4 4 4 5% 7% 37%	114070
	Night	15% 10% 3 7% 4 4 4 3 5% 6% 37%	114910
Transparency	Day	27% 42% 6% 15% 9% 1	117752
	Night	29% 41% 6% 14% 10% 1	119367
Seeing	Day	18% 12% 26% 23% 15% 6%	15980
	Night	19% 9% 26% 29% 15% 2	25801

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	15% 18% 7% 10% 4 3 3 3 4 5% 27%	3617
	11th to 20th	Day	14% 14% 6% 13% 4 5% 4 3 3 4 30%	3857
	21st to end	Day	22% 12% 6% 8% 5% 5% 5% 3 4 4 26%	4466
February	1st to 10th	Day	19% 13% 5% 9% 3 3 4 3 5% 5% 32%	1933
	11th to 20th	Day	16% 12% 5% 10% 5% 4 4 3 5% 5% 32%	1948
	21st to end	Day	18% 13% 6% 8% 4 3 3 3 4 5% 33%	1650
March	1st to 10th	Day	19% 13% 5% 8% 4 4 3 3 3 5% 33%	4480
	11th to 20th	Day	14% 8% 2 7% 4 4 4 3 6% 8% 40%	4976
	21st to end	Day	18% 10% 3 6% 4 4 5% 4 6% 6% 34%	5465
April	1st to 10th	Day	16% 10% 3 7% 4 4 5% 4 5% 8% 35%	2385
	11th to 20th	Day	16% 9% 2 7% 5% 4 4 3 5% 6% 39%	2408
	21st to end	Day	18% 8% 2 7% 5% 4 5% 3 5% 7% 35%	2600
May	1st to 10th	Day	22% 9% 3 7% 5% 5% 5% 4 6% 7% 28%	4212
	11th to 20th	Day	21% 8% 2 5% 4 4 3 3 5% 7% 38%	4081
	21st to end	Day	10% 6% 3 8% 5% 6% 6% 5% 7% 10% 33%	4476
June	1st to 10th	Day	13% 6% 2 6% 5% 5% 5% 4 6% 8% 41%	2497
	11th to 20th	Day	9% 5% 2 6% 5% 5% 5% 4 6% 9% 44%	2660
	21st to end	Day	8% 6% 3 7% 5% 5% 6% 4 6% 9% 41%	2660
July	1st to 10th	Day	14% 7% 2 6% 5% 5% 5% 4 6% 10% 38%	4440
	11th to 20th	Day	7% 5% 2 5% 4 4 5% 4 7% 10% 47%	4326
	21st to end	Day	11% 7% 3 8% 7% 6% 6% 6% 7% 8% 30%	4466
August	1st to 10th	Day	9% 5% 2 5% 4 4 5% 4 7% 10% 45%	4104
	11th to 20th	Day	9% 6% 2 6% 4 5% 5% 4 7% 11% 41%	4018
	21st to end	Day	6% 4 2 5% 4 4 4 4 5% 8% 55%	4180
September	1st to 10th	Day	13% 5% 2 5% 4 4 4 4 6% 6% 46%	2177
	11th to 20th	Day	9% 4 2 6% 4 4 4 4 6% 9% 47%	2090
	21st to end	Day	15% 6% 2 5% 4 3 3 3 5% 6% 48%	2090
October	1st to 10th	Day	10% 5% 2 6% 5% 4 4 4 5% 4 51%	2090
	11th to 20th	Day	13% 7% 2 6% 4 3 3 3 6% 6% 46%	1948
	21st to end	Day	19% 8% 3 6% 3 4 3 3 5% 6% 39%	1925
November	1st to 10th	Day	9% 7% 3 6% 4 3 3 3 3 6% 53%	1762
	11th to 20th	Day	15% 10% 4 8% 4 3 4 3 5% 6% 37%	1800
	21st to end	Day	18% 11% 4 7% 4 3 3 3 4 5% 38%	1800
December	1st to 10th	Day	19% 11% 3 7% 4 3 3 3 3 5% 40%	2930
	11th to 20th	Day	18% 15% 4 9% 5% 5% 5% 4 4 4 27%	3373
	21st to end	Day	24% 18% 4 9% 5% 4 4 3 4 6% 20%	4180
January	1st to 10th	Night	16% 16% 6% 13% 6% 5% 5% 3 4 5% 20%	6348
	11th to 20th	Night	14% 17% 7% 14% 6% 5% 5% 3 3 4 23%	6105
	21st to end	Night	21% 14% 4 7% 4 4 3 2 3 4 32%	6490
February	1st to 10th	Night	21% 12% 4 9% 5% 4 4 3 5% 5% 28%	2465
	11th to 20th	Night	18% 11% 4 9% 5% 4 3 3 5% 6% 31%	2338
	21st to end	Night	21% 13% 4 8% 4 3 3 3 4 6% 31%	1980
March	1st to 10th	Night	20% 13% 2 6% 4 3 3 3 4 5% 37%	5373
	11th to 20th	Night	13% 7% 2 6% 4 4 4 4 6% 6% 44%	4983
	21st to end	Night	17% 9% 3 8% 5% 4 5% 4 5% 6% 34%	5467
April	1st to 10th	Night	16% 10% 2 6% 4 4 4 4 6% 6% 37%	2030
	11th to 20th	Night	14% 9% 3 6% 4 4 3 3 7% 7% 40%	1941
	21st to end	Night	17% 7% 3 6% 5% 4 4 4 6% 8% 37%	1800
May	1st to 10th	Night	21% 11% 4 8% 5% 4 4 4 6% 7% 27%	2903
	11th to 20th	Night	19% 6% 2 6% 4 4 4 3 6% 8% 38%	2818
	21st to end	Night	9% 6% 2 5% 5% 4 5% 4 6% 9% 45%	3079
June	1st to 10th	Night	8% 5% 2 5% 5% 4 4 4 6% 9% 48%	1677
	11th to 20th	Night	7% 6% 2 5% 4 4 4 4 6% 11% 47%	1520
	21st to end	Night	6% 4 1 6% 5% 4 4 5% 7% 10% 47%	1520
July	1st to 10th	Night	10% 6% 2 5% 4 4 4 4 6% 8% 48%	2530
	11th to 20th	Night	6% 4 2 4 4 4 4 4 7% 8% 53%	2623
	21st to end	Night	11% 6% 2 7% 5% 5% 5% 5% 6% 9% 37%	3087
August	1st to 10th	Night	8% 5% 2 7% 4 4 4 4 6% 10% 46%	2825
	11th to 20th	Night	8% 6% 3 6% 4 5% 4 4 7% 8% 47%	2807
	21st to end	Night	4 3 1 4 3 4 3 3 6% 10% 60%	3476
September	1st to 10th	Night	12% 5% 1 4 3 3 4 4 5% 6% 52%	1982
	11th to 20th	Night	8% 4 2 5% 3 3 4 4 5% 8% 54%	2090
	21st to end	Night	15% 6% 2 5% 4 3 4 3 3 6% 48%	2090
October	1st to 10th	Night	8% 5% 1 5% 4 4 3 3 4 7% 56%	2090
	11th to 20th	Night	14% 6% 3 5% 3 3 4 3 5% 7% 46%	2230
	21st to end	Night	18% 8% 2 6% 4 4 3 3 4 6% 41%	2669
November	1st to 10th	Night	13% 7% 2 6% 4 3 3 3 5% 5% 48%	2536
	11th to 20th	Night	16% 8% 3 7% 4 3 4 3 5% 7% 39%	2600
	21st to end	Night	19% 12% 3 7% 5% 4 4 3 4 5% 34%	2600
December	1st to 10th	Night	19% 11% 4 7% 4 3 3 3 4 5% 38%	4250
	11th to 20th	Night	19% 14% 6% 9% 5% 4 4 3 4 5% 28%	4822
	21st to end	Night	23% 19% 6% 10% 5% 4 4 3 4 4 18%	6766

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	45% 27% 6% 15% 6%	3618
	11th to 20th	Day	43% 27% 6% 13% 10% 2	3857
	21st to end	Day	45% 29% 5% 13% 5% 2	4466
February	1st to 10th	Day	21% 62% 4 8% 5%	1933
	11th to 20th	Day	22% 61% 3 9% 6%	1958
	21st to end	Day	19% 62% 4 9% 6% 1	1650
March	1st to 10th	Day	42% 29% 3 11% 11% 3	4480
	11th to 20th	Day	28% 30% 6% 23% 12% 1	4976
	21st to end	Day	33% 31% 9% 15% 10% 1	5465
April	1st to 10th	Day	22% 62% 4 8% 4	2385
	11th to 20th	Day	18% 62% 4 10% 5% 1	2408
	21st to end	Day	17% 62% 5% 10% 5% 1	2600
May	1st to 10th	Day	32% 42% 8% 12% 5%	4250
	11th to 20th	Day	27% 41% 8% 16% 7%	4085
	21st to end	Day	26% 44% 8% 11% 11% 1	4489
June	1st to 10th	Day	15% 65% 4 8% 6% 1	2497
	11th to 20th	Day	15% 66% 3 8% 8%	2660
	21st to end	Day	15% 65% 6% 7% 6% 1	2660
July	1st to 10th	Day	27% 38% 8% 14% 13% 1	4371
	11th to 20th	Day	17% 38% 8% 25% 12%	4251
	21st to end	Day	35% 38% 7% 9% 9% 2	4324
August	1st to 10th	Day	20% 39% 11% 23% 6%	4091
	11th to 20th	Day	22% 39% 7% 23% 9%	4057
	21st to end	Day	15% 38% 10% 22% 13% 1	4179
September	1st to 10th	Day	14% 60% 4 10% 9% 2	2177
	11th to 20th	Day	13% 61% 6% 10% 9% 1	2090
	21st to end	Day	15% 61% 4 12% 8%	2090
October	1st to 10th	Day	11% 37% 4 25% 22% 1	3480
	11th to 20th	Day	15% 38% 5% 19% 21% 1	3214
	21st to end	Day	19% 36% 6% 17% 19% 2	3149
November	1st to 10th	Day	19% 56% 5% 13% 6% 1	1762
	11th to 20th	Day	19% 57% 2 10% 10% 2	1800
	21st to end	Day	24% 57% 5% 10% 4	1800
December	1st to 10th	Day	29% 36% 5% 19% 11%	2930
	11th to 20th	Day	42% 32% 5% 15% 6% 1	3370
	21st to end	Day	51% 28% 7% 9% 5%	4180
January	1st to 10th	Night	50% 29% 5% 9% 7%	6348
	11th to 20th	Night	50% 27% 5% 9% 6% 3	6105
	21st to end	Night	40% 28% 4 17% 8% 1	6490
February	1st to 10th	Night	22% 62% 4 8% 4	2465
	11th to 20th	Night	22% 61% 3 8% 6%	2339
	21st to end	Night	20% 62% 3 7% 7% 1	1980
March	1st to 10th	Night	38% 29% 4 20% 7% 2	5373
	11th to 20th	Night	25% 31% 7% 21% 16% 1	4983
	21st to end	Night	37% 30% 4 19% 9% 1	5467
April	1st to 10th	Night	21% 62% 4 8% 4	2030
	11th to 20th	Night	18% 62% 4 10% 6% 1	1941
	21st to end	Night	17% 62% 4 10% 5% 1	1800
May	1st to 10th	Night	35% 44% 6% 11% 4 1	2930
	11th to 20th	Night	25% 43% 9% 20% 3	2819
	21st to end	Night	19% 43% 8% 18% 9% 2	3087
June	1st to 10th	Night	12% 65% 4 10% 8% 1	1677
	11th to 20th	Night	12% 66% 4 8% 9% 2	1520
	21st to end	Night	11% 65% 7% 10% 7% 1	1520
July	1st to 10th	Night	22% 39% 6% 17% 12% 4	2490
	11th to 20th	Night	15% 39% 9% 24% 13%	2575
	21st to end	Night	30% 38% 9% 13% 7% 3	2988
August	1st to 10th	Night	22% 39% 10% 23% 5%	2814
	11th to 20th	Night	22% 40% 7% 20% 11%	2834
	21st to end	Night	11% 38% 11% 24% 15% 1	3476
September	1st to 10th	Night	11% 60% 6% 11% 9% 2	1982
	11th to 20th	Night	10% 61% 4 12% 11% 2	2090
	21st to end	Night	15% 60% 5% 10% 10% 1	2090
October	1st to 10th	Night	12% 37% 3 17% 31% 1	3480
	11th to 20th	Night	12% 37% 7% 16% 28% 1	3703
	21st to end	Night	18% 37% 5% 15% 22% 3	4395
November	1st to 10th	Night	21% 55% 5% 9% 8% 1	2536
	11th to 20th	Night	19% 57% 4 10% 9% 2	2600
	21st to end	Night	25% 57% 4 10% 4	2600
December	1st to 10th	Night	31% 36% 7% 17% 9%	4250
	11th to 20th	Night	41% 33% 8% 13% 6%	4824
	21st to end	Night	55% 27% 6% 10% 3	6766

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	24% 22% 32% 18% 5%	300
	11th to 20th	Day	28% 26% 30% 13% 2	300
	21st to end	Day	29% 22% 32% 14% 2	330
February	1st to 10th	Day	28% 21% 35% 14% 2	300
	11th to 20th	Day	30% 26% 33% 9% 2 1	300
	21st to end	Day	29% 20% 40% 9% 2	249
March	1st to 10th	Day	22% 23% 38% 14% 2	300
	11th to 20th	Day	26% 18% 35% 15% 5% 1	496
	21st to end	Day	30% 19% 29% 16% 5% 1	550
April	1st to 10th	Day	23% 18% 29% 18% 10% 1	500
	11th to 20th	Day	21% 19% 26% 21% 9% 4	492
	21st to end	Day	22% 15% 33% 18% 8% 3	500
May	1st to 10th	Day	24% 13% 31% 20% 8% 4	503
	11th to 20th	Day	22% 9% 23% 26% 14% 6%	487
	21st to end	Day	13% 8% 23% 28% 17% 11%	529
June	1st to 10th	Day	13% 8% 20% 29% 21% 8%	517
	11th to 20th	Day	10% 5% 23% 34% 21% 7%	680
	21st to end	Day	10% 6% 25% 31% 19% 9%	680
July	1st to 10th	Day	13% 4 15% 25% 24% 18%	671
	11th to 20th	Day	8% 5% 21% 32% 23% 12%	606
	21st to end	Day	11% 4 16% 33% 23% 13%	532
August	1st to 10th	Day	10% 3 16% 27% 27% 17%	486
	11th to 20th	Day	13% 4 17% 33% 22% 12%	479
	21st to end	Day	8% 6% 12% 27% 32% 14%	539
September	1st to 10th	Day	12% 5% 15% 27% 31% 10%	490
	11th to 20th	Day	10% 3 17% 26% 30% 16%	490
	21st to end	Day	19% 7% 20% 23% 23% 8%	490
October	1st to 10th	Day	10% 5% 23% 34% 21% 7%	490
	11th to 20th	Day	14% 16% 30% 23% 14% 3	490
	21st to end	Day	24% 11% 37% 20% 8% 1	374
November	1st to 10th	Day	14% 14% 35% 24% 10% 2	300
	11th to 20th	Day	21% 18% 32% 20% 7% 2	300
	21st to end	Day	25% 13% 36% 18% 7%	300
December	1st to 10th	Day	22% 22% 35% 15% 6% 1	300
	11th to 20th	Day	22% 18% 38% 16% 6%	300
	21st to end	Day	31% 18% 31% 16% 4	330
January	1st to 10th	Night	22% 18% 33% 23% 5%	1000
	11th to 20th	Night	28% 16% 33% 19% 3	888
	21st to end	Night	25% 15% 35% 22% 3	880
February	1st to 10th	Night	28% 13% 34% 24% 2	800
	11th to 20th	Night	25% 18% 34% 21% 2	783
	21st to end	Night	27% 15% 33% 24% 1	660
March	1st to 10th	Night	24% 15% 29% 28% 5%	800
	11th to 20th	Night	23% 14% 33% 23% 7% 1	800
	21st to end	Night	22% 12% 32% 26% 7%	880
April	1st to 10th	Night	20% 11% 35% 27% 8%	630
	11th to 20th	Night	20% 11% 28% 30% 9% 1	593
	21st to end	Night	19% 8% 35% 28% 9% 1	600
May	1st to 10th	Night	21% 6% 31% 29% 12% 1	600
	11th to 20th	Night	17% 6% 21% 34% 18% 3	563
	21st to end	Night	9% 3 20% 38% 26% 4	617
June	1st to 10th	Night	10% 3 20% 42% 22% 2	570
	11th to 20th	Night	9% 4 15% 40% 28% 3	570
	21st to end	Night	6% 2 22% 40% 25% 4	570
July	1st to 10th	Night	12% 2 13% 33% 31% 9%	555
	11th to 20th	Night	7% 3 13% 37% 36% 4	552
	21st to end	Night	12% 3 10% 39% 32% 4	624
August	1st to 10th	Night	9% 2 9% 36% 40% 5%	567
	11th to 20th	Night	9% 1 11% 41% 33% 5%	564
	21st to end	Night	7% 2 9% 35% 39% 9%	627
September	1st to 10th	Night	15% 2 12% 33% 33% 5%	652
	11th to 20th	Night	9% 4 14% 31% 38% 5%	760
	21st to end	Night	18% 3 22% 31% 24% 2	760
October	1st to 10th	Night	10% 5% 21% 36% 23% 5%	760
	11th to 20th	Night	17% 9% 24% 33% 14% 2	760
	21st to end	Night	22% 9% 31% 27% 11% 1	836
November	1st to 10th	Night	16% 9% 28% 31% 15% 1	780
	11th to 20th	Night	20% 14% 29% 26% 10% 1	800
	21st to end	Night	25% 12% 26% 29% 8% 1	800
December	1st to 10th	Night	27% 10% 35% 22% 6%	800
	11th to 20th	Night	25% 12% 33% 23% 6% 1	800
	21st to end	Night	30% 14% 30% 22% 4	1000

Page generated at 2025-05-08 09:34:01. Version ac07

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