AI2 Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the AI2 Observatory clear sky chart from 2005-11-06 to 2026-07-16.

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	17% 6% 2 6% 4 4 5% 4 6% 7% 39%	138260
	Night	17% 7% 2 6% 4 4 4 3 5% 6% 41%	134441
Transparency	Day	42% 5% 12% 23% 15% 2	141777
	Night	42% 5% 11% 23% 16% 3	139012
Seeing	Day	20% 9% 25% 26% 15% 6%	17067
	Night	19% 5% 26% 36% 12% 1	26283

By Month

Percentage Cloud Cover NE (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	13% 7% 2 6% 4 5% 5% 3 6% 6% 42%	4661
	11th to 20th	Day	13% 9% 4 8% 5% 4 5% 4 5% 5% 38%	4687
	21st to end	Day	15% 7% 2 6% 4 5% 5% 4 6% 6% 39%	5780
February	1st to 10th	Day	15% 7% 2 8% 5% 5% 5% 4 6% 7% 37%	1890
	11th to 20th	Day	15% 9% 3 6% 6% 4 4 4 5% 6% 38%	1981
	21st to end	Day	18% 8% 3 7% 5% 5% 4 3 6% 5% 37%	1730
March	1st to 10th	Day	22% 6% 2 5% 4 5% 4 3 5% 6% 40%	4576
	11th to 20th	Day	13% 6% 3 7% 4 5% 4 4 6% 7% 42%	5085
	21st to end	Day	18% 6% 2 5% 3 4 4 3 6% 7% 42%	5928
April	1st to 10th	Day	18% 8% 3 6% 4 4 5% 4 6% 6% 36%	2520
	11th to 20th	Day	17% 6% 2 6% 5% 4 4 4 6% 7% 39%	2494
	21st to end	Day	17% 6% 2 6% 5% 4 4 4 6% 7% 39%	2520
May	1st to 10th	Day	25% 6% 2 5% 4 4 5% 4 5% 7% 33%	7725
	11th to 20th	Day	20% 7% 2 6% 4 4 4 3 5% 7% 38%	8221
	21st to end	Day	24% 6% 2 5% 4 4 4 4 6% 8% 33%	9060
June	1st to 10th	Day	14% 5% 2 6% 4 5% 5% 5% 6% 9% 37%	2940
	11th to 20th	Day	15% 6% 2 6% 5% 5% 5% 4 7% 9% 36%	2940
	21st to end	Day	13% 5% 2 6% 4 4 5% 4 6% 9% 41%	2940
July	1st to 10th	Day	10% 6% 2 7% 5% 5% 6% 4 7% 10% 39%	6500
	11th to 20th	Day	13% 6% 2 6% 5% 5% 6% 5% 8% 11% 33%	6430
	21st to end	Day	14% 6% 2 6% 4 4 5% 4 6% 9% 41%	6892
August	1st to 10th	Day	22% 6% 2 6% 4 5% 5% 5% 6% 7% 32%	4234
	11th to 20th	Day	15% 5% 2 6% 4 5% 5% 3 6% 8% 42%	3886
	21st to end	Day	15% 5% 2 4 3 3 4 3 5% 8% 49%	4312
September	1st to 10th	Day	16% 5% 2 6% 4 5% 4 4 4 7% 43%	2400
	11th to 20th	Day	13% 5% 2 5% 3 3 4 3 5% 7% 51%	2370
	21st to end	Day	20% 6% 2 7% 4 4 5% 3 6% 5% 38%	2196
October	1st to 10th	Day	19% 6% 2 6% 3 3 3 3 6% 7% 43%	2000
	11th to 20th	Day	16% 5% 1 6% 3 4 4 3 5% 7% 45%	2000
	21st to end	Day	20% 5% 2 5% 4 4 5% 3 5% 7% 41%	2110
November	1st to 10th	Day	14% 7% 2 4 3 3 3 2 4 6% 52%	1851
	11th to 20th	Day	17% 6% 1 4 3 4 5% 4 5% 6% 43%	1890
	21st to end	Day	18% 7% 2 5% 4 3 5% 4 5% 7% 41%	1717
December	1st to 10th	Day	18% 6% 2 6% 4 4 4 4 5% 6% 41%	2680
	11th to 20th	Day	23% 8% 2 4 3 3 4 4 5% 6% 38%	3066
	21st to end	Day	19% 7% 3 7% 5% 5% 5% 4 5% 7% 34%	4048
January	1st to 10th	Night	13% 8% 2 7% 4 4 4 4 6% 7% 41%	8234
	11th to 20th	Night	14% 8% 3 7% 4 4 4 4 6% 7% 39%	8193
	21st to end	Night	15% 8% 2 5% 4 3 4 3 5% 6% 44%	8401
February	1st to 10th	Night	15% 9% 3 6% 4 4 4 3 5% 7% 40%	2730
	11th to 20th	Night	17% 8% 3 7% 5% 4 5% 4 4 5% 38%	2525
	21st to end	Night	18% 8% 3 6% 3 4 4 5% 5% 5% 40%	2076
March	1st to 10th	Night	19% 9% 2 6% 4 3 3 3 5% 5% 41%	5484
	11th to 20th	Night	13% 7% 3 5% 4 3 4 3 4 5% 49%	5092
	21st to end	Night	17% 8% 3 7% 4 4 4 3 4 5% 42%	5245
April	1st to 10th	Night	19% 7% 2 7% 6% 4 4 4 5% 6% 36%	2100
	11th to 20th	Night	18% 6% 3 6% 4 4 4 3 5% 8% 39%	2075
	21st to end	Night	20% 5% 2 4 3 3 3 3 5% 6% 44%	2100
May	1st to 10th	Night	27% 7% 2 5% 4 5% 4 3 5% 7% 30%	5228
	11th to 20th	Night	22% 7% 2 6% 4 3 4 3 5% 7% 37%	4639
	21st to end	Night	25% 6% 3 6% 4 4 4 3 5% 7% 35%	5114
June	1st to 10th	Night	16% 7% 2 4 4 3 4 3 6% 7% 43%	1680
	11th to 20th	Night	14% 7% 2 5% 4 3 4 6% 6% 9% 41%	1680
	21st to end	Night	14% 4 1 4 4 4 4 4 7% 10% 43%	1680
July	1st to 10th	Night	13% 6% 2 7% 5% 5% 4 4 6% 9% 38%	3680
	11th to 20th	Night	17% 7% 2 7% 5% 5% 5% 5% 7% 9% 32%	3640
	21st to end	Night	15% 5% 2 5% 3 4 4 4 5% 7% 46%	3943
August	1st to 10th	Night	22% 7% 2 7% 4 3 4 4 5% 8% 33%	2915
	11th to 20th	Night	16% 7% 3 6% 4 4 4 4 5% 6% 42%	3127
	21st to end	Night	13% 4 2 4 3 4 4 3 5% 8% 50%	3586
September	1st to 10th	Night	15% 6% 2 5% 4 4 4 3 5% 6% 46%	2000
	11th to 20th	Night	13% 5% 2 5% 4 3 4 3 4 7% 51%	2030
	21st to end	Night	19% 6% 2 6% 3 3 4 3 4 6% 43%	2200
October	1st to 10th	Night	22% 5% 2 5% 3 3 3 3 4 6% 44%	2400
	11th to 20th	Night	17% 6% 2 5% 3 3 3 3 4 6% 49%	2400
	21st to end	Night	20% 5% 2 4 3 3 3 3 5% 5% 46%	2728
November	1st to 10th	Night	13% 5% 2 5% 3 3 4 4 5% 6% 51%	2667
	11th to 20th	Night	17% 7% 2 5% 4 3 3 3 5% 5% 46%	2730
	21st to end	Night	15% 7% 2 6% 3 4 4 3 6% 7% 43%	2901
December	1st to 10th	Night	14% 8% 3 6% 4 4 4 3 4 6% 44%	4720
	11th to 20th	Night	24% 7% 2 4 3 3 4 3 5% 5% 40%	5348
	21st to end	Night	21% 10% 3 6% 4 3 4 3 4 5% 37%	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	40% 4 11% 27% 16% 2	4662
	11th to 20th	Day	46% 4 10% 20% 15% 5%	4717
	21st to end	Day	44% 5% 13% 23% 15% 1	5780
February	1st to 10th	Day	44% 5% 13% 22% 13% 2	1890
	11th to 20th	Day	46% 5% 9% 24% 14% 2	1990
	21st to end	Day	48% 4 10% 23% 12% 3	1730
March	1st to 10th	Day	44% 4 10% 23% 12% 7%	4579
	11th to 20th	Day	40% 6% 13% 27% 11% 4	5085
	21st to end	Day	41% 5% 12% 15% 23% 5%	5928
April	1st to 10th	Day	46% 5% 9% 20% 15% 5%	2520
	11th to 20th	Day	42% 5% 11% 22% 16% 3	2494
	21st to end	Day	43% 5% 10% 23% 15% 5%	2520
May	1st to 10th	Day	49% 6% 12% 20% 11% 1	7736
	11th to 20th	Day	46% 5% 11% 25% 13% 1	8225
	21st to end	Day	48% 5% 11% 22% 14% 1	9061
June	1st to 10th	Day	41% 7% 12% 24% 15% 1	2940
	11th to 20th	Day	42% 7% 14% 23% 12% 2	2940
	21st to end	Day	38% 8% 14% 26% 14% 1	2940
July	1st to 10th	Day	40% 9% 12% 22% 16% 1	6430
	11th to 20th	Day	42% 8% 17% 22% 10% 1	6355
	21st to end	Day	39% 6% 11% 22% 22% 1	6694
August	1st to 10th	Day	48% 7% 13% 20% 11%	4221
	11th to 20th	Day	39% 6% 13% 29% 12% 1	3923
	21st to end	Day	34% 5% 10% 33% 16% 2	4312
September	1st to 10th	Day	41% 5% 10% 23% 20% 2	2400
	11th to 20th	Day	32% 5% 9% 25% 25% 3	2370
	21st to end	Day	47% 4 9% 19% 19% 2	2196
October	1st to 10th	Day	38% 4 8% 21% 24% 5%	3260
	11th to 20th	Day	30% 3 10% 28% 24% 4	3240
	21st to end	Day	28% 3 14% 26% 25% 4	3387
November	1st to 10th	Day	35% 3 13% 24% 21% 3	1851
	11th to 20th	Day	39% 5% 10% 26% 18% 3	1890
	21st to end	Day	41% 5% 13% 29% 10% 2	1717
December	1st to 10th	Day	43% 4 15% 25% 10% 2	2680
	11th to 20th	Day	46% 4 16% 22% 12% 1	3066
	21st to end	Day	47% 6% 12% 20% 13% 1	4048
January	1st to 10th	Night	40% 5% 14% 23% 16% 3	8234
	11th to 20th	Night	43% 4 12% 20% 16% 5%	8238
	21st to end	Night	41% 3 9% 23% 20% 3	8401
February	1st to 10th	Night	45% 5% 12% 24% 12% 3	2730
	11th to 20th	Night	47% 5% 9% 20% 16% 4	2527
	21st to end	Night	44% 4 8% 24% 17% 3	2076
March	1st to 10th	Night	44% 4 10% 22% 12% 8%	5494
	11th to 20th	Night	37% 5% 14% 25% 14% 5%	5093
	21st to end	Night	45% 4 6% 16% 23% 5%	5245
April	1st to 10th	Night	47% 5% 9% 21% 14% 4	2100
	11th to 20th	Night	43% 7% 10% 19% 19% 2	2075
	21st to end	Night	39% 5% 10% 23% 19% 4	2100
May	1st to 10th	Night	52% 6% 11% 19% 11% 2	5234
	11th to 20th	Night	47% 6% 13% 21% 12% 2	4640
	21st to end	Night	49% 6% 10% 22% 11% 1	5115
June	1st to 10th	Night	39% 5% 10% 24% 20% 1	1680
	11th to 20th	Night	38% 8% 14% 25% 13% 1	1680
	21st to end	Night	35% 7% 15% 28% 14% 1	1680
July	1st to 10th	Night	43% 8% 13% 18% 16% 2	3640
	11th to 20th	Night	47% 8% 13% 21% 11% 1	3592
	21st to end	Night	35% 6% 13% 23% 21% 1	3826
August	1st to 10th	Night	48% 7% 16% 18% 10% 1	2904
	11th to 20th	Night	42% 5% 12% 24% 15% 1	3156
	21st to end	Night	34% 5% 10% 32% 18% 1	3586
September	1st to 10th	Night	39% 4 10% 24% 21% 2	2000
	11th to 20th	Night	33% 5% 9% 23% 26% 3	2030
	21st to end	Night	43% 4 9% 21% 21% 2	2200
October	1st to 10th	Night	41% 4 10% 22% 21% 2	3920
	11th to 20th	Night	28% 4 8% 30% 26% 4	3899
	21st to end	Night	29% 4 12% 31% 23% 2	4401
November	1st to 10th	Night	34% 5% 12% 24% 21% 4	2667
	11th to 20th	Night	40% 4 10% 24% 19% 3	2730
	21st to end	Night	40% 4 12% 28% 14% 2	2901
December	1st to 10th	Night	42% 3 13% 31% 10% 2	4720
	11th to 20th	Night	46% 3 11% 22% 15% 2	5348
	21st to end	Night	49% 5% 10% 20% 15% 2	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	24% 15% 31% 22% 6% 1	220
	11th to 20th	Day	23% 22% 35% 17% 4 1	226
	21st to end	Day	18% 12% 39% 22% 7% 2	363
February	1st to 10th	Day	21% 15% 35% 23% 5% 1	330
	11th to 20th	Day	24% 11% 35% 26% 3	469
	21st to end	Day	24% 9% 31% 31% 4	446
March	1st to 10th	Day	21% 16% 28% 27% 7% 1	540
	11th to 20th	Day	20% 14% 32% 26% 6% 2	540
	21st to end	Day	23% 17% 28% 22% 8% 3	594
April	1st to 10th	Day	24% 13% 31% 24% 8% 1	540
	11th to 20th	Day	20% 14% 30% 22% 11% 4	532
	21st to end	Day	23% 10% 30% 22% 13% 2	540
May	1st to 10th	Day	31% 11% 26% 18% 10% 4	543
	11th to 20th	Day	21% 8% 29% 24% 13% 5%	567
	21st to end	Day	21% 5% 20% 25% 20% 8%	617
June	1st to 10th	Day	15% 9% 23% 26% 20% 7%	570
	11th to 20th	Day	18% 8% 23% 26% 19% 6%	570
	21st to end	Day	18% 5% 21% 30% 21% 6%	570
July	1st to 10th	Day	13% 2 16% 29% 27% 13%	565
	11th to 20th	Day	12% 2 15% 31% 27% 13%	544
	21st to end	Day	16% 2 20% 31% 23% 8%	576
August	1st to 10th	Day	14% 3 16% 28% 27% 11%	526
	11th to 20th	Day	15% 3 13% 34% 23% 13%	519
	21st to end	Day	15% 2 17% 30% 23% 14%	583
September	1st to 10th	Day	18% 5% 18% 24% 23% 12%	530
	11th to 20th	Day	13% 3 17% 28% 23% 16%	530
	21st to end	Day	24% 5% 18% 28% 17% 7%	530
October	1st to 10th	Day	23% 6% 18% 24% 20% 10%	530
	11th to 20th	Day	16% 10% 31% 25% 15% 4	530
	21st to end	Day	22% 7% 31% 27% 11% 2	533
November	1st to 10th	Day	16% 9% 28% 29% 15% 3	425
	11th to 20th	Day	20% 13% 28% 29% 9% 1	430
	21st to end	Day	23% 12% 35% 22% 7% 1	257
December	1st to 10th	Day	23% 17% 33% 19% 8% 1	220
	11th to 20th	Day	24% 16% 30% 23% 7%	220
	21st to end	Day	26% 10% 31% 23% 8% 2	242
January	1st to 10th	Night	18% 8% 38% 30% 5%	840
	11th to 20th	Night	23% 8% 32% 32% 5%	840
	21st to end	Night	18% 5% 41% 33% 3	924
February	1st to 10th	Night	20% 6% 35% 36% 3	840
	11th to 20th	Night	21% 9% 41% 27% 2	823
	21st to end	Night	21% 8% 36% 32% 3	692
March	1st to 10th	Night	21% 6% 36% 31% 5%	840
	11th to 20th	Night	20% 7% 35% 31% 6%	840
	21st to end	Night	21% 6% 34% 32% 7% 1	924
April	1st to 10th	Night	21% 9% 36% 28% 6%	840
	11th to 20th	Night	19% 8% 33% 31% 9%	831
	21st to end	Night	22% 6% 27% 37% 8%	840
May	1st to 10th	Night	25% 7% 25% 33% 8% 1	624
	11th to 20th	Night	20% 3 26% 41% 10%	414
	21st to end	Night	19% 2 14% 45% 18% 1	455
June	1st to 10th	Night	17% 3 16% 42% 21% 1	420
	11th to 20th	Night	15% 3 18% 47% 17% 1	420
	21st to end	Night	14% 2 12% 53% 19%	420
July	1st to 10th	Night	16% 1 11% 42% 27% 2	410
	11th to 20th	Night	13% 3 12% 44% 27% 2	398
	21st to end	Night	12% 2 12% 47% 25% 1	443
August	1st to 10th	Night	14% 2 7% 46% 30% 2	597
	11th to 20th	Night	15% 2 9% 38% 32% 3	744
	21st to end	Night	17% 2 9% 41% 29% 2	880
September	1st to 10th	Night	19% 2 14% 38% 24% 3	800
	11th to 20th	Night	15% 2 10% 41% 28% 5%	800
	21st to end	Night	23% 3 14% 38% 21% 2	800
October	1st to 10th	Night	23% 3 18% 31% 22% 3	800
	11th to 20th	Night	19% 4 24% 39% 13% 1	800
	21st to end	Night	23% 4 27% 36% 10%	880
November	1st to 10th	Night	16% 5% 26% 38% 13% 1	820
	11th to 20th	Night	20% 6% 33% 32% 10% 1	840
	21st to end	Night	17% 7% 32% 36% 7%	840
December	1st to 10th	Night	23% 5% 33% 32% 5% 1	840
	11th to 20th	Night	23% 5% 31% 35% 6%	840
	21st to end	Night	23% 6% 33% 35% 3	924

Page generated at 2026-07-13 03:14:49. Version ac07

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