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 2024-10-20.

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% 7% 2 6% 4 4 5% 4 6% 7% 38%	101224
	Night	18% 7% 2 6% 4 4 4 4 5% 6% 40%	97475
Transparency	Day	43% 6% 12% 23% 14% 2	100981
	Night	43% 6% 11% 23% 15% 2	97338
Seeing	Day	20% 8% 24% 26% 16% 6%	14868
	Night	20% 5% 26% 36% 13% 1	24079

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	15% 9% 2 5% 4 4 4 3 4 5% 44%	2517
	11th to 20th	Day	14% 12% 5% 8% 5% 5% 5% 4 5% 6% 30%	2529
	21st to end	Day	21% 10% 3 7% 4 4 4 4 5% 5% 33%	3124
February	1st to 10th	Day	16% 7% 3 7% 5% 5% 5% 4 6% 6% 36%	1710
	11th to 20th	Day	14% 8% 3 6% 6% 4 4 4 5% 6% 39%	1787
	21st to end	Day	19% 8% 4 7% 5% 5% 4 3 6% 4 35%	1570
March	1st to 10th	Day	26% 7% 2 5% 4 5% 4 2 5% 6% 33%	3116
	11th to 20th	Day	9% 6% 2 7% 5% 5% 5% 4 6% 7% 43%	3475
	21st to end	Day	20% 8% 2 5% 3 3 4 3 4 6% 42%	4052
April	1st to 10th	Day	17% 8% 3 7% 4 4 5% 4 6% 6% 37%	2280
	11th to 20th	Day	17% 7% 2 7% 5% 4 4 3 6% 7% 39%	2254
	21st to end	Day	17% 6% 2 6% 5% 4 4 4 6% 7% 39%	2280
May	1st to 10th	Day	28% 7% 2 6% 4 4 4 3 5% 7% 30%	4151
	11th to 20th	Day	24% 9% 3 7% 5% 5% 4 4 5% 6% 29%	4398
	21st to end	Day	13% 5% 2 5% 3 4 5% 4 7% 10% 43%	4837
June	1st to 10th	Day	14% 5% 2 7% 4 5% 6% 5% 6% 9% 37%	2660
	11th to 20th	Day	14% 6% 2 6% 5% 5% 5% 4 7% 9% 36%	2660
	21st to end	Day	13% 5% 2 6% 5% 4 5% 4 6% 9% 41%	2660
July	1st to 10th	Day	8% 6% 2 7% 5% 6% 6% 5% 7% 10% 38%	4440
	11th to 20th	Day	11% 5% 2 5% 5% 5% 5% 4 7% 10% 40%	4440
	21st to end	Day	15% 8% 3 7% 5% 5% 5% 4 7% 8% 33%	4794
August	1st to 10th	Day	23% 6% 2 6% 4 5% 5% 5% 6% 7% 31%	4104
	11th to 20th	Day	15% 5% 2 6% 4 5% 5% 3 6% 8% 42%	3765
	21st to end	Day	15% 5% 2 4 3 3 4 3 5% 8% 48%	4180
September	1st to 10th	Day	17% 5% 2 6% 4 5% 4 4 4 7% 42%	2280
	11th to 20th	Day	13% 5% 2 5% 3 3 4 3 5% 7% 51%	2252
	21st to end	Day	20% 6% 2 6% 4 4 5% 3 5% 5% 39%	2086
October	1st to 10th	Day	20% 6% 2 7% 3 3 3 3 6% 7% 40%	1900
	11th to 20th	Day	16% 5% 1 5% 3 4 4 3 5% 6% 46%	1890
	21st to end	Day	22% 6% 2 6% 4 4 5% 3 5% 6% 38%	1900
November	1st to 10th	Day	15% 8% 2 4 3 3 3 2 4 5% 52%	1671
	11th to 20th	Day	19% 5% 1 4 4 4 5% 4 5% 6% 43%	1710
	21st to end	Day	18% 6% 2 4 4 3 4 4 5% 7% 42%	1554
December	1st to 10th	Day	20% 7% 3 9% 4 5% 3 3 5% 7% 34%	1520
	11th to 20th	Day	21% 8% 2 5% 3 4 4 4 6% 6% 38%	1906
	21st to end	Day	20% 9% 4 9% 6% 5% 6% 4 5% 6% 27%	2772
January	1st to 10th	Night	16% 7% 2 6% 4 4 3 3 5% 7% 42%	4429
	11th to 20th	Night	17% 9% 3 8% 5% 4 4 4 6% 6% 36%	4426
	21st to end	Night	24% 11% 3 5% 3 3 4 3 4 5% 35%	4532
February	1st to 10th	Night	16% 9% 3 6% 4 4 4 3 5% 7% 39%	2470
	11th to 20th	Night	16% 8% 3 7% 5% 4 5% 4 4 5% 39%	2279
	21st to end	Night	20% 7% 3 6% 3 4 4 5% 5% 5% 40%	1884
March	1st to 10th	Night	23% 10% 2 6% 4 3 3 3 4 5% 36%	3724
	11th to 20th	Night	11% 7% 2 6% 4 4 4 3 5% 6% 47%	3482
	21st to end	Night	17% 9% 3 7% 4 3 3 2 4 4 43%	3579
April	1st to 10th	Night	17% 7% 2 7% 6% 4 4 4 5% 6% 37%	1900
	11th to 20th	Night	18% 6% 3 6% 4 4 4 3 5% 8% 39%	1875
	21st to end	Night	20% 5% 2 5% 3 3 3 4 6% 6% 44%	1900
May	1st to 10th	Night	28% 8% 3 5% 3 4 4 3 5% 7% 30%	2816
	11th to 20th	Night	25% 9% 3 7% 5% 3 4 3 5% 6% 31%	2495
	21st to end	Night	15% 6% 2 6% 4 4 4 4 5% 9% 41%	2738
June	1st to 10th	Night	16% 7% 2 4 4 3 4 3 6% 7% 43%	1520
	11th to 20th	Night	13% 6% 2 5% 4 4 4 5% 6% 9% 42%	1520
	21st to end	Night	13% 4 1 5% 4 4 4 4 7% 10% 44%	1520
July	1st to 10th	Night	11% 6% 2 7% 5% 6% 5% 5% 6% 9% 38%	2520
	11th to 20th	Night	12% 5% 1 6% 5% 5% 5% 4 7% 11% 38%	2520
	21st to end	Night	18% 6% 2 5% 4 4 5% 5% 6% 8% 38%	2756
August	1st to 10th	Night	22% 7% 2 7% 5% 3 4 4 5% 8% 33%	2825
	11th to 20th	Night	16% 7% 3 6% 4 4 4 4 5% 6% 41%	3029
	21st to end	Night	13% 4 1 4 3 4 4 3 5% 8% 49%	3476
September	1st to 10th	Night	15% 6% 2 5% 4 4 4 2 5% 6% 46%	1900
	11th to 20th	Night	13% 5% 2 4 4 3 3 3 4 7% 52%	1928
	21st to end	Night	19% 6% 2 6% 3 3 4 3 4 6% 44%	2090
October	1st to 10th	Night	23% 6% 3 5% 3 3 3 3 4 6% 43%	2280
	11th to 20th	Night	17% 6% 1 5% 3 3 3 3 4 6% 49%	2275
	21st to end	Night	22% 6% 2 4 3 3 3 3 5% 5% 44%	2454
November	1st to 10th	Night	13% 5% 2 5% 3 3 4 4 5% 5% 51%	2407
	11th to 20th	Night	18% 7% 2 5% 3 3 3 3 5% 5% 46%	2470
	21st to end	Night	15% 6% 2 5% 3 4 4 3 5% 7% 44%	2624
December	1st to 10th	Night	20% 7% 3 6% 4 5% 5% 4 5% 6% 36%	2660
	11th to 20th	Night	19% 8% 2 5% 4 4 4 4 6% 6% 39%	3288
	21st to end	Night	21% 10% 3 7% 4 4 4 3 5% 6% 34%	4884

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	41% 5% 12% 29% 11% 2	2518
	11th to 20th	Day	52% 6% 11% 19% 11% 2	2529
	21st to end	Day	52% 5% 11% 22% 9% 1	3124
February	1st to 10th	Day	45% 5% 13% 22% 13% 2	1710
	11th to 20th	Day	44% 5% 10% 24% 15% 2	1796
	21st to end	Day	50% 5% 9% 22% 11% 3	1570
March	1st to 10th	Day	51% 4 9% 26% 8% 2	3119
	11th to 20th	Day	36% 7% 13% 28% 14% 2	3475
	21st to end	Day	44% 4 9% 17% 22% 4	4052
April	1st to 10th	Day	45% 5% 9% 21% 15% 5%	2280
	11th to 20th	Day	43% 6% 10% 23% 15% 4	2254
	21st to end	Day	43% 5% 10% 22% 16% 5%	2280
May	1st to 10th	Day	53% 6% 11% 17% 11% 2	4165
	11th to 20th	Day	55% 6% 11% 18% 9% 2	4402
	21st to end	Day	35% 6% 12% 28% 17% 2	4837
June	1st to 10th	Day	41% 8% 12% 24% 14% 1	2660
	11th to 20th	Day	42% 7% 14% 24% 11% 3	2660
	21st to end	Day	37% 8% 14% 27% 14% 1	2660
July	1st to 10th	Day	39% 9% 13% 22% 15% 2	4370
	11th to 20th	Day	37% 7% 17% 25% 13% 1	4365
	21st to end	Day	47% 7% 10% 19% 16% 1	4641
August	1st to 10th	Day	49% 8% 14% 20% 10%	4091
	11th to 20th	Day	39% 6% 12% 29% 12% 1	3802
	21st to end	Day	35% 5% 10% 32% 16% 2	4180
September	1st to 10th	Day	41% 5% 10% 22% 19% 2	2280
	11th to 20th	Day	33% 5% 9% 24% 26% 3	2252
	21st to end	Day	46% 4 8% 20% 20% 2	2086
October	1st to 10th	Day	42% 4 11% 24% 17% 2	1900
	11th to 20th	Day	39% 3 11% 27% 19% 1	1890
	21st to end	Day	46% 5% 9% 23% 15% 2	1900
November	1st to 10th	Day	36% 3 13% 24% 20% 3	1671
	11th to 20th	Day	40% 5% 10% 25% 18% 2	1710
	21st to end	Day	39% 5% 13% 29% 11% 2	1554
December	1st to 10th	Day	49% 5% 14% 19% 11% 1	1520
	11th to 20th	Day	45% 5% 17% 23% 10% 1	1906
	21st to end	Day	53% 8% 11% 19% 8%	2772
January	1st to 10th	Night	41% 6% 11% 29% 11% 3	4429
	11th to 20th	Night	46% 5% 12% 24% 11% 2	4426
	21st to end	Night	51% 4 9% 23% 11% 2	4532
February	1st to 10th	Night	45% 5% 13% 24% 11% 2	2470
	11th to 20th	Night	46% 5% 9% 19% 16% 4	2281
	21st to end	Night	45% 5% 8% 23% 17% 3	1884
March	1st to 10th	Night	49% 5% 9% 23% 12% 2	3734
	11th to 20th	Night	36% 6% 13% 26% 17% 2	3483
	21st to end	Night	44% 4 6% 17% 22% 6%	3579
April	1st to 10th	Night	45% 6% 9% 22% 15% 3	1900
	11th to 20th	Night	43% 6% 10% 20% 19% 2	1875
	21st to end	Night	39% 5% 10% 22% 19% 4	1900
May	1st to 10th	Night	53% 6% 9% 21% 10% 1	2822
	11th to 20th	Night	54% 7% 13% 14% 10% 1	2496
	21st to end	Night	38% 8% 13% 24% 13% 3	2739
June	1st to 10th	Night	40% 5% 10% 24% 19% 1	1520
	11th to 20th	Night	36% 8% 15% 26% 13% 1	1520
	21st to end	Night	34% 7% 14% 29% 14% 1	1520
July	1st to 10th	Night	42% 9% 14% 17% 15% 2	2480
	11th to 20th	Night	39% 8% 14% 25% 13% 1	2472
	21st to end	Night	42% 8% 13% 17% 18% 1	2668
August	1st to 10th	Night	49% 7% 16% 18% 9% 1	2814
	11th to 20th	Night	43% 5% 12% 24% 15% 1	3058
	21st to end	Night	34% 5% 10% 32% 18% 1	3476
September	1st to 10th	Night	39% 4 10% 24% 21% 2	1900
	11th to 20th	Night	33% 5% 9% 24% 26% 3	1928
	21st to end	Night	42% 4 8% 22% 22% 2	2090
October	1st to 10th	Night	43% 5% 10% 25% 15% 2	2280
	11th to 20th	Night	37% 5% 10% 29% 16% 2	2275
	21st to end	Night	41% 5% 10% 27% 16% 1	2454
November	1st to 10th	Night	35% 5% 12% 24% 21% 4	2407
	11th to 20th	Night	41% 4 10% 24% 18% 3	2470
	21st to end	Night	39% 4 13% 29% 13% 2	2624
December	1st to 10th	Night	48% 5% 11% 23% 12% 1	2660
	11th to 20th	Night	44% 4 13% 20% 15% 3	3288
	21st to end	Night	50% 6% 12% 23% 9%	4884

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	25% 12% 31% 23% 7% 2	180
	11th to 20th	Day	23% 21% 33% 18% 4 1	184
	21st to end	Day	20% 11% 34% 24% 8% 3	297
February	1st to 10th	Day	23% 16% 33% 23% 4 1	270
	11th to 20th	Day	24% 10% 35% 27% 4 1	395
	21st to end	Day	27% 9% 29% 30% 5%	382
March	1st to 10th	Day	20% 16% 28% 28% 8% 1	460
	11th to 20th	Day	19% 14% 32% 27% 6% 2	460
	21st to end	Day	25% 13% 27% 24% 8% 3	506
April	1st to 10th	Day	22% 12% 31% 26% 8% 1	460
	11th to 20th	Day	20% 13% 29% 22% 11% 5%	452
	21st to end	Day	24% 10% 30% 21% 13% 2	460
May	1st to 10th	Day	31% 11% 24% 19% 11% 5%	463
	11th to 20th	Day	22% 7% 28% 25% 14% 5%	487
	21st to end	Day	19% 4 19% 27% 23% 9%	529
June	1st to 10th	Day	16% 9% 23% 26% 19% 7%	490
	11th to 20th	Day	17% 6% 21% 27% 22% 7%	490
	21st to end	Day	17% 5% 21% 30% 21% 6%	490
July	1st to 10th	Day	14% 2 15% 29% 26% 14%	485
	11th to 20th	Day	13% 2 14% 31% 28% 12%	484
	21st to end	Day	17% 2 20% 30% 23% 8%	532
August	1st to 10th	Day	15% 3 16% 29% 26% 11%	486
	11th to 20th	Day	15% 3 13% 33% 23% 12%	479
	21st to end	Day	16% 2 17% 29% 23% 14%	539
September	1st to 10th	Day	19% 5% 18% 23% 23% 12%	490
	11th to 20th	Day	13% 3 17% 27% 22% 18%	490
	21st to end	Day	24% 6% 19% 27% 18% 7%	490
October	1st to 10th	Day	24% 6% 19% 23% 20% 8%	490
	11th to 20th	Day	16% 8% 31% 27% 15% 4	486
	21st to end	Day	25% 6% 32% 25% 11% 2	455
November	1st to 10th	Day	18% 8% 25% 29% 17% 4	365
	11th to 20th	Day	21% 13% 27% 29% 9% 1	370
	21st to end	Day	20% 10% 35% 26% 8% 1	214
December	1st to 10th	Day	24% 17% 32% 18% 8% 2	180
	11th to 20th	Day	24% 15% 28% 25% 7% 1	180
	21st to end	Day	27% 9% 30% 23% 8% 2	198
January	1st to 10th	Night	19% 7% 38% 31% 5%	760
	11th to 20th	Night	24% 7% 31% 32% 5%	760
	21st to end	Night	19% 5% 38% 35% 3	836
February	1st to 10th	Night	20% 5% 35% 37% 3	760
	11th to 20th	Night	20% 8% 43% 27% 2	743
	21st to end	Night	22% 8% 35% 31% 3	628
March	1st to 10th	Night	21% 6% 36% 32% 5%	760
	11th to 20th	Night	20% 7% 36% 31% 6%	760
	21st to end	Night	21% 6% 33% 32% 8% 1	836
April	1st to 10th	Night	20% 8% 37% 29% 6%	760
	11th to 20th	Night	20% 7% 32% 32% 10%	751
	21st to end	Night	22% 5% 27% 36% 9%	760
May	1st to 10th	Night	26% 6% 24% 34% 9% 1	565
	11th to 20th	Night	21% 2 26% 40% 10%	374
	21st to end	Night	16% 2 14% 47% 20% 1	411
June	1st to 10th	Night	18% 3 15% 42% 21% 1	380
	11th to 20th	Night	14% 2 17% 49% 17% 1	380
	21st to end	Night	14% 1 12% 54% 19%	380
July	1st to 10th	Night	15% 1 12% 44% 25% 2	370
	11th to 20th	Night	13% 2 11% 45% 28% 1	368
	21st to end	Night	13% 2 12% 47% 24% 1	421
August	1st to 10th	Night	14% 2 7% 46% 30% 2	567
	11th to 20th	Night	16% 2 9% 39% 31% 3	706
	21st to end	Night	17% 2 8% 41% 30% 2	836
September	1st to 10th	Night	19% 2 14% 38% 24% 3	760
	11th to 20th	Night	15% 2 10% 40% 28% 5%	760
	21st to end	Night	23% 3 14% 38% 21% 2	760
October	1st to 10th	Night	23% 4 19% 31% 20% 3	760
	11th to 20th	Night	19% 3 23% 41% 13% 1	759
	21st to end	Night	25% 3 27% 34% 10% 1	792
November	1st to 10th	Night	17% 4 27% 38% 14% 1	740
	11th to 20th	Night	20% 6% 30% 33% 11% 1	760
	21st to end	Night	17% 6% 31% 37% 8%	760
December	1st to 10th	Night	24% 5% 31% 33% 6% 1	760
	11th to 20th	Night	22% 5% 31% 36% 7%	760
	21st to end	Night	23% 6% 33% 36% 3	836

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