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 2024-11-01.

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%	101221
	Night	15% 10% 3 7% 4 4 4 3 5% 7% 38%	97697
Transparency	Day	24% 49% 6% 13% 8% 1	101023
	Night	24% 49% 5% 13% 7% 1	97565
Seeing	Day	18% 11% 25% 24% 15% 7%	15346
	Night	19% 9% 26% 29% 15% 2	25057

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% 19% 5% 8% 4 4 4 3 4 6% 28%	2527
	11th to 20th	Day	17% 14% 6% 12% 5% 7% 4 3 3 4 25%	2689
	21st to end	Day	28% 16% 7% 8% 4 4 3 3 3 4 20%	3124
February	1st to 10th	Day	19% 13% 4 9% 3 3 4 3 5% 5% 32%	1836
	11th to 20th	Day	15% 12% 5% 10% 5% 4 4 3 5% 5% 33%	1848
	21st to end	Day	19% 14% 6% 7% 4 3 3 3 4 5% 32%	1570
March	1st to 10th	Day	24% 15% 3 8% 4 3 3 3 4 5% 28%	3120
	11th to 20th	Day	14% 8% 2 6% 4 4 3 3 5% 7% 42%	3476
	21st to end	Day	17% 10% 3 6% 4 4 5% 4 6% 6% 35%	3815
April	1st to 10th	Day	15% 11% 3 7% 4 4 5% 4 5% 8% 35%	2266
	11th to 20th	Day	16% 8% 2 7% 5% 4 4 3 5% 6% 39%	2286
	21st to end	Day	18% 8% 2 7% 6% 4 5% 3 5% 7% 36%	2470
May	1st to 10th	Day	22% 9% 3 7% 5% 5% 5% 4 5% 7% 29%	4082
	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 5% 4 3 3 3 3 6% 54%	1672
	11th to 20th	Day	14% 10% 4 8% 4 3 4 3 5% 6% 38%	1710
	21st to end	Day	18% 11% 4 6% 4 3 3 3 4 5% 39%	1710
December	1st to 10th	Day	21% 15% 3 9% 5% 4 4 3 4 5% 28%	1710
	11th to 20th	Day	16% 14% 4 9% 5% 4 4 3 4 5% 30%	2153
	21st to end	Day	21% 20% 5% 8% 6% 4 4 3 4 5% 19%	2929
January	1st to 10th	Night	17% 17% 6% 10% 5% 4 5% 4 4 5% 23%	4428
	11th to 20th	Night	15% 17% 5% 16% 6% 5% 4 2 3 4 23%	4269
	21st to end	Night	26% 18% 5% 8% 4 3 3 2 3 4 24%	4532
February	1st to 10th	Night	21% 12% 4 9% 5% 4 4 3 5% 5% 29%	2342
	11th to 20th	Night	16% 11% 4 9% 5% 4 4 4 5% 6% 32%	2218
	21st to end	Night	21% 13% 4 8% 4 3 3 3 4 5% 30%	1884
March	1st to 10th	Night	23% 13% 2 6% 4 3 3 3 4 5% 33%	3733
	11th to 20th	Night	14% 7% 2 6% 3 4 4 3 5% 7% 46%	3483
	21st to end	Night	17% 8% 2 7% 4 3 5% 3 5% 6% 39%	3817
April	1st to 10th	Night	14% 9% 2 7% 4 4 4 4 6% 6% 38%	1929
	11th to 20th	Night	14% 9% 3 6% 4 4 3 3 7% 7% 40%	1843
	21st to end	Night	18% 7% 3 6% 5% 4 4 4 6% 7% 37%	1710
May	1st to 10th	Night	21% 11% 4 8% 5% 4 4 4 6% 7% 27%	2813
	11th to 20th	Night	19% 6% 2 6% 4 4 4 3 6% 8% 38%	2812
	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 4 4 6% 41%	2669
November	1st to 10th	Night	12% 7% 2 6% 4 3 3 3 5% 5% 48%	2413
	11th to 20th	Night	16% 8% 3 8% 4 3 4 3 5% 7% 40%	2470
	21st to end	Night	19% 11% 2 6% 5% 4 4 3 4 6% 35%	2470
December	1st to 10th	Night	21% 12% 5% 8% 4 4 3 3 5% 6% 30%	2470
	11th to 20th	Night	18% 13% 5% 8% 5% 4 4 3 5% 5% 31%	3042
	21st to end	Night	20% 20% 6% 10% 6% 4 4 3 4 5% 18%	4724

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	38% 38% 7% 10% 7%	2528
	11th to 20th	Day	40% 38% 6% 11% 4	2689
	21st to end	Day	46% 39% 4 9% 2	3124
February	1st to 10th	Day	19% 65% 4 6% 5%	1836
	11th to 20th	Day	19% 64% 2 9% 6%	1858
	21st to end	Day	18% 65% 3 8% 5% 1	1570
March	1st to 10th	Day	40% 40% 3 9% 6% 1	3120
	11th to 20th	Day	21% 41% 6% 19% 12% 1	3476
	21st to end	Day	25% 42% 8% 15% 9%	3815
April	1st to 10th	Day	20% 65% 4 8% 3	2266
	11th to 20th	Day	16% 66% 4 9% 5% 1	2286
	21st to end	Day	16% 65% 4 8% 5% 1	2470
May	1st to 10th	Day	32% 44% 8% 12% 5%	4120
	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	15% 61% 3 12% 8% 1	2090
	11th to 20th	Day	14% 61% 3 11% 11% 1	1948
	21st to end	Day	19% 58% 4 10% 8% 1	1925
November	1st to 10th	Day	17% 59% 5% 12% 6% 1	1672
	11th to 20th	Day	17% 60% 2 10% 10% 1	1710
	21st to end	Day	21% 60% 5% 10% 4	1710
December	1st to 10th	Day	26% 60% 3 7% 4 1	1710
	11th to 20th	Day	27% 48% 4 12% 8% 1	2150
	21st to end	Day	46% 38% 4 6% 5% 1	2929
January	1st to 10th	Night	40% 40% 5% 8% 7%	4428
	11th to 20th	Night	44% 36% 5% 9% 5%	4269
	21st to end	Night	43% 39% 4 10% 4	4532
February	1st to 10th	Night	20% 65% 3 8% 4	2342
	11th to 20th	Night	19% 64% 3 8% 5%	2219
	21st to end	Night	20% 65% 3 6% 5% 1	1884
March	1st to 10th	Night	36% 42% 4 13% 5% 1	3733
	11th to 20th	Night	18% 42% 7% 18% 14% 1	3483
	21st to end	Night	25% 42% 4 19% 9% 1	3817
April	1st to 10th	Night	19% 65% 3 8% 4 1	1929
	11th to 20th	Night	16% 65% 3 8% 6% 1	1843
	21st to end	Night	16% 65% 4 9% 4 1	1710
May	1st to 10th	Night	34% 45% 5% 11% 4 1	2840
	11th to 20th	Night	25% 43% 9% 20% 3	2813
	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% 60% 4 13% 10% 1	2090
	11th to 20th	Night	13% 60% 4 9% 13% 1	2230
	21st to end	Night	18% 59% 4 10% 9% 1	2669
November	1st to 10th	Night	20% 58% 5% 9% 7% 1	2413
	11th to 20th	Night	17% 59% 3 9% 9% 2	2470
	21st to end	Night	22% 60% 4 10% 4	2470
December	1st to 10th	Night	23% 60% 4 7% 5% 1	2470
	11th to 20th	Night	26% 49% 4 13% 7% 1	3044
	21st to end	Night	46% 38% 5% 8% 3	4724

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	22% 21% 33% 18% 5%	270
	11th to 20th	Day	29% 24% 31% 14% 2	270
	21st to end	Day	31% 21% 31% 15% 3	297
February	1st to 10th	Day	28% 21% 33% 15% 3	270
	11th to 20th	Day	27% 25% 35% 10% 2 1	270
	21st to end	Day	30% 19% 40% 8% 3	225
March	1st to 10th	Day	23% 23% 38% 13% 3	270
	11th to 20th	Day	26% 18% 35% 15% 5% 1	456
	21st to end	Day	29% 18% 29% 17% 6% 1	506
April	1st to 10th	Day	22% 18% 29% 18% 11% 2	460
	11th to 20th	Day	21% 18% 25% 22% 9% 4	452
	21st to end	Day	22% 15% 35% 17% 7% 3	460
May	1st to 10th	Day	24% 13% 30% 20% 8% 4	463
	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% 10% 38% 20% 8% 1	374
November	1st to 10th	Day	14% 14% 33% 26% 11% 3	270
	11th to 20th	Day	21% 17% 31% 21% 7% 2	270
	21st to end	Day	25% 12% 34% 20% 8%	270
December	1st to 10th	Day	21% 23% 33% 16% 7% 1	270
	11th to 20th	Day	20% 18% 37% 18% 7%	270
	21st to end	Day	30% 19% 30% 16% 4	297
January	1st to 10th	Night	22% 17% 33% 23% 5%	950
	11th to 20th	Night	29% 16% 32% 19% 3	844
	21st to end	Night	26% 15% 34% 22% 3	836
February	1st to 10th	Night	28% 13% 33% 24% 2	760
	11th to 20th	Night	24% 18% 35% 22% 2	743
	21st to end	Night	29% 14% 34% 23% 1	628
March	1st to 10th	Night	24% 15% 29% 28% 5%	760
	11th to 20th	Night	23% 13% 34% 22% 7% 1	760
	21st to end	Night	23% 11% 32% 27% 7%	836
April	1st to 10th	Night	18% 11% 35% 28% 8%	599
	11th to 20th	Night	20% 11% 28% 30% 10% 1	563
	21st to end	Night	20% 8% 35% 28% 8% 1	570
May	1st to 10th	Night	21% 6% 31% 29% 12% 1	570
	11th to 20th	Night	17% 6% 22% 34% 18% 3	561
	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% 8% 28% 30% 16% 1	743
	11th to 20th	Night	20% 14% 28% 26% 11% 1	760
	21st to end	Night	25% 11% 26% 29% 8% 1	760
December	1st to 10th	Night	27% 10% 34% 23% 6%	760
	11th to 20th	Night	24% 12% 33% 24% 7% 1	760
	21st to end	Night	29% 14% 30% 22% 4	950

Page generated at 2024-10-29 11:20:46. Version ac07

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