Arnold Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Arnold Observatory clear sky chart from 2005-11-06 to 2024-12-02.

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	8% 5% 1 3 2 2 2 2 4 5% 65%	105312
	Night	10% 5% 1 3 2 2 3 2 4 5% 62%	103059
Transparency	Day	22% 4 7% 17% 25% 24%	105062
	Night	26% 4 8% 18% 23% 21%	102898
Seeing	Day	12% 13% 33% 27% 11% 3	15273
	Night	12% 4 18% 36% 24% 5%	24674

By Month

Percentage Cloud Cover SW (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	19% 11% 2 4 3 3 3 2 4 5% 45%	2539
	11th to 20th	Day	20% 8% 2 7% 5% 5% 3 3 4 5% 37%	2735
	21st to end	Day	20% 11% 2 6% 3 4 4 3 4 7% 36%	3146
February	1st to 10th	Day	19% 9% 1 4 3 2 3 3 4 4 48%	1710
	11th to 20th	Day	13% 11% 2 4 3 3 3 3 4 6% 48%	1850
	21st to end	Day	18% 12% 3 4 3 3 3 3 4 5% 43%	1570
March	1st to 10th	Day	15% 16% 3 7% 5% 4 5% 4 6% 6% 28%	3155
	11th to 20th	Day	14% 7% 1 4 2 3 3 2 4 5% 55%	3490
	21st to end	Day	16% 13% 3 6% 5% 4 4 3 5% 5% 35%	4032
April	1st to 10th	Day	14% 8% 2 3 3 3 4 3 5% 6% 48%	2280
	11th to 20th	Day	13% 6% 2 4 3 2 3 3 5% 6% 53%	2249
	21st to end	Day	11% 5% 1 3 3 2 3 2 4 6% 59%	2280
May	1st to 10th	Day	8% 3 1 3 2 2 2 2 4 4 69%	3882
	11th to 20th	Day	4 2 1 2 2 1 2 2 4 4 76%	4284
	21st to end	Day	5% 3 1 2 2 2 2 2 3 5% 73%	4813
June	1st to 10th	Day	5% 3 1 4 3 2 2 3 4 6% 66%	2660
	11th to 20th	Day	2 2 1 2 2 2 1 2 3 4 80%	2660
	21st to end	Day	2 2 1 1 1 2 2 2 2 3 82%	2660
July	1st to 10th	Day	1 1 1 1 1 1 2 93%	4420
	11th to 20th	Day	1 1 1 2 1 2 2 2 3 4 82%	4420
	21st to end	Day	1 1 1 1 1 1 1 2 3 87%	4529
August	1st to 10th	Day	2 2 1 2 1 1 2 2 3 3 82%	4040
	11th to 20th	Day	1 1 1 1 1 2 2 3 88%	3724
	21st to end	Day	2 1 1 1 1 1 1 1 1 2 88%	4180
September	1st to 10th	Day	2 1 1 1 1 1 1 2 2 87%	2280
	11th to 20th	Day	3 2 1 3 1 2 2 2 4 5% 76%	2267
	21st to end	Day	4 2 2 1 1 1 1 3 4 80%	2090
October	1st to 10th	Day	3 3 1 3 1 2 2 3 4 5% 74%	3185
	11th to 20th	Day	5% 2 1 2 2 2 2 2 3 4 77%	3161
	21st to end	Day	6% 2 1 4 2 3 3 2 5% 6% 66%	3448
November	1st to 10th	Day	11% 5% 1 5% 4 4 3 3 5% 6% 54%	1768
	11th to 20th	Day	11% 5% 2 4 3 3 3 3 4 6% 57%	1800
	21st to end	Day	12% 6% 1 4 3 3 3 2 5% 6% 54%	1755
December	1st to 10th	Day	15% 5% 2 4 3 4 3 3 4 5% 52%	1528
	11th to 20th	Day	28% 9% 2 5% 4 4 3 3 3 5% 35%	1928
	21st to end	Day	13% 8% 3 6% 4 4 5% 5% 6% 8% 38%	2794
January	1st to 10th	Night	15% 10% 3 5% 2 3 3 3 4 6% 46%	4406
	11th to 20th	Night	22% 8% 2 6% 4 4 4 5% 5% 5% 36%	4229
	21st to end	Night	21% 9% 2 5% 4 4 4 4 5% 6% 36%	4521
February	1st to 10th	Night	17% 8% 2 5% 3 3 2 2 4 6% 49%	2468
	11th to 20th	Night	15% 9% 2 4 3 3 3 3 5% 7% 48%	2218
	21st to end	Night	16% 10% 2 4 3 2 3 2 4 5% 49%	1884
March	1st to 10th	Night	18% 17% 3 4 4 3 4 3 5% 5% 35%	3716
	11th to 20th	Night	15% 6% 1 3 2 2 2 2 4 4 57%	3490
	21st to end	Night	20% 12% 2 5% 3 3 5% 3 5% 5% 37%	3619
April	1st to 10th	Night	13% 6% 2 5% 3 3 4 3 4 6% 51%	1900
	11th to 20th	Night	11% 6% 1 4 3 3 3 2 4 6% 56%	1879
	21st to end	Night	9% 5% 1 2 2 2 3 3 5% 7% 60%	1900
May	1st to 10th	Night	4 6% 1 2 1 1 2 2 3 6% 71%	3094
	11th to 20th	Night	3 2 1 2 1 1 2 2 4 6% 77%	2594
	21st to end	Night	3 2 1 2 2 1 2 2 3 5% 78%	2762
June	1st to 10th	Night	5% 3 1 2 2 2 3 2 4 7% 69%	1520
	11th to 20th	Night	1 1 1 1 2 2 2 3 4 83%	1520
	21st to end	Night	1 1 1 2 2 2 1 1 2 3 84%	1520
July	1st to 10th	Night	1 1 1 1 1 1 93%	2540
	11th to 20th	Night	3 2 1 3 2 2 2 2 3 3 77%	2540
	21st to end	Night	1 1 1 2 1 2 2 1 2 3 83%	3028
August	1st to 10th	Night	2 2 1 2 1 1 2 2 3 4 79%	2901
	11th to 20th	Night	1 1 1 1 1 2 2 2 3 87%	3126
	21st to end	Night	3 1 1 1 1 1 1 1 2 88%	3498
September	1st to 10th	Night	1 1 1 1 1 2 1 2 2 89%	1900
	11th to 20th	Night	3 1 1 2 2 2 2 2 3 4 79%	1912
	21st to end	Night	2 2 1 2 1 2 2 1 2 4 81%	2090
October	1st to 10th	Night	3 2 1 2 2 2 2 2 3 3 79%	3790
	11th to 20th	Night	3 2 1 2 1 1 2 2 3 4 80%	3777
	21st to end	Night	7% 3 1 2 2 3 3 3 4 5% 68%	4121
November	1st to 10th	Night	11% 5% 1 3 2 2 2 2 4 6% 61%	2530
	11th to 20th	Night	10% 3 1 4 3 2 3 3 5% 7% 58%	2600
	21st to end	Night	13% 5% 1 3 3 3 3 3 3 5% 57%	2642
December	1st to 10th	Night	17% 6% 2 4 2 3 3 2 4 6% 51%	2686
	11th to 20th	Night	28% 8% 2 5% 3 3 3 2 5% 5% 36%	3276
	21st to end	Night	16% 8% 3 7% 5% 5% 4 4 5% 7% 37%	4862

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	42% 4 9% 22% 17% 5%	2539
	11th to 20th	Day	48% 7% 10% 13% 13% 9%	2735
	21st to end	Day	47% 6% 11% 18% 12% 5%	3146
February	1st to 10th	Day	40% 3 9% 20% 17% 10%	1710
	11th to 20th	Day	38% 4 9% 20% 23% 7%	1860
	21st to end	Day	44% 4 8% 19% 19% 5%	1570
March	1st to 10th	Day	50% 10% 12% 12% 11% 4	3155
	11th to 20th	Day	31% 6% 12% 29% 13% 9%	3490
	21st to end	Day	44% 12% 13% 19% 9% 2	4032
April	1st to 10th	Day	36% 6% 13% 22% 18% 5%	2280
	11th to 20th	Day	31% 6% 14% 25% 18% 6%	2252
	21st to end	Day	26% 6% 13% 30% 22% 3	2280
May	1st to 10th	Day	18% 4 10% 30% 23% 14%	3882
	11th to 20th	Day	13% 3 7% 29% 39% 9%	4287
	21st to end	Day	16% 4 8% 34% 35% 3	4814
June	1st to 10th	Day	19% 5% 10% 25% 29% 11%	2660
	11th to 20th	Day	10% 3 7% 22% 37% 21%	2660
	21st to end	Day	10% 2 3 11% 32% 41%	2660
July	1st to 10th	Day	3 1 1 6% 16% 73%	4351
	11th to 20th	Day	7% 2 4 8% 17% 61%	4345
	21st to end	Day	5% 1 3 6% 27% 57%	4386
August	1st to 10th	Day	10% 2 3 7% 26% 53%	4025
	11th to 20th	Day	5% 1 2 6% 35% 51%	3760
	21st to end	Day	7% 1 3 7% 39% 42%	4179
September	1st to 10th	Day	6% 1 3 8% 33% 48%	2280
	11th to 20th	Day	12% 3 6% 20% 39% 20%	2267
	21st to end	Day	12% 2 4 18% 37% 27%	2090
October	1st to 10th	Day	14% 3 5% 10% 27% 42%	3185
	11th to 20th	Day	14% 3 4 18% 41% 21%	3161
	21st to end	Day	19% 4 5% 19% 37% 16%	3448
November	1st to 10th	Day	30% 3 9% 21% 21% 15%	1768
	11th to 20th	Day	29% 4 10% 19% 27% 12%	1800
	21st to end	Day	32% 4 8% 17% 26% 14%	1755
December	1st to 10th	Day	35% 5% 9% 18% 21% 12%	1528
	11th to 20th	Day	54% 4 7% 13% 14% 9%	1928
	21st to end	Day	38% 8% 14% 18% 15% 6%	2794
January	1st to 10th	Night	38% 5% 12% 24% 16% 5%	4406
	11th to 20th	Night	47% 6% 9% 16% 12% 10%	4229
	21st to end	Night	47% 8% 11% 16% 12% 7%	4521
February	1st to 10th	Night	39% 3 7% 18% 21% 12%	2468
	11th to 20th	Night	35% 4 9% 23% 19% 9%	2219
	21st to end	Night	38% 3 11% 22% 20% 6%	1884
March	1st to 10th	Night	50% 6% 10% 16% 13% 5%	3716
	11th to 20th	Night	30% 5% 9% 26% 19% 10%	3490
	21st to end	Night	46% 8% 11% 20% 11% 4	3619
April	1st to 10th	Night	34% 7% 13% 21% 20% 5%	1900
	11th to 20th	Night	29% 6% 13% 26% 18% 8%	1879
	21st to end	Night	24% 6% 12% 35% 18% 5%	1900
May	1st to 10th	Night	16% 4 11% 31% 26% 12%	3094
	11th to 20th	Night	10% 4 9% 27% 36% 13%	2594
	21st to end	Night	12% 4 9% 44% 27% 4	2762
June	1st to 10th	Night	17% 6% 13% 23% 28% 13%	1520
	11th to 20th	Night	6% 4 7% 25% 36% 22%	1520
	21st to end	Night	9% 2 2 13% 33% 41%	1520
July	1st to 10th	Night	3 1 1 5% 17% 73%	2500
	11th to 20th	Night	13% 2 5% 7% 15% 58%	2492
	21st to end	Night	8% 2 3 7% 24% 57%	2933
August	1st to 10th	Night	11% 2 2 8% 25% 51%	2892
	11th to 20th	Night	5% 2 3 7% 31% 53%	3156
	21st to end	Night	8% 2 10% 37% 43%	3498
September	1st to 10th	Night	5% 1 3 8% 34% 49%	1900
	11th to 20th	Night	11% 3 4 18% 41% 22%	1912
	21st to end	Night	10% 2 5% 20% 37% 27%	2090
October	1st to 10th	Night	12% 3 3 11% 27% 44%	3790
	11th to 20th	Night	11% 2 4 21% 39% 23%	3777
	21st to end	Night	19% 4 5% 18% 36% 19%	4121
November	1st to 10th	Night	25% 4 9% 21% 24% 17%	2530
	11th to 20th	Night	26% 4 8% 22% 27% 13%	2600
	21st to end	Night	31% 4 7% 18% 26% 15%	2642
December	1st to 10th	Night	34% 5% 8% 20% 19% 15%	2686
	11th to 20th	Night	50% 4 8% 12% 19% 7%	3276
	21st to end	Night	43% 7% 12% 15% 16% 6%	4862

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	32% 14% 24% 21% 9%	180
	11th to 20th	Day	23% 6% 26% 31% 11% 2	297
	21st to end	Day	22% 10% 27% 25% 14% 3	407
February	1st to 10th	Day	27% 14% 23% 24% 9% 3	370
	11th to 20th	Day	21% 19% 31% 20% 9% 1	455
	21st to end	Day	22% 19% 32% 20% 7% 1	382
March	1st to 10th	Day	29% 21% 28% 16% 6%	460
	11th to 20th	Day	25% 20% 25% 19% 8% 2	460
	21st to end	Day	24% 20% 33% 20% 4	506
April	1st to 10th	Day	21% 19% 33% 20% 7% 1	460
	11th to 20th	Day	20% 22% 31% 20% 6% 1	452
	21st to end	Day	12% 21% 31% 23% 11% 3	460
May	1st to 10th	Day	14% 22% 33% 22% 7% 2	463
	11th to 20th	Day	13% 18% 31% 23% 12% 3	488
	21st to end	Day	12% 16% 28% 29% 12% 5%	529
June	1st to 10th	Day	9% 13% 34% 27% 11% 5%	490
	11th to 20th	Day	5% 17% 40% 26% 7% 5%	490
	21st to end	Day	4 10% 39% 29% 12% 6%	490
July	1st to 10th	Day	2 9% 42% 30% 12% 5%	486
	11th to 20th	Day	1 4 39% 36% 14% 6%	484
	21st to end	Day	2 4 40% 35% 11% 7%	531
August	1st to 10th	Day	3 9% 42% 29% 14% 4	487
	11th to 20th	Day	2 9% 41% 33% 12% 5%	477
	21st to end	Day	2 5% 40% 35% 15% 3	539
September	1st to 10th	Day	3 7% 43% 31% 13% 1	490
	11th to 20th	Day	5% 13% 38% 32% 11% 2	490
	21st to end	Day	5% 11% 34% 39% 9% 2	490
October	1st to 10th	Day	4 14% 34% 33% 12% 3	490
	11th to 20th	Day	7% 11% 31% 33% 14% 3	490
	21st to end	Day	10% 11% 30% 32% 14% 3	539
November	1st to 10th	Day	14% 13% 23% 28% 15% 7%	304
	11th to 20th	Day	13% 10% 25% 32% 15% 6%	300
	21st to end	Day	17% 12% 23% 29% 16% 3	277
December	1st to 10th	Day	19% 8% 24% 26% 17% 6%	182
	11th to 20th	Day	18% 12% 29% 23% 15% 3	180
	21st to end	Day	23% 12% 28% 20% 13% 4	198
January	1st to 10th	Night	23% 5% 24% 25% 18% 6%	760
	11th to 20th	Night	18% 6% 22% 37% 14% 4	760
	21st to end	Night	21% 6% 24% 28% 17% 4	836
February	1st to 10th	Night	20% 6% 24% 29% 18% 4	760
	11th to 20th	Night	19% 8% 21% 30% 17% 6%	744
	21st to end	Night	22% 6% 23% 33% 13% 3	628
March	1st to 10th	Night	25% 8% 20% 30% 15% 3	760
	11th to 20th	Night	21% 8% 18% 28% 20% 5%	760
	21st to end	Night	22% 7% 22% 31% 17% 2	836
April	1st to 10th	Night	17% 5% 20% 38% 18% 2	760
	11th to 20th	Night	15% 7% 23% 34% 17% 4	751
	21st to end	Night	10% 4 24% 37% 21% 5%	760
May	1st to 10th	Night	10% 5% 24% 37% 21% 4	713
	11th to 20th	Night	7% 3 18% 41% 29% 3	426
	21st to end	Night	7% 3 15% 38% 34% 4	411
June	1st to 10th	Night	6% 2 12% 44% 31% 5%	380
	11th to 20th	Night	1 1 14% 46% 34% 3	380
	21st to end	Night	2 1 12% 44% 38% 3	380
July	1st to 10th	Night	1 6% 44% 45% 5%	370
	11th to 20th	Night	1 1 7% 41% 45% 5%	368
	21st to end	Night	2 1 9% 31% 46% 11%	569
August	1st to 10th	Night	3 10% 41% 38% 9%	594
	11th to 20th	Night	1 1 10% 46% 35% 7%	752
	21st to end	Night	2 1 7% 44% 42% 4	836
September	1st to 10th	Night	1 1 8% 46% 38% 6%	760
	11th to 20th	Night	3 1 12% 40% 35% 8%	760
	21st to end	Night	2 3 16% 43% 29% 8%	760
October	1st to 10th	Night	6% 3 16% 46% 24% 6%	760
	11th to 20th	Night	7% 3 17% 39% 29% 5%	760
	21st to end	Night	7% 4 19% 35% 24% 11%	836
November	1st to 10th	Night	14% 4 17% 35% 24% 7%	780
	11th to 20th	Night	11% 5% 25% 35% 19% 5%	800
	21st to end	Night	15% 5% 26% 32% 16% 5%	800
December	1st to 10th	Night	20% 5% 23% 33% 14% 5%	768
	11th to 20th	Night	25% 4 22% 29% 16% 3	760
	21st to end	Night	24% 7% 24% 28% 13% 2	836

Page generated at 2024-11-29 07:43:05. Version ac07

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