Tralfamadore Observatory
Clear Sky Chart
History

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

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	1 1 1 2 2 2 3 3 4 6% 76%	105192
	Night	1 1 1 2 2 2 2 2 4 6% 78%	103271
Transparency	Day	10% 4 6% 15% 29% 36%	104941
	Night	10% 3 6% 19% 31% 31%	103111
Seeing	Day	9% 19% 37% 26% 8% 2	15398
	Night	4 11% 28% 32% 20% 6%	25092

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	1 1 1 2 2 2 3 3 5% 9% 72%	2539
	11th to 20th	Day	1 1 1 2 2 3 4 4 7% 10% 65%	2604
	21st to end	Day	3 3 1 4 2 3 3 3 7% 10% 60%	3288
February	1st to 10th	Day	2 2 1 3 2 2 3 3 5% 6% 72%	1900
	11th to 20th	Day	1 2 1 3 2 2 3 3 6% 9% 68%	1850
	21st to end	Day	2 3 1 2 2 2 3 3 5% 7% 71%	1570
March	1st to 10th	Day	1 3 1 3 3 3 4 4 6% 8% 66%	3138
	11th to 20th	Day	1 3 1 4 2 3 3 4 6% 8% 66%	3259
	21st to end	Day	1 2 1 3 3 3 4 4 6% 9% 64%	4165
April	1st to 10th	Day	1 3 1 2 2 3 2 3 4 7% 72%	2280
	11th to 20th	Day	1 2 2 2 2 3 3 4 6% 75%	2249
	21st to end	Day	1 1 2 1 2 2 3 5% 8% 74%	2280
May	1st to 10th	Day	1 1 1 2 2 2 3 2 4 5% 79%	4039
	11th to 20th	Day	1 1 2 2 2 3 4 5% 7% 72%	4071
	21st to end	Day	1 2 2 2 2 2 2 3 5% 79%	4477
June	1st to 10th	Day	1 1 2 2 2 3 3 4 7% 74%	2651
	11th to 20th	Day	1 1 1 1 2 2 3 5% 83%	2660
	21st to end	Day	1 1 1 2 2 2 2 3 4 83%	2660
July	1st to 10th	Day	1 1 1 1 1 1 2 3 3 85%	4420
	11th to 20th	Day	1 1 1 2 2 3 4 3 5% 6% 73%	4420
	21st to end	Day	1 1 1 2 2 2 3 3 4 6% 75%	4533
August	1st to 10th	Day	2 1 1 2 1 2 2 2 2 3 81%	3783
	11th to 20th	Day	1 1 1 1 1 1 3 3 89%	3724
	21st to end	Day	1 1 1 1 1 1 2 2 89%	4180
September	1st to 10th	Day	1 1 1 2 2 2 3 3 4 4 78%	2280
	11th to 20th	Day	1 1 1 1 2 2 2 4 85%	2144
	21st to end	Day	1 1 1 1 2 2 2 2 4 85%	2090
October	1st to 10th	Day	1 1 1 1 1 2 2 3 6% 82%	3490
	11th to 20th	Day	1 1 1 1 1 2 2 3 4 84%	3224
	21st to end	Day	1 2 1 2 2 2 4 5% 81%	3273
November	1st to 10th	Day	1 1 2 1 1 1 2 4 7% 79%	1764
	11th to 20th	Day	1 1 1 1 1 2 2 4 6% 81%	1800
	21st to end	Day	1 2 2 2 2 3 3 5% 6% 74%	1800
December	1st to 10th	Day	1 1 1 2 1 2 2 3 6% 9% 71%	1755
	11th to 20th	Day	1 2 1 4 3 3 4 3 7% 9% 63%	2038
	21st to end	Day	5% 3 2 4 3 2 3 3 6% 10% 59%	2794
January	1st to 10th	Night	1 2 1 3 2 2 3 2 4 7% 73%	4406
	11th to 20th	Night	1 1 3 3 3 4 3 6% 11% 65%	4354
	21st to end	Night	2 4 1 4 3 3 4 3 6% 8% 61%	4367
February	1st to 10th	Night	1 2 1 3 2 3 3 4 5% 7% 68%	2280
	11th to 20th	Night	1 1 2 2 3 3 3 5% 7% 73%	2218
	21st to end	Night	1 2 1 2 2 2 3 3 5% 7% 71%	1884
March	1st to 10th	Night	1 2 1 2 2 2 3 3 5% 8% 71%	3736
	11th to 20th	Night	1 3 1 3 3 3 4 4 6% 10% 63%	3716
	21st to end	Night	2 2 2 2 3 3 6% 8% 70%	3488
April	1st to 10th	Night	1 1 1 2 3 2 3 3 4 7% 74%	1900
	11th to 20th	Night	1 1 2 2 1 2 2 4 7% 78%	1879
	21st to end	Night	1 1 2 2 1 2 2 3 5% 80%	1900
May	1st to 10th	Night	1 1 1 2 1 3 5% 85%	2929
	11th to 20th	Night	1 1 1 1 1 2 3 6% 84%	2834
	21st to end	Night	1 1 1 1 1 1 1 2 4 88%	3110
June	1st to 10th	Night	1 1 1 1 1 1 2 2 3 7% 80%	1524
	11th to 20th	Night	1 1 1 1 1 1 1 2 4 86%	1520
	21st to end	Night	1 1 1 2 2 2 3 87%	1520
July	1st to 10th	Night	1 1 1 1 1 2 2 91%	2540
	11th to 20th	Night	1 1 1 2 1 2 2 2 3 3 82%	2540
	21st to end	Night	1 2 2 1 2 1 3 3 85%	3022
August	1st to 10th	Night	1 1 1 2 1 2 2 2 2 4 84%	3166
	11th to 20th	Night	1 1 2 3 92%	3126
	21st to end	Night	1 1 1 1 1 2 92%	3498
September	1st to 10th	Night	1 1 1 1 1 1 3 3 87%	1900
	11th to 20th	Night	1 1 2 2 1 2 3 87%	2034
	21st to end	Night	1 1 1 1 2 2 4 88%	2090
October	1st to 10th	Night	1 1 1 1 1 1 3 3 88%	3490
	11th to 20th	Night	1 1 1 1 1 1 1 2 4 88%	3710
	21st to end	Night	1 1 1 1 1 1 2 4 5% 82%	4289
November	1st to 10th	Night	1 1 1 1 1 1 1 2 3 5% 82%	2535
	11th to 20th	Night	1 1 1 1 2 2 2 3 8% 79%	2600
	21st to end	Night	1 1 1 3 2 2 2 2 5% 7% 75%	2600
December	1st to 10th	Night	1 2 1 3 2 3 3 3 5% 8% 69%	2541
	11th to 20th	Night	1 2 1 3 3 4 4 4 6% 13% 60%	3163
	21st to end	Night	6% 3 1 3 2 3 4 4 6% 8% 59%	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	11% 5% 10% 28% 35% 11%	2539
	11th to 20th	Day	15% 8% 16% 25% 25% 12%	2604
	21st to end	Day	20% 6% 12% 26% 27% 10%	3288
February	1st to 10th	Day	14% 5% 8% 24% 29% 20%	1900
	11th to 20th	Day	13% 4 8% 24% 31% 20%	1860
	21st to end	Day	14% 5% 11% 24% 31% 14%	1570
March	1st to 10th	Day	15% 6% 7% 24% 30% 18%	3138
	11th to 20th	Day	15% 6% 9% 29% 29% 13%	3259
	21st to end	Day	15% 7% 10% 26% 28% 14%	4165
April	1st to 10th	Day	14% 4 9% 24% 31% 18%	2280
	11th to 20th	Day	11% 5% 6% 22% 35% 22%	2252
	21st to end	Day	8% 4 8% 23% 40% 17%	2280
May	1st to 10th	Day	7% 5% 4 16% 41% 26%	4039
	11th to 20th	Day	8% 5% 5% 12% 43% 26%	4074
	21st to end	Day	8% 3 3 12% 51% 22%	4477
June	1st to 10th	Day	9% 5% 6% 13% 32% 36%	2651
	11th to 20th	Day	6% 2 3 11% 24% 53%	2660
	21st to end	Day	7% 2 2 5% 20% 64%	2660
July	1st to 10th	Day	6% 2 2 3 7% 80%	4351
	11th to 20th	Day	10% 4 3 7% 15% 62%	4345
	21st to end	Day	10% 4 4 15% 17% 50%	4389
August	1st to 10th	Day	10% 2 2 6% 24% 56%	3770
	11th to 20th	Day	4 2 2 4 16% 73%	3760
	21st to end	Day	4 1 2 4 18% 71%	4179
September	1st to 10th	Day	9% 4 3 7% 19% 57%	2280
	11th to 20th	Day	5% 3 3 8% 30% 51%	2144
	21st to end	Day	6% 2 2 10% 35% 45%	2090
October	1st to 10th	Day	7% 2 2 8% 28% 53%	3490
	11th to 20th	Day	7% 1 1 8% 37% 46%	3224
	21st to end	Day	8% 2 3 10% 42% 35%	3273
November	1st to 10th	Day	8% 4 6% 17% 39% 26%	1764
	11th to 20th	Day	7% 3 6% 23% 37% 23%	1800
	21st to end	Day	12% 3 8% 21% 31% 25%	1800
December	1st to 10th	Day	12% 6% 11% 28% 24% 20%	1755
	11th to 20th	Day	18% 9% 11% 23% 25% 13%	2037
	21st to end	Day	22% 6% 11% 28% 22% 10%	2794
January	1st to 10th	Night	13% 4 9% 26% 35% 13%	4406
	11th to 20th	Night	14% 6% 13% 35% 22% 10%	4354
	21st to end	Night	20% 5% 15% 26% 25% 8%	4367
February	1st to 10th	Night	17% 4 10% 25% 28% 17%	2280
	11th to 20th	Night	12% 4 8% 24% 31% 21%	2219
	21st to end	Night	12% 5% 9% 26% 32% 16%	1884
March	1st to 10th	Night	12% 5% 7% 26% 30% 21%	3736
	11th to 20th	Night	15% 6% 11% 25% 31% 10%	3716
	21st to end	Night	12% 4 9% 33% 34% 9%	3488
April	1st to 10th	Night	13% 3 9% 24% 34% 16%	1900
	11th to 20th	Night	10% 2 8% 26% 33% 21%	1879
	21st to end	Night	9% 3 6% 27% 41% 16%	1900
May	1st to 10th	Night	5% 2 5% 17% 48% 23%	2929
	11th to 20th	Night	5% 2 4 19% 46% 23%	2835
	21st to end	Night	5% 1 4 18% 52% 20%	3110
June	1st to 10th	Night	7% 3 5% 16% 36% 33%	1524
	11th to 20th	Night	6% 2 3 14% 26% 49%	1520
	21st to end	Night	5% 2 2 5% 24% 62%	1520
July	1st to 10th	Night	3 1 1 4 10% 81%	2500
	11th to 20th	Night	8% 2 3 7% 16% 64%	2492
	21st to end	Night	7% 2 2 16% 21% 52%	2928
August	1st to 10th	Night	8% 2 3 7% 31% 49%	3156
	11th to 20th	Night	2 1 2 5% 18% 72%	3156
	21st to end	Night	3 1 1 4 21% 70%	3498
September	1st to 10th	Night	5% 2 2 7% 24% 60%	1900
	11th to 20th	Night	5% 2 3 9% 30% 50%	2034
	21st to end	Night	5% 1 2 9% 34% 49%	2090
October	1st to 10th	Night	5% 2 2 9% 25% 58%	3490
	11th to 20th	Night	5% 1 2 8% 37% 47%	3710
	21st to end	Night	8% 2 2 9% 43% 35%	4289
November	1st to 10th	Night	8% 2 5% 17% 41% 26%	2535
	11th to 20th	Night	8% 3 9% 20% 35% 25%	2600
	21st to end	Night	12% 3 7% 21% 35% 23%	2600
December	1st to 10th	Night	14% 5% 10% 25% 27% 18%	2541
	11th to 20th	Night	18% 10% 12% 25% 23% 12%	3163
	21st to end	Night	23% 6% 10% 29% 22% 10%	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	15% 21% 37% 19% 7% 1	180
	11th to 20th	Day	14% 26% 28% 22% 9% 1	218
	21st to end	Day	13% 23% 32% 23% 8% 2	449
February	1st to 10th	Day	12% 29% 33% 18% 7% 2	460
	11th to 20th	Day	15% 24% 37% 18% 5% 1	455
	21st to end	Day	15% 27% 43% 10% 4	382
March	1st to 10th	Day	18% 28% 33% 16% 4 1	460
	11th to 20th	Day	15% 29% 35% 16% 5% 1	460
	21st to end	Day	18% 29% 39% 11% 2	506
April	1st to 10th	Day	15% 26% 37% 16% 5% 2	460
	11th to 20th	Day	13% 29% 38% 15% 4	452
	21st to end	Day	9% 26% 36% 20% 6% 2	460
May	1st to 10th	Day	11% 28% 37% 17% 6% 1	463
	11th to 20th	Day	7% 22% 35% 27% 7% 2	488
	21st to end	Day	7% 22% 34% 27% 7% 2	529
June	1st to 10th	Day	5% 15% 36% 32% 10% 3	490
	11th to 20th	Day	4 16% 41% 27% 9% 3	490
	21st to end	Day	4 14% 38% 30% 10% 3	490
July	1st to 10th	Day	3 7% 40% 33% 14% 4	486
	11th to 20th	Day	2 4 33% 38% 17% 6%	484
	21st to end	Day	3 5% 34% 37% 14% 6%	531
August	1st to 10th	Day	3 13% 37% 31% 11% 5%	487
	11th to 20th	Day	4 9% 36% 34% 13% 4	477
	21st to end	Day	2 6% 41% 37% 9% 4	539
September	1st to 10th	Day	4 10% 39% 33% 13% 1	490
	11th to 20th	Day	7% 15% 37% 33% 7% 1	490
	21st to end	Day	4 14% 43% 30% 8% 1	490
October	1st to 10th	Day	6% 18% 38% 30% 7% 1	490
	11th to 20th	Day	7% 16% 39% 29% 7% 1	490
	21st to end	Day	8% 19% 35% 27% 10% 1	439
November	1st to 10th	Day	11% 16% 32% 29% 10% 1	300
	11th to 20th	Day	12% 19% 35% 20% 12% 2	300
	21st to end	Day	9% 21% 36% 25% 8% 2	300
December	1st to 10th	Day	8% 18% 38% 26% 9% 1	285
	11th to 20th	Day	9% 24% 35% 20% 10% 2	230
	21st to end	Day	18% 25% 30% 19% 6% 2	198
January	1st to 10th	Night	5% 14% 28% 26% 18% 8%	760
	11th to 20th	Night	4 15% 33% 29% 15% 5%	760
	21st to end	Night	8% 14% 31% 27% 15% 5%	836
February	1st to 10th	Night	5% 15% 33% 25% 17% 6%	760
	11th to 20th	Night	6% 13% 33% 27% 15% 5%	744
	21st to end	Night	7% 18% 34% 26% 11% 3	628
March	1st to 10th	Night	9% 16% 34% 26% 12% 3	760
	11th to 20th	Night	6% 17% 31% 22% 17% 6%	760
	21st to end	Night	7% 19% 35% 25% 11% 3	836
April	1st to 10th	Night	6% 19% 30% 29% 12% 4	760
	11th to 20th	Night	5% 18% 34% 25% 15% 3	751
	21st to end	Night	3 16% 35% 26% 15% 4	760
May	1st to 10th	Night	5% 14% 36% 29% 13% 2	613
	11th to 20th	Night	2 13% 31% 35% 17% 3	561
	21st to end	Night	4 13% 28% 37% 16% 2	617
June	1st to 10th	Night	2 5% 29% 41% 21% 2	384
	11th to 20th	Night	1 8% 32% 37% 18% 3	380
	21st to end	Night	1 4 28% 43% 20% 3	380
July	1st to 10th	Night	1 1 16% 49% 27% 6%	370
	11th to 20th	Night	2 15% 47% 31% 5%	368
	21st to end	Night	2 1 13% 43% 34% 7%	566
August	1st to 10th	Night	2 2 19% 42% 29% 6%	756
	11th to 20th	Night	1 2 18% 40% 31% 9%	752
	21st to end	Night	2 2 17% 41% 31% 8%	836
September	1st to 10th	Night	1 4 16% 35% 36% 8%	760
	11th to 20th	Night	1 6% 22% 36% 28% 7%	760
	21st to end	Night	1 7% 23% 33% 26% 10%	760
October	1st to 10th	Night	7% 29% 40% 17% 6%	760
	11th to 20th	Night	4 8% 23% 34% 21% 10%	760
	21st to end	Night	4 11% 24% 29% 22% 10%	836
November	1st to 10th	Night	5% 10% 24% 30% 22% 9%	780
	11th to 20th	Night	2 10% 30% 32% 20% 7%	800
	21st to end	Night	4 15% 29% 30% 17% 5%	800
December	1st to 10th	Night	5% 12% 32% 29% 16% 5%	782
	11th to 20th	Night	4 13% 34% 31% 15% 4	760
	21st to end	Night	6% 17% 32% 25% 15% 4	836

Page generated at 2024-12-03 03:45:08. Version ac07

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