Big Bird Observatory
Clear Sky Chart
History

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

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	10% 4 1 3 2 2 2 2 4 5% 65%	105274
	Night	13% 4 1 3 2 2 3 2 4 5% 60%	102628
Transparency	Day	24% 3 7% 18% 24% 25%	105027
	Night	28% 4 7% 17% 21% 23%	102464
Seeing	Day	12% 8% 22% 32% 22% 5%	15141
	Night	14% 4 15% 35% 29% 4	24369

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	30% 7% 1 5% 3 3 3 2 3 5% 37%	2539
	11th to 20th	Day	30% 8% 3 7% 4 4 2 2 2 5% 34%	2764
	21st to end	Day	30% 7% 3 5% 2 3 3 2 5% 7% 33%	3146
February	1st to 10th	Day	21% 5% 1 5% 2 3 4 3 4 5% 46%	1710
	11th to 20th	Day	18% 8% 2 4 3 3 3 2 4 6% 46%	1741
	21st to end	Day	20% 5% 1 3 3 3 3 3 5% 6% 47%	1570
March	1st to 10th	Day	28% 12% 3 6% 4 3 3 3 3 4 31%	3279
	11th to 20th	Day	18% 4 1 3 2 2 2 2 3 6% 57%	3490
	21st to end	Day	22% 10% 3 5% 4 4 3 3 6% 7% 34%	3943
April	1st to 10th	Day	16% 6% 2 5% 4 4 3 3 5% 8% 45%	2280
	11th to 20th	Day	14% 5% 2 4 2 2 3 4 5% 6% 54%	2249
	21st to end	Day	7% 4 1 4 3 3 3 4 5% 7% 60%	2280
May	1st to 10th	Day	8% 3 1 2 2 2 2 2 3 5% 70%	4005
	11th to 20th	Day	4 2 1 2 2 2 2 2 3 4 78%	4377
	21st to end	Day	3 2 1 3 2 2 2 3 4 5% 74%	4813
June	1st to 10th	Day	6% 3 1 3 3 3 3 4 5% 8% 62%	2660
	11th to 20th	Day	2 1 2 2 2 2 2 4 5% 78%	2660
	21st to end	Day	2 1 1 1 1 2 2 2 3 85%	2660
July	1st to 10th	Day	1 1 1 1 2 93%	4420
	11th to 20th	Day	1 1 1 1 1 1 1 1 2 3 86%	4420
	21st to end	Day	1 1 1 1 1 1 2 2 90%	4491
August	1st to 10th	Day	2 2 1 2 2 2 2 3 4 5% 76%	4091
	11th to 20th	Day	1 1 1 1 1 1 2 2 90%	3853
	21st to end	Day	1 1 1 1 1 1 1 2 3 89%	4180
September	1st to 10th	Day	1 1 1 1 1 1 1 2 4 87%	2280
	11th to 20th	Day	3 2 1 3 2 2 2 2 3 5% 76%	2280
	21st to end	Day	4 2 1 2 1 1 2 1 3 5% 79%	2034
October	1st to 10th	Day	3 2 1 3 2 2 2 2 3 5% 76%	3180
	11th to 20th	Day	8% 2 1 3 2 2 2 2 4 6% 69%	3161
	21st to end	Day	7% 3 1 4 3 3 4 3 6% 7% 57%	3448
November	1st to 10th	Day	15% 4 2 4 3 2 3 3 5% 7% 52%	1807
	11th to 20th	Day	16% 4 1 4 3 4 3 4 5% 7% 49%	1701
	21st to end	Day	15% 4 2 5% 3 3 3 2 5% 5% 54%	1520
December	1st to 10th	Day	21% 5% 1 4 2 3 3 3 4 6% 50%	1520
	11th to 20th	Day	33% 7% 3 5% 3 3 3 2 3 6% 31%	1928
	21st to end	Day	23% 7% 2 5% 4 4 4 3 5% 7% 37%	2794
January	1st to 10th	Night	20% 7% 2 5% 4 3 3 3 5% 7% 40%	4406
	11th to 20th	Night	28% 6% 2 5% 3 3 3 3 4 6% 36%	4199
	21st to end	Night	30% 8% 2 7% 4 3 3 3 4 5% 31%	4521
February	1st to 10th	Night	20% 5% 2 4 3 2 3 2 5% 6% 49%	2470
	11th to 20th	Night	18% 5% 2 5% 3 4 3 2 5% 6% 47%	2324
	21st to end	Night	19% 6% 1 4 2 3 4 2 4 5% 48%	1884
March	1st to 10th	Night	29% 12% 3 5% 4 2 3 3 4 4 32%	3591
	11th to 20th	Night	20% 5% 1 3 2 2 2 2 4 5% 56%	3490
	21st to end	Night	23% 10% 3 5% 3 4 4 4 5% 5% 35%	3710
April	1st to 10th	Night	12% 7% 2 6% 3 4 3 4 5% 7% 48%	1900
	11th to 20th	Night	12% 5% 1 4 3 3 2 3 4 7% 55%	1879
	21st to end	Night	8% 4 1 3 3 2 3 2 4 6% 64%	1900
May	1st to 10th	Night	6% 4 1 4 2 3 2 3 5% 5% 65%	2964
	11th to 20th	Night	3 1 1 2 2 2 3 2 3 5% 75%	2516
	21st to end	Night	3 2 1 2 1 2 2 2 3 5% 75%	2762
June	1st to 10th	Night	5% 3 1 4 3 3 4 3 5% 7% 62%	1520
	11th to 20th	Night	2 1 1 2 2 2 3 2 3 6% 77%	1520
	21st to end	Night	2 2 1 2 2 2 2 2 3 3 81%	1520
July	1st to 10th	Night	1 1 1 1 1 1 1 93%	2540
	11th to 20th	Night	1 1 1 2 1 2 2 2 2 2 84%	2540
	21st to end	Night	1 1 1 1 1 1 2 3 87%	3067
August	1st to 10th	Night	3 2 1 3 1 2 3 3 3 4 74%	2848
	11th to 20th	Night	1 1 1 1 2 1 2 2 90%	2990
	21st to end	Night	3 2 1 1 1 1 1 2 2 87%	3498
September	1st to 10th	Night	1 1 1 1 1 1 1 3 88%	1900
	11th to 20th	Night	4 2 1 2 2 1 2 2 4 5% 76%	1900
	21st to end	Night	3 1 2 1 1 2 2 3 5% 80%	2142
October	1st to 10th	Night	3 2 1 3 2 2 2 2 2 3 79%	3800
	11th to 20th	Night	5% 2 1 2 1 2 2 2 3 4 75%	3777
	21st to end	Night	10% 2 1 3 2 3 3 3 5% 7% 62%	4121
November	1st to 10th	Night	16% 4 1 3 2 3 3 3 6% 6% 54%	2490
	11th to 20th	Night	14% 4 1 4 3 3 3 3 6% 7% 53%	2481
	21st to end	Night	14% 4 1 4 3 3 3 3 4 6% 55%	2660
December	1st to 10th	Night	22% 4 1 3 2 2 3 2 5% 7% 49%	2660
	11th to 20th	Night	35% 5% 1 4 3 3 3 3 4 7% 32%	3276
	21st to end	Night	24% 6% 2 7% 4 4 3 3 5% 5% 36%	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	51% 3 8% 26% 9% 3	2539
	11th to 20th	Day	56% 4 6% 15% 11% 9%	2764
	21st to end	Day	52% 5% 10% 15% 13% 5%	3146
February	1st to 10th	Day	40% 4 9% 18% 18% 10%	1710
	11th to 20th	Day	40% 3 9% 19% 22% 8%	1750
	21st to end	Day	37% 5% 11% 23% 17% 7%	1570
March	1st to 10th	Day	58% 5% 7% 16% 11% 4	3279
	11th to 20th	Day	32% 3 8% 28% 17% 11%	3490
	21st to end	Day	47% 7% 12% 21% 10% 3	3943
April	1st to 10th	Day	39% 4 11% 23% 18% 5%	2280
	11th to 20th	Day	31% 5% 12% 22% 22% 8%	2252
	21st to end	Day	23% 5% 14% 28% 24% 5%	2280
May	1st to 10th	Day	19% 2 9% 27% 24% 19%	4005
	11th to 20th	Day	12% 2 6% 22% 39% 18%	4380
	21st to end	Day	13% 4 11% 34% 28% 10%	4814
June	1st to 10th	Day	20% 6% 10% 25% 25% 13%	2660
	11th to 20th	Day	10% 3 9% 19% 34% 25%	2660
	21st to end	Day	8% 2 3 15% 34% 39%	2660
July	1st to 10th	Day	3 1 2 6% 15% 73%	4351
	11th to 20th	Day	7% 1 2 7% 18% 64%	4345
	21st to end	Day	4 1 1 5% 29% 60%	4349
August	1st to 10th	Day	11% 2 3 8% 25% 51%	4076
	11th to 20th	Day	4 1 1 8% 42% 43%	3892
	21st to end	Day	6% 1 2 11% 38% 41%	4179
September	1st to 10th	Day	5% 1 2 8% 27% 57%	2280
	11th to 20th	Day	13% 2 6% 21% 35% 24%	2280
	21st to end	Day	11% 2 5% 18% 30% 34%	2034
October	1st to 10th	Day	13% 2 4 12% 25% 45%	3180
	11th to 20th	Day	18% 3 4 21% 31% 23%	3161
	21st to end	Day	24% 5% 6% 20% 28% 17%	3448
November	1st to 10th	Day	31% 4 9% 22% 19% 15%	1807
	11th to 20th	Day	34% 5% 12% 20% 19% 10%	1701
	21st to end	Day	33% 3 8% 19% 22% 14%	1520
December	1st to 10th	Day	38% 4 7% 18% 21% 12%	1520
	11th to 20th	Day	57% 3 6% 14% 15% 5%	1928
	21st to end	Day	48% 5% 14% 17% 12% 4	2794
January	1st to 10th	Night	42% 7% 14% 23% 11% 4	4406
	11th to 20th	Night	50% 6% 11% 10% 14% 9%	4199
	21st to end	Night	55% 5% 10% 11% 12% 6%	4521
February	1st to 10th	Night	38% 3 8% 20% 19% 13%	2470
	11th to 20th	Night	39% 3 9% 18% 22% 9%	2326
	21st to end	Night	38% 4 9% 22% 19% 9%	1884
March	1st to 10th	Night	55% 5% 9% 13% 12% 5%	3591
	11th to 20th	Night	33% 3 7% 23% 22% 12%	3490
	21st to end	Night	49% 6% 11% 20% 10% 3	3710
April	1st to 10th	Night	35% 6% 11% 20% 19% 8%	1900
	11th to 20th	Night	29% 5% 12% 22% 20% 11%	1879
	21st to end	Night	22% 5% 13% 28% 26% 6%	1900
May	1st to 10th	Night	20% 5% 11% 16% 30% 18%	2964
	11th to 20th	Night	13% 4 7% 19% 35% 22%	2516
	21st to end	Night	14% 3 11% 34% 26% 12%	2762
June	1st to 10th	Night	21% 6% 11% 26% 25% 11%	1520
	11th to 20th	Night	11% 4 8% 18% 31% 29%	1520
	21st to end	Night	11% 2 3 15% 29% 41%	1520
July	1st to 10th	Night	3 1 1 7% 14% 74%	2500
	11th to 20th	Night	9% 1 2 7% 16% 65%	2492
	21st to end	Night	6% 1 1 6% 25% 60%	2971
August	1st to 10th	Night	15% 2 2 8% 23% 50%	2839
	11th to 20th	Night	4 1 1 9% 35% 50%	3017
	21st to end	Night	7% 1 2 11% 34% 44%	3498
September	1st to 10th	Night	6% 1 2 7% 27% 57%	1900
	11th to 20th	Night	13% 3 4 17% 38% 25%	1900
	21st to end	Night	10% 2 5% 16% 31% 37%	2142
October	1st to 10th	Night	13% 2 3 11% 18% 53%	3800
	11th to 20th	Night	14% 3 6% 24% 29% 24%	3777
	21st to end	Night	23% 3 6% 19% 29% 21%	4121
November	1st to 10th	Night	31% 4 9% 23% 17% 17%	2490
	11th to 20th	Night	30% 5% 12% 23% 19% 11%	2481
	21st to end	Night	32% 3 7% 21% 23% 14%	2660
December	1st to 10th	Night	37% 5% 8% 18% 17% 14%	2660
	11th to 20th	Night	53% 3 7% 13% 17% 7%	3276
	21st to end	Night	50% 6% 10% 18% 11% 5%	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	47% 2 20% 17% 13% 1	180
	11th to 20th	Day	28% 7% 20% 28% 13% 4	313
	21st to end	Day	24% 6% 21% 28% 18% 4	407
February	1st to 10th	Day	25% 9% 20% 28% 17% 1	370
	11th to 20th	Day	23% 12% 27% 22% 14% 1	401
	21st to end	Day	18% 14% 25% 31% 10% 1	382
March	1st to 10th	Day	30% 18% 25% 19% 8% 1	460
	11th to 20th	Day	27% 11% 24% 19% 16% 2	460
	21st to end	Day	24% 13% 30% 23% 9%	506
April	1st to 10th	Day	21% 13% 28% 23% 13% 2	460
	11th to 20th	Day	18% 15% 26% 26% 13% 3	452
	21st to end	Day	7% 14% 26% 32% 17% 4	460
May	1st to 10th	Day	9% 14% 32% 28% 15% 3	463
	11th to 20th	Day	10% 11% 24% 28% 21% 7%	488
	21st to end	Day	5% 9% 24% 29% 26% 6%	529
June	1st to 10th	Day	5% 12% 21% 30% 26% 7%	490
	11th to 20th	Day	4 8% 28% 37% 17% 5%	490
	21st to end	Day	1 5% 21% 40% 25% 7%	490
July	1st to 10th	Day	2 1 19% 46% 25% 7%	486
	11th to 20th	Day	1 1 17% 44% 31% 7%	484
	21st to end	Day	1 1 15% 47% 30% 7%	531
August	1st to 10th	Day	2 2 17% 44% 27% 8%	487
	11th to 20th	Day	1 3 20% 37% 30% 9%	477
	21st to end	Day	2 2 17% 41% 32% 6%	539
September	1st to 10th	Day	1 4 17% 46% 29% 4	490
	11th to 20th	Day	5% 6% 21% 32% 31% 4	490
	21st to end	Day	2 7% 23% 37% 28% 3	490
October	1st to 10th	Day	5% 8% 24% 29% 29% 5%	490
	11th to 20th	Day	8% 7% 19% 30% 30% 6%	490
	21st to end	Day	12% 7% 20% 32% 22% 6%	539
November	1st to 10th	Day	17% 6% 20% 29% 22% 6%	343
	11th to 20th	Day	15% 4 16% 33% 20% 12%	266
	21st to end	Day	13% 7% 23% 31% 21% 6%	180
December	1st to 10th	Day	30% 4 20% 23% 13% 9%	180
	11th to 20th	Day	27% 5% 19% 26% 13% 9%	180
	21st to end	Day	23% 11% 21% 23% 19% 4	198
January	1st to 10th	Night	26% 4 17% 35% 16% 3	760
	11th to 20th	Night	24% 4 16% 38% 16% 2	760
	21st to end	Night	21% 3 19% 34% 20% 3	836
February	1st to 10th	Night	21% 4 17% 36% 20% 2	760
	11th to 20th	Night	20% 6% 15% 34% 22% 2	744
	21st to end	Night	21% 6% 19% 39% 13% 2	628
March	1st to 10th	Night	28% 8% 16% 28% 18% 3	760
	11th to 20th	Night	23% 5% 15% 29% 25% 2	760
	21st to end	Night	23% 7% 23% 29% 17% 1	836
April	1st to 10th	Night	16% 6% 21% 35% 20% 2	760
	11th to 20th	Night	14% 7% 18% 35% 21% 5%	751
	21st to end	Night	10% 5% 19% 36% 25% 4	760
May	1st to 10th	Night	9% 6% 27% 30% 25% 3	635
	11th to 20th	Night	7% 2 19% 37% 30% 4	374
	21st to end	Night	6% 5% 18% 31% 37% 3	411
June	1st to 10th	Night	6% 3 17% 34% 36% 4	380
	11th to 20th	Night	2 2 20% 37% 35% 4	380
	21st to end	Night	3 2 12% 39% 39% 6%	380
July	1st to 10th	Night	1 1 7% 35% 53% 4	370
	11th to 20th	Night	1 8% 41% 47% 4	368
	21st to end	Night	5% 33% 55% 7%	595
August	1st to 10th	Night	3 1 5% 34% 50% 6%	567
	11th to 20th	Night	1 1 6% 39% 45% 9%	664
	21st to end	Night	1 1 6% 34% 53% 6%	836
September	1st to 10th	Night	1 1 9% 37% 46% 6%	760
	11th to 20th	Night	4 1 10% 34% 41% 10%	760
	21st to end	Night	3 3 14% 36% 35% 10%	760
October	1st to 10th	Night	6% 4 14% 37% 32% 7%	760
	11th to 20th	Night	7% 4 13% 35% 34% 6%	760
	21st to end	Night	11% 4 14% 34% 30% 7%	836
November	1st to 10th	Night	17% 4 15% 30% 28% 5%	780
	11th to 20th	Night	15% 3 15% 40% 23% 3	762
	21st to end	Night	15% 3 18% 37% 24% 4	760
December	1st to 10th	Night	24% 4 15% 34% 21% 3	760
	11th to 20th	Night	29% 4 13% 34% 18% 2	760
	21st to end	Night	26% 4 20% 34% 15% 1	836

Page generated at 2024-11-08 03:44:45. Version ac07

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