Bailyhill Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Bailyhill Observatory clear sky chart from 2005-11-06 to 2024-11-10.

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% 9% 3 8% 5% 5% 5% 4 6% 7% 31%	101394
	Night	17% 10% 3 8% 5% 5% 5% 4 5% 6% 31%	97753
Transparency	Day	48% 7% 13% 20% 11% 1	104906
	Night	48% 8% 14% 20% 10% 1	102330
Seeing	Day	24% 19% 28% 18% 8% 2	14866
	Night	23% 16% 26% 26% 9% 1	23760

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	13% 12% 4 11% 6% 5% 6% 4 6% 6% 27%	2517
	11th to 20th	Day	15% 12% 5% 11% 11% 7% 5% 3 5% 4 22%	2520
	21st to end	Day	16% 13% 8% 12% 5% 5% 4 4 5% 5% 24%	2772
February	1st to 10th	Day	15% 13% 4 9% 6% 6% 5% 4 5% 6% 26%	1677
	11th to 20th	Day	11% 12% 4 13% 6% 5% 5% 4 5% 6% 28%	1848
	21st to end	Day	15% 12% 3 10% 6% 5% 4 4 5% 6% 30%	1570
March	1st to 10th	Day	19% 17% 4 10% 5% 5% 4 3 4 4 25%	3116
	11th to 20th	Day	15% 13% 6% 10% 6% 5% 5% 4 5% 6% 27%	3286
	21st to end	Day	18% 11% 4 10% 5% 4 4 3 4 7% 30%	4097
April	1st to 10th	Day	19% 12% 4 8% 5% 4 4 3 4 6% 30%	2280
	11th to 20th	Day	17% 10% 3 7% 6% 5% 5% 3 5% 6% 34%	2267
	21st to end	Day	21% 10% 3 8% 6% 4 4 4 5% 6% 28%	2470
May	1st to 10th	Day	25% 13% 4 8% 5% 4 4 3 5% 5% 25%	4106
	11th to 20th	Day	18% 11% 3 8% 5% 5% 6% 4 6% 7% 29%	4332
	21st to end	Day	13% 6% 2 6% 5% 5% 5% 4 7% 8% 38%	4837
June	1st to 10th	Day	19% 9% 3 7% 6% 5% 6% 5% 6% 7% 28%	2660
	11th to 20th	Day	17% 8% 3 6% 5% 5% 5% 4 7% 7% 34%	2660
	21st to end	Day	20% 8% 3 9% 6% 5% 5% 4 6% 6% 28%	2660
July	1st to 10th	Day	13% 6% 2 6% 5% 5% 6% 5% 8% 11% 33%	4440
	11th to 20th	Day	11% 5% 2 7% 5% 6% 6% 5% 7% 10% 34%	4440
	21st to end	Day	19% 7% 2 6% 4 5% 5% 5% 6% 8% 34%	4814
August	1st to 10th	Day	15% 7% 3 8% 6% 6% 5% 5% 6% 8% 31%	4297
	11th to 20th	Day	16% 9% 3 8% 5% 5% 5% 5% 7% 8% 28%	3993
	21st to end	Day	10% 5% 3 7% 5% 5% 5% 5% 7% 9% 40%	4180
September	1st to 10th	Day	12% 5% 2 5% 5% 5% 5% 4 6% 7% 43%	2280
	11th to 20th	Day	14% 6% 2 5% 3 4 3 3 5% 5% 49%	2119
	21st to end	Day	16% 6% 2 6% 4 4 4 3 6% 7% 41%	2090
October	1st to 10th	Day	22% 9% 2 7% 5% 4 4 3 4 6% 33%	2090
	11th to 20th	Day	15% 10% 3 8% 6% 4 5% 3 4 6% 38%	1805
	21st to end	Day	25% 12% 3 7% 4 4 4 3 4 5% 28%	1881
November	1st to 10th	Day	12% 8% 4 9% 6% 3 5% 3 5% 7% 37%	1752
	11th to 20th	Day	18% 9% 3 8% 5% 4 5% 4 6% 6% 31%	1710
	21st to end	Day	20% 10% 3 9% 6% 4 6% 4 4 6% 29%	1630
December	1st to 10th	Day	21% 13% 4 11% 7% 6% 4 3 4 6% 22%	1520
	11th to 20th	Day	21% 15% 3 8% 3 4 4 3 4 6% 30%	1906
	21st to end	Day	24% 13% 4 11% 6% 5% 4 3 3 5% 22%	2772
January	1st to 10th	Night	15% 10% 4 10% 6% 5% 5% 4 5% 7% 29%	4429
	11th to 20th	Night	12% 14% 5% 14% 9% 5% 5% 4 5% 6% 22%	4440
	21st to end	Night	18% 15% 5% 11% 6% 6% 4 3 5% 6% 22%	4884
February	1st to 10th	Night	15% 11% 5% 9% 7% 5% 5% 5% 6% 7% 26%	2496
	11th to 20th	Night	11% 10% 4 10% 7% 6% 6% 4 7% 7% 26%	2218
	21st to end	Night	12% 12% 3 11% 6% 5% 6% 4 6% 6% 30%	1884
March	1st to 10th	Night	22% 15% 3 7% 5% 4 5% 4 4 6% 25%	3724
	11th to 20th	Night	13% 13% 5% 10% 7% 6% 4 3 5% 5% 28%	3672
	21st to end	Night	20% 14% 4 8% 4 4 3 3 5% 6% 30%	3532
April	1st to 10th	Night	19% 10% 3 8% 5% 5% 4 3 4 5% 32%	1900
	11th to 20th	Night	20% 11% 2 8% 4 4 4 3 4 6% 33%	1861
	21st to end	Night	19% 9% 4 8% 5% 4 3 3 5% 6% 34%	1710
May	1st to 10th	Night	28% 11% 3 7% 4 4 4 4 4 6% 23%	2834
	11th to 20th	Night	20% 11% 4 8% 6% 6% 6% 4 5% 5% 27%	2576
	21st to end	Night	14% 7% 2 6% 4 5% 5% 4 6% 9% 39%	2738
June	1st to 10th	Night	21% 8% 3 8% 5% 5% 4 4 7% 8% 30%	1520
	11th to 20th	Night	18% 8% 3 6% 4 4 4 4 6% 6% 37%	1520
	21st to end	Night	17% 8% 2 7% 5% 4 4 4 7% 7% 34%	1520
July	1st to 10th	Night	12% 6% 2 7% 5% 6% 7% 5% 7% 8% 35%	2520
	11th to 20th	Night	16% 6% 3 7% 4 4 5% 3 6% 7% 40%	2520
	21st to end	Night	17% 7% 2 6% 5% 4 4 3 6% 7% 38%	2739
August	1st to 10th	Night	20% 9% 3 8% 4 4 4 3 4 6% 34%	2601
	11th to 20th	Night	16% 9% 3 7% 5% 4 4 4 6% 7% 34%	2831
	21st to end	Night	10% 5% 2 6% 6% 4 5% 4 7% 10% 42%	3476
September	1st to 10th	Night	13% 6% 2 6% 4 3 3 3 5% 7% 47%	1900
	11th to 20th	Night	15% 7% 2 5% 4 3 3 3 4 6% 48%	2061
	21st to end	Night	17% 7% 2 5% 4 4 4 3 5% 7% 43%	2090
October	1st to 10th	Night	21% 8% 3 7% 4 4 4 3 6% 6% 34%	2090
	11th to 20th	Night	16% 11% 3 7% 5% 4 5% 4 5% 6% 35%	2371
	21st to end	Night	23% 11% 2 7% 5% 5% 4 3 5% 6% 28%	2717
November	1st to 10th	Night	14% 9% 4 9% 5% 4 5% 3 6% 5% 37%	2531
	11th to 20th	Night	17% 9% 4 8% 6% 4 4 4 5% 7% 32%	2470
	21st to end	Night	17% 10% 3 8% 6% 5% 5% 4 6% 6% 30%	2546
December	1st to 10th	Night	19% 13% 4 11% 6% 6% 5% 4 5% 6% 21%	2660
	11th to 20th	Night	21% 13% 4 8% 5% 4 5% 3 6% 6% 25%	3288
	21st to end	Night	23% 14% 5% 12% 5% 5% 4 4 5% 4 20%	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	48% 9% 17% 21% 5%	2518
	11th to 20th	Day	58% 9% 12% 15% 5% 1	2520
	21st to end	Day	57% 8% 12% 15% 7% 1	2772
February	1st to 10th	Day	54% 6% 13% 17% 8% 2	1677
	11th to 20th	Day	52% 6% 12% 19% 9% 2	1857
	21st to end	Day	52% 7% 11% 16% 12% 2	1570
March	1st to 10th	Day	60% 6% 11% 11% 10% 2	3119
	11th to 20th	Day	50% 9% 13% 15% 10% 3	3286
	21st to end	Day	53% 5% 8% 15% 17% 2	4097
April	1st to 10th	Day	53% 5% 10% 18% 11% 3	2280
	11th to 20th	Day	47% 7% 12% 19% 14% 1	2267
	21st to end	Day	54% 6% 9% 17% 12% 2	2470
May	1st to 10th	Day	58% 8% 10% 17% 6% 1	4120
	11th to 20th	Day	51% 9% 16% 15% 9% 1	4336
	21st to end	Day	37% 10% 15% 24% 13% 1	4837
June	1st to 10th	Day	51% 9% 13% 18% 9%	2660
	11th to 20th	Day	45% 8% 12% 19% 13% 2	2660
	21st to end	Day	52% 8% 12% 17% 9% 1	2660
July	1st to 10th	Day	40% 8% 15% 23% 13%	4370
	11th to 20th	Day	40% 9% 17% 25% 8%	4365
	21st to end	Day	45% 7% 13% 22% 13% 1	4661
August	1st to 10th	Day	46% 9% 16% 19% 10%	4283
	11th to 20th	Day	47% 9% 15% 23% 6%	4032
	21st to end	Day	35% 8% 18% 25% 14% 1	4180
September	1st to 10th	Day	35% 7% 13% 29% 14% 1	2280
	11th to 20th	Day	35% 6% 10% 29% 18% 2	2119
	21st to end	Day	39% 6% 14% 26% 13% 2	2090
October	1st to 10th	Day	46% 7% 10% 22% 14% 1	3480
	11th to 20th	Day	43% 5% 13% 22% 15% 2	2969
	21st to end	Day	41% 4 11% 28% 14% 2	3081
November	1st to 10th	Day	43% 7% 15% 21% 13% 1	1752
	11th to 20th	Day	47% 6% 14% 18% 13% 2	1710
	21st to end	Day	50% 7% 20% 17% 5%	1630
December	1st to 10th	Day	61% 5% 14% 14% 5% 1	1520
	11th to 20th	Day	52% 7% 12% 16% 12% 1	1906
	21st to end	Day	61% 7% 11% 13% 8%	2772
January	1st to 10th	Night	48% 8% 17% 21% 7% 1	4429
	11th to 20th	Night	55% 10% 14% 14% 6% 1	4440
	21st to end	Night	58% 8% 12% 14% 6% 1	4884
February	1st to 10th	Night	51% 8% 14% 16% 9% 1	2496
	11th to 20th	Night	48% 7% 15% 20% 10% 1	2220
	21st to end	Night	47% 7% 15% 19% 8% 3	1884
March	1st to 10th	Night	56% 7% 12% 15% 9% 2	3734
	11th to 20th	Night	51% 9% 11% 16% 11% 2	3673
	21st to end	Night	52% 6% 8% 15% 17% 3	3532
April	1st to 10th	Night	51% 5% 11% 19% 12% 2	1900
	11th to 20th	Night	48% 6% 10% 22% 13% 1	1861
	21st to end	Night	48% 7% 12% 17% 13% 3	1710
May	1st to 10th	Night	58% 9% 10% 15% 8% 1	2840
	11th to 20th	Night	54% 10% 12% 14% 8% 1	2577
	21st to end	Night	39% 10% 18% 19% 13% 2	2739
June	1st to 10th	Night	50% 8% 13% 19% 10%	1520
	11th to 20th	Night	45% 8% 15% 19% 11% 2	1520
	21st to end	Night	46% 9% 13% 20% 10% 1	1520
July	1st to 10th	Night	40% 9% 19% 20% 12%	2480
	11th to 20th	Night	39% 7% 16% 25% 12%	2472
	21st to end	Night	42% 7% 13% 26% 11% 1	2651
August	1st to 10th	Night	50% 7% 12% 21% 9%	2591
	11th to 20th	Night	45% 7% 17% 20% 10%	2858
	21st to end	Night	30% 9% 22% 26% 13%	3476
September	1st to 10th	Night	34% 6% 16% 28% 14%	1900
	11th to 20th	Night	35% 7% 12% 29% 16% 1	2061
	21st to end	Night	39% 6% 13% 25% 15% 1	2090
October	1st to 10th	Night	44% 8% 15% 24% 9% 1	3480
	11th to 20th	Night	38% 8% 14% 22% 16% 3	3946
	21st to end	Night	43% 6% 12% 27% 11% 1	4467
November	1st to 10th	Night	44% 6% 15% 23% 11% 2	2531
	11th to 20th	Night	45% 6% 15% 21% 12% 1	2470
	21st to end	Night	48% 8% 17% 20% 7%	2546
December	1st to 10th	Night	57% 8% 14% 16% 4	2660
	11th to 20th	Night	52% 7% 15% 16% 9% 1	3288
	21st to end	Night	61% 6% 12% 16% 4	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	28% 20% 32% 16% 3 1	180
	11th to 20th	Day	33% 27% 27% 10% 2 1	180
	21st to end	Day	21% 29% 31% 15% 5% 1	198
February	1st to 10th	Day	32% 28% 29% 9% 2	282
	11th to 20th	Day	29% 29% 28% 12% 2	456
	21st to end	Day	32% 24% 26% 14% 3	382
March	1st to 10th	Day	28% 21% 33% 14% 4	460
	11th to 20th	Day	32% 27% 25% 12% 4 1	460
	21st to end	Day	32% 23% 23% 14% 6% 2	506
April	1st to 10th	Day	35% 22% 22% 15% 4 2	460
	11th to 20th	Day	32% 24% 22% 14% 6% 2	452
	21st to end	Day	31% 24% 26% 12% 5% 2	460
May	1st to 10th	Day	32% 22% 26% 13% 5% 2	463
	11th to 20th	Day	21% 21% 33% 17% 6% 2	487
	21st to end	Day	21% 16% 31% 22% 8% 1	529
June	1st to 10th	Day	22% 14% 36% 17% 10% 1	490
	11th to 20th	Day	18% 19% 31% 19% 10% 3	490
	21st to end	Day	22% 18% 29% 19% 10% 2	490
July	1st to 10th	Day	17% 13% 34% 21% 11% 3	485
	11th to 20th	Day	15% 9% 35% 29% 11% 2	484
	21st to end	Day	16% 12% 36% 26% 8% 2	532
August	1st to 10th	Day	12% 14% 31% 24% 14% 3	486
	11th to 20th	Day	18% 13% 28% 22% 14% 5%	479
	21st to end	Day	13% 13% 29% 26% 14% 6%	539
September	1st to 10th	Day	17% 12% 27% 23% 16% 5%	490
	11th to 20th	Day	16% 14% 26% 20% 17% 8%	490
	21st to end	Day	18% 17% 26% 22% 11% 7%	490
October	1st to 10th	Day	27% 15% 22% 21% 12% 3	490
	11th to 20th	Day	25% 18% 22% 23% 9% 2	437
	21st to end	Day	36% 17% 22% 18% 7%	429
November	1st to 10th	Day	24% 19% 23% 22% 10% 1	392
	11th to 20th	Day	28% 21% 26% 18% 6% 1	370
	21st to end	Day	31% 21% 26% 17% 4	290
December	1st to 10th	Day	29% 24% 27% 16% 3	180
	11th to 20th	Day	36% 21% 32% 9% 2	180
	21st to end	Day	33% 22% 26% 16% 3 1	198
January	1st to 10th	Night	24% 21% 29% 23% 3	760
	11th to 20th	Night	28% 20% 26% 23% 3	760
	21st to end	Night	18% 24% 33% 21% 4	836
February	1st to 10th	Night	22% 21% 34% 20% 2	760
	11th to 20th	Night	23% 24% 32% 19% 2	743
	21st to end	Night	25% 19% 28% 24% 3	628
March	1st to 10th	Night	26% 18% 31% 21% 4	760
	11th to 20th	Night	25% 22% 25% 22% 5%	760
	21st to end	Night	28% 19% 25% 22% 6% 1	836
April	1st to 10th	Night	27% 20% 27% 23% 4	760
	11th to 20th	Night	23% 23% 25% 22% 7%	737
	21st to end	Night	27% 18% 28% 22% 5%	570
May	1st to 10th	Night	28% 14% 27% 24% 7%	570
	11th to 20th	Night	18% 15% 29% 31% 7%	429
	21st to end	Night	21% 9% 24% 36% 10% 1	411
June	1st to 10th	Night	23% 9% 24% 33% 10% 1	380
	11th to 20th	Night	19% 10% 26% 32% 13%	380
	21st to end	Night	19% 8% 24% 39% 10% 1	380
July	1st to 10th	Night	15% 5% 22% 41% 16%	370
	11th to 20th	Night	17% 5% 20% 38% 20% 2	368
	21st to end	Night	16% 6% 21% 40% 17% 1	416
August	1st to 10th	Night	17% 3 17% 43% 19%	434
	11th to 20th	Night	16% 7% 26% 31% 18% 2	585
	21st to end	Night	16% 7% 20% 31% 23% 2	836
September	1st to 10th	Night	16% 9% 23% 31% 19% 2	760
	11th to 20th	Night	16% 11% 21% 27% 22% 4	760
	21st to end	Night	19% 12% 21% 30% 17% 2	760
October	1st to 10th	Night	25% 11% 21% 27% 15% 1	760
	11th to 20th	Night	23% 16% 23% 27% 11% 1	760
	21st to end	Night	29% 16% 25% 24% 6%	836
November	1st to 10th	Night	20% 16% 27% 28% 8% 1	779
	11th to 20th	Night	23% 20% 26% 23% 7% 1	760
	21st to end	Night	23% 20% 26% 24% 6% 1	760
December	1st to 10th	Night	26% 20% 31% 19% 4	760
	11th to 20th	Night	29% 19% 28% 20% 3	760
	21st to end	Night	28% 22% 28% 19% 3	836

Page generated at 2024-11-07 06:55:46. Version ac07

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