Bobservatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Bobservatory clear sky chart from 2005-11-06 to 2024-11-05.

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	16% 9% 3 7% 5% 5% 5% 4 6% 7% 33%	101339
	Night	17% 9% 3 7% 5% 5% 5% 4 5% 7% 34%	97720
Transparency	Day	46% 7% 12% 21% 12% 1	104864
	Night	46% 8% 13% 21% 12% 1	102284
Seeing	Day	23% 19% 30% 18% 8% 2	14880
	Night	22% 18% 28% 24% 8% 1	23784

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	12% 11% 4 8% 5% 5% 5% 4 6% 6% 34%	2517
	11th to 20th	Day	15% 10% 4 10% 6% 6% 6% 5% 5% 6% 27%	2520
	21st to end	Day	16% 13% 5% 9% 5% 5% 4 4 6% 6% 26%	2772
February	1st to 10th	Day	15% 11% 4 8% 6% 5% 6% 4 6% 6% 30%	1698
	11th to 20th	Day	10% 11% 4 10% 6% 6% 5% 3 6% 7% 32%	1848
	21st to end	Day	15% 13% 3 8% 5% 4 6% 4 5% 6% 31%	1570
March	1st to 10th	Day	20% 18% 4 8% 5% 4 4 3 3 4 26%	3116
	11th to 20th	Day	14% 11% 5% 8% 5% 5% 5% 5% 6% 6% 30%	3279
	21st to end	Day	18% 11% 4 8% 5% 5% 5% 3 4 6% 31%	4119
April	1st to 10th	Day	18% 11% 3 8% 5% 5% 4 3 4 6% 33%	2280
	11th to 20th	Day	17% 7% 2 7% 5% 5% 5% 5% 6% 7% 35%	2254
	21st to end	Day	21% 9% 3 8% 5% 5% 4 3 5% 6% 31%	2470
May	1st to 10th	Day	23% 12% 4 9% 5% 4 4 4 5% 6% 26%	4106
	11th to 20th	Day	19% 9% 3 7% 5% 5% 5% 4 6% 7% 30%	4325
	21st to end	Day	13% 5% 2 6% 5% 5% 5% 4 7% 9% 40%	4837
June	1st to 10th	Day	18% 8% 3 8% 6% 5% 6% 4 6% 8% 30%	2660
	11th to 20th	Day	17% 6% 3 6% 6% 5% 4 4 6% 9% 35%	2660
	21st to end	Day	19% 8% 3 8% 6% 5% 6% 5% 5% 7% 29%	2660
July	1st to 10th	Day	13% 6% 3 6% 5% 5% 5% 5% 7% 10% 35%	4440
	11th to 20th	Day	11% 5% 2 6% 5% 5% 6% 6% 8% 10% 36%	4440
	21st to end	Day	17% 7% 3 7% 5% 5% 5% 4 6% 7% 35%	4814
August	1st to 10th	Day	15% 6% 2 8% 6% 6% 6% 4 7% 9% 32%	4281
	11th to 20th	Day	14% 8% 3 8% 5% 5% 6% 4 7% 8% 32%	3959
	21st to end	Day	10% 4 2 6% 5% 5% 6% 5% 8% 9% 41%	4180
September	1st to 10th	Day	12% 5% 1 6% 4 5% 4 3 6% 8% 45%	2280
	11th to 20th	Day	14% 5% 2 4 3 3 4 3 5% 6% 50%	2123
	21st to end	Day	15% 6% 2 6% 4 4 4 3 6% 8% 43%	2090
October	1st to 10th	Day	21% 9% 4 7% 4 3 4 3 5% 5% 36%	2090
	11th to 20th	Day	15% 7% 3 6% 5% 4 4 4 5% 8% 39%	1815
	21st to end	Day	21% 11% 2 8% 4 4 5% 4 4 5% 32%	1881
November	1st to 10th	Day	13% 7% 3 7% 5% 4 4 3 5% 8% 42%	1708
	11th to 20th	Day	17% 7% 3 6% 5% 5% 5% 3 5% 6% 38%	1710
	21st to end	Day	18% 10% 3 8% 4 4 4 4 5% 6% 33%	1639
December	1st to 10th	Day	18% 12% 3 8% 6% 6% 6% 3 5% 7% 25%	1520
	11th to 20th	Day	20% 12% 3 6% 5% 4 3 4 4 6% 33%	1906
	21st to end	Day	22% 14% 3 9% 5% 5% 5% 3 4 5% 24%	2772
January	1st to 10th	Night	15% 8% 3 7% 5% 4 5% 4 6% 7% 36%	4429
	11th to 20th	Night	12% 9% 3 11% 8% 6% 6% 5% 7% 7% 26%	4440
	21st to end	Night	19% 14% 5% 10% 6% 4 4 4 5% 5% 25%	4884
February	1st to 10th	Night	15% 10% 4 8% 5% 6% 6% 5% 6% 8% 29%	2475
	11th to 20th	Night	11% 9% 4 8% 6% 5% 6% 5% 7% 8% 31%	2218
	21st to end	Night	14% 12% 3 7% 4 4 5% 4 6% 7% 35%	1884
March	1st to 10th	Night	23% 12% 3 7% 5% 5% 3 3 5% 5% 28%	3724
	11th to 20th	Night	12% 11% 4 8% 6% 6% 6% 5% 5% 6% 33%	3679
	21st to end	Night	20% 12% 3 8% 4 4 3 3 5% 6% 32%	3512
April	1st to 10th	Night	18% 9% 3 7% 5% 5% 5% 3 5% 6% 33%	1900
	11th to 20th	Night	17% 11% 3 7% 4 4 3 3 5% 6% 37%	1875
	21st to end	Night	19% 9% 3 6% 5% 4 4 3 5% 7% 35%	1710
May	1st to 10th	Night	26% 12% 3 8% 5% 4 5% 4 4 6% 25%	2834
	11th to 20th	Night	17% 10% 3 9% 7% 6% 6% 4 6% 6% 27%	2596
	21st to end	Night	15% 6% 2 6% 5% 5% 4 4 6% 8% 40%	2738
June	1st to 10th	Night	20% 6% 2 8% 5% 5% 5% 3 6% 7% 33%	1520
	11th to 20th	Night	19% 6% 3 6% 4 4 6% 3 5% 6% 39%	1520
	21st to end	Night	19% 6% 2 7% 5% 5% 5% 5% 7% 7% 34%	1520
July	1st to 10th	Night	11% 6% 2 7% 5% 5% 6% 5% 7% 8% 39%	2520
	11th to 20th	Night	13% 6% 2 6% 4 4 5% 3 6% 7% 43%	2520
	21st to end	Night	17% 7% 2 5% 4 4 4 4 5% 7% 40%	2739
August	1st to 10th	Night	21% 7% 3 7% 5% 4 4 3 5% 7% 35%	2616
	11th to 20th	Night	14% 8% 4 7% 5% 5% 4 3 6% 7% 38%	2864
	21st to end	Night	9% 4 1 5% 4 4 4 4 7% 10% 48%	3476
September	1st to 10th	Night	13% 5% 2 5% 4 3 3 3 5% 6% 51%	1900
	11th to 20th	Night	13% 6% 2 5% 4 3 3 3 4 6% 50%	2057
	21st to end	Night	15% 5% 2 5% 4 4 4 3 6% 7% 45%	2090
October	1st to 10th	Night	20% 7% 3 7% 4 5% 4 3 5% 5% 37%	2090
	11th to 20th	Night	15% 8% 2 8% 5% 5% 4 4 5% 6% 39%	2363
	21st to end	Night	22% 10% 2 6% 5% 4 4 3 5% 6% 32%	2717
November	1st to 10th	Night	14% 7% 3 7% 5% 4 4 3 5% 7% 41%	2470
	11th to 20th	Night	16% 8% 3 6% 5% 5% 4 4 5% 6% 39%	2470
	21st to end	Night	15% 9% 4 7% 4 5% 5% 4 5% 7% 35%	2538
December	1st to 10th	Night	17% 10% 5% 9% 6% 5% 5% 4 5% 6% 28%	2660
	11th to 20th	Night	19% 11% 3 6% 5% 4 5% 4 6% 6% 31%	3288
	21st to end	Night	22% 12% 4 10% 5% 4 4 3 5% 6% 25%	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	46% 7% 12% 27% 7% 1	2518
	11th to 20th	Day	52% 10% 12% 19% 6% 1	2520
	21st to end	Day	56% 7% 10% 17% 7% 2	2772
February	1st to 10th	Day	51% 7% 13% 18% 8% 2	1698
	11th to 20th	Day	49% 7% 11% 19% 11% 2	1857
	21st to end	Day	51% 7% 11% 18% 12% 1	1570
March	1st to 10th	Day	61% 6% 9% 12% 10% 2	3119
	11th to 20th	Day	47% 9% 15% 15% 11% 2	3279
	21st to end	Day	51% 7% 7% 15% 17% 3	4119
April	1st to 10th	Day	51% 6% 9% 19% 13% 3	2280
	11th to 20th	Day	46% 8% 11% 19% 15% 2	2254
	21st to end	Day	53% 6% 8% 16% 14% 3	2470
May	1st to 10th	Day	57% 7% 10% 17% 7% 1	4120
	11th to 20th	Day	51% 8% 15% 16% 8% 2	4329
	21st to end	Day	36% 10% 14% 25% 14% 1	4837
June	1st to 10th	Day	49% 9% 13% 19% 10% 1	2660
	11th to 20th	Day	44% 8% 12% 18% 15% 2	2660
	21st to end	Day	51% 9% 12% 16% 11% 1	2660
July	1st to 10th	Day	41% 8% 14% 23% 14% 1	4370
	11th to 20th	Day	38% 9% 17% 26% 11%	4365
	21st to end	Day	46% 7% 13% 20% 14% 1	4661
August	1st to 10th	Day	45% 8% 14% 21% 11%	4267
	11th to 20th	Day	45% 8% 16% 24% 7%	3998
	21st to end	Day	34% 8% 14% 28% 14% 1	4180
September	1st to 10th	Day	36% 7% 14% 26% 16% 1	2280
	11th to 20th	Day	35% 6% 9% 28% 20% 2	2123
	21st to end	Day	39% 6% 13% 25% 15% 2	2090
October	1st to 10th	Day	45% 6% 9% 22% 17% 1	3480
	11th to 20th	Day	40% 5% 13% 24% 15% 3	2992
	21st to end	Day	39% 4 8% 31% 14% 3	3081
November	1st to 10th	Day	41% 5% 15% 23% 15% 1	1708
	11th to 20th	Day	44% 7% 13% 19% 15% 2	1710
	21st to end	Day	48% 7% 17% 18% 9% 1	1639
December	1st to 10th	Day	54% 8% 15% 14% 8% 1	1520
	11th to 20th	Day	50% 7% 12% 17% 13% 1	1906
	21st to end	Day	60% 6% 10% 14% 9%	2772
January	1st to 10th	Night	44% 8% 14% 23% 10% 1	4429
	11th to 20th	Night	49% 12% 13% 17% 7% 2	4440
	21st to end	Night	58% 8% 10% 15% 8% 1	4884
February	1st to 10th	Night	48% 10% 12% 18% 10% 1	2475
	11th to 20th	Night	46% 9% 13% 19% 12% 1	2220
	21st to end	Night	46% 7% 14% 21% 9% 3	1884
March	1st to 10th	Night	55% 6% 10% 17% 9% 2	3734
	11th to 20th	Night	48% 9% 13% 18% 11% 2	3680
	21st to end	Night	50% 7% 7% 13% 19% 4	3512
April	1st to 10th	Night	49% 7% 10% 18% 14% 2	1900
	11th to 20th	Night	47% 5% 11% 20% 15% 1	1875
	21st to end	Night	47% 6% 12% 16% 15% 3	1710
May	1st to 10th	Night	57% 9% 10% 15% 8% 1	2840
	11th to 20th	Night	52% 12% 13% 14% 8% 2	2597
	21st to end	Night	40% 9% 16% 21% 13% 2	2739
June	1st to 10th	Night	48% 8% 13% 20% 11%	1520
	11th to 20th	Night	44% 9% 15% 19% 12% 2	1520
	21st to end	Night	46% 10% 12% 21% 11% 1	1520
July	1st to 10th	Night	39% 8% 16% 23% 13% 1	2480
	11th to 20th	Night	38% 8% 15% 26% 14%	2472
	21st to end	Night	42% 6% 11% 28% 12% 1	2651
August	1st to 10th	Night	49% 7% 13% 21% 9%	2606
	11th to 20th	Night	45% 7% 17% 21% 10%	2891
	21st to end	Night	28% 7% 18% 32% 15% 1	3476
September	1st to 10th	Night	33% 7% 14% 27% 18%	1900
	11th to 20th	Night	35% 6% 11% 28% 20% 1	2057
	21st to end	Night	37% 6% 12% 28% 15% 2	2090
October	1st to 10th	Night	43% 6% 15% 23% 11% 1	3480
	11th to 20th	Night	36% 6% 12% 24% 19% 3	3925
	21st to end	Night	42% 5% 10% 30% 12% 1	4467
November	1st to 10th	Night	43% 6% 13% 22% 14% 2	2470
	11th to 20th	Night	43% 6% 14% 21% 14% 2	2470
	21st to end	Night	44% 9% 16% 21% 9% 1	2538
December	1st to 10th	Night	52% 9% 14% 18% 7%	2660
	11th to 20th	Night	49% 8% 14% 15% 13% 1	3288
	21st to end	Night	58% 7% 12% 17% 6%	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	26% 26% 30% 13% 4	180
	11th to 20th	Day	30% 26% 32% 10% 1 1	180
	21st to end	Day	23% 33% 29% 12% 4	198
February	1st to 10th	Day	31% 32% 26% 10% 1	293
	11th to 20th	Day	30% 30% 27% 12% 2	456
	21st to end	Day	28% 29% 28% 13% 2	382
March	1st to 10th	Day	31% 21% 33% 12% 3	460
	11th to 20th	Day	30% 29% 25% 12% 3 1	460
	21st to end	Day	32% 22% 27% 11% 5% 2	506
April	1st to 10th	Day	35% 20% 27% 11% 6% 2	460
	11th to 20th	Day	30% 23% 27% 12% 6% 1	452
	21st to end	Day	28% 24% 29% 11% 6% 2	460
May	1st to 10th	Day	29% 24% 27% 14% 5% 2	463
	11th to 20th	Day	20% 21% 37% 17% 5% 2	487
	21st to end	Day	19% 16% 33% 23% 8% 1	529
June	1st to 10th	Day	20% 17% 37% 16% 10% 1	490
	11th to 20th	Day	18% 18% 32% 20% 10% 2	490
	21st to end	Day	24% 16% 33% 18% 8% 1	490
July	1st to 10th	Day	16% 13% 31% 24% 13% 2	485
	11th to 20th	Day	15% 8% 35% 30% 9% 2	484
	21st to end	Day	13% 12% 38% 28% 7% 2	532
August	1st to 10th	Day	12% 12% 32% 27% 14% 2	486
	11th to 20th	Day	15% 11% 32% 24% 12% 5%	479
	21st to end	Day	12% 13% 30% 26% 13% 5%	539
September	1st to 10th	Day	16% 13% 28% 23% 15% 5%	490
	11th to 20th	Day	17% 15% 26% 18% 15% 9%	490
	21st to end	Day	18% 16% 28% 21% 12% 5%	490
October	1st to 10th	Day	26% 15% 25% 18% 12% 3	490
	11th to 20th	Day	22% 20% 28% 21% 7% 2	445
	21st to end	Day	32% 18% 26% 18% 6% 1	429
November	1st to 10th	Day	23% 23% 23% 20% 10% 1	378
	11th to 20th	Day	29% 23% 25% 17% 6% 1	370
	21st to end	Day	33% 21% 26% 16% 4 1	299
December	1st to 10th	Day	27% 30% 24% 16% 3	180
	11th to 20th	Day	33% 28% 27% 11% 1	180
	21st to end	Day	35% 24% 26% 11% 4 1	198
January	1st to 10th	Night	22% 24% 32% 20% 2	760
	11th to 20th	Night	26% 23% 30% 18% 2	760
	21st to end	Night	20% 25% 31% 20% 3	836
February	1st to 10th	Night	25% 23% 33% 17% 3	760
	11th to 20th	Night	22% 26% 35% 15% 2	743
	21st to end	Night	26% 22% 30% 19% 2	628
March	1st to 10th	Night	27% 21% 31% 19% 2	760
	11th to 20th	Night	26% 22% 28% 20% 4	760
	21st to end	Night	27% 21% 27% 20% 6%	836
April	1st to 10th	Night	27% 19% 29% 21% 4	760
	11th to 20th	Night	23% 25% 27% 19% 6%	751
	21st to end	Night	26% 18% 30% 22% 4	570
May	1st to 10th	Night	27% 15% 32% 22% 5%	570
	11th to 20th	Night	19% 17% 31% 28% 5%	436
	21st to end	Night	18% 9% 29% 33% 10%	411
June	1st to 10th	Night	21% 10% 27% 31% 10% 1	380
	11th to 20th	Night	19% 10% 29% 31% 10%	380
	21st to end	Night	19% 9% 25% 38% 8%	380
July	1st to 10th	Night	15% 6% 28% 38% 12% 1	370
	11th to 20th	Night	13% 6% 24% 39% 16% 2	368
	21st to end	Night	14% 6% 27% 38% 14% 1	416
August	1st to 10th	Night	15% 5% 22% 42% 16%	436
	11th to 20th	Night	13% 8% 30% 32% 15% 3	605
	21st to end	Night	15% 8% 22% 33% 19% 2	836
September	1st to 10th	Night	14% 10% 28% 28% 18% 2	760
	11th to 20th	Night	15% 11% 24% 28% 18% 4	760
	21st to end	Night	17% 10% 27% 28% 16% 2	760
October	1st to 10th	Night	24% 14% 20% 27% 13% 1	760
	11th to 20th	Night	19% 18% 27% 24% 11% 1	760
	21st to end	Night	27% 19% 26% 21% 6%	836
November	1st to 10th	Night	19% 20% 27% 25% 8% 1	760
	11th to 20th	Night	22% 23% 28% 19% 7% 1	760
	21st to end	Night	24% 21% 29% 20% 5% 1	760
December	1st to 10th	Night	24% 24% 33% 15% 4	760
	11th to 20th	Night	27% 23% 30% 18% 3	760
	21st to end	Night	28% 25% 28% 17% 2	836

Page generated at 2024-11-03 02:11:17. Version ac07

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