Boulder Heights Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Boulder Heights Observatory clear sky chart from 2005-11-06 to 2024-12-24.

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	4 5% 2 5% 4 4 4 4 6% 8% 53%	105607
	Night	3 5% 2 5% 3 3 3 3 5% 8% 59%	103226
Transparency	Day	26% 7% 13% 24% 22% 8%	105351
	Night	22% 6% 12% 27% 24% 9%	103072
Seeing	Day	8% 9% 32% 30% 16% 4	15756
	Night	8% 8% 33% 33% 15% 2	25863

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	4 2 7% 4 5% 4 4 5% 7% 58%	2538
	11th to 20th	Day	5% 2 8% 5% 3 3 3 5% 9% 57%	2618
	21st to end	Day	1 5% 3 6% 5% 5% 4 3 5% 7% 56%	3146
February	1st to 10th	Day	1 6% 3 9% 4 3 3 4 5% 7% 56%	1710
	11th to 20th	Day	4 2 5% 4 3 4 3 7% 9% 60%	1800
	21st to end	Day	1 7% 2 7% 4 3 3 4 5% 7% 58%	1570
March	1st to 10th	Day	2 8% 3 7% 4 3 4 4 6% 7% 54%	3130
	11th to 20th	Day	3 12% 4 7% 4 6% 4 3 5% 6% 47%	3480
	21st to end	Day	3 9% 2 7% 4 4 4 5% 7% 9% 46%	4036
April	1st to 10th	Day	4 6% 2 4 3 3 4 4 6% 10% 54%	2280
	11th to 20th	Day	7% 9% 3 7% 4 4 4 4 6% 8% 43%	2251
	21st to end	Day	8% 8% 2 6% 4 5% 5% 5% 7% 8% 43%	2280
May	1st to 10th	Day	8% 8% 3 7% 5% 5% 5% 5% 6% 9% 39%	4041
	11th to 20th	Day	9% 6% 2 6% 4 5% 6% 5% 8% 9% 39%	4388
	21st to end	Day	5% 4 1 3 3 4 5% 5% 8% 9% 52%	4825
June	1st to 10th	Day	5% 3 1 4 4 5% 5% 5% 8% 11% 48%	2660
	11th to 20th	Day	6% 3 2 5% 3 5% 5% 5% 8% 9% 51%	2660
	21st to end	Day	4 3 2 4 5% 5% 5% 5% 8% 9% 49%	2660
July	1st to 10th	Day	6% 3 1 4 3 4 4 5% 7% 8% 55%	4430
	11th to 20th	Day	4 3 1 4 5% 6% 6% 6% 10% 10% 45%	4430
	21st to end	Day	5% 3 1 5% 4 4 5% 5% 7% 8% 53%	4711
August	1st to 10th	Day	10% 4 2 5% 5% 5% 6% 6% 8% 9% 39%	4092
	11th to 20th	Day	4 3 1 4 4 5% 5% 4 8% 9% 53%	3764
	21st to end	Day	4 2 1 4 4 5% 5% 5% 7% 8% 53%	4180
September	1st to 10th	Day	4 4 1 4 3 4 4 4 6% 8% 58%	2280
	11th to 20th	Day	5% 4 1 3 3 3 4 4 6% 8% 59%	2271
	21st to end	Day	7% 4 1 4 3 3 4 3 4 5% 64%	2081
October	1st to 10th	Day	3 4 1 2 2 2 3 2 4 5% 72%	3170
	11th to 20th	Day	6% 3 2 5% 3 2 3 3 5% 6% 61%	3150
	21st to end	Day	4 7% 2 4 2 2 3 3 4 6% 65%	3328
November	1st to 10th	Day	2 4 1 4 3 3 2 2 6% 8% 65%	1763
	11th to 20th	Day	2 4 2 3 3 3 2 4 6% 9% 63%	1800
	21st to end	Day	2 5% 3 6% 3 3 3 4 5% 9% 59%	1660
December	1st to 10th	Day	1 4 2 6% 2 4 3 3 5% 9% 61%	1600
	11th to 20th	Day	2 4 2 3 3 3 3 4 6% 10% 61%	2006
	21st to end	Day	2 7% 3 8% 5% 4 4 3 5% 7% 54%	2818
January	1st to 10th	Night	5% 4 7% 4 4 3 3 5% 8% 56%	4417
	11th to 20th	Night	2 1 5% 3 3 3 3 8% 10% 61%	4370
	21st to end	Night	1 7% 3 7% 4 4 3 3 4 6% 58%	4521
February	1st to 10th	Night	1 5% 3 6% 4 4 4 4 5% 7% 57%	2470
	11th to 20th	Night	1 5% 2 6% 4 4 4 4 6% 6% 59%	2266
	21st to end	Night	1 7% 3 5% 3 3 4 3 4 7% 61%	1884
March	1st to 10th	Night	1 7% 3 6% 3 2 3 3 5% 8% 58%	3734
	11th to 20th	Night	3 14% 3 4 3 2 3 3 4 6% 54%	3498
	21st to end	Night	3 8% 3 6% 4 3 4 3 5% 8% 53%	3609
April	1st to 10th	Night	3 7% 2 5% 3 3 4 4 6% 7% 57%	1900
	11th to 20th	Night	7% 8% 3 5% 3 3 3 3 5% 7% 53%	1877
	21st to end	Night	10% 8% 2 5% 3 3 4 3 5% 7% 49%	1900
May	1st to 10th	Night	9% 6% 2 6% 4 4 4 4 5% 7% 49%	2922
	11th to 20th	Night	11% 7% 2 5% 3 4 4 4 6% 8% 47%	2517
	21st to end	Night	5% 5% 2 4 3 3 4 3 5% 6% 61%	2762
June	1st to 10th	Night	5% 3 1 3 3 3 4 3 6% 10% 59%	1520
	11th to 20th	Night	6% 3 1 3 3 2 4 3 6% 8% 63%	1520
	21st to end	Night	4 3 1 3 2 3 3 4 7% 9% 63%	1520
July	1st to 10th	Night	4 4 1 3 3 3 3 3 5% 8% 62%	2540
	11th to 20th	Night	3 2 1 3 3 4 4 4 6% 10% 61%	2540
	21st to end	Night	4 2 1 3 3 4 4 3 8% 8% 61%	2848
August	1st to 10th	Night	8% 5% 1 3 3 4 5% 5% 8% 10% 48%	2847
	11th to 20th	Night	3 2 1 3 3 4 4 4 7% 10% 58%	3075
	21st to end	Night	2 2 1 3 3 4 4 4 7% 9% 60%	3487
September	1st to 10th	Night	6% 3 1 4 3 3 3 2 4 7% 63%	1900
	11th to 20th	Night	6% 3 1 2 2 2 3 2 4 6% 68%	1907
	21st to end	Night	7% 2 1 3 2 3 4 3 4 6% 65%	2095
October	1st to 10th	Night	3 3 1 2 2 2 2 2 3 4 77%	3800
	11th to 20th	Night	4 4 1 3 2 3 3 3 5% 6% 65%	3778
	21st to end	Night	4 6% 2 4 2 2 2 2 3 6% 65%	4199
November	1st to 10th	Night	2 4 1 3 2 2 3 3 5% 7% 66%	2535
	11th to 20th	Night	1 5% 2 4 3 2 2 3 4 7% 67%	2600
	21st to end	Night	2 5% 2 5% 3 3 4 3 5% 6% 62%	2735
December	1st to 10th	Night	1 4 2 5% 3 3 3 2 5% 8% 64%	2800
	11th to 20th	Night	1 5% 2 5% 4 4 3 3 6% 10% 56%	3411
	21st to end	Night	2 8% 3 7% 4 3 4 3 5% 7% 54%	4922

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	22% 5% 16% 31% 21% 4	2539
	11th to 20th	Day	23% 7% 18% 31% 16% 5%	2618
	21st to end	Day	25% 4 16% 29% 22% 4	3146
February	1st to 10th	Day	28% 6% 17% 26% 17% 5%	1710
	11th to 20th	Day	21% 6% 13% 31% 24% 5%	1810
	21st to end	Day	26% 3 12% 28% 23% 8%	1570
March	1st to 10th	Day	26% 4 13% 24% 27% 4	3130
	11th to 20th	Day	31% 9% 14% 31% 12% 2	3480
	21st to end	Day	29% 10% 16% 30% 12% 3	4036
April	1st to 10th	Day	25% 6% 14% 29% 20% 7%	2280
	11th to 20th	Day	35% 6% 14% 22% 17% 5%	2254
	21st to end	Day	34% 7% 15% 27% 16% 1	2280
May	1st to 10th	Day	36% 8% 14% 24% 15% 3	4041
	11th to 20th	Day	33% 10% 17% 24% 13% 3	4392
	21st to end	Day	22% 8% 17% 28% 18% 6%	4825
June	1st to 10th	Day	25% 9% 18% 23% 16% 7%	2660
	11th to 20th	Day	26% 8% 13% 18% 22% 13%	2660
	21st to end	Day	24% 7% 13% 22% 23% 11%	2660
July	1st to 10th	Day	23% 7% 10% 17% 28% 16%	4361
	11th to 20th	Day	27% 9% 11% 22% 19% 12%	4355
	21st to end	Day	25% 7% 11% 19% 19% 18%	4562
August	1st to 10th	Day	36% 9% 15% 18% 14% 7%	4078
	11th to 20th	Day	23% 9% 16% 19% 20% 13%	3800
	21st to end	Day	25% 6% 11% 20% 27% 11%	4179
September	1st to 10th	Day	22% 5% 9% 23% 24% 17%	2280
	11th to 20th	Day	21% 5% 11% 22% 27% 13%	2271
	21st to end	Day	22% 3 7% 22% 35% 10%	2081
October	1st to 10th	Day	16% 3 7% 15% 36% 23%	3170
	11th to 20th	Day	23% 4 8% 30% 27% 7%	3150
	21st to end	Day	20% 4 7% 26% 35% 8%	3328
November	1st to 10th	Day	18% 5% 13% 25% 28% 11%	1763
	11th to 20th	Day	17% 4 9% 32% 28% 9%	1800
	21st to end	Day	23% 5% 10% 27% 26% 11%	1660
December	1st to 10th	Day	21% 5% 15% 31% 22% 6%	1600
	11th to 20th	Day	18% 6% 12% 32% 29% 3	2004
	21st to end	Day	27% 7% 11% 25% 25% 5%	2818
January	1st to 10th	Night	26% 6% 11% 32% 23% 3	4417
	11th to 20th	Night	16% 8% 18% 30% 21% 7%	4370
	21st to end	Night	27% 5% 13% 32% 16% 6%	4521
February	1st to 10th	Night	25% 7% 15% 28% 21% 5%	2470
	11th to 20th	Night	25% 8% 9% 30% 23% 5%	2266
	21st to end	Night	24% 5% 8% 29% 24% 10%	1884
March	1st to 10th	Night	24% 6% 8% 26% 31% 5%	3734
	11th to 20th	Night	30% 7% 13% 26% 21% 3	3498
	21st to end	Night	26% 9% 13% 33% 17% 2	3609
April	1st to 10th	Night	25% 7% 13% 25% 23% 8%	1900
	11th to 20th	Night	30% 5% 12% 27% 19% 6%	1877
	21st to end	Night	33% 6% 10% 27% 19% 3	1900
May	1st to 10th	Night	29% 10% 13% 23% 21% 3	2922
	11th to 20th	Night	30% 10% 18% 24% 16% 2	2518
	21st to end	Night	21% 8% 20% 24% 19% 7%	2762
June	1st to 10th	Night	21% 7% 19% 30% 14% 9%	1520
	11th to 20th	Night	19% 5% 13% 20% 23% 20%	1520
	21st to end	Night	17% 7% 12% 24% 28% 13%	1520
July	1st to 10th	Night	20% 6% 10% 19% 29% 15%	2500
	11th to 20th	Night	17% 5% 14% 30% 22% 11%	2492
	21st to end	Night	18% 6% 15% 26% 19% 17%	2759
August	1st to 10th	Night	30% 7% 17% 22% 18% 6%	2838
	11th to 20th	Night	19% 7% 20% 22% 21% 11%	3105
	21st to end	Night	18% 5% 14% 24% 30% 9%	3487
September	1st to 10th	Night	21% 5% 10% 22% 23% 19%	1900
	11th to 20th	Night	18% 4 11% 24% 26% 17%	1907
	21st to end	Night	20% 4 9% 23% 31% 13%	2095
October	1st to 10th	Night	13% 3 6% 20% 32% 25%	3800
	11th to 20th	Night	20% 5% 9% 29% 29% 9%	3778
	21st to end	Night	22% 3 5% 26% 32% 12%	4199
November	1st to 10th	Night	18% 5% 9% 27% 28% 12%	2535
	11th to 20th	Night	18% 4 8% 31% 30% 9%	2600
	21st to end	Night	21% 4 11% 24% 30% 9%	2735
December	1st to 10th	Night	20% 5% 15% 28% 24% 8%	2800
	11th to 20th	Night	20% 6% 11% 33% 26% 4	3412
	21st to end	Night	26% 8% 12% 25% 24% 5%	4922

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	10% 8% 36% 31% 11% 4	270
	11th to 20th	Day	6% 13% 48% 26% 7% 1	270
	21st to end	Day	3 9% 44% 31% 11% 1	297
February	1st to 10th	Day	9% 19% 46% 21% 5%	270
	11th to 20th	Day	9% 11% 49% 27% 4 1	270
	21st to end	Day	9% 14% 52% 24% 1	225
March	1st to 10th	Day	7% 13% 44% 23% 11% 1	270
	11th to 20th	Day	13% 13% 36% 28% 9% 1	450
	21st to end	Day	14% 15% 44% 21% 6% 1	506
April	1st to 10th	Day	12% 20% 34% 28% 5% 1	460
	11th to 20th	Day	15% 16% 36% 26% 6% 1	452
	21st to end	Day	13% 13% 37% 26% 10% 1	460
May	1st to 10th	Day	16% 9% 36% 27% 10% 2	574
	11th to 20th	Day	18% 7% 26% 30% 16% 3	675
	21st to end	Day	10% 7% 30% 27% 22% 3	734
June	1st to 10th	Day	5% 5% 23% 34% 25% 9%	680
	11th to 20th	Day	6% 6% 33% 32% 18% 4	680
	21st to end	Day	6% 5% 29% 33% 20% 6%	680
July	1st to 10th	Day	7% 5% 18% 30% 30% 10%	671
	11th to 20th	Day	4 3 16% 38% 31% 8%	668
	21st to end	Day	6% 5% 16% 32% 31% 11%	684
August	1st to 10th	Day	6% 5% 23% 38% 21% 7%	487
	11th to 20th	Day	4 4 25% 35% 25% 7%	477
	21st to end	Day	5% 4 26% 37% 22% 6%	539
September	1st to 10th	Day	6% 5% 24% 35% 24% 7%	490
	11th to 20th	Day	5% 7% 27% 37% 18% 6%	481
	21st to end	Day	8% 6% 25% 31% 21% 8%	300
October	1st to 10th	Day	9% 9% 24% 26% 21% 11%	300
	11th to 20th	Day	4 9% 37% 34% 14% 3	300
	21st to end	Day	8% 15% 36% 29% 9% 2	330
November	1st to 10th	Day	7% 10% 40% 29% 12% 3	300
	11th to 20th	Day	11% 10% 39% 29% 9% 2	300
	21st to end	Day	8% 12% 41% 30% 8% 1	300
December	1st to 10th	Day	8% 14% 38% 28% 10% 2	300
	11th to 20th	Day	9% 14% 44% 28% 4	300
	21st to end	Day	9% 14% 41% 28% 7% 2	306
January	1st to 10th	Night	10% 11% 40% 28% 9% 2	950
	11th to 20th	Night	10% 15% 43% 23% 8% 1	916
	21st to end	Night	9% 12% 44% 27% 6% 1	836
February	1st to 10th	Night	10% 13% 51% 20% 6%	760
	11th to 20th	Night	8% 13% 47% 25% 7%	744
	21st to end	Night	8% 12% 51% 26% 3	628
March	1st to 10th	Night	7% 9% 47% 29% 7% 1	760
	11th to 20th	Night	10% 9% 34% 36% 9% 1	760
	21st to end	Night	10% 8% 43% 32% 5% 1	707
April	1st to 10th	Night	10% 8% 44% 33% 5%	570
	11th to 20th	Night	12% 8% 40% 29% 9% 1	563
	21st to end	Night	14% 7% 35% 32% 11% 1	570
May	1st to 10th	Night	14% 7% 31% 34% 13% 1	570
	11th to 20th	Night	19% 4 17% 40% 17% 3	561
	21st to end	Night	8% 5% 24% 37% 22% 3	617
June	1st to 10th	Night	6% 3 15% 47% 24% 5%	570
	11th to 20th	Night	6% 2 23% 46% 20% 2	570
	21st to end	Night	5% 3 16% 43% 30% 3	570
July	1st to 10th	Night	6% 2 11% 35% 39% 6%	555
	11th to 20th	Night	2 2 9% 41% 39% 7%	552
	21st to end	Night	7% 3 5% 36% 43% 7%	624
August	1st to 10th	Night	7% 2 6% 42% 39% 4	567
	11th to 20th	Night	5% 1 10% 44% 34% 6%	564
	21st to end	Night	5% 1 12% 41% 33% 7%	627
September	1st to 10th	Night	6% 2 13% 42% 31% 7%	570
	11th to 20th	Night	8% 4 19% 42% 21% 6%	570
	21st to end	Night	7% 4 23% 43% 19% 5%	575
October	1st to 10th	Night	9% 6% 28% 36% 17% 4	760
	11th to 20th	Night	7% 6% 34% 39% 11% 3	760
	21st to end	Night	9% 8% 39% 31% 9% 3	879
November	1st to 10th	Night	6% 8% 42% 32% 10% 2	975
	11th to 20th	Night	8% 12% 46% 28% 5% 1	1000
	21st to end	Night	7% 9% 44% 33% 7% 1	1000
December	1st to 10th	Night	10% 12% 43% 30% 6%	1000
	11th to 20th	Night	8% 12% 45% 28% 7% 1	1000
	21st to end	Night	9% 14% 44% 25% 7% 1	1063

Page generated at 2024-12-21 02:13:46. Version ac07

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