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 2026-04-17.

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% 54%	140423
	Night	4 5% 2 5% 3 3 3 3 5% 7% 60%	139254
Transparency	Day	25% 7% 12% 25% 23% 9%	135363
	Night	23% 5% 11% 26% 25% 10%	135321
Seeing	Day	9% 10% 32% 30% 15% 4	17508
	Night	8% 8% 35% 32% 14% 2	27670

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 6% 4 7% 3 3 3 3 4 6% 60%	4720
	11th to 20th	Day	3 2 7% 5% 3 4 3 4 7% 62%	4902
	21st to end	Day	3 2 6% 5% 5% 3 3 4 6% 62%	5844
February	1st to 10th	Day	1 6% 2 8% 4 3 3 4 5% 8% 57%	1890
	11th to 20th	Day	4 2 5% 4 4 4 3 7% 9% 59%	1996
	21st to end	Day	1 6% 2 7% 3 3 3 4 5% 7% 60%	1730
March	1st to 10th	Day	2 9% 3 6% 4 3 3 3 5% 7% 56%	4600
	11th to 20th	Day	2 9% 4 8% 4 5% 4 3 5% 7% 49%	5099
	21st to end	Day	3 7% 3 6% 3 4 4 4 6% 9% 50%	5916
April	1st to 10th	Day	4 7% 2 4 3 3 4 4 6% 9% 54%	2520
	11th to 20th	Day	7% 9% 3 7% 4 4 4 4 6% 9% 44%	2444
	21st to end	Day	8% 8% 2 6% 4 5% 5% 5% 7% 8% 44%	2400
May	1st to 10th	Day	11% 8% 2 6% 4 4 5% 4 6% 8% 42%	5791
	11th to 20th	Day	8% 6% 2 5% 4 5% 6% 4 7% 8% 45%	6298
	21st to end	Day	8% 6% 2 4 3 4 5% 5% 8% 8% 48%	6925
June	1st to 10th	Day	5% 4 2 4 4 5% 5% 5% 8% 11% 47%	2800
	11th to 20th	Day	6% 3 2 5% 3 5% 5% 5% 8% 9% 51%	2800
	21st to end	Day	4 3 2 4 5% 5% 5% 5% 8% 9% 49%	2800
July	1st to 10th	Day	5% 3 1 3 3 4 4 4 6% 9% 58%	6339
	11th to 20th	Day	3 3 1 4 5% 6% 6% 6% 10% 10% 44%	6340
	21st to end	Day	6% 3 1 6% 5% 5% 6% 6% 7% 9% 47%	6770
August	1st to 10th	Day	8% 3 2 4 4 5% 6% 6% 8% 10% 45%	5779
	11th to 20th	Day	3 2 1 3 4 4 5% 4 7% 9% 56%	5417
	21st to end	Day	8% 4 1 5% 5% 5% 5% 5% 7% 8% 46%	5984
September	1st to 10th	Day	4 4 1 4 3 4 4 4 6% 8% 58%	2400
	11th to 20th	Day	5% 4 1 3 3 3 4 4 6% 8% 58%	2390
	21st to end	Day	7% 4 1 4 3 3 3 3 4 5% 63%	2190
October	1st to 10th	Day	3 4 1 2 2 2 3 2 4 5% 71%	3270
	11th to 20th	Day	6% 3 2 5% 3 2 3 3 5% 6% 62%	3250
	21st to end	Day	4 7% 2 4 2 2 3 3 4 6% 65%	3435
November	1st to 10th	Day	2 4 1 4 3 3 2 2 5% 8% 66%	1853
	11th to 20th	Day	2 4 2 3 3 3 3 3 6% 9% 62%	1890
	21st to end	Day	2 5% 3 6% 3 3 3 3 5% 9% 59%	1743
December	1st to 10th	Day	3 1 4 2 3 2 2 4 7% 71%	2700
	11th to 20th	Day	1 4 2 3 2 2 2 3 4 8% 69%	3106
	21st to end	Day	2 6% 2 7% 4 4 4 3 5% 7% 56%	4092
January	1st to 10th	Night	1 6% 3 6% 3 3 3 3 4 7% 61%	8201
	11th to 20th	Night	3 2 7% 4 3 3 3 6% 7% 63%	8134
	21st to end	Night	1 4 2 6% 4 4 3 3 4 5% 64%	8370
February	1st to 10th	Night	1 5% 3 5% 4 3 4 4 5% 8% 58%	2730
	11th to 20th	Night	1 5% 2 6% 4 4 4 3 6% 6% 59%	2510
	21st to end	Night	1 7% 3 4 3 3 3 3 4 7% 63%	2076
March	1st to 10th	Night	1 8% 3 6% 3 3 3 3 4 7% 57%	5494
	11th to 20th	Night	3 11% 2 4 3 3 3 3 5% 8% 55%	5118
	21st to end	Night	5% 7% 3 6% 3 3 3 3 5% 8% 54%	5286
April	1st to 10th	Night	3 6% 2 5% 3 3 4 3 6% 7% 58%	2100
	11th to 20th	Night	7% 7% 2 6% 3 3 3 3 5% 7% 53%	2037
	21st to end	Night	10% 8% 2 5% 3 4 4 3 5% 7% 49%	2000
May	1st to 10th	Night	14% 6% 2 5% 3 3 4 4 5% 7% 47%	4182
	11th to 20th	Night	8% 7% 2 6% 3 4 4 3 6% 7% 49%	3617
	21st to end	Night	8% 6% 2 4 2 3 3 3 4 5% 60%	3972
June	1st to 10th	Night	6% 3 1 3 3 3 4 3 6% 9% 59%	1600
	11th to 20th	Night	6% 2 1 3 3 2 4 3 5% 8% 64%	1600
	21st to end	Night	4 3 1 3 2 3 3 3 6% 9% 63%	1600
July	1st to 10th	Night	3 3 1 3 3 3 3 3 6% 9% 63%	3640
	11th to 20th	Night	5% 2 1 3 3 3 4 4 7% 11% 58%	3640
	21st to end	Night	4 3 1 3 4 4 4 4 8% 9% 57%	4099
August	1st to 10th	Night	6% 4 1 3 2 4 5% 5% 8% 10% 51%	4033
	11th to 20th	Night	3 2 1 2 2 3 4 3 6% 10% 63%	4432
	21st to end	Night	6% 4 2 4 3 4 4 4 6% 9% 55%	4993
September	1st to 10th	Night	6% 3 1 4 3 3 3 2 4 7% 63%	2000
	11th to 20th	Night	6% 3 1 2 2 3 3 2 4 7% 67%	2008
	21st to end	Night	7% 2 1 4 2 2 3 3 4 6% 65%	2205
October	1st to 10th	Night	3 3 1 2 2 2 2 2 3 4 76%	3920
	11th to 20th	Night	4 4 1 3 2 3 3 3 5% 6% 66%	3898
	21st to end	Night	4 6% 2 4 2 2 2 2 3 6% 65%	4333
November	1st to 10th	Night	2 4 1 3 2 2 3 3 5% 7% 68%	2665
	11th to 20th	Night	1 5% 2 4 3 2 2 3 4 8% 66%	2730
	21st to end	Night	2 5% 2 5% 3 3 4 3 5% 7% 61%	2872
December	1st to 10th	Night	1 2 1 3 2 2 2 1 4 5% 75%	4710
	11th to 20th	Night	1 3 2 4 3 3 2 3 5% 8% 66%	5321
	21st to end	Night	2 8% 3 6% 4 3 3 3 5% 7% 55%	7128

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	24% 4 10% 33% 24% 5%	4723
	11th to 20th	Day	22% 5% 12% 28% 23% 10%	4902
	21st to end	Day	22% 4 10% 26% 25% 13%	5844
February	1st to 10th	Day	26% 5% 16% 27% 19% 6%	1890
	11th to 20th	Day	21% 6% 13% 32% 23% 5%	2006
	21st to end	Day	24% 3 11% 27% 26% 9%	1730
March	1st to 10th	Day	26% 4 9% 21% 30% 9%	4600
	11th to 20th	Day	29% 8% 10% 26% 22% 5%	5099
	21st to end	Day	25% 9% 15% 35% 14% 2	5916
April	1st to 10th	Day	25% 6% 13% 29% 20% 7%	2520
	11th to 20th	Day	34% 6% 13% 23% 18% 6%	2447
	21st to end	Day	34% 6% 15% 27% 16% 1	2400
May	1st to 10th	Day	35% 8% 13% 27% 15% 2	5791
	11th to 20th	Day	30% 9% 13% 20% 18% 9%	6302
	21st to end	Day	28% 9% 15% 25% 19% 5%	6926
June	1st to 10th	Day	26% 10% 18% 23% 17% 7%	2800
	11th to 20th	Day	26% 8% 13% 18% 22% 14%	2800
	21st to end	Day	25% 7% 13% 22% 23% 11%	2800
July	1st to 10th	Day	20% 6% 8% 17% 27% 22%	6271
	11th to 20th	Day	27% 10% 13% 25% 17% 10%	6265
	21st to end	Day	30% 8% 11% 18% 18% 15%	6574
August	1st to 10th	Day	36% 9% 15% 18% 15% 7%	4208
	11th to 20th	Day	23% 9% 15% 19% 21% 13%	3921
	21st to end	Day	25% 7% 12% 19% 26% 11%	4311
September	1st to 10th	Day	22% 5% 9% 24% 24% 16%	2400
	11th to 20th	Day	22% 5% 10% 23% 27% 13%	2390
	21st to end	Day	22% 3 7% 23% 34% 10%	2190
October	1st to 10th	Day	16% 3 7% 15% 36% 23%	3270
	11th to 20th	Day	23% 4 8% 30% 27% 7%	3250
	21st to end	Day	20% 4 7% 26% 36% 8%	3435
November	1st to 10th	Day	17% 5% 13% 25% 29% 11%	1853
	11th to 20th	Day	18% 5% 9% 32% 28% 8%	1890
	21st to end	Day	23% 5% 10% 28% 25% 10%	1743
December	1st to 10th	Day	16% 3 10% 35% 28% 8%	2700
	11th to 20th	Day	14% 5% 8% 36% 30% 6%	3104
	21st to end	Day	25% 7% 9% 27% 25% 7%	4092
January	1st to 10th	Night	24% 4 7% 32% 25% 7%	8202
	11th to 20th	Night	22% 5% 11% 26% 23% 13%	8134
	21st to end	Night	23% 4 9% 24% 25% 14%	8370
February	1st to 10th	Night	24% 6% 14% 27% 23% 6%	2730
	11th to 20th	Night	25% 7% 10% 30% 22% 6%	2510
	21st to end	Night	23% 5% 8% 28% 25% 12%	2076
March	1st to 10th	Night	27% 6% 6% 21% 32% 9%	5494
	11th to 20th	Night	27% 6% 10% 20% 26% 12%	5118
	21st to end	Night	27% 7% 12% 34% 17% 2	5286
April	1st to 10th	Night	24% 7% 13% 25% 23% 8%	2100
	11th to 20th	Night	30% 5% 12% 27% 20% 6%	2037
	21st to end	Night	33% 6% 10% 27% 20% 3	2000
May	1st to 10th	Night	33% 9% 13% 26% 17% 2	4179
	11th to 20th	Night	31% 7% 15% 22% 18% 6%	3618
	21st to end	Night	24% 7% 18% 25% 20% 5%	3972
June	1st to 10th	Night	21% 7% 18% 30% 15% 9%	1600
	11th to 20th	Night	19% 5% 12% 21% 23% 20%	1600
	21st to end	Night	17% 6% 11% 24% 29% 13%	1600
July	1st to 10th	Night	18% 5% 9% 17% 32% 18%	3600
	11th to 20th	Night	19% 6% 17% 30% 19% 9%	3592
	21st to end	Night	21% 7% 16% 23% 18% 15%	3983
August	1st to 10th	Night	29% 7% 17% 22% 19% 7%	2928
	11th to 20th	Night	19% 7% 20% 22% 22% 11%	3204
	21st to end	Night	19% 6% 14% 24% 29% 8%	3597
September	1st to 10th	Night	21% 5% 10% 23% 24% 18%	2000
	11th to 20th	Night	18% 4 11% 25% 27% 16%	2008
	21st to end	Night	20% 4 8% 23% 30% 13%	2205
October	1st to 10th	Night	14% 3 6% 20% 32% 24%	3920
	11th to 20th	Night	20% 5% 8% 29% 30% 9%	3898
	21st to end	Night	22% 3 5% 26% 33% 12%	4333
November	1st to 10th	Night	17% 5% 9% 28% 29% 13%	2665
	11th to 20th	Night	18% 4 8% 32% 30% 8%	2730
	21st to end	Night	21% 4 11% 25% 29% 9%	2872
December	1st to 10th	Night	13% 3 9% 31% 35% 7%	4710
	11th to 20th	Night	16% 4 7% 34% 34% 4	5322
	21st to end	Night	26% 7% 10% 25% 27% 6%	7128

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	11% 11% 36% 29% 9% 3	330
	11th to 20th	Day	7% 14% 47% 24% 7% 1	330
	21st to end	Day	5% 14% 42% 28% 10% 1	363
February	1st to 10th	Day	8% 24% 43% 19% 5%	330
	11th to 20th	Day	8% 12% 50% 25% 3 1	330
	21st to end	Day	11% 16% 48% 21% 3	273
March	1st to 10th	Day	8% 15% 44% 22% 10% 1	330
	11th to 20th	Day	15% 16% 35% 25% 8% 1	529
	21st to end	Day	13% 18% 43% 19% 5% 1	594
April	1st to 10th	Day	12% 20% 35% 26% 5% 1	540
	11th to 20th	Day	13% 19% 37% 24% 6% 1	517
	21st to end	Day	15% 13% 36% 26% 9% 1	500
May	1st to 10th	Day	16% 9% 36% 27% 10% 2	620
	11th to 20th	Day	17% 9% 26% 30% 15% 3	725
	21st to end	Day	11% 9% 30% 27% 21% 3	789
June	1st to 10th	Day	5% 5% 24% 34% 24% 8%	730
	11th to 20th	Day	6% 7% 32% 32% 18% 4	730
	21st to end	Day	6% 6% 30% 32% 20% 6%	730
July	1st to 10th	Day	6% 5% 19% 31% 29% 10%	721
	11th to 20th	Day	3 4 18% 37% 29% 8%	718
	21st to end	Day	6% 5% 17% 33% 29% 11%	736
August	1st to 10th	Day	6% 7% 24% 36% 20% 7%	527
	11th to 20th	Day	4 3 26% 36% 24% 7%	517
	21st to end	Day	6% 4 25% 38% 22% 5%	583
September	1st to 10th	Day	6% 5% 25% 34% 23% 7%	530
	11th to 20th	Day	5% 7% 28% 37% 17% 6%	520
	21st to end	Day	8% 7% 25% 31% 22% 8%	330
October	1st to 10th	Day	10% 10% 24% 25% 20% 11%	330
	11th to 20th	Day	5% 11% 36% 32% 13% 3	330
	21st to end	Day	7% 15% 39% 29% 9% 2	363
November	1st to 10th	Day	7% 12% 40% 28% 12% 2	330
	11th to 20th	Day	12% 10% 38% 30% 8% 2	330
	21st to end	Day	7% 14% 40% 31% 7% 1	330
December	1st to 10th	Day	8% 17% 39% 26% 9% 2	330
	11th to 20th	Day	11% 16% 42% 27% 4 1	330
	21st to end	Day	10% 15% 42% 26% 6% 1	363
January	1st to 10th	Night	10% 12% 42% 26% 8% 2	1050
	11th to 20th	Night	10% 16% 44% 22% 7% 1	1012
	21st to end	Night	9% 13% 47% 26% 6% 1	924
February	1st to 10th	Night	10% 14% 52% 19% 5%	840
	11th to 20th	Night	8% 15% 47% 24% 7%	824
	21st to end	Night	9% 13% 52% 24% 3	692
March	1st to 10th	Night	6% 9% 48% 29% 7% 1	840
	11th to 20th	Night	10% 11% 37% 33% 8% 1	840
	21st to end	Night	10% 9% 45% 30% 4 1	781
April	1st to 10th	Night	9% 8% 45% 32% 5%	630
	11th to 20th	Night	12% 9% 42% 28% 8% 1	611
	21st to end	Night	14% 7% 35% 32% 11% 1	600
May	1st to 10th	Night	14% 7% 31% 35% 12% 1	600
	11th to 20th	Night	18% 4 20% 39% 17% 3	591
	21st to end	Night	9% 6% 25% 37% 21% 3	650
June	1st to 10th	Night	7% 3 16% 46% 23% 5%	600
	11th to 20th	Night	6% 2 24% 47% 19% 2	600
	21st to end	Night	5% 3 19% 42% 28% 3	600
July	1st to 10th	Night	6% 2 12% 36% 38% 6%	585
	11th to 20th	Night	2 2 10% 42% 37% 7%	582
	21st to end	Night	7% 2 7% 36% 41% 7%	657
August	1st to 10th	Night	6% 2 9% 41% 37% 4	597
	11th to 20th	Night	5% 1 10% 46% 33% 5%	594
	21st to end	Night	5% 2 13% 41% 33% 7%	660
September	1st to 10th	Night	6% 2 15% 43% 29% 7%	600
	11th to 20th	Night	8% 4 21% 41% 20% 6%	600
	21st to end	Night	7% 4 23% 43% 18% 5%	605
October	1st to 10th	Night	10% 6% 30% 35% 16% 4	800
	11th to 20th	Night	7% 7% 35% 37% 11% 3	800
	21st to end	Night	9% 8% 41% 31% 9% 2	925
November	1st to 10th	Night	6% 8% 43% 31% 9% 2	1025
	11th to 20th	Night	7% 13% 47% 27% 5% 1	1050
	21st to end	Night	7% 9% 44% 33% 7% 1	1050
December	1st to 10th	Night	10% 12% 44% 29% 5%	1050
	11th to 20th	Night	8% 13% 45% 27% 6% 1	1050
	21st to end	Night	9% 15% 45% 24% 7% 1	1155

Page generated at 2026-04-14 00:44:26. Version ac07

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