Boulder
Clear Sky Chart
History

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

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%	105454
	Night	4 5% 2 4 3 3 3 3 5% 7% 59%	102967
Transparency	Day	26% 6% 11% 23% 24% 10%	105198
	Night	24% 5% 11% 24% 26% 10%	102813
Seeing	Day	8% 8% 26% 33% 19% 6%	15701
	Night	6% 8% 25% 41% 17% 3	25765

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	5% 2 7% 4 4 3 3 5% 7% 59%	2538
	11th to 20th	Day	1 4 2 6% 5% 5% 3 3 6% 8% 57%	2613
	21st to end	Day	1 6% 3 6% 3 4 3 4 5% 8% 58%	3146
February	1st to 10th	Day	2 5% 3 8% 3 4 5% 4 5% 6% 55%	1710
	11th to 20th	Day	4 2 5% 4 4 3 4 6% 9% 59%	1806
	21st to end	Day	2 6% 2 8% 4 3 3 3 5% 5% 59%	1570
March	1st to 10th	Day	3 8% 3 5% 3 3 4 3 6% 6% 54%	3130
	11th to 20th	Day	4 11% 3 7% 3 3 3 3 5% 8% 50%	3475
	21st to end	Day	4 8% 3 7% 4 4 4 5% 7% 9% 47%	4041
April	1st to 10th	Day	5% 7% 2 4 2 3 3 3 6% 10% 56%	2280
	11th to 20th	Day	8% 10% 2 6% 3 3 4 4 6% 9% 46%	2251
	21st to end	Day	8% 8% 2 4 4 5% 5% 4 6% 8% 47%	2280
May	1st to 10th	Day	8% 8% 3 6% 5% 4 5% 5% 7% 9% 40%	4023
	11th to 20th	Day	10% 6% 2 5% 4 5% 5% 5% 7% 10% 41%	4388
	21st to end	Day	5% 4 1 4 4 4 5% 5% 8% 10% 51%	4825
June	1st to 10th	Day	5% 3 2 4 4 4 6% 5% 8% 11% 49%	2660
	11th to 20th	Day	6% 3 1 3 4 4 6% 5% 8% 9% 51%	2660
	21st to end	Day	3 3 1 4 4 4 6% 6% 8% 10% 50%	2660
July	1st to 10th	Day	5% 3 1 4 3 3 4 4 7% 9% 55%	4430
	11th to 20th	Day	3 2 1 4 5% 7% 7% 7% 9% 10% 45%	4430
	21st to end	Day	4 3 1 5% 4 4 5% 4 7% 8% 55%	4715
August	1st to 10th	Day	10% 4 2 6% 5% 6% 7% 7% 8% 9% 39%	4092
	11th to 20th	Day	4 2 1 4 3 5% 6% 6% 8% 9% 52%	3757
	21st to end	Day	4 3 2 4 4 4 5% 5% 8% 10% 52%	4180
September	1st to 10th	Day	4 3 1 3 3 4 4 3 6% 8% 60%	2280
	11th to 20th	Day	5% 3 2 3 3 4 4 3 6% 8% 60%	2268
	21st to end	Day	7% 4 2 3 2 2 3 2 4 6% 65%	2089
October	1st to 10th	Day	3 3 1 2 2 2 2 2 4 4 74%	3170
	11th to 20th	Day	6% 3 2 4 4 3 3 3 4 6% 62%	3150
	21st to end	Day	4 6% 1 4 2 2 2 3 4 6% 65%	3324
November	1st to 10th	Day	2 5% 1 4 2 2 2 3 5% 7% 66%	1763
	11th to 20th	Day	3 4 1 3 3 2 3 4 6% 8% 63%	1800
	21st to end	Day	3 5% 3 4 3 3 3 3 6% 8% 60%	1670
December	1st to 10th	Day	1 5% 2 6% 3 2 3 3 4 9% 62%	1560
	11th to 20th	Day	2 5% 2 3 3 3 3 3 4 9% 62%	1926
	21st to end	Day	2 9% 3 6% 4 4 3 3 5% 7% 54%	2794
January	1st to 10th	Night	1 7% 3 7% 4 4 3 3 4 8% 56%	4417
	11th to 20th	Night	1 3 1 4 3 4 4 3 6% 9% 62%	4384
	21st to end	Night	2 8% 3 7% 4 3 3 2 4 6% 58%	4521
February	1st to 10th	Night	1 8% 3 6% 4 3 3 3 5% 7% 57%	2470
	11th to 20th	Night	1 6% 3 7% 5% 4 4 3 6% 6% 57%	2260
	21st to end	Night	2 7% 3 6% 4 2 3 2 4 6% 60%	1884
March	1st to 10th	Night	3 8% 3 7% 3 3 2 3 5% 7% 57%	3734
	11th to 20th	Night	7% 12% 2 4 3 2 3 2 5% 6% 54%	3503
	21st to end	Night	4 9% 2 5% 4 3 3 3 5% 8% 53%	3605
April	1st to 10th	Night	4 7% 1 4 3 4 3 4 5% 6% 58%	1900
	11th to 20th	Night	9% 7% 2 5% 3 3 3 3 5% 6% 53%	1877
	21st to end	Night	12% 8% 2 4 4 3 3 3 4 7% 50%	1900
May	1st to 10th	Night	11% 6% 2 5% 3 4 3 4 6% 8% 48%	2916
	11th to 20th	Night	12% 5% 2 5% 3 3 3 4 6% 9% 47%	2517
	21st to end	Night	7% 4 1 4 2 4 4 2 5% 7% 60%	2762
June	1st to 10th	Night	6% 3 1 3 3 4 3 3 6% 9% 59%	1520
	11th to 20th	Night	6% 3 1 2 2 3 3 4 6% 7% 63%	1520
	21st to end	Night	4 2 1 3 2 3 3 4 7% 8% 62%	1520
July	1st to 10th	Night	4 2 1 3 3 3 4 3 6% 7% 63%	2540
	11th to 20th	Night	2 2 1 4 3 3 4 4 6% 10% 60%	2540
	21st to end	Night	4 2 1 3 2 4 4 4 7% 8% 60%	2844
August	1st to 10th	Night	8% 5% 1 5% 4 5% 5% 5% 8% 10% 46%	2847
	11th to 20th	Night	3 2 1 3 3 3 5% 4 7% 11% 57%	3080
	21st to end	Night	2 2 1 3 3 4 5% 4 8% 9% 59%	3487
September	1st to 10th	Night	6% 2 1 4 3 3 4 3 5% 7% 63%	1900
	11th to 20th	Night	6% 3 1 2 2 2 2 2 5% 7% 67%	1910
	21st to end	Night	7% 3 1 3 2 3 4 2 5% 5% 65%	2090
October	1st to 10th	Night	4 3 1 2 1 2 2 2 3 4 77%	3797
	11th to 20th	Night	5% 3 1 3 3 3 3 2 4 7% 64%	3778
	21st to end	Night	5% 6% 2 3 2 2 2 2 4 6% 65%	4202
November	1st to 10th	Night	2 5% 2 3 2 2 2 3 5% 7% 66%	2535
	11th to 20th	Night	2 6% 2 4 3 2 2 2 5% 6% 67%	2600
	21st to end	Night	3 6% 2 4 3 2 3 3 5% 7% 62%	2726
December	1st to 10th	Night	1 5% 2 6% 3 3 3 3 4 8% 63%	2737
	11th to 20th	Night	1 5% 2 4 4 3 3 3 6% 9% 58%	3271
	21st to end	Night	3 9% 3 6% 4 3 3 3 5% 7% 53%	4873

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	23% 5% 11% 27% 23% 10%	2539
	11th to 20th	Day	24% 6% 14% 30% 18% 8%	2613
	21st to end	Day	24% 7% 10% 31% 24% 5%	3146
February	1st to 10th	Day	29% 5% 12% 26% 19% 9%	1710
	11th to 20th	Day	22% 5% 10% 27% 29% 7%	1816
	21st to end	Day	26% 5% 10% 22% 26% 12%	1570
March	1st to 10th	Day	27% 7% 9% 20% 29% 8%	3130
	11th to 20th	Day	32% 9% 11% 29% 16% 3	3475
	21st to end	Day	29% 11% 11% 28% 18% 4	4041
April	1st to 10th	Day	25% 6% 12% 25% 22% 10%	2280
	11th to 20th	Day	35% 6% 11% 21% 19% 8%	2254
	21st to end	Day	34% 8% 12% 25% 20% 2	2280
May	1st to 10th	Day	36% 10% 10% 22% 14% 7%	4023
	11th to 20th	Day	34% 11% 14% 24% 16% 2	4392
	21st to end	Day	24% 11% 13% 26% 21% 6%	4825
June	1st to 10th	Day	26% 10% 16% 23% 18% 8%	2660
	11th to 20th	Day	27% 9% 11% 18% 22% 14%	2660
	21st to end	Day	24% 8% 12% 20% 24% 11%	2660
July	1st to 10th	Day	25% 6% 8% 15% 30% 17%	4361
	11th to 20th	Day	29% 6% 11% 21% 22% 12%	4355
	21st to end	Day	25% 6% 9% 19% 21% 19%	4566
August	1st to 10th	Day	37% 7% 14% 19% 15% 7%	4078
	11th to 20th	Day	26% 6% 14% 18% 22% 14%	3793
	21st to end	Day	26% 5% 10% 20% 26% 13%	4179
September	1st to 10th	Day	22% 6% 8% 21% 26% 18%	2280
	11th to 20th	Day	21% 6% 9% 21% 28% 15%	2268
	21st to end	Day	22% 4 6% 20% 37% 12%	2089
October	1st to 10th	Day	15% 4 6% 15% 36% 25%	3170
	11th to 20th	Day	25% 2 7% 28% 29% 9%	3150
	21st to end	Day	21% 2 5% 22% 40% 10%	3324
November	1st to 10th	Day	19% 4 9% 24% 30% 14%	1763
	11th to 20th	Day	18% 4 8% 27% 33% 10%	1800
	21st to end	Day	23% 3 8% 23% 30% 12%	1670
December	1st to 10th	Day	20% 6% 12% 31% 21% 9%	1560
	11th to 20th	Day	20% 5% 10% 28% 30% 7%	1924
	21st to end	Day	28% 4 12% 21% 27% 8%	2794
January	1st to 10th	Night	27% 4 9% 26% 28% 6%	4417
	11th to 20th	Night	17% 5% 14% 29% 25% 11%	4384
	21st to end	Night	27% 5% 11% 30% 21% 6%	4521
February	1st to 10th	Night	28% 3 11% 26% 24% 7%	2470
	11th to 20th	Night	27% 4 7% 28% 25% 8%	2260
	21st to end	Night	26% 3 6% 24% 27% 14%	1884
March	1st to 10th	Night	27% 4 6% 20% 34% 9%	3734
	11th to 20th	Night	31% 6% 12% 21% 24% 4	3503
	21st to end	Night	28% 5% 11% 34% 19% 3	3605
April	1st to 10th	Night	26% 5% 11% 23% 23% 11%	1900
	11th to 20th	Night	32% 4 10% 24% 21% 8%	1877
	21st to end	Night	35% 5% 9% 24% 22% 4	1900
May	1st to 10th	Night	32% 9% 11% 22% 16% 11%	2916
	11th to 20th	Night	31% 9% 16% 23% 19% 2	2518
	21st to end	Night	24% 6% 17% 24% 21% 8%	2762
June	1st to 10th	Night	21% 7% 17% 31% 15% 9%	1520
	11th to 20th	Night	19% 6% 13% 19% 23% 20%	1520
	21st to end	Night	17% 7% 11% 22% 29% 13%	1520
July	1st to 10th	Night	20% 5% 10% 18% 31% 15%	2500
	11th to 20th	Night	19% 4 14% 28% 23% 11%	2492
	21st to end	Night	20% 5% 13% 25% 20% 17%	2755
August	1st to 10th	Night	31% 6% 15% 24% 17% 7%	2838
	11th to 20th	Night	21% 5% 20% 21% 21% 12%	3110
	21st to end	Night	21% 4 14% 23% 30% 9%	3487
September	1st to 10th	Night	22% 5% 9% 22% 24% 18%	1900
	11th to 20th	Night	18% 4 9% 24% 27% 19%	1910
	21st to end	Night	21% 4 7% 21% 32% 14%	2090
October	1st to 10th	Night	14% 4 7% 18% 31% 27%	3797
	11th to 20th	Night	21% 2 8% 25% 33% 10%	3778
	21st to end	Night	23% 2 4 22% 35% 14%	4202
November	1st to 10th	Night	19% 4 9% 25% 29% 15%	2535
	11th to 20th	Night	20% 3 7% 27% 34% 10%	2600
	21st to end	Night	23% 3 9% 22% 32% 12%	2726
December	1st to 10th	Night	21% 4 13% 26% 26% 10%	2737
	11th to 20th	Night	22% 5% 10% 26% 31% 6%	3272
	21st to end	Night	28% 5% 12% 23% 24% 9%	4873

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	4 11% 24% 41% 14% 4	270
	11th to 20th	Day	5% 11% 38% 36% 9% 1	270
	21st to end	Day	4 10% 34% 35% 16% 1	297
February	1st to 10th	Day	5% 21% 36% 31% 5% 2	270
	11th to 20th	Day	4 12% 40% 37% 6% 1	270
	21st to end	Day	6% 15% 38% 36% 4	225
March	1st to 10th	Day	5% 13% 35% 32% 13% 3	270
	11th to 20th	Day	11% 13% 33% 31% 11% 2	445
	21st to end	Day	10% 17% 39% 24% 8% 1	506
April	1st to 10th	Day	9% 19% 31% 30% 9% 2	460
	11th to 20th	Day	12% 20% 32% 28% 8% 2	452
	21st to end	Day	12% 13% 33% 29% 11% 2	460
May	1st to 10th	Day	15% 9% 31% 32% 11% 2	577
	11th to 20th	Day	18% 6% 24% 30% 18% 4	675
	21st to end	Day	10% 7% 27% 30% 23% 4	734
June	1st to 10th	Day	6% 4 20% 34% 27% 10%	680
	11th to 20th	Day	6% 5% 26% 38% 20% 5%	680
	21st to end	Day	5% 4 25% 36% 23% 8%	680
July	1st to 10th	Day	7% 2 17% 30% 33% 12%	671
	11th to 20th	Day	5% 1 15% 33% 35% 11%	668
	21st to end	Day	6% 3 13% 30% 35% 13%	688
August	1st to 10th	Day	7% 3 20% 35% 25% 9%	487
	11th to 20th	Day	5% 2 23% 35% 27% 8%	477
	21st to end	Day	6% 2 21% 38% 26% 7%	539
September	1st to 10th	Day	6% 3 21% 33% 28% 9%	490
	11th to 20th	Day	6% 3 24% 36% 23% 7%	478
	21st to end	Day	7% 3 24% 32% 23% 11%	300
October	1st to 10th	Day	8% 8% 19% 29% 24% 12%	300
	11th to 20th	Day	3 7% 28% 38% 19% 4	300
	21st to end	Day	8% 11% 31% 32% 15% 2	330
November	1st to 10th	Day	5% 8% 28% 37% 18% 4	300
	11th to 20th	Day	8% 11% 27% 39% 13% 2	300
	21st to end	Day	7% 10% 29% 39% 11% 3	300
December	1st to 10th	Day	6% 15% 31% 33% 13% 3	285
	11th to 20th	Day	4 13% 31% 42% 9%	270
	21st to end	Day	6% 15% 33% 36% 8% 2	297
January	1st to 10th	Night	3 18% 28% 39% 10% 3	950
	11th to 20th	Night	3 21% 35% 30% 10% 2	917
	21st to end	Night	4 14% 31% 41% 8% 1	836
February	1st to 10th	Night	4 17% 38% 34% 7% 1	760
	11th to 20th	Night	3 16% 34% 38% 8%	744
	21st to end	Night	5% 13% 35% 43% 4	628
March	1st to 10th	Night	4 11% 33% 42% 8% 1	760
	11th to 20th	Night	9% 9% 26% 43% 11% 1	760
	21st to end	Night	9% 9% 32% 43% 6% 1	703
April	1st to 10th	Night	9% 9% 33% 42% 6%	570
	11th to 20th	Night	13% 8% 32% 36% 10% 1	563
	21st to end	Night	15% 6% 28% 38% 12% 1	570
May	1st to 10th	Night	13% 6% 26% 40% 14% 2	570
	11th to 20th	Night	19% 3 14% 43% 18% 4	561
	21st to end	Night	9% 3 23% 39% 24% 3	617
June	1st to 10th	Night	7% 1 13% 47% 27% 5%	570
	11th to 20th	Night	7% 2 19% 50% 22% 1	570
	21st to end	Night	4 2 14% 44% 34% 3	570
July	1st to 10th	Night	6% 3 10% 35% 42% 5%	555
	11th to 20th	Night	2 8% 39% 45% 5%	552
	21st to end	Night	7% 6% 33% 46% 7%	624
August	1st to 10th	Night	6% 1 8% 40% 40% 5%	567
	11th to 20th	Night	4 1 9% 45% 35% 6%	564
	21st to end	Night	4 1 8% 44% 35% 8%	627
September	1st to 10th	Night	6% 1 10% 43% 33% 7%	570
	11th to 20th	Night	7% 2 16% 45% 23% 7%	570
	21st to end	Night	7% 2 18% 47% 21% 5%	570
October	1st to 10th	Night	10% 4 21% 43% 19% 4	757
	11th to 20th	Night	6% 6% 22% 48% 15% 3	760
	21st to end	Night	8% 8% 29% 39% 13% 3	882
November	1st to 10th	Night	5% 8% 29% 44% 13% 2	975
	11th to 20th	Night	5% 13% 32% 41% 7% 2	1000
	21st to end	Night	6% 9% 30% 46% 10% 1	1000
December	1st to 10th	Night	4 15% 29% 42% 9%	978
	11th to 20th	Night	4 12% 34% 40% 9% 1	950
	21st to end	Night	6% 16% 31% 37% 9% 2	1045

Page generated at 2024-12-03 04:45:15. Version ac07

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