Berthoud
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Berthoud clear sky chart from 2005-11-06 to 2025-07-21.

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	5% 5% 2 5% 4 4 4 4 6% 8% 53%	124156
	Night	5% 6% 2 5% 3 3 3 3 5% 7% 57%	123881
Transparency	Day	26% 7% 11% 23% 23% 10%	123900
	Night	26% 5% 11% 24% 24% 9%	123726
Seeing	Day	8% 6% 23% 35% 21% 7%	16620
	Night	7% 4 25% 45% 17% 2	26619

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	2 7% 3 8% 4 3 3 3 5% 7% 55%	3638
	11th to 20th	Day	1 6% 2 9% 6% 6% 4 3 5% 6% 52%	3704
	21st to end	Day	1 5% 3 5% 3 3 4 3 5% 7% 61%	4510
February	1st to 10th	Day	3 6% 3 7% 4 4 4 3 5% 9% 52%	1800
	11th to 20th	Day	1 6% 2 6% 4 4 4 3 5% 9% 56%	1909
	21st to end	Day	3 6% 2 6% 3 2 4 3 6% 6% 59%	1650
March	1st to 10th	Day	5% 7% 2 4 2 3 3 3 5% 7% 60%	4500
	11th to 20th	Day	4 7% 3 7% 4 4 5% 4 6% 8% 48%	4942
	21st to end	Day	5% 7% 2 5% 4 4 4 4 7% 9% 48%	5827
April	1st to 10th	Day	5% 7% 2 4 3 3 3 3 6% 8% 54%	2400
	11th to 20th	Day	10% 9% 2 6% 4 3 4 3 6% 7% 46%	2371
	21st to end	Day	9% 8% 2 6% 4 5% 4 4 6% 8% 44%	2400
May	1st to 10th	Day	11% 6% 3 6% 5% 4 5% 4 5% 6% 45%	5821
	11th to 20th	Day	8% 5% 2 6% 4 5% 5% 5% 7% 8% 44%	6298
	21st to end	Day	9% 4 2 5% 4 5% 5% 4 7% 9% 46%	6925
June	1st to 10th	Day	5% 4 2 4 4 5% 5% 5% 8% 11% 48%	2800
	11th to 20th	Day	6% 3 1 3 4 4 6% 5% 7% 9% 52%	2800
	21st to end	Day	4 3 1 4 4 5% 6% 6% 8% 9% 52%	2800
July	1st to 10th	Day	5% 3 1 5% 3 4 5% 4 6% 7% 57%	4570
	11th to 20th	Day	4 2 1 6% 5% 6% 7% 6% 9% 9% 46%	4570
	21st to end	Day	4 3 1 4 4 5% 5% 4 7% 7% 56%	4757
August	1st to 10th	Day	11% 4 2 6% 5% 5% 6% 5% 8% 9% 39%	4092
	11th to 20th	Day	4 3 1 5% 4 4 6% 5% 8% 9% 51%	3755
	21st to end	Day	4 3 2 4 3 4 5% 5% 7% 9% 54%	4180
September	1st to 10th	Day	6% 4 1 4 3 3 4 4 5% 7% 59%	2280
	11th to 20th	Day	6% 2 1 4 3 3 4 3 5% 7% 61%	2260
	21st to end	Day	7% 4 2 3 2 2 3 3 3 5% 65%	2090
October	1st to 10th	Day	5% 3 1 3 2 2 2 2 3 4 71%	3181
	11th to 20th	Day	7% 4 1 4 3 4 3 3 5% 7% 60%	3150
	21st to end	Day	6% 5% 2 4 2 2 2 3 5% 7% 62%	3293
November	1st to 10th	Day	3 4 2 3 3 3 3 3 6% 8% 62%	1763
	11th to 20th	Day	3 5% 2 4 3 3 3 3 5% 7% 63%	1800
	21st to end	Day	3 5% 3 5% 3 3 3 3 6% 8% 58%	1670
December	1st to 10th	Day	1 4 1 4 2 2 3 3 3 7% 68%	2620
	11th to 20th	Day	2 4 2 3 3 2 3 3 4 6% 69%	3026
	21st to end	Day	4 6% 2 7% 4 4 4 3 5% 7% 52%	4004
January	1st to 10th	Night	3 9% 3 7% 4 3 3 2 5% 7% 54%	6327
	11th to 20th	Night	1 5% 3 10% 5% 3 3 3 5% 6% 56%	6319
	21st to end	Night	2 6% 3 8% 4 3 3 2 4 5% 60%	6468
February	1st to 10th	Night	2 9% 4 7% 3 3 4 3 5% 8% 53%	2600
	11th to 20th	Night	2 8% 3 7% 5% 4 4 3 5% 6% 51%	2377
	21st to end	Night	2 9% 3 6% 4 3 3 3 4 6% 58%	1980
March	1st to 10th	Night	4 10% 2 5% 4 4 3 3 4 5% 55%	5374
	11th to 20th	Night	8% 8% 2 4 3 3 4 3 4 6% 54%	5054
	21st to end	Night	9% 9% 3 6% 4 3 3 3 4 7% 49%	5132
April	1st to 10th	Night	6% 7% 2 5% 4 3 4 3 5% 7% 54%	2000
	11th to 20th	Night	11% 7% 2 6% 3 4 3 3 5% 7% 49%	1977
	21st to end	Night	14% 8% 2 5% 4 4 3 3 4 8% 46%	2000
May	1st to 10th	Night	14% 5% 2 6% 4 3 4 3 6% 7% 45%	4143
	11th to 20th	Night	9% 7% 2 7% 4 5% 4 4 6% 8% 45%	3617
	21st to end	Night	12% 5% 2 5% 3 3 3 3 4 6% 53%	3972
June	1st to 10th	Night	7% 2 2 4 4 3 2 4 6% 9% 57%	1600
	11th to 20th	Night	6% 2 1 4 3 3 3 3 6% 7% 61%	1600
	21st to end	Night	4 3 2 4 3 3 5% 4 6% 8% 60%	1600
July	1st to 10th	Night	5% 3 1 3 3 3 4 3 6% 8% 60%	2620
	11th to 20th	Night	3 1 1 3 3 3 5% 5% 8% 11% 57%	2620
	21st to end	Night	4 2 1 4 4 3 5% 4 7% 8% 57%	2791
August	1st to 10th	Night	11% 4 2 5% 4 4 5% 5% 8% 9% 44%	2847
	11th to 20th	Night	4 3 1 4 3 4 5% 5% 8% 9% 54%	3084
	21st to end	Night	3 3 2 4 3 4 5% 4 8% 10% 55%	3487
September	1st to 10th	Night	6% 3 1 3 3 3 4 4 4 7% 60%	1900
	11th to 20th	Night	7% 2 1 4 2 2 3 3 5% 7% 65%	1919
	21st to end	Night	8% 3 1 3 3 3 3 2 4 6% 64%	2090
October	1st to 10th	Night	7% 3 1 2 2 2 2 2 3 4 73%	3786
	11th to 20th	Night	7% 4 2 5% 3 2 3 3 4 6% 60%	3778
	21st to end	Night	7% 6% 2 3 2 2 3 2 3 6% 64%	4232
November	1st to 10th	Night	4 5% 1 4 2 4 2 3 4 7% 63%	2535
	11th to 20th	Night	3 5% 1 4 3 2 3 3 5% 6% 65%	2600
	21st to end	Night	3 7% 2 4 3 3 3 3 5% 7% 60%	2727
December	1st to 10th	Night	2 4 1 4 2 3 2 2 4 5% 71%	4570
	11th to 20th	Night	2 5% 2 4 3 2 3 3 5% 9% 63%	5181
	21st to end	Night	5% 10% 3 6% 4 3 4 3 5% 6% 52%	6974

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	26% 6% 9% 32% 20% 7%	3639
	11th to 20th	Day	28% 8% 10% 21% 24% 8%	3704
	21st to end	Day	21% 6% 11% 26% 26% 12%	4510
February	1st to 10th	Day	29% 5% 13% 27% 19% 6%	1800
	11th to 20th	Day	24% 5% 12% 29% 25% 5%	1919
	21st to end	Day	24% 6% 10% 24% 25% 11%	1650
March	1st to 10th	Day	24% 4 8% 20% 32% 11%	4500
	11th to 20th	Day	30% 9% 11% 21% 24% 4	4942
	21st to end	Day	27% 9% 13% 35% 14% 2	5827
April	1st to 10th	Day	26% 6% 13% 25% 21% 8%	2400
	11th to 20th	Day	35% 7% 11% 21% 18% 7%	2374
	21st to end	Day	34% 9% 13% 24% 19% 2	2400
May	1st to 10th	Day	33% 9% 13% 26% 15% 4	5821
	11th to 20th	Day	30% 11% 12% 19% 18% 9%	6302
	21st to end	Day	30% 9% 15% 26% 17% 4	6926
June	1st to 10th	Day	24% 12% 16% 24% 17% 7%	2800
	11th to 20th	Day	25% 8% 13% 19% 23% 14%	2800
	21st to end	Day	23% 9% 12% 21% 22% 12%	2800
July	1st to 10th	Day	24% 6% 8% 15% 30% 16%	4501
	11th to 20th	Day	30% 6% 10% 21% 19% 13%	4495
	21st to end	Day	24% 6% 11% 18% 21% 20%	4607
August	1st to 10th	Day	38% 7% 14% 18% 15% 8%	4078
	11th to 20th	Day	27% 5% 15% 19% 21% 13%	3791
	21st to end	Day	25% 5% 10% 19% 26% 15%	4179
September	1st to 10th	Day	22% 5% 10% 19% 26% 17%	2280
	11th to 20th	Day	20% 6% 10% 20% 27% 16%	2260
	21st to end	Day	21% 4 7% 19% 36% 12%	2090
October	1st to 10th	Day	19% 3 6% 16% 33% 24%	3181
	11th to 20th	Day	26% 3 9% 26% 28% 8%	3150
	21st to end	Day	23% 3 6% 25% 33% 10%	3293
November	1st to 10th	Day	19% 6% 11% 25% 27% 12%	1763
	11th to 20th	Day	20% 5% 10% 25% 32% 9%	1800
	21st to end	Day	23% 3 10% 23% 30% 9%	1670
December	1st to 10th	Day	17% 4 8% 33% 28% 10%	2620
	11th to 20th	Day	16% 4 9% 33% 33% 5%	3024
	21st to end	Day	29% 4 11% 26% 22% 8%	4004
January	1st to 10th	Night	28% 5% 11% 27% 21% 7%	6327
	11th to 20th	Night	25% 9% 11% 22% 22% 12%	6319
	21st to end	Night	26% 6% 10% 24% 25% 10%	6468
February	1st to 10th	Night	28% 6% 12% 27% 22% 6%	2600
	11th to 20th	Night	31% 5% 12% 27% 21% 5%	2377
	21st to end	Night	27% 4 8% 23% 26% 12%	1980
March	1st to 10th	Night	29% 5% 6% 17% 35% 8%	5374
	11th to 20th	Night	29% 6% 10% 16% 28% 12%	5054
	21st to end	Night	33% 6% 12% 29% 18% 2	5132
April	1st to 10th	Night	27% 7% 11% 24% 22% 9%	2000
	11th to 20th	Night	32% 5% 13% 24% 18% 6%	1977
	21st to end	Night	37% 5% 10% 25% 19% 4	2000
May	1st to 10th	Night	33% 9% 15% 25% 13% 5%	4140
	11th to 20th	Night	33% 9% 15% 20% 16% 8%	3618
	21st to end	Night	29% 8% 16% 26% 15% 5%	3972
June	1st to 10th	Night	22% 8% 19% 29% 14% 8%	1600
	11th to 20th	Night	21% 6% 14% 20% 21% 18%	1600
	21st to end	Night	20% 7% 11% 24% 29% 10%	1600
July	1st to 10th	Night	22% 6% 10% 21% 28% 12%	2580
	11th to 20th	Night	19% 4 17% 26% 24% 9%	2572
	21st to end	Night	22% 6% 12% 26% 19% 15%	2704
August	1st to 10th	Night	34% 6% 15% 23% 17% 5%	2838
	11th to 20th	Night	25% 6% 18% 22% 18% 11%	3114
	21st to end	Night	23% 4 14% 24% 26% 10%	3487
September	1st to 10th	Night	22% 6% 11% 21% 21% 19%	1900
	11th to 20th	Night	17% 5% 12% 24% 24% 18%	1919
	21st to end	Night	21% 4 9% 22% 31% 14%	2090
October	1st to 10th	Night	17% 4 7% 18% 31% 24%	3786
	11th to 20th	Night	26% 3 8% 26% 27% 9%	3778
	21st to end	Night	24% 3 5% 25% 32% 11%	4232
November	1st to 10th	Night	20% 5% 11% 25% 26% 13%	2535
	11th to 20th	Night	20% 4 9% 28% 30% 9%	2600
	21st to end	Night	22% 5% 11% 23% 31% 9%	2727
December	1st to 10th	Night	17% 3 8% 35% 30% 7%	4570
	11th to 20th	Night	19% 4 7% 36% 30% 4	5182
	21st to end	Night	32% 5% 9% 23% 26% 6%	6974

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	6% 8% 15% 47% 18% 5%	300
	11th to 20th	Day	6% 6% 27% 45% 15% 1	300
	21st to end	Day	3 3 30% 45% 17% 2	330
February	1st to 10th	Day	5% 9% 35% 43% 6% 2	300
	11th to 20th	Day	3 6% 37% 44% 9% 1	300
	21st to end	Day	6% 9% 35% 41% 9% 1	249
March	1st to 10th	Day	6% 7% 35% 32% 17% 3	300
	11th to 20th	Day	12% 11% 30% 32% 13% 3	468
	21st to end	Day	10% 13% 39% 27% 9% 3	550
April	1st to 10th	Day	10% 12% 33% 32% 12% 2	500
	11th to 20th	Day	14% 17% 33% 27% 8% 1	492
	21st to end	Day	15% 12% 27% 32% 13% 2	500
May	1st to 10th	Day	14% 8% 26% 34% 14% 3	647
	11th to 20th	Day	18% 5% 19% 31% 20% 6%	725
	21st to end	Day	10% 6% 25% 29% 25% 5%	789
June	1st to 10th	Day	6% 4 16% 35% 28% 11%	730
	11th to 20th	Day	6% 2 24% 37% 25% 5%	730
	21st to end	Day	5% 4 20% 37% 27% 8%	730
July	1st to 10th	Day	7% 2 14% 31% 34% 12%	721
	11th to 20th	Day	5% 1 14% 33% 34% 13%	718
	21st to end	Day	6% 2 13% 27% 38% 15%	718
August	1st to 10th	Day	7% 2 18% 38% 25% 10%	487
	11th to 20th	Day	3 3 18% 36% 30% 10%	477
	21st to end	Day	6% 2 20% 35% 28% 9%	539
September	1st to 10th	Day	6% 3 18% 34% 27% 11%	490
	11th to 20th	Day	5% 2 21% 34% 29% 9%	470
	21st to end	Day	8% 2 23% 28% 25% 14%	300
October	1st to 10th	Day	12% 4 17% 26% 28% 14%	300
	11th to 20th	Day	3 6% 23% 43% 18% 8%	300
	21st to end	Day	9% 5% 27% 35% 19% 4	330
November	1st to 10th	Day	7% 6% 23% 40% 17% 8%	300
	11th to 20th	Day	7% 5% 24% 44% 16% 5%	300
	21st to end	Day	7% 3 24% 49% 15% 3	300
December	1st to 10th	Day	5% 7% 22% 44% 17% 5%	300
	11th to 20th	Day	4 7% 21% 56% 11% 1	300
	21st to end	Day	6% 8% 25% 48% 9% 4	330
January	1st to 10th	Night	4 7% 32% 44% 10% 3	1000
	11th to 20th	Night	4 7% 36% 42% 11% 1	992
	21st to end	Night	5% 3 32% 50% 9% 1	880
February	1st to 10th	Night	6% 4 43% 40% 7% 1	800
	11th to 20th	Night	4 5% 37% 45% 9%	784
	21st to end	Night	6% 5% 39% 46% 4	660
March	1st to 10th	Night	4 4 35% 47% 10%	800
	11th to 20th	Night	10% 4 28% 46% 12% 1	800
	21st to end	Night	9% 7% 29% 46% 8% 1	730
April	1st to 10th	Night	9% 6% 33% 47% 5%	600
	11th to 20th	Night	15% 5% 32% 39% 10% 1	593
	21st to end	Night	17% 5% 27% 37% 13% 1	600
May	1st to 10th	Night	15% 4 26% 39% 15% 1	600
	11th to 20th	Night	17% 3 16% 41% 20% 3	591
	21st to end	Night	10% 4 22% 39% 24% 2	650
June	1st to 10th	Night	7% 2 13% 48% 27% 3	600
	11th to 20th	Night	6% 2 19% 55% 18% 1	600
	21st to end	Night	5% 2 14% 50% 28% 2	600
July	1st to 10th	Night	8% 2 10% 38% 38% 4	585
	11th to 20th	Night	3 1 6% 47% 40% 3	582
	21st to end	Night	6% 1 8% 35% 46% 5%	624
August	1st to 10th	Night	6% 1 10% 39% 40% 4	567
	11th to 20th	Night	4 1 7% 47% 37% 3	564
	21st to end	Night	4 1 7% 45% 38% 5%	627
September	1st to 10th	Night	7% 2 8% 45% 31% 7%	570
	11th to 20th	Night	7% 1 12% 48% 28% 5%	570
	21st to end	Night	8% 1 16% 49% 24% 3	570
October	1st to 10th	Night	10% 1 17% 46% 21% 4	746
	11th to 20th	Night	8% 2 22% 49% 18% 2	760
	21st to end	Night	11% 3 23% 46% 14% 3	899
November	1st to 10th	Night	6% 2 27% 49% 14% 2	975
	11th to 20th	Night	5% 6% 32% 48% 8% 1	1000
	21st to end	Night	6% 4 27% 49% 13% 1	1000
December	1st to 10th	Night	5% 5% 30% 49% 11%	1000
	11th to 20th	Night	5% 4 35% 46% 9% 1	1000
	21st to end	Night	7% 4 35% 45% 8% 1	1100

Page generated at 2025-07-17 23:10:03. Version ac07

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