Burland Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Burland Observatory 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	4 6% 2 6% 4 4 5% 4 6% 8% 51%	124104
	Night	2 5% 2 5% 3 3 3 3 5% 7% 61%	123913
Transparency	Day	28% 7% 10% 22% 23% 10%	123852
	Night	22% 6% 11% 26% 26% 10%	123754
Seeing	Day	10% 16% 34% 26% 10% 2	16535
	Night	6% 12% 39% 30% 11% 1	26548

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 3 8% 5% 3 3 3 4 7% 61%	3638
	11th to 20th	Day	3 2 8% 6% 5% 5% 3 5% 8% 54%	3847
	21st to end	Day	4 3 7% 4 3 4 3 5% 6% 60%	4510
February	1st to 10th	Day	1 6% 3 8% 4 3 3 4 5% 8% 56%	1800
	11th to 20th	Day	4 2 5% 3 3 4 3 6% 7% 61%	1910
	21st to end	Day	1 6% 2 6% 4 3 3 3 6% 6% 59%	1650
March	1st to 10th	Day	1 7% 4 6% 3 3 3 3 5% 6% 58%	4500
	11th to 20th	Day	2 9% 7% 9% 5% 4 5% 4 6% 7% 43%	5000
	21st to end	Day	1 7% 3 6% 4 4 5% 4 7% 10% 49%	5783
April	1st to 10th	Day	3 7% 2 6% 4 4 5% 4 6% 8% 51%	2400
	11th to 20th	Day	6% 10% 3 7% 5% 5% 5% 4 5% 7% 43%	2371
	21st to end	Day	6% 12% 3 6% 5% 5% 5% 4 7% 8% 41%	2400
May	1st to 10th	Day	8% 11% 3 7% 5% 4 5% 4 6% 8% 39%	5675
	11th to 20th	Day	6% 7% 2 5% 5% 5% 6% 5% 7% 8% 42%	6298
	21st to end	Day	5% 5% 2 5% 4 5% 6% 5% 9% 9% 45%	6925
June	1st to 10th	Day	5% 5% 2 5% 4 5% 6% 5% 9% 9% 45%	2800
	11th to 20th	Day	5% 5% 2 4 4 5% 5% 5% 7% 9% 50%	2800
	21st to end	Day	5% 4 2 6% 5% 6% 7% 6% 8% 9% 43%	2800
July	1st to 10th	Day	5% 4 2 4 5% 5% 6% 5% 7% 7% 52%	4570
	11th to 20th	Day	4 4 2 6% 7% 7% 8% 6% 8% 10% 37%	4570
	21st to end	Day	7% 5% 2 6% 5% 5% 7% 5% 7% 8% 43%	4631
August	1st to 10th	Day	7% 6% 2 8% 7% 7% 7% 6% 8% 9% 31%	4092
	11th to 20th	Day	4 5% 1 6% 5% 6% 6% 6% 9% 9% 43%	3738
	21st to end	Day	5% 5% 2 6% 5% 5% 6% 5% 8% 9% 44%	4180
September	1st to 10th	Day	6% 6% 3 6% 4 5% 4 4 5% 6% 52%	2280
	11th to 20th	Day	6% 4 2 4 5% 4 4 4 5% 7% 54%	2274
	21st to end	Day	5% 4 2 5% 3 4 3 3 4 7% 61%	2081
October	1st to 10th	Day	3 5% 2 5% 3 3 3 2 3 5% 66%	3170
	11th to 20th	Day	4 3 1 4 3 4 4 4 7% 8% 58%	3150
	21st to end	Day	3 5% 2 4 2 2 2 2 4 6% 67%	3359
November	1st to 10th	Day	1 4 2 4 2 2 3 3 5% 6% 66%	1763
	11th to 20th	Day	1 4 2 4 3 3 2 2 5% 6% 66%	1800
	21st to end	Day	1 5% 3 7% 4 3 3 3 5% 7% 59%	1689
December	1st to 10th	Day	3 3 6% 3 2 2 2 3 6% 69%	2620
	11th to 20th	Day	4 1 3 2 2 2 3 4 8% 70%	3026
	21st to end	Day	1 5% 3 7% 4 3 4 3 6% 9% 55%	4004
January	1st to 10th	Night	4 3 10% 4 3 3 3 4 7% 59%	6327
	11th to 20th	Night	2 1 4 4 3 5% 3 6% 7% 64%	6174
	21st to end	Night	6% 2 6% 4 3 3 3 4 5% 64%	6468
February	1st to 10th	Night	5% 3 5% 3 3 3 3 4 9% 62%	2600
	11th to 20th	Night	1 4 2 6% 4 3 3 4 6% 6% 62%	2374
	21st to end	Night	6% 2 5% 3 3 4 3 5% 6% 64%	1980
March	1st to 10th	Night	1 7% 3 6% 3 3 3 3 5% 7% 61%	5358
	11th to 20th	Night	1 11% 4 6% 3 3 3 2 4 7% 56%	5000
	21st to end	Night	2 7% 3 5% 3 3 3 3 5% 8% 57%	5177
April	1st to 10th	Night	2 6% 3 5% 3 3 3 3 5% 9% 60%	2000
	11th to 20th	Night	5% 10% 3 6% 2 3 2 3 4 7% 55%	1977
	21st to end	Night	6% 10% 3 5% 4 3 3 3 4 7% 53%	2000
May	1st to 10th	Night	7% 10% 2 6% 3 3 4 4 6% 8% 46%	4287
	11th to 20th	Night	6% 6% 2 4 3 3 4 4 5% 7% 54%	3617
	21st to end	Night	4 5% 2 5% 3 3 3 3 5% 7% 62%	3972
June	1st to 10th	Night	4 4 1 3 2 3 3 4 7% 9% 60%	1600
	11th to 20th	Night	4 3 1 3 2 3 3 3 7% 8% 64%	1600
	21st to end	Night	3 3 1 4 3 4 5% 4 8% 9% 57%	1600
July	1st to 10th	Night	4 4 1 4 3 2 3 4 5% 7% 62%	2620
	11th to 20th	Night	1 2 1 3 3 3 4 4 8% 12% 58%	2620
	21st to end	Night	3 3 1 3 3 4 5% 5% 7% 10% 57%	2916
August	1st to 10th	Night	4 3 1 5% 4 5% 6% 5% 9% 11% 47%	2847
	11th to 20th	Night	3 2 1 4 3 4 5% 4 8% 11% 55%	3100
	21st to end	Night	3 3 2 5% 4 4 5% 5% 7% 10% 51%	3487
September	1st to 10th	Night	6% 4 2 4 4 3 4 3 6% 8% 56%	1900
	11th to 20th	Night	6% 2 1 3 3 3 4 3 5% 6% 63%	1905
	21st to end	Night	5% 5% 2 4 2 3 3 2 5% 5% 63%	2095
October	1st to 10th	Night	3 2 1 2 2 2 3 2 3 5% 74%	3800
	11th to 20th	Night	3 4 1 3 2 3 3 3 5% 7% 66%	3778
	21st to end	Night	2 5% 2 4 2 2 2 2 4 5% 70%	4165
November	1st to 10th	Night	1 4 2 3 3 3 3 2 5% 8% 68%	2535
	11th to 20th	Night	1 4 1 4 3 2 3 3 4 7% 68%	2600
	21st to end	Night	1 5% 2 5% 3 3 3 3 5% 7% 62%	2709
December	1st to 10th	Night	1 2 1 3 3 2 2 2 3 7% 75%	4570
	11th to 20th	Night	1 3 2 4 2 2 2 3 5% 7% 69%	5181
	21st to end	Night	1 8% 3 7% 4 4 3 3 5% 7% 55%	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	23% 4 10% 27% 27% 9%	3639
	11th to 20th	Day	27% 6% 10% 22% 24% 10%	3847
	21st to end	Day	24% 4 9% 25% 21% 16%	4510
February	1st to 10th	Day	27% 4 14% 26% 21% 8%	1800
	11th to 20th	Day	22% 4 11% 28% 28% 7%	1920
	21st to end	Day	24% 4 10% 23% 25% 13%	1650
March	1st to 10th	Day	23% 4 7% 19% 30% 16%	4500
	11th to 20th	Day	35% 8% 9% 23% 21% 5%	5000
	21st to end	Day	25% 5% 14% 39% 15% 2	5783
April	1st to 10th	Day	30% 7% 11% 24% 19% 9%	2400
	11th to 20th	Day	39% 6% 11% 17% 19% 7%	2374
	21st to end	Day	38% 8% 11% 25% 17% 2	2400
May	1st to 10th	Day	38% 8% 11% 23% 16% 5%	5675
	11th to 20th	Day	32% 10% 11% 19% 18% 11%	6302
	21st to end	Day	28% 8% 12% 20% 23% 8%	6926
June	1st to 10th	Day	30% 10% 15% 20% 18% 8%	2800
	11th to 20th	Day	27% 8% 10% 18% 21% 16%	2800
	21st to end	Day	31% 9% 12% 18% 19% 11%	2800
July	1st to 10th	Day	28% 8% 7% 15% 23% 19%	4501
	11th to 20th	Day	35% 11% 11% 20% 16% 8%	4495
	21st to end	Day	35% 9% 11% 16% 16% 14%	4485
August	1st to 10th	Day	44% 11% 13% 16% 12% 3	4078
	11th to 20th	Day	34% 8% 14% 15% 16% 12%	3774
	21st to end	Day	33% 6% 12% 18% 22% 9%	4179
September	1st to 10th	Day	31% 7% 9% 17% 23% 13%	2280
	11th to 20th	Day	28% 5% 10% 19% 28% 11%	2274
	21st to end	Day	23% 5% 8% 22% 32% 10%	2081
October	1st to 10th	Day	22% 4 5% 15% 30% 23%	3170
	11th to 20th	Day	22% 4 10% 28% 30% 6%	3150
	21st to end	Day	19% 3 6% 19% 41% 12%	3359
November	1st to 10th	Day	18% 5% 11% 21% 34% 12%	1763
	11th to 20th	Day	18% 4 8% 30% 28% 11%	1800
	21st to end	Day	23% 4 9% 25% 27% 12%	1689
December	1st to 10th	Day	18% 4 9% 28% 32% 10%	2620
	11th to 20th	Day	14% 4 9% 36% 25% 12%	3024
	21st to end	Day	23% 7% 8% 25% 28% 10%	4004
January	1st to 10th	Night	23% 5% 8% 26% 30% 8%	6327
	11th to 20th	Night	18% 5% 11% 26% 23% 17%	6174
	21st to end	Night	21% 5% 9% 24% 24% 16%	6468
February	1st to 10th	Night	21% 5% 12% 31% 22% 10%	2600
	11th to 20th	Night	21% 5% 10% 30% 26% 8%	2374
	21st to end	Night	20% 4 8% 27% 27% 13%	1980
March	1st to 10th	Night	23% 5% 6% 18% 35% 13%	5358
	11th to 20th	Night	28% 4 7% 21% 26% 13%	5000
	21st to end	Night	24% 6% 13% 38% 18% 2	5177
April	1st to 10th	Night	22% 7% 12% 28% 23% 7%	2000
	11th to 20th	Night	30% 5% 11% 27% 21% 6%	1977
	21st to end	Night	32% 6% 13% 28% 19% 3	2000
May	1st to 10th	Night	32% 8% 14% 26% 13% 7%	4284
	11th to 20th	Night	26% 7% 15% 25% 20% 7%	3618
	21st to end	Night	21% 6% 16% 26% 26% 5%	3972
June	1st to 10th	Night	19% 9% 20% 28% 15% 9%	1600
	11th to 20th	Night	17% 8% 13% 20% 23% 18%	1600
	21st to end	Night	20% 7% 13% 23% 25% 11%	1600
July	1st to 10th	Night	21% 6% 12% 19% 27% 16%	2580
	11th to 20th	Night	16% 7% 16% 34% 19% 8%	2572
	21st to end	Night	19% 9% 16% 29% 11% 16%	2825
August	1st to 10th	Night	29% 9% 21% 24% 14% 3	2838
	11th to 20th	Night	22% 7% 24% 20% 19% 9%	3130
	21st to end	Night	27% 8% 16% 21% 23% 5%	3487
September	1st to 10th	Night	25% 6% 11% 23% 23% 12%	1900
	11th to 20th	Night	21% 5% 12% 23% 25% 14%	1905
	21st to end	Night	22% 5% 9% 24% 31% 9%	2095
October	1st to 10th	Night	15% 5% 8% 21% 34% 17%	3800
	11th to 20th	Night	18% 4 9% 33% 27% 8%	3778
	21st to end	Night	18% 2 6% 25% 35% 14%	4165
November	1st to 10th	Night	16% 5% 11% 24% 32% 13%	2535
	11th to 20th	Night	16% 4 9% 28% 31% 12%	2600
	21st to end	Night	21% 4 10% 24% 29% 12%	2709
December	1st to 10th	Night	12% 4 9% 30% 36% 9%	4570
	11th to 20th	Night	14% 4 9% 27% 35% 11%	5182
	21st to end	Night	25% 7% 9% 23% 27% 10%	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	9% 15% 42% 24% 9% 1	300
	11th to 20th	Day	6% 24% 44% 22% 4	300
	21st to end	Day	4 20% 49% 19% 6% 2	330
February	1st to 10th	Day	14% 33% 39% 12% 1 1	300
	11th to 20th	Day	10% 30% 44% 13% 3	300
	21st to end	Day	12% 23% 46% 17% 2	249
March	1st to 10th	Day	11% 22% 40% 20% 7% 1	300
	11th to 20th	Day	14% 22% 39% 20% 4 1	500
	21st to end	Day	16% 29% 34% 17% 3	550
April	1st to 10th	Day	17% 26% 32% 21% 4	500
	11th to 20th	Day	23% 31% 29% 13% 3 1	492
	21st to end	Day	17% 24% 34% 19% 4 1	500
May	1st to 10th	Day	15% 23% 36% 21% 5% 1	591
	11th to 20th	Day	17% 14% 33% 27% 8% 1	725
	21st to end	Day	12% 16% 29% 28% 13% 2	789
June	1st to 10th	Day	7% 11% 31% 33% 14% 3	730
	11th to 20th	Day	7% 13% 35% 30% 13% 2	730
	21st to end	Day	5% 11% 39% 27% 14% 3	730
July	1st to 10th	Day	7% 6% 25% 39% 19% 5%	721
	11th to 20th	Day	6% 4 25% 38% 22% 6%	718
	21st to end	Day	9% 6% 20% 36% 23% 6%	643
August	1st to 10th	Day	11% 6% 29% 32% 17% 3	487
	11th to 20th	Day	7% 6% 31% 37% 15% 3	477
	21st to end	Day	8% 9% 31% 36% 13% 3	539
September	1st to 10th	Day	9% 9% 32% 32% 15% 3	490
	11th to 20th	Day	8% 13% 35% 31% 12% 2	484
	21st to end	Day	8% 13% 32% 28% 13% 5%	300
October	1st to 10th	Day	8% 18% 28% 26% 14% 6%	300
	11th to 20th	Day	5% 21% 37% 26% 8% 2	300
	21st to end	Day	9% 19% 39% 26% 6% 1	330
November	1st to 10th	Day	9% 20% 43% 18% 9% 1	300
	11th to 20th	Day	11% 19% 39% 25% 6% 1	300
	21st to end	Day	7% 21% 44% 22% 6%	300
December	1st to 10th	Day	11% 22% 39% 19% 8% 1	300
	11th to 20th	Day	8% 18% 52% 18% 4	300
	21st to end	Day	13% 25% 40% 15% 7% 1	330
January	1st to 10th	Night	5% 19% 42% 26% 7% 2	1000
	11th to 20th	Night	6% 22% 45% 19% 6% 1	925
	21st to end	Night	4 14% 54% 23% 5%	880
February	1st to 10th	Night	6% 19% 56% 16% 4	800
	11th to 20th	Night	4 18% 52% 21% 4	784
	21st to end	Night	4 20% 51% 23% 2	660
March	1st to 10th	Night	4 16% 51% 24% 5% 1	800
	11th to 20th	Night	6% 13% 46% 27% 7% 1	800
	21st to end	Night	5% 15% 51% 23% 5%	749
April	1st to 10th	Night	7% 15% 50% 24% 3	600
	11th to 20th	Night	10% 17% 41% 25% 6% 1	593
	21st to end	Night	10% 16% 41% 26% 7% 1	600
May	1st to 10th	Night	12% 8% 38% 31% 10% 1	600
	11th to 20th	Night	13% 6% 32% 38% 10% 2	591
	21st to end	Night	6% 9% 32% 36% 14% 2	650
June	1st to 10th	Night	5% 4 26% 46% 17% 2	600
	11th to 20th	Night	6% 5% 35% 40% 12% 2	600
	21st to end	Night	3 2 26% 45% 23% 2	600
July	1st to 10th	Night	7% 2 16% 43% 29% 2	585
	11th to 20th	Night	2 1 17% 42% 36% 3	582
	21st to end	Night	6% 2 9% 46% 33% 4	624
August	1st to 10th	Night	6% 1 15% 45% 30% 3	567
	11th to 20th	Night	6% 2 19% 45% 25% 3	564
	21st to end	Night	5% 2 20% 44% 25% 4	627
September	1st to 10th	Night	6% 2 20% 44% 23% 5%	570
	11th to 20th	Night	6% 2 28% 42% 18% 3	570
	21st to end	Night	7% 6% 30% 39% 16% 3	575
October	1st to 10th	Night	6% 8% 39% 32% 12% 3	760
	11th to 20th	Night	4 9% 41% 34% 9% 3	760
	21st to end	Night	7% 10% 44% 27% 11% 1	857
November	1st to 10th	Night	3 13% 47% 27% 9% 1	975
	11th to 20th	Night	6% 17% 49% 22% 6% 1	1000
	21st to end	Night	5% 15% 46% 27% 7% 1	1000
December	1st to 10th	Night	6% 18% 44% 27% 5%	1000
	11th to 20th	Night	5% 17% 52% 20% 5% 1	1000
	21st to end	Night	7% 20% 47% 21% 5% 1	1100

Page generated at 2025-07-17 23:10:00. Version ac07

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