Blackwell
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Blackwell clear sky chart from 2005-11-06 to 2024-11-28.

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	7% 4 2 4 3 3 3 3 5% 7% 59%	101151
	Night	7% 4 1 4 3 3 3 3 5% 7% 61%	98509
Transparency	Day	25% 5% 9% 21% 26% 14%	105035
	Night	23% 4 9% 22% 27% 16%	102753
Seeing	Day	9% 7% 24% 31% 23% 6%	15090
	Night	9% 7% 31% 38% 15% 1	25047

By Month

Percentage Cloud Cover SE (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	8% 6% 2 5% 2 3 2 2 4 6% 61%	2847
	11th to 20th	Day	5% 4 2 3 2 2 2 2 4 6% 67%	2886
	21st to end	Day	11% 7% 2 4 2 2 3 3 5% 6% 55%	3487
February	1st to 10th	Day	9% 5% 1 4 3 3 4 2 5% 7% 58%	1900
	11th to 20th	Day	8% 7% 2 6% 4 3 4 3 3 4 57%	1849
	21st to end	Day	9% 7% 2 4 3 1 3 2 4 5% 59%	1570
March	1st to 10th	Day	9% 5% 2 4 3 4 4 3 5% 7% 55%	3147
	11th to 20th	Day	15% 7% 2 5% 3 4 3 3 5% 5% 48%	3480
	21st to end	Day	5% 6% 2 5% 4 4 4 4 5% 8% 54%	3814
April	1st to 10th	Day	7% 4 2 4 3 3 3 4 5% 7% 59%	2201
	11th to 20th	Day	10% 6% 2 5% 3 3 3 3 5% 5% 55%	2254
	21st to end	Day	8% 5% 1 4 4 3 3 3 4 5% 61%	2309
May	1st to 10th	Day	10% 6% 3 5% 3 4 3 3 4 6% 53%	4082
	11th to 20th	Day	13% 6% 2 5% 4 3 3 3 4 5% 53%	4082
	21st to end	Day	9% 5% 2 5% 4 4 4 3 5% 7% 53%	4477
June	1st to 10th	Day	3 3 1 4 3 3 4 4 5% 8% 62%	2470
	11th to 20th	Day	4 4 1 3 3 4 3 3 6% 8% 60%	2470
	21st to end	Day	6% 4 1 3 3 3 4 3 6% 7% 60%	2470
July	1st to 10th	Day	4 3 1 4 3 3 4 4 6% 9% 57%	4120
	11th to 20th	Day	2 2 1 3 3 3 4 3 6% 9% 65%	4120
	21st to end	Day	4 3 1 4 4 4 4 4 5% 8% 60%	4465
August	1st to 10th	Day	1 1 1 2 2 3 3 4 5% 7% 70%	3822
	11th to 20th	Day	3 3 1 3 3 3 3 3 5% 8% 65%	3726
	21st to end	Day	3 3 1 5% 3 4 5% 4 6% 9% 57%	4180
September	1st to 10th	Day	10% 5% 2 5% 4 4 5% 4 5% 6% 51%	2154
	11th to 20th	Day	10% 5% 2 5% 4 5% 4 3 5% 8% 50%	2090
	21st to end	Day	9% 3 2 4 3 4 3 3 5% 6% 57%	2090
October	1st to 10th	Day	4 4 1 4 3 3 3 3 5% 7% 64%	2090
	11th to 20th	Day	9% 4 2 4 2 2 2 2 3 5% 66%	2081
	21st to end	Day	11% 3 1 3 2 2 2 2 3 4 67%	2076
November	1st to 10th	Day	9% 4 1 4 2 4 2 2 4 6% 62%	1763
	11th to 20th	Day	4 4 1 3 3 2 3 3 4 6% 66%	1800
	21st to end	Day	9% 5% 2 3 3 3 4 2 4 6% 59%	1773
December	1st to 10th	Day	12% 5% 1 4 2 3 2 3 4 7% 56%	1710
	11th to 20th	Day	8% 5% 2 5% 4 4 4 4 4 5% 56%	2161
	21st to end	Day	10% 5% 2 4 3 2 2 2 3 5% 63%	3135
January	1st to 10th	Night	8% 5% 2 5% 2 3 2 2 4 6% 62%	4108
	11th to 20th	Night	7% 5% 1 3 2 2 2 2 4 5% 68%	4111
	21st to end	Night	10% 5% 2 4 3 3 3 3 5% 7% 56%	4180
February	1st to 10th	Night	8% 6% 3 4 3 3 2 3 3 6% 60%	2280
	11th to 20th	Night	10% 6% 2 4 3 4 3 3 4 6% 55%	2218
	21st to end	Night	8% 6% 2 4 2 2 2 2 3 6% 61%	1884
March	1st to 10th	Night	6% 5% 2 5% 3 3 4 3 4 6% 58%	3714
	11th to 20th	Night	12% 6% 2 4 3 3 3 3 6% 6% 51%	3480
	21st to end	Night	6% 4 1 4 3 3 5% 3 6% 7% 58%	3817
April	1st to 10th	Night	5% 4 2 3 4 3 3 3 5% 7% 61%	1973
	11th to 20th	Night	9% 4 2 3 2 3 4 3 5% 8% 56%	1877
	21st to end	Night	9% 3 2 3 2 2 2 2 4 6% 64%	1867
May	1st to 10th	Night	12% 6% 2 5% 3 3 2 3 4 5% 55%	2823
	11th to 20th	Night	6% 4 2 4 3 3 4 3 5% 7% 59%	2823
	21st to end	Night	7% 4 1 4 3 2 3 3 6% 8% 58%	3090
June	1st to 10th	Night	2 2 1 3 3 4 4 3 8% 11% 60%	1710
	11th to 20th	Night	3 2 1 3 2 2 4 4 6% 9% 63%	1710
	21st to end	Night	4 3 1 2 2 3 3 3 6% 7% 66%	1710
July	1st to 10th	Night	3 2 1 4 3 3 3 4 6% 10% 62%	2850
	11th to 20th	Night	1 1 2 2 2 2 3 5% 10% 71%	2850
	21st to end	Night	2 2 1 3 2 3 4 4 6% 7% 65%	3098
August	1st to 10th	Night	1 1 1 3 2 3 3 3 4 8% 72%	3113
	11th to 20th	Night	2 3 1 2 3 3 3 2 5% 8% 68%	3113
	21st to end	Night	3 2 1 4 3 4 4 4 7% 9% 61%	3487
September	1st to 10th	Night	6% 4 2 4 3 4 3 3 6% 8% 57%	2024
	11th to 20th	Night	7% 5% 2 5% 3 4 3 4 6% 8% 53%	2090
	21st to end	Night	8% 4 1 3 3 3 3 3 5% 8% 60%	2090
October	1st to 10th	Night	4 2 1 3 2 3 3 3 5% 8% 66%	2090
	11th to 20th	Night	8% 3 1 3 3 2 2 2 3 5% 68%	2090
	21st to end	Night	8% 3 1 3 2 2 3 2 3 5% 67%	2504
November	1st to 10th	Night	9% 4 1 4 2 2 2 2 3 5% 64%	2535
	11th to 20th	Night	5% 2 1 3 2 3 3 3 4 7% 67%	2600
	21st to end	Night	11% 4 2 4 3 3 3 2 4 5% 57%	2562
December	1st to 10th	Night	9% 4 2 3 3 2 2 3 4 7% 60%	2470
	11th to 20th	Night	9% 5% 2 5% 4 3 3 3 4 6% 55%	3036
	21st to end	Night	8% 6% 2 5% 3 2 3 3 3 5% 60%	4532

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% 3 6% 16% 30% 19%	2848
	11th to 20th	Day	20% 3 8% 18% 34% 16%	2886
	21st to end	Day	30% 4 6% 18% 31% 10%	3487
February	1st to 10th	Day	26% 4 7% 17% 27% 19%	1900
	11th to 20th	Day	32% 3 5% 21% 22% 16%	1859
	21st to end	Day	29% 2 5% 12% 30% 22%	1570
March	1st to 10th	Day	29% 4 6% 11% 24% 27%	3147
	11th to 20th	Day	37% 6% 10% 11% 24% 12%	3480
	21st to end	Day	27% 5% 10% 15% 25% 18%	3814
April	1st to 10th	Day	24% 5% 10% 18% 22% 21%	2201
	11th to 20th	Day	32% 4 8% 13% 24% 19%	2254
	21st to end	Day	26% 4 8% 18% 27% 16%	2309
May	1st to 10th	Day	32% 6% 7% 15% 17% 23%	4120
	11th to 20th	Day	33% 5% 9% 17% 25% 12%	4086
	21st to end	Day	30% 6% 12% 20% 16% 16%	4489
June	1st to 10th	Day	19% 7% 16% 30% 22% 7%	2470
	11th to 20th	Day	21% 5% 11% 28% 26% 9%	2470
	21st to end	Day	23% 6% 12% 25% 24% 10%	2470
July	1st to 10th	Day	21% 8% 15% 35% 17% 4	4056
	11th to 20th	Day	16% 5% 16% 35% 25% 4	4051
	21st to end	Day	19% 5% 12% 32% 27% 4	4325
August	1st to 10th	Day	13% 4 11% 31% 36% 6%	3808
	11th to 20th	Day	17% 6% 14% 34% 26% 3	3762
	21st to end	Day	22% 8% 16% 25% 23% 5%	4179
September	1st to 10th	Day	32% 7% 9% 20% 22% 11%	2154
	11th to 20th	Day	33% 7% 10% 21% 23% 7%	2090
	21st to end	Day	28% 4 9% 19% 26% 14%	2090
October	1st to 10th	Day	13% 3 7% 14% 31% 32%	3480
	11th to 20th	Day	22% 3 5% 20% 24% 26%	3437
	21st to end	Day	17% 2 3 25% 39% 14%	3403
November	1st to 10th	Day	25% 4 6% 21% 26% 17%	1763
	11th to 20th	Day	20% 3 9% 22% 28% 19%	1800
	21st to end	Day	28% 3 6% 19% 28% 16%	1773
December	1st to 10th	Day	29% 4 7% 19% 30% 12%	1710
	11th to 20th	Day	30% 4 8% 14% 25% 18%	2159
	21st to end	Day	27% 2 4 19% 30% 19%	3135
January	1st to 10th	Night	25% 2 5% 17% 33% 18%	4108
	11th to 20th	Night	20% 3 7% 21% 29% 21%	4111
	21st to end	Night	29% 4 7% 17% 27% 15%	4180
February	1st to 10th	Night	29% 3 6% 16% 26% 21%	2280
	11th to 20th	Night	31% 4 6% 18% 22% 19%	2218
	21st to end	Night	27% 3 4 14% 27% 25%	1884
March	1st to 10th	Night	26% 4 6% 13% 28% 23%	3714
	11th to 20th	Night	32% 6% 11% 17% 20% 14%	3480
	21st to end	Night	23% 6% 11% 19% 27% 14%	3817
April	1st to 10th	Night	23% 5% 10% 21% 17% 24%	1973
	11th to 20th	Night	27% 6% 9% 17% 21% 20%	1877
	21st to end	Night	23% 4 9% 22% 28% 14%	1867
May	1st to 10th	Night	30% 6% 9% 16% 19% 20%	2850
	11th to 20th	Night	24% 8% 11% 21% 22% 14%	2824
	21st to end	Night	23% 7% 12% 23% 19% 17%	3099
June	1st to 10th	Night	17% 7% 17% 31% 21% 6%	1710
	11th to 20th	Night	16% 6% 12% 30% 27% 9%	1710
	21st to end	Night	17% 6% 12% 30% 24% 11%	1710
July	1st to 10th	Night	17% 6% 15% 38% 19% 4	2805
	11th to 20th	Night	10% 3 11% 40% 29% 7%	2796
	21st to end	Night	16% 6% 11% 32% 31% 3	2999
August	1st to 10th	Night	11% 4 9% 29% 40% 7%	3103
	11th to 20th	Night	15% 5% 14% 35% 26% 5%	3143
	21st to end	Night	19% 7% 20% 26% 23% 5%	3487
September	1st to 10th	Night	25% 7% 10% 24% 21% 13%	2024
	11th to 20th	Night	29% 5% 13% 23% 23% 7%	2090
	21st to end	Night	23% 4 8% 24% 24% 17%	2090
October	1st to 10th	Night	10% 3 7% 14% 31% 35%	3480
	11th to 20th	Night	19% 3 7% 16% 28% 27%	3460
	21st to end	Night	17% 2 4 23% 39% 15%	4128
November	1st to 10th	Night	25% 3 7% 20% 25% 20%	2535
	11th to 20th	Night	18% 4 8% 20% 29% 20%	2600
	21st to end	Night	30% 3 5% 15% 30% 17%	2562
December	1st to 10th	Night	25% 3 7% 20% 27% 17%	2470
	11th to 20th	Night	31% 3 5% 18% 25% 18%	3037
	21st to end	Night	28% 3 4 18% 29% 19%	4532

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	13% 11% 39% 30% 7% 1	270
	11th to 20th	Day	13% 15% 31% 29% 10% 2	270
	21st to end	Day	10% 17% 37% 26% 9% 1	297
February	1st to 10th	Day	16% 11% 38% 28% 6% 1	270
	11th to 20th	Day	17% 14% 40% 22% 6% 1	270
	21st to end	Day	16% 18% 44% 19% 3	225
March	1st to 10th	Day	12% 15% 40% 29% 4	270
	11th to 20th	Day	18% 13% 36% 26% 7% 1	270
	21st to end	Day	9% 18% 43% 25% 5%	297
April	1st to 10th	Day	9% 16% 48% 21% 7%	381
	11th to 20th	Day	12% 17% 47% 21% 3 1	452
	21st to end	Day	8% 19% 41% 24% 7% 1	489
May	1st to 10th	Day	11% 12% 37% 33% 7%	653
	11th to 20th	Day	13% 6% 33% 32% 13% 3	675
	21st to end	Day	12% 5% 32% 37% 13% 1	734
June	1st to 10th	Day	4 3 18% 48% 24% 4	680
	11th to 20th	Day	4 1 22% 48% 22% 3	680
	21st to end	Day	5% 1 13% 34% 39% 8%	680
July	1st to 10th	Day	6% 1 6% 32% 43% 12%	671
	11th to 20th	Day	2 4 35% 46% 13%	668
	21st to end	Day	2 5% 27% 49% 17%	739
August	1st to 10th	Day	2 1 3 28% 50% 17%	511
	11th to 20th	Day	5% 1 5% 35% 41% 13%	477
	21st to end	Day	4 1 6% 33% 42% 14%	539
September	1st to 10th	Day	10% 2 5% 25% 39% 19%	364
	11th to 20th	Day	6% 2 7% 28% 41% 16%	300
	21st to end	Day	12% 2 10% 26% 35% 14%	300
October	1st to 10th	Day	3 2 18% 37% 31% 9%	300
	11th to 20th	Day	10% 7% 29% 33% 19% 3	300
	21st to end	Day	16% 8% 24% 31% 18% 4	330
November	1st to 10th	Day	14% 6% 30% 33% 15% 2	300
	11th to 20th	Day	11% 13% 29% 30% 14% 4	300
	21st to end	Day	12% 18% 34% 21% 12% 3	291
December	1st to 10th	Day	17% 9% 30% 31% 10% 1	270
	11th to 20th	Day	7% 15% 40% 30% 9%	270
	21st to end	Day	15% 12% 38% 27% 8%	297
January	1st to 10th	Night	11% 8% 39% 32% 9%	950
	11th to 20th	Night	11% 11% 33% 36% 8%	941
	21st to end	Night	11% 11% 37% 35% 7%	836
February	1st to 10th	Night	13% 11% 42% 28% 6%	760
	11th to 20th	Night	14% 11% 42% 28% 5%	744
	21st to end	Night	13% 13% 49% 23% 2	628
March	1st to 10th	Night	12% 10% 44% 30% 4	753
	11th to 20th	Night	13% 12% 39% 31% 5%	570
	21st to end	Night	8% 13% 49% 28% 3	627
April	1st to 10th	Night	6% 12% 53% 25% 4	570
	11th to 20th	Night	9% 13% 51% 26% 1	563
	21st to end	Night	9% 10% 50% 27% 5%	570
May	1st to 10th	Night	8% 7% 48% 32% 5%	570
	11th to 20th	Night	8% 5% 40% 36% 10% 1	561
	21st to end	Night	8% 4 41% 37% 9%	617
June	1st to 10th	Night	3 3 28% 49% 16%	570
	11th to 20th	Night	3 1 35% 52% 8%	570
	21st to end	Night	4 2 20% 48% 24% 1	570
July	1st to 10th	Night	4 9% 50% 34% 3	555
	11th to 20th	Night	3 7% 53% 36% 1	552
	21st to end	Night	2 5% 54% 38% 1	624
August	1st to 10th	Night	1 5% 47% 46% 1	567
	11th to 20th	Night	5% 1 7% 47% 38% 2	564
	21st to end	Night	3 1 6% 44% 43% 3	627
September	1st to 10th	Night	6% 1 7% 44% 38% 3	570
	11th to 20th	Night	9% 2 9% 46% 33% 2	570
	21st to end	Night	8% 1 14% 45% 31% 1	570
October	1st to 10th	Night	4 2 21% 50% 21% 1	570
	11th to 20th	Night	10% 5% 26% 43% 15% 1	570
	21st to end	Night	9% 9% 29% 37% 15% 2	832
November	1st to 10th	Night	10% 6% 28% 40% 15% 1	975
	11th to 20th	Night	7% 7% 33% 40% 12% 1	1000
	21st to end	Night	14% 10% 31% 33% 12% 1	986
December	1st to 10th	Night	12% 8% 31% 39% 9% 1	950
	11th to 20th	Night	13% 11% 37% 32% 7%	950
	21st to end	Night	10% 9% 38% 34% 9%	1045

Page generated at 2024-11-24 23:22:21. Version ac07

Copyright © 2024 Attilla Danko.