Ashby
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Ashby clear sky chart from 2005-11-06 to 2025-03-27.

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% 6% 2 5% 4 4 4 3 5% 7% 54%	112698
	Night	7% 6% 2 6% 4 4 4 3 5% 7% 52%	115438
Transparency	Day	30% 4 13% 28% 20% 5%	112442
	Night	32% 4 13% 28% 19% 4	115284
Seeing	Day	11% 8% 32% 31% 16% 3	16244
	Night	10% 8% 38% 35% 8% 1	26388

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	3 9% 4 10% 7% 4 4 3 4 5% 48%	3638
	11th to 20th	Day	3 9% 4 9% 4 4 4 4 7% 7% 46%	3640
	21st to end	Day	3 4 2 6% 4 5% 4 3 5% 6% 58%	4004
February	1st to 10th	Day	5% 10% 4 8% 6% 4 4 4 5% 7% 42%	1891
	11th to 20th	Day	6% 9% 3 8% 4 4 4 3 4 7% 47%	1949
	21st to end	Day	6% 8% 2 4 3 3 4 3 6% 6% 55%	1650
March	1st to 10th	Day	6% 9% 2 4 3 3 4 3 4 7% 55%	3230
	11th to 20th	Day	8% 7% 2 4 3 3 3 2 4 5% 60%	3398
	21st to end	Day	10% 9% 4 7% 4 4 5% 3 6% 7% 41%	4132
April	1st to 10th	Day	11% 8% 2 5% 5% 3 3 3 6% 7% 47%	2280
	11th to 20th	Day	13% 8% 3 6% 5% 4 4 4 5% 6% 44%	2251
	21st to end	Day	14% 8% 2 6% 4 4 3 3 5% 7% 45%	2447
May	1st to 10th	Day	12% 5% 2 5% 5% 4 5% 4 6% 8% 44%	4120
	11th to 20th	Day	9% 5% 2 4 3 4 4 4 5% 9% 51%	4164
	21st to end	Day	12% 5% 2 6% 4 4 4 3 5% 6% 47%	4825
June	1st to 10th	Day	6% 4 2 5% 3 4 4 3 6% 9% 55%	2660
	11th to 20th	Day	8% 5% 1 5% 4 3 4 4 7% 9% 52%	2660
	21st to end	Day	3 2 1 3 3 4 4 3 6% 8% 62%	2660
July	1st to 10th	Day	7% 4 2 4 4 4 4 4 6% 7% 54%	4430
	11th to 20th	Day	3 3 1 3 4 4 5% 5% 7% 10% 55%	4430
	21st to end	Day	3 3 1 3 3 4 4 4 6% 10% 59%	4802
August	1st to 10th	Day	10% 5% 2 4 3 4 4 3 6% 8% 51%	4209
	11th to 20th	Day	7% 5% 2 5% 4 4 5% 4 6% 8% 50%	3891
	21st to end	Day	4 3 1 4 3 3 4 3 6% 8% 61%	4180
September	1st to 10th	Day	6% 3 1 4 2 3 3 3 4 6% 62%	2280
	11th to 20th	Day	6% 3 1 4 3 3 4 3 5% 7% 63%	2127
	21st to end	Day	8% 3 2 4 2 3 3 2 4 6% 63%	2090
October	1st to 10th	Day	7% 4 1 2 1 1 2 1 2 4 74%	3480
	11th to 20th	Day	6% 5% 1 5% 3 3 3 3 4 7% 60%	3066
	21st to end	Day	7% 7% 2 5% 4 3 4 3 5% 8% 54%	3091
November	1st to 10th	Day	6% 6% 2 5% 3 3 3 2 6% 8% 57%	1763
	11th to 20th	Day	4 5% 2 4 3 3 4 3 6% 6% 59%	1800
	21st to end	Day	6% 6% 2 4 3 3 3 3 6% 7% 57%	1800
December	1st to 10th	Day	3 5% 2 5% 4 3 4 3 4 6% 62%	2630
	11th to 20th	Day	3 3 1 4 3 3 4 3 6% 8% 61%	3026
	21st to end	Day	7% 11% 3 6% 4 4 4 3 4 6% 47%	4004
January	1st to 10th	Night	5% 11% 4 10% 6% 5% 4 3 4 6% 42%	6327
	11th to 20th	Night	2 6% 3 9% 5% 5% 4 4 6% 7% 48%	6340
	21st to end	Night	2 4 2 7% 5% 5% 4 4 5% 6% 55%	6974
February	1st to 10th	Night	7% 11% 3 9% 4 4 4 4 6% 7% 43%	2526
	11th to 20th	Night	5% 10% 3 9% 4 4 4 4 5% 8% 44%	2338
	21st to end	Night	5% 8% 3 7% 3 3 4 3 4 7% 52%	1980
March	1st to 10th	Night	9% 13% 2 5% 3 3 3 3 3 5% 50%	3854
	11th to 20th	Night	7% 7% 2 6% 4 3 3 2 4 7% 54%	3773
	21st to end	Night	9% 9% 3 9% 6% 4 4 4 6% 7% 41%	3643
April	1st to 10th	Night	11% 7% 2 6% 4 4 4 4 7% 8% 43%	1900
	11th to 20th	Night	11% 8% 3 5% 4 4 4 3 5% 6% 48%	1877
	21st to end	Night	15% 7% 1 6% 4 4 3 4 6% 7% 43%	1731
May	1st to 10th	Night	15% 6% 3 7% 4 4 4 4 5% 7% 41%	2850
	11th to 20th	Night	9% 4 2 4 3 4 5% 4 6% 9% 51%	2738
	21st to end	Night	14% 5% 2 5% 5% 5% 4 4 7% 7% 42%	2762
June	1st to 10th	Night	7% 4 2 3 4 3 5% 4 6% 10% 51%	1520
	11th to 20th	Night	11% 5% 2 4 4 4 4 5% 7% 8% 47%	1520
	21st to end	Night	6% 3 1 5% 5% 4 6% 4 7% 10% 49%	1520
July	1st to 10th	Night	9% 4 2 4 3 3 4 3 6% 8% 54%	2540
	11th to 20th	Night	4 3 1 4 4 4 5% 4 8% 10% 54%	2540
	21st to end	Night	3 4 2 5% 4 4 5% 4 6% 8% 54%	2761
August	1st to 10th	Night	7% 5% 2 7% 5% 5% 6% 5% 8% 9% 41%	2730
	11th to 20th	Night	9% 6% 2 6% 5% 4 4 4 6% 8% 47%	2946
	21st to end	Night	4 3 2 4 4 5% 5% 4 6% 8% 55%	3487
September	1st to 10th	Night	8% 4 2 5% 3 3 3 2 5% 6% 60%	1900
	11th to 20th	Night	7% 4 2 4 2 3 3 2 4 7% 61%	2052
	21st to end	Night	10% 3 1 4 3 3 2 3 4 6% 62%	2090
October	1st to 10th	Night	7% 3 1 3 2 1 2 1 2 3 74%	3480
	11th to 20th	Night	8% 4 1 3 3 2 2 2 4 6% 64%	3827
	21st to end	Night	7% 7% 3 5% 4 3 3 3 5% 6% 55%	4467
November	1st to 10th	Night	6% 5% 2 4 3 3 3 4 7% 7% 56%	2535
	11th to 20th	Night	4 5% 2 6% 5% 3 4 3 6% 7% 56%	2600
	21st to end	Night	4 6% 2 5% 3 3 3 3 5% 8% 57%	2600
December	1st to 10th	Night	2 4 2 5% 4 3 3 3 5% 6% 63%	4555
	11th to 20th	Night	3 5% 3 6% 4 4 4 4 6% 8% 54%	5181
	21st to end	Night	8% 10% 3 6% 4 4 4 3 5% 7% 45%	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	41% 3 8% 30% 17% 1	3639
	11th to 20th	Day	36% 5% 12% 25% 20% 2	3640
	21st to end	Day	28% 3 17% 27% 20% 4	4004
February	1st to 10th	Day	42% 5% 14% 21% 13% 4	1891
	11th to 20th	Day	38% 4 13% 24% 17% 4	1959
	21st to end	Day	29% 5% 13% 32% 18% 5%	1650
March	1st to 10th	Day	29% 4 8% 28% 26% 4	3230
	11th to 20th	Day	28% 3 15% 33% 19% 2	3398
	21st to end	Day	42% 5% 18% 26% 7% 1	4132
April	1st to 10th	Day	36% 4 15% 27% 14% 4	2280
	11th to 20th	Day	41% 5% 12% 23% 16% 3	2254
	21st to end	Day	40% 3 13% 26% 17% 1	2447
May	1st to 10th	Day	35% 8% 15% 22% 16% 4	4120
	11th to 20th	Day	31% 5% 17% 33% 12% 2	4168
	21st to end	Day	36% 5% 16% 28% 11% 3	4825
June	1st to 10th	Day	27% 4 21% 32% 14% 2	2660
	11th to 20th	Day	28% 5% 14% 30% 17% 5%	2660
	21st to end	Day	20% 5% 12% 32% 26% 6%	2660
July	1st to 10th	Day	28% 4 8% 25% 29% 5%	4361
	11th to 20th	Day	23% 5% 13% 31% 22% 7%	4355
	21st to end	Day	19% 5% 11% 25% 27% 13%	4651
August	1st to 10th	Day	30% 5% 13% 27% 20% 5%	4194
	11th to 20th	Day	31% 6% 13% 23% 22% 4	3930
	21st to end	Day	22% 5% 13% 24% 24% 13%	4179
September	1st to 10th	Day	22% 4 9% 28% 24% 13%	2280
	11th to 20th	Day	22% 3 8% 30% 27% 9%	2127
	21st to end	Day	24% 3 8% 30% 29% 6%	2090
October	1st to 10th	Day	17% 2 7% 21% 34% 20%	3480
	11th to 20th	Day	26% 3 11% 35% 21% 4	3066
	21st to end	Day	31% 2 11% 31% 19% 6%	3091
November	1st to 10th	Day	26% 4 14% 30% 21% 5%	1763
	11th to 20th	Day	25% 3 12% 29% 26% 5%	1800
	21st to end	Day	27% 3 13% 31% 21% 4	1800
December	1st to 10th	Day	24% 3 15% 33% 22% 3	2630
	11th to 20th	Day	22% 3 17% 39% 16% 3	3024
	21st to end	Day	39% 3 7% 35% 14% 1	4004
January	1st to 10th	Night	45% 3 11% 24% 15% 3	6327
	11th to 20th	Night	34% 4 15% 25% 20% 1	6340
	21st to end	Night	31% 3 13% 28% 21% 4	6974
February	1st to 10th	Night	40% 5% 13% 26% 13% 3	2526
	11th to 20th	Night	38% 4 15% 26% 14% 4	2338
	21st to end	Night	32% 3 13% 28% 19% 5%	1980
March	1st to 10th	Night	36% 5% 7% 22% 27% 3	3854
	11th to 20th	Night	32% 3 12% 29% 22% 2	3773
	21st to end	Night	41% 6% 19% 25% 7% 1	3643
April	1st to 10th	Night	37% 6% 16% 22% 16% 4	1900
	11th to 20th	Night	37% 5% 13% 25% 16% 3	1877
	21st to end	Night	40% 5% 15% 28% 11% 1	1731
May	1st to 10th	Night	40% 7% 14% 22% 15% 2	2850
	11th to 20th	Night	29% 6% 18% 36% 9% 2	2739
	21st to end	Night	39% 7% 17% 27% 8% 1	2762
June	1st to 10th	Night	27% 7% 24% 27% 12% 2	1520
	11th to 20th	Night	33% 6% 18% 25% 13% 5%	1520
	21st to end	Night	28% 7% 16% 28% 19% 3	1520
July	1st to 10th	Night	30% 4 16% 27% 21% 3	2500
	11th to 20th	Night	25% 5% 16% 30% 21% 5%	2492
	21st to end	Night	26% 6% 14% 23% 24% 8%	2674
August	1st to 10th	Night	37% 6% 16% 24% 14% 2	2722
	11th to 20th	Night	36% 5% 14% 25% 17% 3	2973
	21st to end	Night	27% 6% 13% 23% 21% 10%	3487
September	1st to 10th	Night	25% 4 10% 25% 28% 8%	1900
	11th to 20th	Night	26% 3 11% 28% 25% 7%	2052
	21st to end	Night	25% 3 7% 30% 27% 8%	2090
October	1st to 10th	Night	18% 2 5% 20% 43% 12%	3480
	11th to 20th	Night	23% 3 10% 36% 22% 5%	3827
	21st to end	Night	31% 3 12% 28% 20% 7%	4467
November	1st to 10th	Night	25% 4 13% 29% 23% 6%	2535
	11th to 20th	Night	27% 3 12% 28% 24% 6%	2600
	21st to end	Night	26% 3 12% 31% 24% 4	2600
December	1st to 10th	Night	23% 3 16% 35% 19% 4	4555
	11th to 20th	Night	30% 3 13% 36% 16% 2	5182
	21st to end	Night	39% 4 10% 32% 15% 2	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	8% 8% 41% 28% 12% 4	300
	11th to 20th	Day	11% 12% 38% 31% 7% 1	300
	21st to end	Day	7% 9% 52% 21% 9% 3	330
February	1st to 10th	Day	11% 12% 52% 21% 3	300
	11th to 20th	Day	8% 11% 50% 22% 7% 2	300
	21st to end	Day	9% 14% 50% 22% 5%	249
March	1st to 10th	Day	12% 11% 35% 29% 12% 1	300
	11th to 20th	Day	12% 10% 39% 25% 11% 3	300
	21st to end	Day	15% 11% 41% 24% 7% 2	464
April	1st to 10th	Day	16% 17% 33% 25% 8%	460
	11th to 20th	Day	18% 17% 35% 21% 8% 1	452
	21st to end	Day	19% 11% 28% 28% 12% 2	627
May	1st to 10th	Day	17% 9% 29% 29% 12% 3	653
	11th to 20th	Day	19% 7% 27% 29% 16% 3	675
	21st to end	Day	17% 7% 30% 29% 13% 4	734
June	1st to 10th	Day	7% 5% 26% 31% 24% 7%	680
	11th to 20th	Day	9% 7% 34% 33% 14% 2	680
	21st to end	Day	6% 3 32% 35% 20% 4	680
July	1st to 10th	Day	8% 4 21% 39% 23% 5%	671
	11th to 20th	Day	4 2 21% 43% 25% 4	668
	21st to end	Day	7% 1 18% 44% 25% 5%	739
August	1st to 10th	Day	8% 2 20% 40% 23% 7%	676
	11th to 20th	Day	6% 2 26% 40% 22% 3	580
	21st to end	Day	4 4 25% 36% 25% 7%	539
September	1st to 10th	Day	7% 4 25% 37% 21% 5%	490
	11th to 20th	Day	4 3 24% 38% 22% 9%	337
	21st to end	Day	10% 7% 27% 31% 21% 4	300
October	1st to 10th	Day	15% 7% 25% 25% 22% 6%	300
	11th to 20th	Day	9% 13% 35% 28% 13% 1	300
	21st to end	Day	12% 13% 38% 23% 12% 2	330
November	1st to 10th	Day	10% 9% 39% 23% 15% 3	300
	11th to 20th	Day	8% 12% 38% 32% 8% 2	300
	21st to end	Day	10% 7% 39% 32% 8% 4	300
December	1st to 10th	Day	10% 9% 46% 27% 8%	300
	11th to 20th	Day	9% 12% 43% 28% 7%	300
	21st to end	Day	15% 12% 42% 21% 8% 1	330
January	1st to 10th	Night	9% 12% 39% 30% 10% 1	1000
	11th to 20th	Night	7% 14% 44% 28% 6%	1000
	21st to end	Night	9% 11% 43% 30% 6%	1100
February	1st to 10th	Night	12% 12% 45% 26% 4	845
	11th to 20th	Night	10% 9% 49% 27% 5%	784
	21st to end	Night	8% 9% 53% 26% 3	660
March	1st to 10th	Night	14% 9% 40% 33% 5%	800
	11th to 20th	Night	12% 8% 41% 32% 6% 1	718
	21st to end	Night	12% 10% 40% 31% 7%	647
April	1st to 10th	Night	14% 14% 39% 28% 5%	570
	11th to 20th	Night	15% 11% 45% 26% 3	563
	21st to end	Night	17% 11% 32% 32% 7%	570
May	1st to 10th	Night	13% 8% 35% 33% 10% 1	570
	11th to 20th	Night	18% 5% 34% 30% 12% 1	561
	21st to end	Night	17% 6% 34% 32% 10%	617
June	1st to 10th	Night	9% 4 34% 37% 15% 2	570
	11th to 20th	Night	13% 7% 37% 36% 6% 1	570
	21st to end	Night	5% 3 31% 49% 11% 1	570
July	1st to 10th	Night	8% 2 29% 44% 16% 1	555
	11th to 20th	Night	6% 2 29% 52% 11% 1	552
	21st to end	Night	6% 1 27% 49% 17% 1	624
August	1st to 10th	Night	8% 2 22% 51% 15% 1	567
	11th to 20th	Night	6% 1 28% 52% 13%	564
	21st to end	Night	6% 3 28% 49% 14%	627
September	1st to 10th	Night	9% 3 28% 48% 10% 1	570
	11th to 20th	Night	9% 5% 29% 41% 14% 1	570
	21st to end	Night	12% 3 34% 41% 8% 1	570
October	1st to 10th	Night	15% 5% 32% 36% 12%	570
	11th to 20th	Night	10% 7% 34% 37% 10% 1	784
	21st to end	Night	11% 9% 36% 34% 8% 1	1045
November	1st to 10th	Night	9% 7% 39% 37% 7% 1	975
	11th to 20th	Night	7% 12% 42% 32% 7% 1	1000
	21st to end	Night	8% 7% 40% 37% 8% 1	1000
December	1st to 10th	Night	6% 14% 46% 28% 7%	1000
	11th to 20th	Night	8% 12% 43% 31% 6%	1000
	21st to end	Night	12% 13% 40% 30% 5% 1	1100

Page generated at 2025-03-24 07:04:26. Version ac07

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