Ashton Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Ashton Observatory clear sky chart from 2005-11-06 to 2024-04-03.

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	13% 8% 4 7% 6% 3 5% 5% 5% 7% 38%	78879
	Night	12% 9% 4 7% 5% 3 4 4 5% 7% 41%	81305
Transparency	Day	45% 7% 13% 22% 12% 1	78590
	Night	43% 6% 13% 23% 14% 2	81115
Seeing	Day	16% 10% 29% 27% 15% 4	14917
	Night	16% 9% 32% 32% 11% 1	24997

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% 10% 6% 19% 11% 5% 6% 5% 4 5% 20%	2527
	11th to 20th	Day	8% 13% 6% 10% 9% 5% 7% 6% 6% 6% 24%	2788
	21st to end	Day	12% 15% 8% 16% 8% 3 5% 4 4 6% 18%	3135
February	1st to 10th	Day	11% 13% 6% 8% 6% 4 5% 5% 6% 6% 30%	1710
	11th to 20th	Day	12% 10% 4 7% 6% 3 6% 5% 5% 7% 36%	1664
	21st to end	Day	14% 10% 5% 8% 6% 4 4 4 4 5% 35%	1521
March	1st to 10th	Day	14% 10% 5% 8% 6% 3 4 5% 4 7% 35%	2076
	11th to 20th	Day	14% 10% 4 8% 7% 3 4 4 5% 8% 34%	2090
	21st to end	Day	21% 14% 5% 6% 5% 3 4 4 4 6% 28%	2308
April	1st to 10th	Day	18% 9% 5% 6% 5% 3 4 5% 6% 7% 33%	2184
	11th to 20th	Day	18% 9% 3 7% 5% 2 4 4 6% 6% 35%	2134
	21st to end	Day	22% 8% 3 5% 6% 2 4 5% 5% 6% 33%	2259
May	1st to 10th	Day	18% 8% 3 6% 6% 4 5% 5% 7% 7% 32%	2340
	11th to 20th	Day	18% 8% 3 5% 5% 3 5% 4 5% 8% 36%	2467
	21st to end	Day	16% 6% 3 6% 6% 3 6% 6% 5% 7% 36%	2725
June	1st to 10th	Day	10% 5% 2 5% 6% 3 5% 4 6% 8% 45%	2520
	11th to 20th	Day	9% 4 2 5% 4 3 4 5% 6% 10% 47%	2520
	21st to end	Day	11% 6% 2 5% 6% 3 6% 6% 6% 9% 41%	2520
July	1st to 10th	Day	8% 4 3 4 4 3 5% 5% 6% 9% 51%	2520
	11th to 20th	Day	7% 4 2 5% 5% 3 5% 5% 6% 8% 50%	2520
	21st to end	Day	7% 4 2 4 5% 3 5% 6% 7% 12% 45%	2562
August	1st to 10th	Day	8% 4 2 5% 6% 4 6% 6% 8% 9% 42%	2328
	11th to 20th	Day	9% 5% 3 5% 5% 3 4 4 7% 9% 46%	2301
	21st to end	Day	10% 5% 2 5% 4 3 4 5% 5% 7% 51%	2405
September	1st to 10th	Day	13% 5% 2 4 4 2 3 3 4 7% 52%	2160
	11th to 20th	Day	10% 4 2 4 4 3 4 5% 6% 7% 51%	2131
	21st to end	Day	12% 4 2 5% 4 3 4 4 5% 8% 50%	1895
October	1st to 10th	Day	14% 7% 3 6% 5% 3 3 4 5% 6% 45%	1800
	11th to 20th	Day	16% 8% 3 6% 5% 3 3 3 4 5% 43%	1800
	21st to end	Day	15% 10% 3 5% 4 3 4 4 4 6% 41%	1980
November	1st to 10th	Day	10% 6% 2 5% 5% 3 4 6% 6% 8% 45%	1728
	11th to 20th	Day	13% 10% 5% 6% 5% 3 4 4 4 8% 40%	1520
	21st to end	Day	16% 10% 3 5% 6% 2 4 4 5% 7% 39%	1520
December	1st to 10th	Day	13% 11% 4 8% 6% 3 6% 4 4 7% 33%	1520
	11th to 20th	Day	12% 10% 4 9% 8% 3 4 5% 5% 7% 33%	1918
	21st to end	Day	22% 15% 8% 12% 6% 2 4 4 4 5% 18%	2783
January	1st to 10th	Night	10% 11% 6% 18% 9% 4 5% 4 4 5% 25%	4428
	11th to 20th	Night	10% 15% 6% 10% 7% 3 6% 5% 8% 6% 23%	4180
	21st to end	Night	14% 16% 7% 13% 7% 3 5% 3 5% 6% 22%	4532
February	1st to 10th	Night	11% 11% 5% 9% 7% 3 5% 4 4 7% 34%	2470
	11th to 20th	Night	12% 9% 4 6% 5% 3 5% 5% 5% 6% 40%	2403
	21st to end	Night	13% 9% 4 7% 6% 3 4 3 3 5% 42%	1932
March	1st to 10th	Night	15% 9% 4 5% 4 2 3 3 4 6% 44%	2119
	11th to 20th	Night	14% 8% 4 6% 6% 2 4 4 5% 7% 41%	2090
	21st to end	Night	18% 12% 4 9% 7% 3 5% 5% 5% 6% 27%	2290
April	1st to 10th	Night	16% 7% 4 6% 4 3 4 4 4 8% 41%	1820
	11th to 20th	Night	14% 9% 4 6% 6% 3 4 4 4 6% 40%	1775
	21st to end	Night	18% 6% 3 5% 5% 2 4 4 4 6% 42%	1697
May	1st to 10th	Night	16% 4 3 5% 5% 3 5% 5% 6% 7% 41%	1620
	11th to 20th	Night	15% 7% 3 5% 4 3 4 5% 5% 8% 40%	1426
	21st to end	Night	12% 7% 2 6% 5% 3 5% 3 4 9% 43%	1551
June	1st to 10th	Night	7% 4 2 4 6% 3 4 4 5% 9% 51%	1440
	11th to 20th	Night	6% 4 2 4 5% 2 4 5% 6% 10% 51%	1440
	21st to end	Night	8% 4 2 5% 5% 2 5% 5% 6% 9% 50%	1440
July	1st to 10th	Night	5% 2 1 3 3 3 4 5% 6% 11% 56%	1440
	11th to 20th	Night	4 3 1 3 3 2 3 5% 6% 9% 62%	1440
	21st to end	Night	5% 2 1 5% 5% 3 4 6% 6% 9% 56%	1753
August	1st to 10th	Night	6% 5% 2 5% 4 3 5% 6% 6% 9% 49%	1611
	11th to 20th	Night	6% 4 2 5% 4 3 3 4 5% 8% 57%	1600
	21st to end	Night	9% 3 2 4 3 3 4 4 5% 6% 57%	1949
September	1st to 10th	Night	9% 5% 2 4 4 3 4 4 4 6% 56%	1800
	11th to 20th	Night	11% 5% 1 4 3 2 3 4 6% 7% 54%	1826
	21st to end	Night	10% 4 1 4 3 2 3 4 4 7% 57%	2047
October	1st to 10th	Night	13% 6% 3 4 3 2 3 3 4 6% 52%	2160
	11th to 20th	Night	15% 6% 3 5% 4 2 4 3 4 6% 48%	2160
	21st to end	Night	16% 8% 3 5% 4 2 4 3 4 5% 45%	2376
November	1st to 10th	Night	11% 5% 3 4 4 2 3 4 5% 6% 54%	2347
	11th to 20th	Night	13% 7% 4 6% 6% 3 4 4 4 6% 43%	2660
	21st to end	Night	16% 8% 3 6% 5% 3 4 4 5% 6% 42%	2660
December	1st to 10th	Night	10% 11% 4 8% 6% 3 5% 5% 5% 6% 36%	2660
	11th to 20th	Night	12% 10% 3 8% 6% 3 4 5% 4 6% 38%	3279
	21st to end	Night	19% 16% 7% 11% 7% 3 4 4 4 4 21%	4884

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	64% 7% 10% 12% 6%	2528
	11th to 20th	Day	59% 5% 11% 19% 6%	2788
	21st to end	Day	68% 6% 11% 12% 4	3135
February	1st to 10th	Day	54% 5% 14% 18% 9% 1	1710
	11th to 20th	Day	49% 6% 12% 21% 10% 2	1673
	21st to end	Day	51% 6% 12% 18% 11% 3	1521
March	1st to 10th	Day	50% 6% 14% 20% 10% 1	2076
	11th to 20th	Day	50% 6% 11% 21% 10% 1	2090
	21st to end	Day	57% 6% 12% 16% 7% 2	2308
April	1st to 10th	Day	49% 7% 12% 23% 9%	2184
	11th to 20th	Day	48% 7% 11% 22% 11% 1	2134
	21st to end	Day	51% 6% 11% 20% 11% 1	2259
May	1st to 10th	Day	50% 8% 13% 19% 9% 1	2340
	11th to 20th	Day	47% 7% 13% 22% 10% 1	2471
	21st to end	Day	46% 8% 12% 19% 13% 2	2725
June	1st to 10th	Day	35% 8% 16% 23% 15% 2	2520
	11th to 20th	Day	32% 8% 15% 23% 20% 2	2520
	21st to end	Day	38% 9% 15% 22% 14% 2	2520
July	1st to 10th	Day	29% 8% 16% 30% 14% 3	2451
	11th to 20th	Day	31% 8% 16% 25% 17% 3	2445
	21st to end	Day	28% 9% 19% 26% 17% 2	2420
August	1st to 10th	Day	35% 9% 17% 24% 13% 1	2314
	11th to 20th	Day	34% 7% 15% 28% 14% 1	2301
	21st to end	Day	32% 6% 12% 26% 20% 3	2405
September	1st to 10th	Day	33% 5% 13% 23% 21% 4	2160
	11th to 20th	Day	30% 7% 14% 31% 17% 1	2131
	21st to end	Day	33% 5% 11% 32% 18% 1	1895
October	1st to 10th	Day	40% 4 11% 27% 16% 2	1800
	11th to 20th	Day	44% 3 9% 24% 18% 2	1800
	21st to end	Day	44% 4 12% 24% 14% 2	1980
November	1st to 10th	Day	36% 7% 15% 23% 18% 2	1728
	11th to 20th	Day	44% 5% 13% 23% 15% 1	1520
	21st to end	Day	46% 5% 15% 24% 10% 1	1520
December	1st to 10th	Day	51% 5% 16% 18% 9% 1	1520
	11th to 20th	Day	49% 6% 18% 19% 8% 1	1915
	21st to end	Day	68% 6% 9% 13% 4 1	2783
January	1st to 10th	Night	60% 5% 12% 14% 9%	4428
	11th to 20th	Night	58% 4 13% 14% 10% 1	4180
	21st to end	Night	63% 5% 11% 14% 6% 1	4532
February	1st to 10th	Night	50% 5% 12% 22% 10% 1	2470
	11th to 20th	Night	44% 5% 11% 26% 12% 2	2404
	21st to end	Night	46% 5% 11% 21% 15% 3	1932
March	1st to 10th	Night	41% 4 13% 28% 12% 1	2119
	11th to 20th	Night	44% 6% 12% 22% 14% 2	2090
	21st to end	Night	57% 6% 15% 13% 7% 1	2290
April	1st to 10th	Night	43% 5% 11% 24% 15% 2	1820
	11th to 20th	Night	46% 5% 10% 22% 15% 3	1775
	21st to end	Night	43% 5% 14% 23% 13% 2	1697
May	1st to 10th	Night	40% 9% 15% 21% 14% 1	1620
	11th to 20th	Night	41% 7% 15% 24% 12% 1	1427
	21st to end	Night	40% 6% 13% 22% 14% 3	1552
June	1st to 10th	Night	31% 7% 15% 28% 16% 3	1440
	11th to 20th	Night	27% 7% 16% 21% 23% 6%	1440
	21st to end	Night	30% 7% 14% 27% 18% 4	1440
July	1st to 10th	Night	21% 8% 18% 33% 16% 4	1400
	11th to 20th	Night	18% 7% 18% 34% 18% 4	1392
	21st to end	Night	23% 6% 17% 31% 19% 3	1656
August	1st to 10th	Night	29% 9% 16% 30% 13% 3	1601
	11th to 20th	Night	26% 7% 13% 33% 18% 2	1600
	21st to end	Night	26% 6% 12% 25% 23% 7%	1949
September	1st to 10th	Night	29% 7% 10% 26% 25% 4	1800
	11th to 20th	Night	30% 5% 10% 32% 21% 2	1826
	21st to end	Night	28% 5% 10% 32% 23% 3	2047
October	1st to 10th	Night	35% 3 11% 26% 22% 3	2160
	11th to 20th	Night	38% 4 12% 23% 19% 5%	2160
	21st to end	Night	42% 5% 11% 22% 18% 2	2376
November	1st to 10th	Night	31% 5% 15% 26% 19% 4	2347
	11th to 20th	Night	42% 5% 13% 25% 14% 2	2660
	21st to end	Night	45% 3 12% 26% 12% 1	2660
December	1st to 10th	Night	48% 6% 12% 23% 9% 1	2660
	11th to 20th	Night	47% 5% 14% 27% 7% 1	3281
	21st to end	Night	66% 6% 8% 14% 6%	4884

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	20% 12% 35% 23% 9% 2	270
	11th to 20th	Day	21% 13% 42% 20% 3	270
	21st to end	Day	21% 10% 38% 25% 4 1	297
February	1st to 10th	Day	21% 14% 41% 19% 4 1	270
	11th to 20th	Day	17% 17% 37% 23% 7% 1	270
	21st to end	Day	24% 14% 42% 19% 2	225
March	1st to 10th	Day	18% 17% 43% 14% 7%	270
	11th to 20th	Day	23% 16% 33% 20% 7% 1	270
	21st to end	Day	36% 17% 29% 14% 3	306
April	1st to 10th	Day	26% 14% 34% 19% 6% 1	428
	11th to 20th	Day	23% 15% 30% 24% 8%	412
	21st to end	Day	25% 14% 32% 19% 6% 3	519
May	1st to 10th	Day	24% 12% 32% 22% 9% 1	603
	11th to 20th	Day	22% 13% 29% 22% 12% 3	625
	21st to end	Day	17% 8% 27% 27% 17% 3	679
June	1st to 10th	Day	8% 6% 24% 32% 23% 7%	630
	11th to 20th	Day	10% 5% 28% 36% 18% 4	630
	21st to end	Day	11% 7% 31% 32% 14% 5%	630
July	1st to 10th	Day	9% 3 16% 31% 29% 12%	621
	11th to 20th	Day	7% 4 26% 34% 24% 6%	618
	21st to end	Day	6% 2 20% 40% 25% 5%	684
August	1st to 10th	Day	8% 3 21% 36% 27% 5%	626
	11th to 20th	Day	11% 4 17% 36% 25% 7%	615
	21st to end	Day	9% 4 19% 36% 25% 6%	524
September	1st to 10th	Day	16% 6% 21% 30% 20% 7%	450
	11th to 20th	Day	8% 3 25% 35% 22% 6%	421
	21st to end	Day	12% 9% 30% 28% 18% 3	270
October	1st to 10th	Day	19% 6% 30% 26% 14% 5%	270
	11th to 20th	Day	16% 24% 35% 17% 7% 1	270
	21st to end	Day	23% 15% 30% 21% 7% 3	297
November	1st to 10th	Day	15% 10% 43% 23% 8% 2	270
	11th to 20th	Day	18% 17% 34% 21% 9% 1	270
	21st to end	Day	24% 13% 37% 21% 5%	270
December	1st to 10th	Day	12% 13% 42% 20% 11% 1	270
	11th to 20th	Day	16% 13% 36% 24% 8% 2	270
	21st to end	Day	26% 17% 40% 14% 2	297
January	1st to 10th	Night	14% 12% 39% 28% 7%	950
	11th to 20th	Night	17% 13% 35% 28% 7%	950
	21st to end	Night	19% 10% 36% 28% 6%	1045
February	1st to 10th	Night	18% 11% 37% 29% 5%	950
	11th to 20th	Night	16% 12% 39% 28% 5%	929
	21st to end	Night	18% 12% 42% 24% 3 1	676
March	1st to 10th	Night	19% 13% 38% 22% 7% 1	599
	11th to 20th	Night	16% 11% 39% 26% 7% 1	570
	21st to end	Night	23% 12% 40% 19% 5% 1	627
April	1st to 10th	Night	19% 14% 40% 24% 3	546
	11th to 20th	Night	18% 14% 35% 26% 6% 1	533
	21st to end	Night	19% 10% 36% 25% 8% 1	540
May	1st to 10th	Night	19% 9% 33% 31% 8%	540
	11th to 20th	Night	18% 8% 33% 31% 11%	531
	21st to end	Night	15% 5% 28% 36% 15%	584
June	1st to 10th	Night	9% 4 24% 45% 17% 1	540
	11th to 20th	Night	7% 2 29% 49% 12% 1	540
	21st to end	Night	9% 3 31% 41% 15%	540
July	1st to 10th	Night	5% 2 14% 46% 30% 4	525
	11th to 20th	Night	5% 2 20% 46% 25% 2	522
	21st to end	Night	6% 1 18% 50% 24% 1	591
August	1st to 10th	Night	6% 2 16% 46% 26% 2	537
	11th to 20th	Night	8% 2 17% 43% 28% 2	534
	21st to end	Night	9% 2 20% 42% 25% 2	594
September	1st to 10th	Night	13% 3 21% 41% 20% 3	540
	11th to 20th	Night	14% 3 22% 36% 22% 3	540
	21st to end	Night	12% 6% 30% 36% 15% 1	607
October	1st to 10th	Night	14% 7% 32% 33% 13% 1	720
	11th to 20th	Night	18% 11% 33% 30% 8%	720
	21st to end	Night	20% 11% 33% 28% 7%	792
November	1st to 10th	Night	14% 10% 37% 30% 9%	740
	11th to 20th	Night	17% 13% 40% 23% 6% 1	950
	21st to end	Night	20% 10% 36% 26% 8%	950
December	1st to 10th	Night	16% 12% 38% 27% 5% 1	950
	11th to 20th	Night	20% 9% 35% 29% 6%	950
	21st to end	Night	24% 13% 34% 24% 5%	1045

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