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 2024-11-04.

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	12% 8% 3 7% 5% 4 4 4 6% 7% 39%	101441
	Night	12% 9% 3 8% 4 4 4 3 5% 7% 42%	97669
Transparency	Day	42% 6% 13% 23% 14% 2	105080
	Night	41% 5% 12% 24% 15% 2	102171
Seeing	Day	16% 10% 30% 27% 15% 4	15850
	Night	15% 9% 32% 32% 11% 1	25683

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% 11% 6% 22% 8% 7% 5% 4 5% 5% 20%	2527
	11th to 20th	Day	8% 13% 5% 11% 8% 7% 7% 5% 6% 6% 24%	2788
	21st to end	Day	12% 16% 7% 18% 6% 5% 4 3 4 6% 18%	3135
February	1st to 10th	Day	11% 14% 5% 9% 5% 6% 5% 3 6% 6% 30%	1710
	11th to 20th	Day	11% 11% 4 7% 5% 5% 6% 3 5% 7% 35%	1664
	21st to end	Day	14% 10% 5% 9% 5% 5% 4 3 5% 5% 35%	1521
March	1st to 10th	Day	17% 10% 3 7% 4 3 3 3 3 5% 41%	3422
	11th to 20th	Day	10% 7% 3 8% 5% 4 4 4 5% 6% 44%	3480
	21st to end	Day	23% 15% 5% 8% 5% 4 4 3 4 6% 22%	3835
April	1st to 10th	Day	17% 11% 4 6% 5% 4 4 4 6% 6% 34%	2280
	11th to 20th	Day	17% 9% 3 7% 4 3 4 4 6% 6% 36%	2254
	21st to end	Day	22% 8% 3 6% 5% 3 4 4 5% 6% 33%	2385
May	1st to 10th	Day	19% 8% 3 6% 5% 5% 5% 4 6% 8% 32%	4082
	11th to 20th	Day	16% 7% 2 6% 5% 4 4 4 5% 8% 39%	4387
	21st to end	Day	14% 6% 2 6% 4 4 4 4 5% 7% 42%	4824
June	1st to 10th	Day	10% 5% 2 5% 5% 4 5% 4 6% 8% 45%	2660
	11th to 20th	Day	9% 5% 2 5% 4 5% 5% 4 6% 10% 46%	2660
	21st to end	Day	11% 7% 1 6% 5% 4 5% 5% 7% 8% 41%	2660
July	1st to 10th	Day	11% 6% 3 7% 5% 5% 5% 4 7% 9% 37%	4440
	11th to 20th	Day	6% 4 2 5% 3 4 4 3 6% 8% 56%	4440
	21st to end	Day	5% 4 2 5% 5% 6% 6% 6% 8% 11% 42%	4481
August	1st to 10th	Day	6% 5% 2 6% 5% 4 6% 4 7% 10% 45%	4104
	11th to 20th	Day	10% 7% 3 7% 4 5% 5% 4 7% 9% 39%	4041
	21st to end	Day	7% 5% 2 6% 4 5% 5% 4 6% 8% 48%	4223
September	1st to 10th	Day	13% 5% 2 5% 3 3 3 2 5% 7% 54%	2280
	11th to 20th	Day	9% 4 2 5% 4 4 4 4 6% 7% 50%	2249
	21st to end	Day	11% 4 2 5% 4 4 4 3 5% 7% 50%	2000
October	1st to 10th	Day	13% 8% 2 6% 4 3 3 3 4 6% 48%	1900
	11th to 20th	Day	15% 8% 3 7% 4 4 3 3 4 4 45%	1900
	21st to end	Day	15% 9% 3 6% 4 4 4 3 4 6% 42%	2090
November	1st to 10th	Day	10% 7% 2 6% 4 4 5% 4 6% 8% 44%	1758
	11th to 20th	Day	13% 10% 4 7% 4 3 4 3 5% 8% 39%	1520
	21st to end	Day	16% 11% 3 5% 5% 3 3 3 5% 7% 38%	1520
December	1st to 10th	Day	13% 11% 4 10% 5% 4 5% 3 5% 7% 33%	1520
	11th to 20th	Day	11% 10% 4 10% 6% 4 4 4 5% 7% 33%	1918
	21st to end	Day	22% 15% 8% 12% 5% 3 3 3 4 5% 18%	2783
January	1st to 10th	Night	10% 12% 5% 20% 7% 5% 5% 3 4 5% 25%	4428
	11th to 20th	Night	10% 16% 5% 12% 6% 4 6% 4 8% 6% 23%	4180
	21st to end	Night	13% 16% 7% 14% 6% 5% 4 3 5% 6% 22%	4532
February	1st to 10th	Night	11% 12% 4 9% 6% 4 4 3 4 7% 34%	2470
	11th to 20th	Night	11% 10% 3 7% 5% 4 5% 4 5% 6% 40%	2403
	21st to end	Night	13% 10% 4 7% 5% 5% 3 3 4 5% 42%	1932
March	1st to 10th	Night	15% 9% 3 6% 3 3 3 2 4 6% 47%	3459
	11th to 20th	Night	11% 7% 2 6% 3 3 3 3 5% 7% 50%	3480
	21st to end	Night	20% 12% 5% 13% 5% 5% 4 3 5% 5% 22%	3796
April	1st to 10th	Night	15% 8% 3 6% 4 4 4 3 5% 7% 41%	1900
	11th to 20th	Night	14% 9% 4 7% 5% 4 4 3 4 6% 41%	1875
	21st to end	Night	17% 7% 2 6% 4 4 4 3 4 6% 42%	1791
May	1st to 10th	Night	19% 6% 2 6% 4 4 4 3 5% 6% 40%	2823
	11th to 20th	Night	12% 7% 3 6% 4 4 4 3 4 7% 46%	2516
	21st to end	Night	11% 7% 2 6% 4 4 4 3 5% 8% 45%	2743
June	1st to 10th	Night	7% 5% 2 6% 5% 5% 4 4 6% 8% 50%	1520
	11th to 20th	Night	6% 5% 2 4 4 3 3 4 7% 10% 50%	1520
	21st to end	Night	8% 5% 2 6% 4 3 5% 4 6% 8% 48%	1520
July	1st to 10th	Night	9% 4 2 5% 4 4 5% 5% 7% 9% 45%	2530
	11th to 20th	Night	3 2 1 4 3 3 4 3 6% 8% 62%	2530
	21st to end	Night	6% 3 2 6% 5% 4 5% 5% 7% 9% 49%	3065
August	1st to 10th	Night	4 4 2 5% 3 4 5% 4 6% 9% 54%	2836
	11th to 20th	Night	8% 5% 2 5% 3 3 4 3 5% 7% 56%	2798
	21st to end	Night	8% 5% 2 5% 3 3 3 4 5% 7% 54%	3432
September	1st to 10th	Night	9% 4 2 5% 3 3 3 3 4 6% 58%	1900
	11th to 20th	Night	11% 5% 2 5% 3 3 4 3 6% 6% 54%	1928
	21st to end	Night	10% 4 1 4 3 3 3 3 4 7% 57%	2161
October	1st to 10th	Night	12% 7% 2 5% 3 3 3 2 4 6% 53%	2280
	11th to 20th	Night	14% 6% 3 5% 3 3 3 3 4 6% 49%	2280
	21st to end	Night	16% 9% 3 6% 3 3 4 3 5% 5% 44%	2508
November	1st to 10th	Night	11% 5% 2 5% 3 3 3 3 5% 6% 54%	2390
	11th to 20th	Night	13% 8% 4 7% 5% 3 4 3 4 6% 42%	2660
	21st to end	Night	16% 9% 2 6% 4 3 4 3 5% 6% 42%	2660
December	1st to 10th	Night	10% 12% 4 9% 6% 4 5% 4 5% 6% 36%	2660
	11th to 20th	Night	12% 11% 3 9% 5% 4 4 4 5% 6% 38%	3279
	21st to end	Night	18% 16% 7% 13% 5% 4 4 3 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	46% 5% 11% 23% 14% 1	3422
	11th to 20th	Day	38% 8% 11% 25% 16% 2	3480
	21st to end	Day	62% 7% 13% 13% 5% 1	3835
April	1st to 10th	Day	48% 7% 11% 24% 9%	2280
	11th to 20th	Day	47% 7% 11% 21% 12% 1	2254
	21st to end	Day	50% 6% 11% 20% 12% 1	2385
May	1st to 10th	Day	47% 8% 14% 17% 12% 1	4120
	11th to 20th	Day	41% 7% 12% 27% 12% 1	4391
	21st to end	Day	39% 7% 12% 24% 17% 1	4837
June	1st to 10th	Day	35% 8% 16% 23% 15% 2	2660
	11th to 20th	Day	32% 8% 15% 23% 20% 2	2660
	21st to end	Day	38% 9% 15% 23% 14% 2	2660
July	1st to 10th	Day	41% 7% 16% 26% 8% 2	4371
	11th to 20th	Day	25% 6% 15% 32% 19% 2	4365
	21st to end	Day	30% 10% 20% 22% 17% 2	4340
August	1st to 10th	Day	32% 7% 17% 28% 15% 1	4090
	11th to 20th	Day	41% 6% 14% 25% 13% 1	4080
	21st to end	Day	32% 7% 16% 26% 16% 2	4222
September	1st to 10th	Day	32% 5% 13% 24% 22% 4	2280
	11th to 20th	Day	30% 7% 14% 31% 17% 1	2249
	21st to end	Day	33% 5% 11% 31% 19% 1	2000
October	1st to 10th	Day	23% 3 6% 28% 37% 2	3160
	11th to 20th	Day	30% 2 9% 30% 24% 5%	3140
	21st to end	Day	39% 4 10% 29% 15% 3	3428
November	1st to 10th	Day	36% 7% 15% 23% 17% 2	1758
	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	40% 4 10% 27% 17% 2	3459
	11th to 20th	Night	33% 6% 11% 31% 14% 4	3480
	21st to end	Night	61% 9% 16% 9% 5% 1	3796
April	1st to 10th	Night	42% 5% 11% 25% 14% 2	1900
	11th to 20th	Night	44% 5% 10% 22% 16% 3	1875
	21st to end	Night	42% 5% 14% 23% 13% 2	1791
May	1st to 10th	Night	42% 7% 12% 19% 18% 1	2850
	11th to 20th	Night	37% 7% 12% 26% 15% 3	2517
	21st to end	Night	36% 8% 13% 22% 19% 2	2751
June	1st to 10th	Night	31% 7% 14% 27% 16% 4	1520
	11th to 20th	Night	27% 7% 15% 22% 23% 6%	1520
	21st to end	Night	31% 7% 14% 26% 18% 3	1520
July	1st to 10th	Night	33% 7% 15% 31% 12% 2	2490
	11th to 20th	Night	19% 6% 19% 34% 20% 2	2482
	21st to end	Night	28% 9% 19% 25% 18% 2	2968
August	1st to 10th	Night	26% 7% 16% 34% 15% 2	2826
	11th to 20th	Night	29% 6% 16% 31% 16% 1	2825
	21st to end	Night	28% 6% 15% 28% 17% 5%	3432
September	1st to 10th	Night	28% 6% 9% 27% 26% 4	1900
	11th to 20th	Night	30% 5% 10% 32% 20% 2	1928
	21st to end	Night	28% 5% 9% 31% 24% 3	2161
October	1st to 10th	Night	22% 2 7% 24% 42% 3	3800
	11th to 20th	Night	27% 2 9% 26% 28% 8%	3779
	21st to end	Night	42% 4 10% 24% 17% 3	4120
November	1st to 10th	Night	31% 5% 16% 26% 19% 4	2390
	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% 30% 14% 3	306
April	1st to 10th	Day	26% 14% 35% 19% 5% 1	460
	11th to 20th	Day	23% 16% 30% 23% 7%	452
	21st to end	Day	26% 14% 33% 20% 6% 2	565
May	1st to 10th	Day	23% 14% 33% 21% 9% 1	653
	11th to 20th	Day	21% 13% 29% 23% 12% 3	675
	21st to end	Day	17% 8% 28% 27% 16% 3	734
June	1st to 10th	Day	8% 7% 26% 31% 22% 6%	680
	11th to 20th	Day	10% 6% 29% 34% 16% 4	680
	21st to end	Day	11% 8% 32% 31% 13% 4	680
July	1st to 10th	Day	9% 3 18% 32% 28% 11%	671
	11th to 20th	Day	7% 4 26% 33% 24% 6%	668
	21st to end	Day	6% 2 20% 40% 26% 5%	739
August	1st to 10th	Day	8% 3 22% 36% 26% 5%	676
	11th to 20th	Day	12% 4 18% 35% 24% 6%	665
	21st to end	Day	9% 4 19% 38% 24% 6%	569
September	1st to 10th	Day	14% 6% 20% 30% 22% 7%	490
	11th to 20th	Day	8% 4 24% 35% 23% 6%	459
	21st to end	Day	12% 9% 30% 29% 18% 2	300
October	1st to 10th	Day	17% 6% 32% 25% 15% 6%	300
	11th to 20th	Day	15% 23% 35% 19% 7% 1	300
	21st to end	Day	22% 15% 32% 21% 7% 3	330
November	1st to 10th	Day	16% 9% 42% 23% 8% 2	279
	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% 41% 24% 3	570
	11th to 20th	Night	18% 14% 35% 26% 6% 1	563
	21st to end	Night	19% 10% 36% 25% 8% 1	570
May	1st to 10th	Night	19% 9% 33% 31% 8%	570
	11th to 20th	Night	17% 8% 32% 31% 11%	561
	21st to end	Night	15% 6% 28% 36% 14%	617
June	1st to 10th	Night	9% 5% 25% 44% 16% 1	570
	11th to 20th	Night	8% 3 29% 48% 12% 1	570
	21st to end	Night	10% 4 31% 40% 15%	570
July	1st to 10th	Night	6% 2 14% 46% 29% 3	555
	11th to 20th	Night	5% 2 19% 47% 25% 2	552
	21st to end	Night	6% 1 18% 49% 24% 1	624
August	1st to 10th	Night	6% 3 17% 47% 25% 2	567
	11th to 20th	Night	9% 2 18% 44% 26% 2	564
	21st to end	Night	9% 2 19% 42% 25% 2	627
September	1st to 10th	Night	12% 3 20% 42% 20% 3	570
	11th to 20th	Night	14% 2 22% 38% 22% 2	570
	21st to end	Night	11% 6% 29% 37% 15% 1	641
October	1st to 10th	Night	14% 8% 31% 33% 14% 1	760
	11th to 20th	Night	17% 11% 34% 30% 8%	760
	21st to end	Night	20% 11% 33% 28% 7%	836
November	1st to 10th	Night	14% 10% 37% 30% 9%	755
	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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