Allenford Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Allenford Observatory clear sky chart from 2005-11-06 to 2026-09-14.

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	15% 11% 4 9% 5% 4 4 3 5% 7% 32%	149392
	Night	15% 14% 6% 12% 6% 5% 4 3 4 5% 27%	141749
Transparency	Day	52% 5% 11% 20% 11% 1	152939
	Night	58% 5% 10% 17% 9% 1	146290
Seeing	Day	22% 13% 29% 23% 10% 4	17608
	Night	22% 11% 27% 27% 11% 1	26730

By Month

Percentage Cloud Cover NE (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	13% 21% 12% 18% 10% 4 4 2 3 4 10%	4168
	11th to 20th	Day	8% 24% 11% 27% 12% 5% 3 2 2 2 3	4830
	21st to end	Day	10% 17% 9% 21% 12% 9% 5% 3 4 4 8%	5780
February	1st to 10th	Day	13% 21% 7% 19% 9% 6% 4 3 3 3 12%	1890
	11th to 20th	Day	13% 16% 7% 16% 10% 5% 5% 3 5% 4 14%	1843
	21st to end	Day	14% 20% 6% 14% 7% 5% 4 4 4 4 18%	1689
March	1st to 10th	Day	16% 13% 4 12% 7% 5% 4 3 4 5% 25%	4895
	11th to 20th	Day	12% 16% 7% 10% 5% 5% 5% 4 5% 6% 24%	5090
	21st to end	Day	18% 16% 6% 10% 5% 4 4 3 4 6% 25%	5586
April	1st to 10th	Day	18% 15% 4 8% 5% 4 4 3 5% 6% 29%	2416
	11th to 20th	Day	19% 11% 3 7% 4 3 3 2 5% 6% 37%	2702
	21st to end	Day	19% 9% 4 7% 4 3 4 3 5% 7% 34%	2730
May	1st to 10th	Day	15% 9% 2 6% 4 4 5% 4 5% 7% 39%	7642
	11th to 20th	Day	15% 9% 2 7% 4 4 4 3 6% 7% 38%	7922
	21st to end	Day	19% 7% 2 5% 3 3 3 3 5% 6% 44%	9060
June	1st to 10th	Day	14% 7% 3 6% 4 4 4 5% 5% 8% 41%	3083
	11th to 20th	Day	12% 5% 2 7% 4 4 5% 5% 7% 8% 43%	3150
	21st to end	Day	15% 7% 2 6% 4 4 4 4 6% 7% 41%	3150
July	1st to 10th	Day	15% 7% 2 6% 4 4 4 3 6% 8% 42%	8879
	11th to 20th	Day	12% 4 1 4 3 3 4 3 5% 9% 51%	8630
	21st to end	Day	10% 5% 2 4 3 4 4 4 6% 8% 49%	8340
August	1st to 10th	Day	15% 6% 2 5% 3 5% 4 4 6% 7% 41%	6014
	11th to 20th	Day	14% 6% 2 5% 4 3 5% 4 6% 9% 43%	5951
	21st to end	Day	17% 6% 2 6% 4 4 4 4 6% 9% 38%	6552
September	1st to 10th	Day	13% 5% 3 7% 4 5% 5% 4 6% 8% 39%	2489
	11th to 20th	Day	12% 6% 2 5% 4 3 4 3 6% 9% 45%	2233
	21st to end	Day	17% 8% 3 7% 4 5% 5% 4 5% 7% 35%	2200
October	1st to 10th	Day	19% 8% 3 8% 5% 5% 4 3 5% 7% 35%	2030
	11th to 20th	Day	25% 13% 5% 8% 6% 5% 4 3 5% 5% 21%	2000
	21st to end	Day	29% 14% 4 9% 6% 4 3 3 4 5% 18%	2124
November	1st to 10th	Day	19% 19% 4 9% 6% 5% 4 3 4 6% 21%	1815
	11th to 20th	Day	16% 22% 5% 10% 6% 4 5% 4 4 6% 16%	1680
	21st to end	Day	19% 29% 8% 10% 6% 3 4 3 4 5% 10%	1680
December	1st to 10th	Day	14% 31% 11% 16% 7% 4 4 2 2 3 6%	2680
	11th to 20th	Day	19% 26% 6% 16% 6% 4 4 2 5% 4 10%	2916
	21st to end	Day	26% 22% 5% 15% 5% 5% 3 2 4 3 10%	3553
January	1st to 10th	Night	13% 20% 12% 23% 7% 5% 4 3 3 3 7%	8753
	11th to 20th	Night	10% 23% 11% 24% 10% 5% 4 2 2 3 4	8060
	21st to end	Night	9% 17% 9% 19% 12% 8% 5% 3 3 4 10%	8401
February	1st to 10th	Night	13% 20% 6% 18% 10% 5% 4 3 4 4 13%	2730
	11th to 20th	Night	13% 16% 8% 17% 8% 6% 4 3 4 4 18%	2663
	21st to end	Night	14% 18% 5% 13% 7% 5% 4 4 4 4 22%	2113
March	1st to 10th	Night	15% 13% 5% 10% 6% 5% 4 3 5% 6% 28%	5164
	11th to 20th	Night	10% 14% 6% 9% 5% 5% 5% 4 5% 6% 30%	5089
	21st to end	Night	20% 12% 6% 11% 6% 4 3 3 4 4 27%	5588
April	1st to 10th	Night	19% 13% 4 9% 5% 4 4 3 5% 6% 29%	2211
	11th to 20th	Night	18% 10% 3 7% 4 5% 4 3 4 6% 37%	1876
	21st to end	Night	20% 10% 4 8% 4 4 4 4 4 6% 33%	1890
May	1st to 10th	Night	15% 8% 3 7% 4 5% 4 4 5% 7% 38%	5257
	11th to 20th	Night	17% 7% 3 6% 4 4 4 3 5% 6% 42%	4937
	21st to end	Night	19% 5% 2 6% 3 3 4 3 5% 5% 44%	5114
June	1st to 10th	Night	14% 7% 2 7% 5% 4 4 4 5% 7% 42%	1532
	11th to 20th	Night	11% 6% 2 7% 4 3 4 5% 8% 8% 40%	1470
	21st to end	Night	14% 5% 2 7% 4 4 4 5% 6% 7% 41%	1470
July	1st to 10th	Night	12% 8% 3 6% 4 4 4 3 5% 8% 43%	4110
	11th to 20th	Night	10% 5% 2 6% 3 3 4 3 6% 8% 50%	4323
	21st to end	Night	11% 6% 2 6% 4 3 4 3 5% 8% 49%	5737
August	1st to 10th	Night	14% 8% 3 7% 4 4 4 3 5% 7% 41%	4135
	11th to 20th	Night	15% 6% 2 6% 4 3 4 3 5% 7% 46%	4095
	21st to end	Night	17% 7% 3 7% 4 3 4 3 5% 7% 40%	4532
September	1st to 10th	Night	11% 5% 2 6% 3 4 4 4 7% 8% 46%	2123
	11th to 20th	Night	11% 4 3 5% 4 4 4 4 5% 7% 50%	2234
	21st to end	Night	16% 9% 3 7% 5% 5% 5% 4 4 7% 36%	2200
October	1st to 10th	Night	18% 8% 3 6% 5% 4 5% 3 5% 6% 36%	2370
	11th to 20th	Night	25% 12% 4 9% 5% 4 4 3 5% 6% 24%	2400
	21st to end	Night	26% 14% 4 9% 6% 4 4 3 4 5% 21%	2715
November	1st to 10th	Night	18% 18% 4 9% 5% 4 4 3 5% 5% 24%	2703
	11th to 20th	Night	17% 20% 6% 11% 7% 5% 4 3 5% 5% 17%	2940
	21st to end	Night	22% 25% 6% 10% 6% 4 3 3 3 4 13%	2940
December	1st to 10th	Night	16% 27% 9% 15% 7% 5% 4 3 4 4 7%	4720
	11th to 20th	Night	14% 24% 9% 17% 6% 5% 4 3 4 3 10%	5487
	21st to end	Night	21% 23% 8% 15% 6% 5% 4 3 3 4 9%	7667

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	78% 5% 8% 5% 4 1	4169
	11th to 20th	Day	90% 3 3 3 1	4861
	21st to end	Day	80% 4 7% 5% 3	5780
February	1st to 10th	Day	77% 5% 6% 10% 2	1890
	11th to 20th	Day	73% 4 9% 10% 4	1852
	21st to end	Day	69% 5% 8% 13% 4	1689
March	1st to 10th	Day	61% 4 15% 13% 7% 1	4898
	11th to 20th	Day	60% 5% 11% 16% 6% 1	5090
	21st to end	Day	60% 4 11% 16% 8% 1	5586
April	1st to 10th	Day	57% 5% 10% 18% 10%	2416
	11th to 20th	Day	49% 4 9% 22% 15% 1	2702
	21st to end	Day	49% 5% 12% 20% 11% 2	2730
May	1st to 10th	Day	43% 6% 11% 22% 14% 3	7651
	11th to 20th	Day	45% 6% 10% 21% 17% 1	7926
	21st to end	Day	42% 4 9% 26% 16% 4	9061
June	1st to 10th	Day	41% 7% 10% 25% 17% 1	3083
	11th to 20th	Day	37% 6% 12% 26% 17% 1	3150
	21st to end	Day	42% 6% 12% 25% 13% 2	3150
July	1st to 10th	Day	40% 5% 15% 23% 16% 2	8806
	11th to 20th	Day	30% 5% 11% 38% 16% 1	8549
	21st to end	Day	33% 6% 12% 32% 16% 1	8156
August	1st to 10th	Day	41% 6% 13% 22% 16% 2	6001
	11th to 20th	Day	38% 6% 18% 29% 9%	5990
	21st to end	Day	42% 6% 15% 28% 9%	6552
September	1st to 10th	Day	41% 7% 13% 25% 12% 1	2489
	11th to 20th	Day	36% 5% 14% 31% 14%	2233
	21st to end	Day	47% 8% 11% 25% 9% 1	2200
October	1st to 10th	Day	45% 6% 11% 25% 12% 1	3303
	11th to 20th	Day	52% 5% 12% 22% 9%	3240
	21st to end	Day	62% 5% 12% 16% 5%	3413
November	1st to 10th	Day	64% 5% 10% 16% 5%	1815
	11th to 20th	Day	66% 7% 13% 9% 5%	1680
	21st to end	Day	76% 6% 10% 7% 1	1680
December	1st to 10th	Day	83% 6% 7% 4 1	2680
	11th to 20th	Day	78% 4 9% 7% 2	2915
	21st to end	Day	79% 5% 7% 6% 3	3553
January	1st to 10th	Night	80% 4 6% 6% 3	8753
	11th to 20th	Night	85% 4 5% 4 2	8104
	21st to end	Night	77% 4 9% 4 5%	8401
February	1st to 10th	Night	75% 5% 7% 11% 2	2730
	11th to 20th	Night	70% 5% 8% 12% 4	2665
	21st to end	Night	64% 5% 11% 13% 6%	2113
March	1st to 10th	Night	57% 4 14% 15% 9% 1	5174
	11th to 20th	Night	53% 5% 16% 17% 8% 1	5090
	21st to end	Night	59% 4 9% 16% 11% 1	5588
April	1st to 10th	Night	57% 4 9% 19% 11% 1	2211
	11th to 20th	Night	50% 3 8% 21% 17% 1	1876
	21st to end	Night	52% 5% 9% 17% 14% 2	1890
May	1st to 10th	Night	45% 5% 8% 22% 15% 5%	5264
	11th to 20th	Night	43% 6% 8% 24% 18% 1	4938
	21st to end	Night	41% 5% 7% 27% 18% 2	5115
June	1st to 10th	Night	41% 5% 9% 22% 19% 3	1532
	11th to 20th	Night	37% 7% 13% 25% 17% 1	1470
	21st to end	Night	39% 7% 12% 25% 15% 2	1470
July	1st to 10th	Night	39% 5% 14% 23% 15% 3	4075
	11th to 20th	Night	31% 5% 12% 33% 18% 1	4281
	21st to end	Night	34% 5% 10% 31% 18% 1	5606
August	1st to 10th	Night	43% 5% 10% 24% 16% 1	4124
	11th to 20th	Night	37% 7% 16% 29% 10% 1	4122
	21st to end	Night	44% 6% 12% 27% 10% 1	4532
September	1st to 10th	Night	34% 8% 13% 31% 13% 2	2123
	11th to 20th	Night	33% 6% 14% 31% 16%	2234
	21st to end	Night	46% 7% 13% 23% 9% 1	2200
October	1st to 10th	Night	39% 6% 10% 26% 18% 1	3877
	11th to 20th	Night	51% 6% 14% 18% 11%	3899
	21st to end	Night	56% 5% 15% 19% 5%	4376
November	1st to 10th	Night	61% 6% 10% 18% 5%	2703
	11th to 20th	Night	67% 6% 12% 10% 4 1	2940
	21st to end	Night	74% 5% 11% 9% 1	2940
December	1st to 10th	Night	78% 7% 11% 4 1	4720
	11th to 20th	Night	79% 4 9% 7% 1	5487
	21st to end	Night	80% 6% 7% 6% 2	7667

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	25% 20% 34% 16% 3 2	232
	11th to 20th	Day	25% 23% 34% 12% 5% 1	330
	21st to end	Day	25% 20% 29% 19% 6%	363
February	1st to 10th	Day	28% 23% 30% 15% 3	330
	11th to 20th	Day	23% 24% 36% 14% 2	330
	21st to end	Day	30% 23% 35% 10% 2	273
March	1st to 10th	Day	26% 18% 35% 14% 6%	466
	11th to 20th	Day	27% 17% 31% 19% 5% 1	540
	21st to end	Day	30% 14% 32% 17% 6% 1	594
April	1st to 10th	Day	28% 18% 28% 18% 7% 1	540
	11th to 20th	Day	33% 14% 26% 14% 9% 5%	532
	21st to end	Day	28% 11% 32% 21% 6% 2	540
May	1st to 10th	Day	24% 12% 32% 22% 8% 3	543
	11th to 20th	Day	20% 11% 29% 25% 12% 4	567
	21st to end	Day	19% 10% 29% 24% 14% 4	617
June	1st to 10th	Day	16% 11% 32% 26% 12% 2	713
	11th to 20th	Day	15% 13% 33% 27% 10% 3	780
	21st to end	Day	19% 8% 35% 26% 9% 2	780
July	1st to 10th	Day	12% 8% 30% 31% 15% 4	771
	11th to 20th	Day	15% 8% 31% 28% 12% 6%	753
	21st to end	Day	14% 6% 30% 30% 14% 6%	620
August	1st to 10th	Day	13% 7% 25% 31% 16% 8%	566
	11th to 20th	Day	14% 6% 22% 30% 18% 9%	559
	21st to end	Day	17% 8% 22% 29% 17% 7%	627
September	1st to 10th	Day	15% 10% 24% 27% 16% 8%	570
	11th to 20th	Day	10% 7% 20% 29% 21% 13%	542
	21st to end	Day	18% 10% 25% 26% 15% 6%	530
October	1st to 10th	Day	23% 13% 22% 25% 13% 4	530
	11th to 20th	Day	26% 17% 30% 20% 5% 1	530
	21st to end	Day	33% 15% 30% 17% 5%	507
November	1st to 10th	Day	37% 15% 28% 16% 5%	311
	11th to 20th	Day	29% 21% 28% 17% 3 1	220
	21st to end	Day	33% 23% 29% 12% 2	220
December	1st to 10th	Day	32% 16% 31% 16% 4	220
	11th to 20th	Day	34% 23% 26% 10% 5%	220
	21st to end	Day	41% 14% 29% 12% 3	242
January	1st to 10th	Night	24% 18% 31% 21% 5%	1050
	11th to 20th	Night	29% 16% 30% 18% 6% 1	991
	21st to end	Night	19% 14% 32% 26% 7% 1	924
February	1st to 10th	Night	22% 18% 34% 22% 5%	840
	11th to 20th	Night	22% 15% 32% 25% 5%	823
	21st to end	Night	24% 17% 34% 22% 3	692
March	1st to 10th	Night	21% 14% 35% 23% 6%	840
	11th to 20th	Night	23% 15% 32% 24% 6% 1	840
	21st to end	Night	24% 12% 30% 25% 8% 1	924
April	1st to 10th	Night	25% 13% 31% 25% 5%	840
	11th to 20th	Night	24% 11% 29% 25% 9% 2	632
	21st to end	Night	26% 8% 25% 29% 11%	630
May	1st to 10th	Night	20% 6% 26% 34% 13% 1	630
	11th to 20th	Night	19% 8% 21% 38% 12% 1	519
	21st to end	Night	16% 3 26% 40% 15% 1	455
June	1st to 10th	Night	16% 7% 27% 37% 11% 2	420
	11th to 20th	Night	12% 7% 29% 39% 13%	420
	21st to end	Night	16% 6% 23% 38% 17%	420
July	1st to 10th	Night	10% 5% 19% 48% 17% 1	410
	11th to 20th	Night	13% 2 23% 39% 23% 1	470
	21st to end	Night	10% 3 20% 45% 22% 1	690
August	1st to 10th	Night	12% 3 18% 39% 29% 1	627
	11th to 20th	Night	13% 3 14% 40% 27% 4	624
	21st to end	Night	16% 4 18% 34% 25% 2	693
September	1st to 10th	Night	13% 7% 20% 34% 21% 5%	835
	11th to 20th	Night	13% 4 16% 38% 24% 4	813
	21st to end	Night	20% 6% 22% 29% 19% 4	800
October	1st to 10th	Night	21% 7% 27% 30% 14% 2	800
	11th to 20th	Night	30% 12% 25% 25% 7% 1	800
	21st to end	Night	33% 14% 26% 23% 4	880
November	1st to 10th	Night	30% 13% 27% 22% 7% 1	820
	11th to 20th	Night	27% 18% 29% 18% 8% 1	840
	21st to end	Night	34% 15% 31% 17% 3	840
December	1st to 10th	Night	32% 18% 30% 15% 5%	840
	11th to 20th	Night	24% 19% 37% 16% 3 1	903
	21st to end	Night	34% 14% 31% 17% 4	1155

Page generated at 2026-09-11 04:11:55. Version ac07

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