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 2025-05-11.

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% 12% 5% 10% 5% 4 4 3 5% 6% 30%	113679
	Night	15% 14% 6% 12% 6% 5% 4 3 4 5% 25%	115285
Transparency	Day	54% 6% 11% 19% 10% 1	117269
	Night	59% 6% 11% 16% 9% 1	119858
Seeing	Day	22% 12% 29% 23% 10% 4	15742
	Night	22% 11% 27% 27% 11% 1	25118

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	11% 20% 13% 19% 11% 5% 4 2 3 3 10%	3218
	11th to 20th	Day	9% 23% 11% 27% 11% 5% 3 2 2 2 4	3701
	21st to end	Day	13% 19% 10% 21% 8% 7% 5% 3 3 3 8%	4466
February	1st to 10th	Day	13% 21% 7% 19% 9% 6% 4 3 3 3 11%	1800
	11th to 20th	Day	13% 16% 8% 16% 10% 5% 5% 3 5% 4 15%	1753
	21st to end	Day	14% 20% 6% 13% 7% 5% 4 4 4 4 18%	1611
March	1st to 10th	Day	17% 13% 5% 12% 6% 5% 4 3 4 5% 25%	4789
	11th to 20th	Day	12% 16% 7% 10% 5% 5% 5% 4 5% 6% 24%	4980
	21st to end	Day	18% 16% 6% 10% 5% 4 4 3 4 6% 26%	5465
April	1st to 10th	Day	18% 15% 4 8% 5% 4 4 3 5% 6% 29%	2301
	11th to 20th	Day	19% 11% 3 7% 4 3 3 3 5% 6% 37%	2572
	21st to end	Day	20% 10% 4 7% 4 3 4 3 5% 7% 34%	2600
May	1st to 10th	Day	17% 9% 2 7% 4 4 4 3 5% 7% 38%	4238
	11th to 20th	Day	15% 8% 2 6% 4 4 4 3 6% 8% 39%	4237
	21st to end	Day	15% 5% 2 5% 4 3 4 3 5% 7% 46%	4837
June	1st to 10th	Day	14% 7% 3 6% 5% 4 4 5% 5% 8% 40%	2789
	11th to 20th	Day	11% 5% 2 7% 4 4 5% 4 7% 8% 44%	2850
	21st to end	Day	16% 7% 2 6% 4 4 4 4 6% 7% 41%	2850
July	1st to 10th	Day	14% 6% 2 6% 4 4 5% 4 7% 8% 39%	4760
	11th to 20th	Day	12% 5% 1 5% 3 3 4 4 6% 9% 48%	4622
	21st to end	Day	10% 5% 2 5% 4 4 5% 3 6% 9% 47%	4466
August	1st to 10th	Day	12% 6% 2 5% 3 5% 5% 4 7% 9% 41%	4104
	11th to 20th	Day	15% 6% 2 6% 4 3 5% 4 6% 9% 40%	4044
	21st to end	Day	11% 5% 2 5% 4 4 5% 4 6% 9% 45%	4505
September	1st to 10th	Day	14% 5% 3 8% 4 4 5% 4 6% 8% 39%	2252
	11th to 20th	Day	12% 7% 2 5% 4 3 4 3 6% 8% 44%	2090
	21st to end	Day	17% 8% 3 7% 4 5% 5% 4 5% 7% 34%	2090
October	1st to 10th	Day	19% 9% 3 8% 5% 5% 4 3 5% 7% 33%	1929
	11th to 20th	Day	25% 13% 5% 9% 6% 5% 4 3 5% 5% 20%	1900
	21st to end	Day	30% 15% 4 9% 5% 4 3 3 4 5% 17%	2018
November	1st to 10th	Day	18% 19% 4 9% 6% 5% 4 3 4 6% 22%	1727
	11th to 20th	Day	16% 22% 5% 10% 6% 4 5% 4 4 6% 17%	1600
	21st to end	Day	20% 28% 8% 10% 6% 3 4 3 4 5% 10%	1600
December	1st to 10th	Day	14% 31% 11% 15% 7% 4 3 2 2 3 6%	2600
	11th to 20th	Day	19% 26% 6% 16% 6% 4 4 2 5% 4 10%	2839
	21st to end	Day	26% 23% 5% 15% 5% 5% 3 2 4 3 10%	3476
January	1st to 10th	Night	13% 21% 11% 23% 7% 4 3 3 3 3 7%	6744
	11th to 20th	Night	11% 19% 11% 25% 10% 6% 4 3 3 3 5%	6182
	21st to end	Night	12% 20% 10% 18% 8% 7% 5% 3 4 4 9%	6490
February	1st to 10th	Night	13% 20% 6% 19% 10% 5% 4 3 3 4 12%	2600
	11th to 20th	Night	13% 15% 8% 17% 8% 5% 4 3 4 4 17%	2533
	21st to end	Night	14% 17% 5% 13% 7% 5% 5% 4 4 4 22%	2015
March	1st to 10th	Night	15% 13% 5% 10% 6% 5% 4 3 4 6% 28%	5051
	11th to 20th	Night	10% 14% 6% 9% 5% 5% 5% 4 5% 6% 30%	4979
	21st to end	Night	20% 12% 6% 11% 6% 4 3 3 4 4 27%	5467
April	1st to 10th	Night	20% 13% 4 9% 5% 4 4 3 5% 6% 29%	2106
	11th to 20th	Night	18% 10% 3 7% 3 4 4 3 4 6% 37%	1786
	21st to end	Night	20% 11% 4 8% 4 4 4 4 5% 6% 32%	1800
May	1st to 10th	Night	15% 7% 2 7% 4 5% 5% 4 4 7% 39%	2923
	11th to 20th	Night	15% 7% 2 6% 5% 4 5% 3 4 7% 41%	2661
	21st to end	Night	12% 5% 2 6% 4 4 4 4 5% 6% 47%	2738
June	1st to 10th	Night	14% 7% 2 8% 5% 4 4 4 5% 7% 40%	1386
	11th to 20th	Night	10% 6% 2 6% 4 3 5% 5% 8% 9% 41%	1330
	21st to end	Night	15% 5% 1 7% 4 4 4 5% 6% 7% 41%	1330
July	1st to 10th	Night	10% 8% 3 7% 4 4 3 3 6% 7% 46%	2210
	11th to 20th	Night	12% 5% 2 5% 3 3 4 3 5% 7% 51%	2326
	21st to end	Night	7% 7% 2 6% 4 4 5% 4 6% 7% 48%	3087
August	1st to 10th	Night	12% 7% 3 6% 4 4 5% 3 5% 8% 43%	2825
	11th to 20th	Night	14% 6% 2 7% 4 3 5% 4 5% 8% 40%	2785
	21st to end	Night	13% 5% 2 6% 4 3 4 4 5% 7% 47%	3124
September	1st to 10th	Night	11% 5% 2 6% 4 4 4 4 6% 8% 46%	1920
	11th to 20th	Night	12% 5% 3 5% 4 4 4 4 5% 7% 48%	2090
	21st to end	Night	16% 8% 2 7% 5% 5% 5% 4 4 7% 36%	2090
October	1st to 10th	Night	19% 8% 3 7% 5% 4 5% 3 5% 6% 35%	2251
	11th to 20th	Night	25% 12% 4 9% 5% 4 4 3 5% 6% 23%	2280
	21st to end	Night	27% 14% 4 10% 6% 4 4 4 4 5% 20%	2579
November	1st to 10th	Night	19% 17% 4 8% 5% 4 4 3 5% 5% 25%	2571
	11th to 20th	Night	17% 19% 6% 10% 7% 5% 5% 4 5% 5% 18%	2800
	21st to end	Night	22% 24% 6% 10% 6% 4 3 3 3 5% 14%	2800
December	1st to 10th	Night	16% 27% 9% 14% 6% 5% 4 3 4 4 7%	4580
	11th to 20th	Night	14% 24% 9% 17% 6% 5% 4 3 3 3 10%	5344
	21st to end	Night	22% 23% 8% 14% 6% 5% 4 2 3 4 9%	7502

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	79% 5% 7% 4 5% 1	3219
	11th to 20th	Day	89% 3 4 3 1	3732
	21st to end	Day	81% 5% 7% 5% 2	4466
February	1st to 10th	Day	78% 6% 6% 9% 1	1800
	11th to 20th	Day	73% 4 9% 10% 4	1762
	21st to end	Day	69% 5% 8% 13% 4	1611
March	1st to 10th	Day	61% 4 15% 13% 7% 1	4792
	11th to 20th	Day	60% 5% 11% 17% 6% 1	4980
	21st to end	Day	59% 5% 11% 17% 8% 1	5465
April	1st to 10th	Day	57% 5% 10% 18% 10%	2301
	11th to 20th	Day	49% 4 9% 22% 15% 1	2572
	21st to end	Day	50% 5% 12% 19% 12% 2	2600
May	1st to 10th	Day	44% 6% 11% 20% 17% 2	4250
	11th to 20th	Day	42% 7% 14% 25% 11% 1	4241
	21st to end	Day	37% 5% 11% 22% 19% 6%	4837
June	1st to 10th	Day	41% 7% 10% 25% 16% 1	2789
	11th to 20th	Day	36% 6% 12% 26% 18% 1	2850
	21st to end	Day	42% 6% 12% 25% 14% 2	2850
July	1st to 10th	Day	41% 6% 16% 20% 15% 2	4686
	11th to 20th	Day	33% 6% 13% 35% 12% 1	4541
	21st to end	Day	33% 7% 11% 30% 18% 2	4324
August	1st to 10th	Day	38% 7% 14% 25% 15% 1	4091
	11th to 20th	Day	41% 7% 16% 25% 10%	4083
	21st to end	Day	35% 6% 15% 32% 12% 1	4505
September	1st to 10th	Day	41% 7% 13% 25% 12% 2	2252
	11th to 20th	Day	38% 5% 14% 29% 14%	2090
	21st to end	Day	47% 8% 11% 25% 9% 1	2090
October	1st to 10th	Day	46% 6% 11% 24% 12% 1	3202
	11th to 20th	Day	52% 5% 12% 22% 8%	3140
	21st to end	Day	63% 5% 12% 16% 4	3307
November	1st to 10th	Day	64% 5% 10% 16% 6%	1727
	11th to 20th	Day	66% 7% 12% 9% 5%	1600
	21st to end	Day	76% 6% 10% 7% 1	1600
December	1st to 10th	Day	83% 6% 7% 4 1	2600
	11th to 20th	Day	78% 4 10% 7% 2	2838
	21st to end	Day	79% 5% 6% 6% 3	3476
January	1st to 10th	Night	81% 4 6% 6% 3	6744
	11th to 20th	Night	83% 5% 6% 4 2	6226
	21st to end	Night	77% 5% 10% 5% 3	6490
February	1st to 10th	Night	77% 5% 7% 10% 2	2600
	11th to 20th	Night	71% 5% 8% 12% 4	2535
	21st to end	Night	63% 5% 12% 13% 6%	2015
March	1st to 10th	Night	57% 4 15% 15% 9% 1	5061
	11th to 20th	Night	53% 5% 16% 17% 8% 1	4980
	21st to end	Night	60% 4 9% 16% 11% 1	5467
April	1st to 10th	Night	56% 4 9% 19% 11% 1	2106
	11th to 20th	Night	49% 3 8% 20% 17% 2	1786
	21st to end	Night	53% 6% 8% 16% 15% 2	1800
May	1st to 10th	Night	44% 6% 11% 20% 16% 3	2930
	11th to 20th	Night	42% 6% 10% 24% 16% 2	2662
	21st to end	Night	35% 7% 10% 24% 20% 3	2739
June	1st to 10th	Night	43% 6% 10% 22% 18% 3	1386
	11th to 20th	Night	36% 7% 13% 25% 18% 1	1330
	21st to end	Night	39% 7% 12% 25% 15% 2	1330
July	1st to 10th	Night	37% 5% 12% 23% 17% 6%	2175
	11th to 20th	Night	32% 5% 11% 36% 14% 1	2284
	21st to end	Night	34% 6% 13% 26% 20% 1	2988
August	1st to 10th	Night	40% 6% 12% 28% 15% 1	2814
	11th to 20th	Night	41% 9% 16% 21% 14% 1	2812
	21st to end	Night	36% 7% 12% 31% 14% 1	3124
September	1st to 10th	Night	34% 8% 12% 30% 14% 2	1920
	11th to 20th	Night	34% 6% 13% 30% 16%	2090
	21st to end	Night	46% 7% 13% 24% 9% 1	2090
October	1st to 10th	Night	40% 6% 10% 26% 18% 1	3758
	11th to 20th	Night	51% 6% 14% 18% 11%	3779
	21st to end	Night	56% 5% 15% 19% 5%	4240
November	1st to 10th	Night	60% 6% 10% 18% 5%	2571
	11th to 20th	Night	67% 6% 12% 10% 4 1	2800
	21st to end	Night	73% 6% 11% 9% 1	2800
December	1st to 10th	Night	78% 7% 11% 4 1	4580
	11th to 20th	Night	79% 4 9% 7% 1	5344
	21st to end	Night	80% 6% 6% 6% 2	7502

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	23% 20% 35% 17% 4 2	211
	11th to 20th	Day	23% 21% 36% 14% 5% 1	300
	21st to end	Day	27% 18% 29% 18% 6%	330
February	1st to 10th	Day	31% 23% 30% 14% 3	300
	11th to 20th	Day	23% 25% 37% 13% 2	300
	21st to end	Day	30% 24% 35% 10% 2	249
March	1st to 10th	Day	26% 17% 35% 15% 6%	430
	11th to 20th	Day	25% 15% 33% 21% 5% 1	500
	21st to end	Day	29% 13% 33% 19% 6% 1	550
April	1st to 10th	Day	27% 18% 29% 19% 8% 1	500
	11th to 20th	Day	32% 14% 26% 14% 9% 5%	492
	21st to end	Day	29% 11% 31% 21% 7% 2	500
May	1st to 10th	Day	25% 10% 31% 22% 9% 3	503
	11th to 20th	Day	20% 9% 28% 25% 14% 5%	487
	21st to end	Day	17% 8% 28% 27% 15% 5%	529
June	1st to 10th	Day	16% 11% 32% 27% 13% 3	619
	11th to 20th	Day	13% 12% 32% 29% 11% 3	680
	21st to end	Day	19% 7% 34% 27% 10% 3	680
July	1st to 10th	Day	12% 7% 28% 31% 16% 4	671
	11th to 20th	Day	15% 7% 30% 28% 13% 7%	655
	21st to end	Day	14% 5% 28% 31% 15% 7%	532
August	1st to 10th	Day	14% 6% 23% 32% 16% 9%	486
	11th to 20th	Day	15% 6% 19% 30% 20% 10%	479
	21st to end	Day	17% 7% 21% 30% 17% 8%	539
September	1st to 10th	Day	16% 9% 24% 28% 16% 8%	490
	11th to 20th	Day	11% 7% 21% 28% 21% 13%	490
	21st to end	Day	18% 10% 25% 26% 16% 6%	490
October	1st to 10th	Day	24% 14% 21% 24% 12% 5%	490
	11th to 20th	Day	26% 17% 30% 20% 5% 1	490
	21st to end	Day	34% 15% 30% 16% 5%	467
November	1st to 10th	Day	35% 16% 28% 16% 6%	283
	11th to 20th	Day	28% 21% 29% 19% 3 2	200
	21st to end	Day	32% 23% 30% 13% 3	200
December	1st to 10th	Day	33% 16% 31% 17% 5% 1	200
	11th to 20th	Day	33% 23% 27% 11% 6% 1	200
	21st to end	Day	40% 13% 31% 12% 3	220
January	1st to 10th	Night	24% 17% 31% 22% 5%	1000
	11th to 20th	Night	28% 16% 30% 18% 6% 1	944
	21st to end	Night	20% 14% 32% 26% 8% 1	880
February	1st to 10th	Night	22% 18% 34% 21% 4	800
	11th to 20th	Night	22% 15% 32% 25% 5%	783
	21st to end	Night	23% 17% 34% 23% 3	660
March	1st to 10th	Night	21% 14% 35% 24% 6% 1	800
	11th to 20th	Night	21% 15% 33% 24% 6% 1	800
	21st to end	Night	24% 10% 30% 26% 9% 1	880
April	1st to 10th	Night	25% 13% 31% 26% 5%	800
	11th to 20th	Night	24% 11% 28% 25% 10% 2	602
	21st to end	Night	26% 8% 26% 29% 11% 1	600
May	1st to 10th	Night	21% 5% 27% 34% 12% 1	600
	11th to 20th	Night	19% 7% 19% 40% 13% 1	471
	21st to end	Night	14% 3 27% 40% 16% 1	411
June	1st to 10th	Night	17% 7% 28% 36% 11% 2	380
	11th to 20th	Night	12% 6% 26% 41% 14% 1	380
	21st to end	Night	17% 5% 23% 37% 18%	380
July	1st to 10th	Night	11% 5% 18% 48% 18% 1	370
	11th to 20th	Night	12% 2 22% 38% 24% 1	424
	21st to end	Night	10% 3 19% 45% 23% 1	624
August	1st to 10th	Night	12% 2 17% 38% 29% 1	567
	11th to 20th	Night	12% 2 14% 38% 29% 4	564
	21st to end	Night	16% 4 18% 35% 25% 3	627
September	1st to 10th	Night	13% 6% 21% 34% 20% 5%	755
	11th to 20th	Night	14% 4 17% 38% 23% 5%	760
	21st to end	Night	20% 7% 22% 29% 19% 4	760
October	1st to 10th	Night	22% 7% 26% 29% 14% 2	760
	11th to 20th	Night	30% 12% 24% 25% 8% 1	760
	21st to end	Night	33% 14% 26% 23% 4	836
November	1st to 10th	Night	30% 12% 27% 23% 7% 1	780
	11th to 20th	Night	26% 18% 29% 18% 8% 1	800
	21st to end	Night	33% 15% 31% 18% 3	800
December	1st to 10th	Night	33% 17% 30% 15% 5%	800
	11th to 20th	Night	24% 18% 37% 17% 3 1	860
	21st to end	Night	34% 14% 31% 18% 4	1100

Page generated at 2025-05-08 09:33:55. Version ac07

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