Anstruther Lake Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Anstruther Lake 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 8% 5% 4 4 4 5% 7% 32%	149753
	Night	15% 12% 4 10% 5% 5% 4 3 5% 6% 31%	141300
Transparency	Day	50% 5% 11% 21% 11% 1	153308
	Night	52% 6% 11% 19% 11% 1	145820
Seeing	Day	23% 16% 31% 19% 8% 3	16743
	Night	22% 13% 31% 24% 9% 1	25939

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	14% 15% 4 12% 6% 5% 5% 4 5% 6% 24%	4661
	11th to 20th	Day	10% 24% 10% 16% 7% 7% 5% 3 3 3 13%	4710
	21st to end	Day	10% 16% 7% 17% 8% 7% 5% 3 4 4 18%	5780
February	1st to 10th	Day	13% 17% 6% 14% 7% 4 5% 3 4 4 22%	1890
	11th to 20th	Day	13% 15% 6% 13% 7% 5% 5% 3 4 4 25%	1843
	21st to end	Day	16% 16% 6% 11% 7% 5% 5% 3 4 5% 23%	1625
March	1st to 10th	Day	19% 13% 5% 10% 6% 4 3 3 4 6% 27%	4938
	11th to 20th	Day	12% 15% 5% 10% 6% 5% 5% 4 6% 7% 26%	5090
	21st to end	Day	19% 12% 5% 8% 4 3 3 3 3 5% 36%	5650
April	1st to 10th	Day	20% 13% 3 9% 5% 5% 4 3 5% 5% 29%	2520
	11th to 20th	Day	23% 10% 2 6% 4 4 3 2 5% 4 35%	2515
	21st to end	Day	22% 10% 3 8% 5% 4 4 3 4 5% 32%	2730
May	1st to 10th	Day	19% 8% 3 7% 5% 5% 4 4 6% 7% 34%	7956
	11th to 20th	Day	18% 8% 3 7% 5% 4 4 3 5% 7% 35%	8221
	21st to end	Day	18% 8% 3 6% 4 4 5% 3 5% 6% 38%	9060
June	1st to 10th	Day	19% 8% 2 7% 6% 5% 4 3 5% 7% 33%	2940
	11th to 20th	Day	14% 7% 3 6% 4 6% 5% 5% 6% 9% 34%	2940
	21st to end	Day	17% 7% 3 6% 5% 5% 5% 4 6% 9% 33%	2940
July	1st to 10th	Day	11% 6% 3 7% 5% 4 5% 4 7% 10% 37%	8280
	11th to 20th	Day	9% 4 2 5% 4 4 5% 4 6% 9% 49%	8280
	21st to end	Day	9% 5% 2 6% 4 5% 5% 5% 8% 9% 42%	8877
August	1st to 10th	Day	16% 9% 3 7% 4 4 5% 4 5% 6% 35%	6014
	11th to 20th	Day	13% 6% 2 7% 5% 4 5% 4 7% 9% 37%	5951
	21st to end	Day	13% 7% 3 7% 5% 5% 6% 5% 7% 10% 34%	6242
September	1st to 10th	Day	14% 7% 2 6% 4 4 4 5% 5% 9% 40%	2520
	11th to 20th	Day	12% 5% 2 6% 5% 4 4 4 4 7% 48%	2411
	21st to end	Day	18% 7% 2 7% 5% 5% 4 4 6% 6% 37%	2101
October	1st to 10th	Day	19% 9% 3 7% 4 3 4 3 5% 8% 34%	2000
	11th to 20th	Day	23% 11% 3 8% 5% 5% 5% 3 5% 6% 27%	2000
	21st to end	Day	25% 13% 4 8% 5% 4 3 3 5% 5% 24%	2178
November	1st to 10th	Day	17% 11% 4 8% 5% 5% 4 4 5% 7% 31%	1736
	11th to 20th	Day	16% 14% 5% 9% 4 4 4 4 6% 7% 27%	1680
	21st to end	Day	20% 19% 5% 9% 6% 5% 5% 4 4 5% 18%	1680
December	1st to 10th	Day	17% 23% 7% 12% 6% 4 4 2 4 6% 15%	2680
	11th to 20th	Day	18% 20% 6% 14% 4 3 3 3 3 4 21%	3066
	21st to end	Day	23% 20% 5% 12% 4 3 3 2 3 4 20%	4048
January	1st to 10th	Night	13% 17% 7% 15% 6% 5% 4 3 5% 5% 19%	8234
	11th to 20th	Night	10% 18% 8% 18% 10% 6% 5% 3 4 4 13%	8158
	21st to end	Night	11% 14% 7% 14% 8% 6% 5% 3 5% 5% 22%	8401
February	1st to 10th	Night	12% 16% 6% 10% 7% 6% 6% 4 6% 6% 22%	2730
	11th to 20th	Night	13% 14% 6% 12% 6% 6% 4 3 5% 5% 26%	2663
	21st to end	Night	14% 16% 4 9% 6% 5% 5% 4 6% 6% 25%	2179
March	1st to 10th	Night	17% 13% 4 10% 6% 5% 5% 3 5% 5% 29%	5120
	11th to 20th	Night	10% 12% 5% 10% 6% 5% 5% 4 6% 7% 29%	5089
	21st to end	Night	20% 8% 3 8% 4 4 4 3 5% 6% 33%	5524
April	1st to 10th	Night	20% 11% 4 7% 5% 4 4 4 5% 6% 30%	2100
	11th to 20th	Night	16% 11% 3 8% 4 4 3 3 5% 6% 36%	2071
	21st to end	Night	17% 10% 2 7% 5% 5% 4 4 6% 5% 33%	1890
May	1st to 10th	Night	21% 9% 2 6% 4 4 4 3 5% 7% 34%	4941
	11th to 20th	Night	15% 7% 3 6% 4 4 4 3 6% 6% 40%	4639
	21st to end	Night	18% 7% 2 5% 4 4 4 4 4 7% 40%	5114
June	1st to 10th	Night	15% 8% 2 6% 4 4 5% 3 6% 9% 38%	1680
	11th to 20th	Night	13% 6% 2 6% 4 5% 4 4 6% 8% 42%	1680
	21st to end	Night	13% 7% 2 8% 5% 5% 4 4 6% 7% 39%	1680
July	1st to 10th	Night	9% 6% 3 8% 5% 5% 5% 4 6% 9% 40%	4680
	11th to 20th	Night	10% 6% 2 5% 3 4 4 3 5% 8% 50%	4680
	21st to end	Night	9% 7% 2 6% 4 4 4 4 6% 7% 47%	5196
August	1st to 10th	Night	14% 6% 2 6% 4 4 5% 4 6% 7% 42%	4135
	11th to 20th	Night	12% 6% 2 7% 4 3 3 3 5% 7% 47%	4095
	21st to end	Night	11% 7% 3 7% 4 5% 4 4 6% 8% 42%	4836
September	1st to 10th	Night	11% 5% 2 6% 4 4 4 3 7% 9% 44%	2100
	11th to 20th	Night	11% 5% 1 4 4 3 4 3 5% 6% 52%	2052
	21st to end	Night	19% 6% 2 6% 4 4 5% 3 4 5% 42%	2298
October	1st to 10th	Night	16% 7% 3 8% 4 4 5% 3 5% 6% 39%	2400
	11th to 20th	Night	21% 10% 4 8% 5% 4 4 4 5% 6% 30%	2400
	21st to end	Night	21% 13% 3 8% 5% 4 4 4 4 5% 28%	2660
November	1st to 10th	Night	18% 10% 4 7% 5% 4 4 3 5% 6% 34%	2777
	11th to 20th	Night	18% 13% 4 7% 5% 4 5% 3 5% 6% 29%	2940
	21st to end	Night	20% 14% 4 10% 6% 5% 5% 4 6% 6% 21%	2940
December	1st to 10th	Night	20% 18% 7% 11% 6% 4 4 4 4 6% 16%	4720
	11th to 20th	Night	16% 20% 6% 15% 6% 5% 4 3 4 4 18%	5348
	21st to end	Night	24% 19% 5% 14% 5% 4 3 2 3 4 16%	7150

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	59% 5% 10% 17% 7% 1	4662
	11th to 20th	Day	76% 4 6% 7% 6%	4740
	21st to end	Day	69% 4 7% 10% 8% 1	5780
February	1st to 10th	Day	66% 4 8% 15% 6% 1	1890
	11th to 20th	Day	63% 5% 8% 13% 10% 2	1852
	21st to end	Day	63% 4 8% 14% 9% 1	1625
March	1st to 10th	Day	59% 4 11% 11% 14% 1	4941
	11th to 20th	Day	57% 5% 13% 16% 7% 2	5090
	21st to end	Day	52% 3 7% 20% 16% 2	5650
April	1st to 10th	Day	57% 4 7% 18% 11% 2	2520
	11th to 20th	Day	52% 4 7% 20% 14% 1	2515
	21st to end	Day	54% 4 8% 20% 12% 3	2730
May	1st to 10th	Day	49% 5% 10% 21% 12% 4	7965
	11th to 20th	Day	48% 6% 10% 19% 16% 1	8225
	21st to end	Day	46% 5% 10% 24% 13% 2	9061
June	1st to 10th	Day	50% 5% 10% 19% 15% 1	2940
	11th to 20th	Day	45% 6% 11% 24% 12% 2	2940
	21st to end	Day	46% 7% 13% 22% 11% 1	2940
July	1st to 10th	Day	39% 7% 17% 25% 12% 1	8210
	11th to 20th	Day	31% 6% 13% 33% 16% 1	8205
	21st to end	Day	35% 7% 15% 33% 9%	8680
August	1st to 10th	Day	48% 6% 12% 18% 13% 1	6001
	11th to 20th	Day	39% 8% 17% 29% 7%	5990
	21st to end	Day	41% 8% 16% 27% 8% 1	6242
September	1st to 10th	Day	41% 6% 12% 29% 11% 1	2520
	11th to 20th	Day	36% 5% 13% 32% 13% 1	2411
	21st to end	Day	47% 6% 14% 23% 11%	2101
October	1st to 10th	Day	43% 5% 12% 29% 11% 1	3260
	11th to 20th	Day	50% 5% 13% 20% 12% 1	3240
	21st to end	Day	57% 6% 12% 19% 6%	3492
November	1st to 10th	Day	52% 5% 14% 21% 7% 1	1736
	11th to 20th	Day	56% 6% 13% 14% 9% 1	1680
	21st to end	Day	68% 5% 11% 10% 5% 1	1680
December	1st to 10th	Day	72% 5% 11% 8% 3	2680
	11th to 20th	Day	68% 5% 7% 13% 7%	3066
	21st to end	Day	69% 4 7% 12% 7%	4048
January	1st to 10th	Night	67% 4 8% 13% 7%	8234
	11th to 20th	Night	74% 5% 7% 7% 6% 1	8203
	21st to end	Night	64% 5% 8% 11% 12% 1	8401
February	1st to 10th	Night	61% 5% 10% 16% 6% 1	2730
	11th to 20th	Night	60% 4 9% 18% 8% 1	2665
	21st to end	Night	57% 5% 10% 19% 9%	2179
March	1st to 10th	Night	56% 5% 8% 15% 11% 5%	5130
	11th to 20th	Night	52% 7% 16% 17% 8% 1	5090
	21st to end	Night	50% 4 9% 21% 14% 2	5524
April	1st to 10th	Night	54% 5% 8% 20% 12% 2	2100
	11th to 20th	Night	49% 4 9% 21% 15% 1	2071
	21st to end	Night	50% 6% 8% 18% 16% 2	1890
May	1st to 10th	Night	49% 5% 9% 20% 13% 4	4948
	11th to 20th	Night	42% 7% 12% 22% 16% 2	4640
	21st to end	Night	43% 5% 11% 23% 16% 2	5115
June	1st to 10th	Night	42% 5% 12% 23% 16% 1	1680
	11th to 20th	Night	38% 7% 11% 25% 17% 2	1680
	21st to end	Night	43% 6% 15% 22% 12% 1	1680
July	1st to 10th	Night	37% 8% 18% 24% 13% 1	4640
	11th to 20th	Night	31% 6% 14% 33% 16%	4632
	21st to end	Night	35% 7% 11% 32% 15%	5078
August	1st to 10th	Night	40% 7% 14% 21% 16% 2	4124
	11th to 20th	Night	35% 7% 20% 27% 10% 1	4122
	21st to end	Night	38% 7% 15% 30% 9% 1	4836
September	1st to 10th	Night	35% 7% 16% 28% 13% 1	2100
	11th to 20th	Night	33% 5% 13% 35% 14% 1	2052
	21st to end	Night	45% 5% 11% 27% 12%	2298
October	1st to 10th	Night	37% 5% 11% 29% 18%	3920
	11th to 20th	Night	47% 5% 13% 25% 10%	3899
	21st to end	Night	51% 6% 14% 21% 8%	4284
November	1st to 10th	Night	50% 6% 12% 22% 9% 1	2777
	11th to 20th	Night	55% 6% 12% 18% 8% 1	2940
	21st to end	Night	63% 7% 12% 14% 4	2940
December	1st to 10th	Night	68% 6% 11% 11% 3	4720
	11th to 20th	Night	69% 5% 9% 11% 6%	5348
	21st to end	Night	72% 4 6% 10% 7%	7150

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	22% 25% 33% 15% 5%	220
	11th to 20th	Day	26% 23% 34% 14% 3	231
	21st to end	Day	25% 22% 35% 13% 5% 1	363
February	1st to 10th	Day	30% 21% 35% 11% 3	330
	11th to 20th	Day	29% 26% 38% 6% 1	330
	21st to end	Day	34% 17% 37% 11% 1	341
March	1st to 10th	Day	26% 19% 37% 15% 3	540
	11th to 20th	Day	30% 20% 31% 17% 2 1	540
	21st to end	Day	33% 18% 31% 12% 6% 1	594
April	1st to 10th	Day	29% 21% 32% 11% 6%	540
	11th to 20th	Day	36% 19% 21% 15% 7% 2	532
	21st to end	Day	30% 18% 32% 14% 5% 1	540
May	1st to 10th	Day	27% 16% 31% 16% 6% 4	543
	11th to 20th	Day	21% 16% 35% 17% 8% 3	567
	21st to end	Day	23% 14% 33% 18% 9% 3	617
June	1st to 10th	Day	23% 14% 33% 21% 9% 1	570
	11th to 20th	Day	18% 14% 33% 23% 9% 3	570
	21st to end	Day	21% 11% 36% 22% 8% 2	570
July	1st to 10th	Day	13% 10% 33% 30% 12% 3	565
	11th to 20th	Day	14% 11% 32% 26% 13% 5%	564
	21st to end	Day	12% 12% 33% 29% 11% 3	620
August	1st to 10th	Day	15% 8% 29% 30% 13% 6%	566
	11th to 20th	Day	16% 6% 28% 30% 14% 6%	559
	21st to end	Day	15% 9% 33% 23% 14% 6%	627
September	1st to 10th	Day	17% 12% 28% 24% 13% 6%	570
	11th to 20th	Day	11% 10% 24% 23% 21% 12%	542
	21st to end	Day	21% 12% 26% 25% 10% 5%	530
October	1st to 10th	Day	24% 15% 25% 18% 14% 4	530
	11th to 20th	Day	25% 23% 30% 15% 6% 1	530
	21st to end	Day	32% 18% 29% 16% 5%	570
November	1st to 10th	Day	30% 17% 30% 15% 6% 1	310
	11th to 20th	Day	29% 25% 31% 9% 4 1	220
	21st to end	Day	34% 19% 34% 12% 1	220
December	1st to 10th	Day	30% 21% 30% 18% 2	220
	11th to 20th	Day	36% 22% 27% 12% 2 1	220
	21st to end	Day	34% 21% 29% 12% 3 1	242
January	1st to 10th	Night	27% 19% 32% 17% 5%	840
	11th to 20th	Night	29% 16% 31% 18% 5%	840
	21st to end	Night	19% 16% 35% 23% 6% 1	924
February	1st to 10th	Night	22% 20% 35% 21% 2	840
	11th to 20th	Night	21% 19% 38% 17% 4	823
	21st to end	Night	23% 20% 33% 20% 4	692
March	1st to 10th	Night	24% 19% 35% 19% 3	840
	11th to 20th	Night	23% 18% 34% 19% 4 1	840
	21st to end	Night	22% 14% 34% 22% 7% 1	924
April	1st to 10th	Night	27% 15% 32% 21% 5%	840
	11th to 20th	Night	23% 18% 31% 20% 7% 1	827
	21st to end	Night	22% 12% 33% 23% 10%	630
May	1st to 10th	Night	23% 8% 29% 25% 13% 2	520
	11th to 20th	Night	17% 9% 35% 29% 9%	414
	21st to end	Night	15% 8% 32% 34% 10% 1	455
June	1st to 10th	Night	17% 7% 33% 31% 11%	420
	11th to 20th	Night	15% 10% 29% 35% 10%	420
	21st to end	Night	16% 7% 28% 36% 11%	420
July	1st to 10th	Night	6% 6% 30% 42% 15% 1	410
	11th to 20th	Night	12% 6% 27% 37% 17% 1	408
	21st to end	Night	11% 4 28% 41% 15% 1	496
August	1st to 10th	Night	12% 3 22% 36% 24% 2	627
	11th to 20th	Night	11% 5% 21% 38% 23% 2	624
	21st to end	Night	13% 5% 29% 32% 19% 2	828
September	1st to 10th	Night	12% 7% 32% 29% 18% 3	840
	11th to 20th	Night	13% 4 24% 31% 23% 5%	813
	21st to end	Night	21% 10% 21% 29% 17% 3	800
October	1st to 10th	Night	20% 11% 27% 24% 16% 2	800
	11th to 20th	Night	27% 14% 30% 20% 8% 1	800
	21st to end	Night	28% 15% 31% 21% 5%	880
November	1st to 10th	Night	25% 14% 28% 26% 7%	820
	11th to 20th	Night	27% 14% 35% 17% 6% 2	840
	21st to end	Night	30% 16% 29% 20% 4	840
December	1st to 10th	Night	31% 17% 32% 15% 5%	840
	11th to 20th	Night	25% 19% 36% 16% 3 1	840
	21st to end	Night	34% 14% 31% 17% 3	924

Page generated at 2026-09-11 04:11:51. Version ac07

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