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 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	16% 11% 4 9% 5% 4 4 4 5% 7% 31%	114016
	Night	16% 12% 4 10% 5% 5% 4 3 5% 6% 30%	114900
Transparency	Day	52% 5% 11% 20% 10% 1	117618
	Night	53% 6% 11% 19% 10% 1	119449
Seeing	Day	23% 15% 31% 19% 8% 3	14973
	Night	22% 13% 31% 24% 9% 1	24374

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	12% 16% 4 12% 5% 4 5% 3 5% 6% 27%	3597
	11th to 20th	Day	10% 20% 10% 16% 7% 7% 5% 4 4 3 15%	3616
	21st to end	Day	13% 19% 7% 16% 7% 5% 4 3 4 4 19%	4466
February	1st to 10th	Day	13% 17% 7% 15% 8% 4 5% 3 4 4 21%	1800
	11th to 20th	Day	13% 15% 6% 13% 7% 5% 4 3 4 4 25%	1753
	21st to end	Day	16% 15% 6% 11% 7% 5% 5% 3 4 5% 23%	1550
March	1st to 10th	Day	19% 13% 5% 10% 6% 4 3 3 4 6% 27%	4831
	11th to 20th	Day	12% 15% 5% 10% 6% 5% 5% 4 6% 7% 26%	4980
	21st to end	Day	19% 12% 5% 8% 4 3 3 3 3 4 37%	5528
April	1st to 10th	Day	19% 14% 3 9% 5% 5% 4 3 5% 5% 30%	2400
	11th to 20th	Day	22% 11% 2 6% 4 4 3 2 5% 5% 35%	2394
	21st to end	Day	22% 10% 3 8% 5% 4 4 3 4 5% 32%	2600
May	1st to 10th	Day	19% 8% 3 7% 5% 5% 4 3 5% 7% 34%	4419
	11th to 20th	Day	17% 10% 3 8% 5% 4 4 4 5% 9% 32%	4398
	21st to end	Day	14% 7% 2 6% 4 4 4 4 6% 7% 42%	4837
June	1st to 10th	Day	19% 9% 2 7% 5% 5% 4 4 5% 8% 32%	2660
	11th to 20th	Day	14% 7% 3 6% 4 6% 5% 5% 6% 9% 35%	2660
	21st to end	Day	17% 7% 2 6% 5% 5% 5% 4 6% 9% 34%	2660
July	1st to 10th	Day	11% 6% 3 7% 5% 5% 5% 4 8% 10% 36%	4440
	11th to 20th	Day	11% 5% 2 5% 5% 5% 5% 4 6% 8% 43%	4440
	21st to end	Day	8% 5% 2 7% 5% 5% 5% 5% 8% 9% 42%	4754
August	1st to 10th	Day	16% 9% 3 7% 4 4 6% 4 6% 8% 34%	4104
	11th to 20th	Day	15% 8% 3 8% 5% 5% 5% 4 7% 9% 33%	4044
	21st to end	Day	10% 5% 2 6% 5% 5% 5% 4 6% 11% 40%	4286
September	1st to 10th	Day	14% 7% 3 6% 5% 4 4 4 5% 9% 40%	2280
	11th to 20th	Day	12% 5% 2 6% 4 4 4 4 4 7% 46%	2256
	21st to end	Day	17% 7% 2 7% 5% 5% 4 4 6% 6% 37%	1996
October	1st to 10th	Day	20% 10% 3 8% 4 4 5% 3 5% 7% 33%	1900
	11th to 20th	Day	23% 11% 3 8% 5% 5% 5% 3 5% 6% 26%	1900
	21st to end	Day	26% 13% 4 8% 5% 5% 3 3 5% 5% 23%	2069
November	1st to 10th	Day	17% 10% 4 8% 5% 5% 4 4 5% 7% 31%	1652
	11th to 20th	Day	16% 15% 4 9% 4 4 4 4 6% 7% 27%	1600
	21st to end	Day	21% 19% 5% 9% 6% 5% 5% 4 4 5% 18%	1600
December	1st to 10th	Day	18% 24% 7% 12% 6% 4 4 2 4 6% 14%	2600
	11th to 20th	Day	18% 20% 6% 15% 4 3 3 3 3 3 22%	2986
	21st to end	Day	23% 20% 5% 12% 4 3 3 2 3 4 20%	3960
January	1st to 10th	Night	14% 17% 6% 12% 6% 5% 4 3 5% 5% 21%	6349
	11th to 20th	Night	11% 15% 7% 18% 10% 7% 6% 4 4 5% 13%	6252
	21st to end	Night	13% 16% 9% 15% 7% 5% 4 3 4 5% 19%	6490
February	1st to 10th	Night	12% 16% 6% 11% 8% 6% 6% 4 6% 6% 21%	2600
	11th to 20th	Night	13% 14% 5% 12% 6% 6% 4 3 5% 5% 27%	2533
	21st to end	Night	14% 15% 4 9% 5% 5% 5% 4 6% 6% 26%	2078
March	1st to 10th	Night	18% 13% 4 10% 6% 5% 5% 3 5% 5% 28%	5007
	11th to 20th	Night	10% 12% 5% 10% 6% 5% 5% 4 6% 7% 29%	4979
	21st to end	Night	20% 8% 3 8% 4 4 4 3 5% 6% 33%	5404
April	1st to 10th	Night	19% 11% 4 7% 5% 5% 4 4 5% 6% 30%	2000
	11th to 20th	Night	17% 11% 3 8% 4 4 3 3 5% 6% 36%	1971
	21st to end	Night	18% 10% 2 7% 6% 5% 4 4 6% 5% 33%	1800
May	1st to 10th	Night	19% 8% 3 7% 5% 4 4 3 5% 5% 37%	2740
	11th to 20th	Night	16% 7% 2 7% 5% 5% 4 4 6% 6% 36%	2501
	21st to end	Night	11% 7% 2 6% 4 4 5% 4 4 8% 45%	2738
June	1st to 10th	Night	15% 8% 2 6% 4 4 5% 3 6% 9% 37%	1520
	11th to 20th	Night	12% 6% 1 6% 5% 5% 5% 4 6% 8% 43%	1520
	21st to end	Night	13% 8% 3 7% 5% 5% 4 4 6% 7% 39%	1520
July	1st to 10th	Night	8% 5% 2 8% 5% 5% 4 4 6% 8% 44%	2520
	11th to 20th	Night	14% 6% 2 5% 3 4 4 3 5% 7% 48%	2520
	21st to end	Night	9% 6% 2 5% 4 4 4 4 6% 8% 48%	2795
August	1st to 10th	Night	12% 5% 2 6% 4 4 5% 4 5% 8% 47%	2825
	11th to 20th	Night	14% 6% 2 6% 4 4 4 3 6% 7% 43%	2785
	21st to end	Night	11% 5% 2 5% 4 4 4 4 5% 8% 48%	3337
September	1st to 10th	Night	11% 6% 3 6% 4 4 3 4 7% 9% 43%	1900
	11th to 20th	Night	12% 5% 1 5% 4 3 4 3 6% 6% 51%	1920
	21st to end	Night	19% 6% 2 6% 4 4 5% 3 5% 5% 42%	2183
October	1st to 10th	Night	17% 7% 3 8% 5% 4 5% 3 5% 6% 38%	2280
	11th to 20th	Night	21% 10% 4 8% 5% 4 4 4 5% 6% 30%	2280
	21st to end	Night	22% 13% 3 8% 5% 5% 4 4 4 5% 26%	2527
November	1st to 10th	Night	18% 10% 3 7% 5% 4 4 3 5% 6% 35%	2642
	11th to 20th	Night	18% 13% 3 7% 5% 4 5% 3 5% 6% 29%	2800
	21st to end	Night	20% 13% 4 10% 6% 5% 5% 4 6% 6% 22%	2800
December	1st to 10th	Night	20% 18% 7% 11% 6% 4 4 4 4 6% 16%	4580
	11th to 20th	Night	16% 20% 6% 15% 6% 5% 4 3 4 4 17%	5208
	21st to end	Night	24% 19% 5% 14% 5% 4 3 2 3 4 16%	6996

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	57% 5% 10% 19% 9% 1	3598
	11th to 20th	Day	73% 5% 8% 8% 6% 1	3646
	21st to end	Day	69% 4 6% 10% 9% 1	4466
February	1st to 10th	Day	67% 4 8% 14% 5% 1	1800
	11th to 20th	Day	62% 5% 8% 13% 10% 2	1762
	21st to end	Day	63% 4 9% 14% 9% 1	1550
March	1st to 10th	Day	59% 4 11% 11% 14% 1	4834
	11th to 20th	Day	56% 5% 13% 16% 7% 2	4980
	21st to end	Day	52% 3 7% 20% 16% 2	5528
April	1st to 10th	Day	56% 4 7% 19% 11% 2	2400
	11th to 20th	Day	52% 4 8% 20% 15% 1	2394
	21st to end	Day	55% 4 8% 19% 12% 3	2600
May	1st to 10th	Day	49% 4 9% 23% 11% 2	4431
	11th to 20th	Day	48% 7% 12% 22% 11% 1	4402
	21st to end	Day	41% 6% 13% 22% 15% 3	4837
June	1st to 10th	Day	51% 6% 11% 18% 13% 1	2660
	11th to 20th	Day	44% 6% 11% 24% 13% 2	2660
	21st to end	Day	47% 6% 12% 22% 11% 1	2660
July	1st to 10th	Day	40% 7% 16% 24% 13% 1	4370
	11th to 20th	Day	36% 7% 15% 31% 10%	4365
	21st to end	Day	35% 8% 11% 33% 12%	4602
August	1st to 10th	Day	47% 7% 13% 23% 10% 1	4091
	11th to 20th	Day	44% 8% 14% 26% 8%	4083
	21st to end	Day	37% 7% 12% 33% 10% 1	4286
September	1st to 10th	Day	41% 6% 12% 28% 12% 1	2280
	11th to 20th	Day	37% 5% 13% 30% 14% 1	2256
	21st to end	Day	47% 6% 14% 23% 11%	1996
October	1st to 10th	Day	44% 6% 12% 28% 10% 1	3160
	11th to 20th	Day	50% 5% 13% 20% 12% 1	3140
	21st to end	Day	57% 6% 12% 19% 6%	3383
November	1st to 10th	Day	52% 5% 14% 21% 7% 1	1652
	11th to 20th	Day	57% 6% 12% 15% 9% 2	1600
	21st to end	Day	68% 5% 11% 10% 5% 1	1600
December	1st to 10th	Day	72% 5% 12% 8% 3	2600
	11th to 20th	Day	68% 5% 6% 13% 6%	2986
	21st to end	Day	69% 4 7% 12% 8%	3960
January	1st to 10th	Night	64% 5% 8% 15% 8%	6349
	11th to 20th	Night	71% 6% 8% 8% 5% 1	6297
	21st to end	Night	67% 5% 8% 9% 11% 1	6490
February	1st to 10th	Night	62% 5% 10% 16% 6% 1	2600
	11th to 20th	Night	59% 4 10% 18% 9% 1	2535
	21st to end	Night	56% 5% 10% 19% 9%	2078
March	1st to 10th	Night	57% 5% 8% 15% 11% 5%	5017
	11th to 20th	Night	51% 7% 16% 17% 8% 1	4980
	21st to end	Night	50% 4 9% 21% 14% 2	5404
April	1st to 10th	Night	55% 5% 8% 20% 11% 2	2000
	11th to 20th	Night	49% 4 9% 21% 15% 1	1971
	21st to end	Night	51% 6% 8% 17% 16% 2	1800
May	1st to 10th	Night	48% 5% 10% 19% 15% 3	2747
	11th to 20th	Night	45% 8% 12% 23% 11% 2	2502
	21st to end	Night	38% 5% 13% 23% 19% 2	2739
June	1st to 10th	Night	43% 5% 13% 23% 14% 2	1520
	11th to 20th	Night	37% 7% 12% 24% 18% 2	1520
	21st to end	Night	43% 6% 14% 22% 12% 1	1520
July	1st to 10th	Night	36% 6% 16% 24% 16% 1	2480
	11th to 20th	Night	36% 5% 11% 34% 14%	2472
	21st to end	Night	33% 7% 14% 32% 14%	2706
August	1st to 10th	Night	35% 7% 14% 26% 17% 1	2814
	11th to 20th	Night	38% 7% 18% 24% 12% 1	2812
	21st to end	Night	33% 6% 13% 34% 11% 2	3337
September	1st to 10th	Night	36% 7% 16% 27% 14% 1	1900
	11th to 20th	Night	34% 5% 13% 34% 14% 1	1920
	21st to end	Night	44% 5% 11% 28% 11%	2183
October	1st to 10th	Night	38% 5% 11% 29% 18%	3800
	11th to 20th	Night	47% 6% 14% 25% 9%	3779
	21st to end	Night	52% 6% 14% 21% 7%	4151
November	1st to 10th	Night	50% 6% 12% 23% 9% 1	2642
	11th to 20th	Night	55% 6% 12% 18% 8% 1	2800
	21st to end	Night	62% 7% 12% 14% 4	2800
December	1st to 10th	Night	69% 6% 12% 11% 3	4580
	11th to 20th	Night	70% 5% 9% 11% 6%	5208
	21st to end	Night	72% 4 6% 10% 7%	6996

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	21% 26% 34% 15% 5% 1	200
	11th to 20th	Day	24% 21% 36% 15% 3	210
	21st to end	Day	25% 22% 35% 12% 5% 1	330
February	1st to 10th	Day	30% 22% 35% 9% 3	300
	11th to 20th	Day	30% 27% 37% 5% 1	300
	21st to end	Day	34% 17% 36% 11% 1	314
March	1st to 10th	Day	26% 18% 37% 15% 3	500
	11th to 20th	Day	27% 20% 33% 18% 2 1	500
	21st to end	Day	33% 17% 31% 12% 6% 1	550
April	1st to 10th	Day	28% 21% 32% 12% 6%	500
	11th to 20th	Day	34% 20% 21% 16% 7% 2	492
	21st to end	Day	30% 18% 32% 13% 6% 1	500
May	1st to 10th	Day	26% 15% 31% 17% 6% 5%	503
	11th to 20th	Day	20% 15% 34% 19% 9% 3	487
	21st to end	Day	20% 12% 36% 19% 10% 4	529
June	1st to 10th	Day	23% 14% 33% 21% 8% 1	490
	11th to 20th	Day	18% 12% 33% 24% 10% 3	490
	21st to end	Day	22% 10% 36% 21% 9% 2	490
July	1st to 10th	Day	13% 8% 32% 31% 13% 4	485
	11th to 20th	Day	14% 10% 31% 25% 14% 5%	484
	21st to end	Day	13% 11% 31% 30% 11% 4	532
August	1st to 10th	Day	15% 8% 29% 29% 13% 6%	486
	11th to 20th	Day	16% 5% 26% 31% 15% 6%	479
	21st to end	Day	15% 8% 32% 24% 14% 7%	539
September	1st to 10th	Day	15% 12% 30% 23% 13% 6%	490
	11th to 20th	Day	11% 10% 25% 22% 20% 11%	490
	21st to end	Day	20% 12% 26% 25% 11% 6%	490
October	1st to 10th	Day	25% 16% 25% 17% 14% 4	490
	11th to 20th	Day	25% 23% 29% 16% 6% 1	490
	21st to end	Day	33% 19% 28% 15% 5%	527
November	1st to 10th	Day	28% 17% 31% 16% 7% 1	286
	11th to 20th	Day	30% 23% 34% 9% 4 2	200
	21st to end	Day	34% 18% 34% 13% 2 1	200
December	1st to 10th	Day	30% 22% 29% 18% 3	200
	11th to 20th	Day	36% 22% 27% 13% 2 1	200
	21st to end	Day	32% 22% 30% 12% 3 1	220
January	1st to 10th	Night	28% 19% 32% 17% 5%	800
	11th to 20th	Night	28% 16% 32% 19% 5%	800
	21st to end	Night	20% 15% 35% 23% 6% 1	880
February	1st to 10th	Night	22% 21% 36% 19% 2	800
	11th to 20th	Night	21% 19% 39% 17% 4	783
	21st to end	Night	23% 21% 32% 20% 4	660
March	1st to 10th	Night	24% 19% 34% 19% 3	800
	11th to 20th	Night	22% 17% 35% 20% 4 1	800
	21st to end	Night	21% 14% 34% 23% 8% 1	880
April	1st to 10th	Night	27% 15% 32% 22% 5%	800
	11th to 20th	Night	23% 18% 31% 20% 8% 1	787
	21st to end	Night	23% 12% 33% 22% 11%	600
May	1st to 10th	Night	24% 7% 29% 25% 12% 2	495
	11th to 20th	Night	16% 10% 34% 30% 9%	376
	21st to end	Night	13% 8% 32% 35% 11% 1	411
June	1st to 10th	Night	17% 7% 34% 31% 12%	380
	11th to 20th	Night	15% 9% 28% 37% 11% 1	380
	21st to end	Night	17% 7% 28% 36% 12%	380
July	1st to 10th	Night	6% 6% 29% 42% 16% 1	370
	11th to 20th	Night	12% 5% 27% 37% 18% 1	368
	21st to end	Night	11% 4 27% 41% 16% 1	448
August	1st to 10th	Night	12% 3 23% 35% 25% 2	567
	11th to 20th	Night	11% 5% 21% 37% 25% 2	564
	21st to end	Night	13% 5% 29% 31% 20% 2	749
September	1st to 10th	Night	12% 7% 32% 29% 18% 3	760
	11th to 20th	Night	13% 4 24% 31% 23% 5%	760
	21st to end	Night	21% 9% 20% 29% 17% 3	760
October	1st to 10th	Night	21% 12% 27% 23% 15% 2	760
	11th to 20th	Night	28% 14% 29% 20% 8% 1	760
	21st to end	Night	29% 15% 31% 20% 5%	836
November	1st to 10th	Night	24% 13% 28% 26% 8%	780
	11th to 20th	Night	27% 14% 34% 17% 6% 2	800
	21st to end	Night	30% 16% 30% 20% 4 1	800
December	1st to 10th	Night	32% 17% 32% 15% 5%	800
	11th to 20th	Night	25% 19% 36% 16% 4 1	800
	21st to end	Night	33% 14% 31% 18% 3	880

Page generated at 2025-05-08 09:33:59. Version ac07

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