Lake Holiday Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Lake Holiday Observatory clear sky chart from 2005-11-06 to 2025-07-21.

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	13% 6% 2 6% 5% 5% 5% 4 6% 8% 40%	120141
	Night	13% 6% 2 6% 4 4 5% 4 6% 7% 42%	118971
Transparency	Day	39% 7% 12% 23% 15% 3	123783
	Night	38% 7% 12% 24% 16% 3	123467
Seeing	Day	20% 14% 26% 21% 14% 5%	15785
	Night	17% 13% 27% 29% 12% 1	25384

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	13% 6% 2 7% 6% 5% 5% 4 5% 7% 40%	3889
	11th to 20th	Day	11% 9% 3 7% 5% 5% 5% 3 5% 7% 40%	4060
	21st to end	Day	13% 6% 3 7% 5% 5% 5% 4 5% 6% 40%	4466
February	1st to 10th	Day	13% 10% 2 8% 5% 5% 4 4 5% 6% 37%	1800
	11th to 20th	Day	15% 8% 2 7% 5% 5% 5% 3 4 7% 37%	1753
	21st to end	Day	15% 9% 3 6% 5% 4 4 4 5% 6% 39%	1670
March	1st to 10th	Day	16% 7% 2 6% 5% 4 5% 3 5% 6% 40%	4935
	11th to 20th	Day	10% 6% 2 6% 4 4 4 3 6% 8% 47%	4980
	21st to end	Day	13% 8% 2 7% 5% 5% 5% 4 6% 7% 38%	5465
April	1st to 10th	Day	13% 7% 3 6% 5% 5% 6% 4 6% 8% 37%	2383
	11th to 20th	Day	12% 7% 2 5% 4 5% 5% 4 5% 7% 44%	2374
	21st to end	Day	17% 6% 2 6% 5% 4 5% 3 5% 6% 39%	2458
May	1st to 10th	Day	17% 7% 3 7% 5% 5% 6% 4 7% 9% 30%	5862
	11th to 20th	Day	22% 7% 2 6% 4 4 5% 4 7% 9% 29%	5951
	21st to end	Day	15% 5% 2 6% 4 5% 5% 4 8% 9% 36%	6934
June	1st to 10th	Day	12% 5% 2 5% 4 5% 5% 4 6% 9% 42%	2800
	11th to 20th	Day	11% 6% 2 6% 5% 6% 6% 5% 6% 10% 38%	2800
	21st to end	Day	9% 4 2 5% 4 4 6% 4 8% 10% 44%	2800
July	1st to 10th	Day	5% 4 2 6% 6% 5% 6% 5% 7% 11% 44%	4580
	11th to 20th	Day	8% 5% 2 6% 5% 5% 5% 5% 7% 11% 41%	4267
	21st to end	Day	10% 7% 3 7% 5% 6% 6% 4 6% 8% 39%	4466
August	1st to 10th	Day	16% 5% 2 6% 4 5% 7% 5% 7% 9% 34%	4104
	11th to 20th	Day	8% 5% 2 6% 6% 5% 6% 5% 7% 9% 42%	3989
	21st to end	Day	10% 4 2 5% 4 4 5% 4 6% 8% 48%	4180
September	1st to 10th	Day	15% 6% 2 6% 3 3 5% 3 6% 7% 45%	2280
	11th to 20th	Day	12% 6% 2 5% 3 4 4 4 6% 7% 47%	2227
	21st to end	Day	20% 6% 2 7% 4 4 4 3 5% 6% 38%	2011
October	1st to 10th	Day	16% 5% 2 5% 4 4 4 3 6% 7% 46%	1900
	11th to 20th	Day	15% 5% 2 5% 4 4 4 4 5% 7% 47%	1900
	21st to end	Day	14% 5% 2 5% 4 4 4 3 6% 6% 49%	2090
November	1st to 10th	Day	10% 6% 2 5% 4 4 4 3 6% 6% 52%	1932
	11th to 20th	Day	15% 6% 2 5% 3 3 4 4 5% 7% 45%	1685
	21st to end	Day	14% 6% 2 6% 4 4 5% 4 5% 8% 42%	1600
December	1st to 10th	Day	12% 6% 3 4 4 4 5% 4 7% 7% 45%	2600
	11th to 20th	Day	18% 8% 3 8% 4 4 4 3 4 7% 38%	2990
	21st to end	Day	16% 7% 3 8% 5% 5% 6% 5% 7% 9% 30%	3960
January	1st to 10th	Night	9% 7% 3 7% 5% 4 5% 4 6% 8% 43%	6054
	11th to 20th	Night	11% 8% 2 6% 4 4 4 4 5% 6% 45%	5900
	21st to end	Night	13% 7% 3 6% 4 4 4 4 5% 6% 44%	6490
February	1st to 10th	Night	13% 8% 3 7% 4 4 4 4 6% 7% 41%	2600
	11th to 20th	Night	15% 9% 2 8% 4 5% 5% 3 6% 5% 38%	2533
	21st to end	Night	13% 10% 3 7% 5% 4 4 3 6% 7% 37%	1956
March	1st to 10th	Night	15% 6% 2 5% 4 4 4 4 4 6% 46%	4918
	11th to 20th	Night	9% 4 2 6% 4 4 4 4 6% 9% 48%	4980
	21st to end	Night	10% 6% 2 7% 5% 4 5% 4 5% 7% 46%	5467
April	1st to 10th	Night	15% 8% 2 6% 6% 4 4 4 6% 7% 37%	2016
	11th to 20th	Night	13% 6% 2 7% 4 5% 4 5% 6% 7% 41%	1975
	21st to end	Night	16% 4 2 5% 4 4 4 4 5% 7% 44%	1958
May	1st to 10th	Night	21% 7% 3 8% 5% 4 5% 4 6% 8% 29%	4033
	11th to 20th	Night	25% 7% 2 5% 4 4 5% 4 6% 7% 31%	3941
	21st to end	Night	16% 7% 2 7% 4 5% 5% 4 6% 8% 35%	3920
June	1st to 10th	Night	10% 5% 2 5% 4 5% 4 4 7% 9% 46%	1600
	11th to 20th	Night	11% 5% 2 5% 5% 5% 5% 4 6% 9% 42%	1600
	21st to end	Night	8% 4 2 5% 5% 5% 4 5% 7% 11% 44%	1600
July	1st to 10th	Night	6% 5% 2 6% 5% 5% 6% 6% 7% 10% 42%	2600
	11th to 20th	Night	5% 6% 3 7% 7% 5% 6% 5% 7% 9% 39%	2894
	21st to end	Night	9% 6% 2 7% 5% 5% 5% 6% 7% 9% 37%	3087
August	1st to 10th	Night	15% 6% 2 7% 5% 4 6% 5% 7% 7% 35%	2825
	11th to 20th	Night	9% 5% 2 6% 4 5% 5% 4 6% 8% 45%	2807
	21st to end	Night	13% 5% 2 5% 4 4 4 4 6% 8% 47%	3476
September	1st to 10th	Night	15% 4 2 5% 4 4 5% 4 5% 6% 46%	1900
	11th to 20th	Night	12% 6% 2 6% 3 3 3 3 5% 7% 50%	1949
	21st to end	Night	20% 5% 2 5% 4 4 3 4 4 6% 42%	2167
October	1st to 10th	Night	19% 6% 2 4 3 2 4 3 5% 7% 46%	2280
	11th to 20th	Night	13% 5% 2 4 4 3 3 3 6% 6% 51%	2280
	21st to end	Night	14% 5% 1 5% 3 4 4 3 5% 6% 50%	2508
November	1st to 10th	Night	9% 7% 2 5% 3 4 4 3 6% 7% 51%	2366
	11th to 20th	Night	14% 7% 2 5% 4 4 4 3 6% 7% 45%	2710
	21st to end	Night	13% 6% 2 6% 5% 5% 5% 4 6% 8% 42%	2800
December	1st to 10th	Night	9% 5% 2 5% 4 4 4 4 6% 8% 48%	4580
	11th to 20th	Night	17% 8% 3 6% 4 5% 5% 4 6% 6% 36%	5205
	21st to end	Night	14% 8% 3 7% 5% 6% 6% 5% 7% 9% 31%	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	39% 8% 7% 26% 15% 5%	3890
	11th to 20th	Day	41% 6% 9% 19% 19% 6%	4060
	21st to end	Day	42% 7% 11% 25% 13% 4	4466
February	1st to 10th	Day	46% 5% 14% 21% 12% 2	1800
	11th to 20th	Day	47% 5% 11% 20% 15% 2	1762
	21st to end	Day	45% 6% 11% 20% 14% 4	1670
March	1st to 10th	Day	42% 6% 9% 21% 14% 7%	4935
	11th to 20th	Day	33% 6% 11% 26% 19% 5%	4980
	21st to end	Day	44% 6% 10% 18% 17% 5%	5465
April	1st to 10th	Day	42% 8% 12% 17% 16% 4	2383
	11th to 20th	Day	38% 6% 11% 22% 19% 5%	2374
	21st to end	Day	44% 6% 10% 22% 15% 3	2458
May	1st to 10th	Day	47% 9% 14% 19% 10% 2	5897
	11th to 20th	Day	48% 8% 10% 24% 8% 1	5955
	21st to end	Day	40% 9% 14% 21% 15% 2	6949
June	1st to 10th	Day	37% 8% 13% 25% 16% 1	2800
	11th to 20th	Day	39% 10% 14% 22% 13% 2	2800
	21st to end	Day	32% 9% 16% 25% 16% 2	2800
July	1st to 10th	Day	33% 9% 17% 25% 11% 4	4510
	11th to 20th	Day	34% 9% 16% 29% 11% 1	4197
	21st to end	Day	42% 7% 10% 21% 18% 2	4324
August	1st to 10th	Day	44% 8% 15% 24% 8%	4091
	11th to 20th	Day	35% 8% 14% 27% 15% 1	4028
	21st to end	Day	33% 7% 16% 26% 17% 1	4180
September	1st to 10th	Day	38% 7% 11% 19% 21% 4	2280
	11th to 20th	Day	35% 6% 10% 27% 19% 3	2227
	21st to end	Day	45% 6% 11% 19% 17% 2	2011
October	1st to 10th	Day	34% 5% 8% 21% 27% 5%	3160
	11th to 20th	Day	26% 5% 11% 28% 22% 8%	3140
	21st to end	Day	25% 3 11% 30% 26% 4	3428
November	1st to 10th	Day	33% 5% 12% 24% 23% 4	1932
	11th to 20th	Day	38% 5% 12% 21% 21% 4	1685
	21st to end	Day	39% 5% 13% 27% 15% 1	1600
December	1st to 10th	Day	35% 8% 14% 28% 11% 4	2600
	11th to 20th	Day	46% 5% 13% 24% 10% 2	2986
	21st to end	Day	45% 9% 17% 17% 12% 1	3960
January	1st to 10th	Night	34% 9% 9% 27% 19% 3	6054
	11th to 20th	Night	37% 6% 10% 24% 18% 6%	5900
	21st to end	Night	38% 6% 12% 27% 16% 2	6490
February	1st to 10th	Night	41% 6% 14% 24% 13% 1	2600
	11th to 20th	Night	45% 6% 10% 21% 15% 3	2535
	21st to end	Night	44% 6% 12% 20% 13% 5%	1956
March	1st to 10th	Night	37% 6% 10% 26% 14% 7%	4918
	11th to 20th	Night	31% 7% 13% 29% 15% 5%	4980
	21st to end	Night	36% 6% 12% 23% 18% 4	5467
April	1st to 10th	Night	44% 7% 10% 20% 16% 3	2016
	11th to 20th	Night	38% 8% 12% 21% 16% 5%	1975
	21st to end	Night	38% 6% 12% 26% 13% 3	1958
May	1st to 10th	Night	51% 8% 13% 16% 9% 4	4060
	11th to 20th	Night	49% 8% 12% 21% 9% 1	3942
	21st to end	Night	43% 8% 15% 23% 7% 2	3927
June	1st to 10th	Night	34% 9% 11% 27% 17% 3	1600
	11th to 20th	Night	37% 10% 14% 25% 14% 2	1600
	21st to end	Night	32% 8% 15% 25% 17% 2	1600
July	1st to 10th	Night	36% 9% 20% 18% 13% 5%	2560
	11th to 20th	Night	38% 9% 17% 22% 12% 2	2842
	21st to end	Night	39% 10% 15% 20% 16% 1	2988
August	1st to 10th	Night	45% 9% 15% 23% 8% 1	2814
	11th to 20th	Night	35% 7% 14% 26% 17% 1	2834
	21st to end	Night	34% 6% 15% 29% 14% 2	3476
September	1st to 10th	Night	37% 7% 13% 19% 20% 4	1900
	11th to 20th	Night	34% 5% 10% 27% 23% 2	1949
	21st to end	Night	42% 7% 11% 21% 18% 2	2167
October	1st to 10th	Night	37% 4 11% 24% 22% 2	3800
	11th to 20th	Night	23% 5% 11% 24% 30% 7%	3779
	21st to end	Night	23% 5% 11% 31% 26% 5%	4120
November	1st to 10th	Night	31% 5% 12% 26% 21% 5%	2366
	11th to 20th	Night	38% 5% 8% 25% 19% 5%	2710
	21st to end	Night	39% 6% 12% 23% 17% 3	2800
December	1st to 10th	Night	32% 6% 14% 31% 15% 1	4580
	11th to 20th	Night	46% 5% 13% 20% 14% 2	5208
	21st to end	Night	43% 9% 15% 18% 14%	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	31% 30% 23% 11% 4	263
	11th to 20th	Day	32% 27% 27% 11% 3	300
	21st to end	Day	22% 29% 32% 13% 4	330
February	1st to 10th	Day	28% 25% 34% 9% 5%	300
	11th to 20th	Day	38% 28% 29% 5%	300
	21st to end	Day	31% 22% 28% 16% 2	396
March	1st to 10th	Day	26% 23% 27% 21% 3	500
	11th to 20th	Day	25% 24% 28% 15% 6% 1	500
	21st to end	Day	22% 24% 33% 15% 5% 1	550
April	1st to 10th	Day	25% 24% 25% 18% 7% 1	500
	11th to 20th	Day	21% 17% 30% 18% 10% 4	492
	21st to end	Day	24% 16% 33% 17% 8% 2	500
May	1st to 10th	Day	20% 20% 28% 18% 11% 3	503
	11th to 20th	Day	23% 16% 25% 20% 12% 4	527
	21st to end	Day	21% 10% 22% 22% 18% 6%	573
June	1st to 10th	Day	16% 10% 28% 21% 19% 6%	530
	11th to 20th	Day	15% 10% 25% 26% 18% 5%	530
	21st to end	Day	12% 6% 28% 29% 19% 6%	530
July	1st to 10th	Day	10% 5% 21% 28% 24% 12%	525
	11th to 20th	Day	6% 4 22% 30% 27% 11%	524
	21st to end	Day	11% 7% 20% 32% 23% 8%	532
August	1st to 10th	Day	14% 3 23% 23% 24% 12%	486
	11th to 20th	Day	11% 3 22% 28% 25% 11%	479
	21st to end	Day	9% 4 20% 27% 26% 14%	539
September	1st to 10th	Day	19% 6% 19% 22% 24% 10%	490
	11th to 20th	Day	16% 3 18% 28% 24% 11%	490
	21st to end	Day	27% 10% 19% 24% 15% 4	490
October	1st to 10th	Day	18% 8% 21% 28% 19% 6%	490
	11th to 20th	Day	18% 15% 26% 25% 13% 3	490
	21st to end	Day	18% 13% 34% 20% 13% 3	539
November	1st to 10th	Day	16% 14% 28% 24% 14% 3	482
	11th to 20th	Day	21% 17% 32% 20% 9% 1	285
	21st to end	Day	23% 22% 31% 19% 6% 1	200
December	1st to 10th	Day	20% 28% 34% 13% 6%	200
	11th to 20th	Day	24% 22% 27% 23% 5%	200
	21st to end	Day	20% 21% 30% 20% 9%	220
January	1st to 10th	Night	20% 22% 37% 19% 3	800
	11th to 20th	Night	24% 19% 29% 25% 4	800
	21st to end	Night	19% 20% 39% 21% 2	880
February	1st to 10th	Night	18% 19% 35% 26% 2	800
	11th to 20th	Night	25% 25% 33% 16% 1	783
	21st to end	Night	24% 20% 34% 20% 2	660
March	1st to 10th	Night	20% 19% 37% 21% 4	800
	11th to 20th	Night	23% 21% 28% 22% 5% 1	800
	21st to end	Night	18% 18% 39% 20% 5% 1	880
April	1st to 10th	Night	21% 21% 31% 21% 5%	800
	11th to 20th	Night	18% 15% 32% 26% 8%	791
	21st to end	Night	19% 13% 30% 30% 6%	758
May	1st to 10th	Night	21% 8% 27% 29% 13% 1	600
	11th to 20th	Night	21% 8% 26% 32% 13% 1	552
	21st to end	Night	14% 4 24% 36% 21% 1	433
June	1st to 10th	Night	11% 5% 25% 37% 22% 1	400
	11th to 20th	Night	13% 5% 20% 42% 19% 2	400
	21st to end	Night	7% 3 25% 44% 20% 1	400
July	1st to 10th	Night	11% 5% 15% 37% 29% 4	390
	11th to 20th	Night	5% 2 13% 40% 35% 5%	561
	21st to end	Night	10% 3 14% 42% 28% 3	624
August	1st to 10th	Night	11% 3 14% 41% 28% 3	567
	11th to 20th	Night	12% 3 13% 38% 30% 4	573
	21st to end	Night	12% 3 15% 39% 27% 4	836
September	1st to 10th	Night	17% 3 16% 33% 27% 4	760
	11th to 20th	Night	13% 5% 17% 33% 29% 3	760
	21st to end	Night	23% 4 21% 31% 19% 2	760
October	1st to 10th	Night	20% 8% 20% 29% 22% 2	760
	11th to 20th	Night	16% 12% 25% 34% 13% 2	760
	21st to end	Night	17% 13% 30% 29% 11%	836
November	1st to 10th	Night	13% 11% 31% 33% 11% 1	780
	11th to 20th	Night	18% 19% 26% 29% 7% 1	800
	21st to end	Night	18% 18% 31% 26% 7%	800
December	1st to 10th	Night	20% 16% 35% 24% 6%	800
	11th to 20th	Night	18% 15% 36% 28% 4	800
	21st to end	Night	18% 18% 30% 33% 2	880

Page generated at 2025-07-18 01:10:00. Version ac07

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