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 2024-04-03.

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	14% 6% 3 5% 5% 3 5% 5% 6% 8% 40%	78904
	Night	14% 6% 2 5% 5% 3 5% 5% 6% 7% 42%	81227
Transparency	Day	41% 8% 13% 22% 15% 3	78619
	Night	39% 7% 13% 23% 15% 2	81032
Seeing	Day	19% 14% 25% 21% 14% 5%	14043
	Night	17% 13% 27% 29% 12% 1	23794

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	15% 6% 3 6% 6% 4 4 6% 5% 7% 40%	2711
	11th to 20th	Day	12% 10% 4 7% 7% 4 6% 5% 5% 6% 34%	2840
	21st to end	Day	16% 8% 4 7% 7% 4 5% 5% 5% 6% 34%	3124
February	1st to 10th	Day	13% 10% 2 7% 6% 4 5% 5% 5% 6% 38%	1710
	11th to 20th	Day	14% 8% 3 6% 6% 4 6% 3 5% 7% 38%	1663
	21st to end	Day	16% 8% 3 5% 6% 3 5% 5% 4 6% 38%	1588
March	1st to 10th	Day	14% 6% 2 5% 5% 3 4 4 5% 6% 45%	2090
	11th to 20th	Day	15% 7% 3 6% 6% 3 5% 4 5% 8% 39%	2090
	21st to end	Day	15% 7% 4 6% 5% 3 5% 5% 5% 7% 37%	2299
April	1st to 10th	Day	11% 6% 4 5% 5% 4 5% 6% 6% 8% 38%	2168
	11th to 20th	Day	13% 6% 3 5% 4 3 5% 5% 5% 7% 44%	2134
	21st to end	Day	18% 6% 2 5% 6% 4 5% 5% 5% 6% 38%	2212
May	1st to 10th	Day	17% 6% 3 6% 5% 3 5% 5% 6% 9% 35%	2340
	11th to 20th	Day	20% 5% 3 5% 5% 3 5% 5% 6% 8% 34%	2335
	21st to end	Day	15% 5% 2 5% 5% 4 5% 5% 6% 9% 38%	2725
June	1st to 10th	Day	12% 4 2 5% 4 4 6% 5% 6% 9% 42%	2520
	11th to 20th	Day	11% 6% 2 6% 6% 4 7% 7% 6% 10% 37%	2520
	21st to end	Day	9% 3 2 5% 5% 3 6% 6% 8% 11% 42%	2520
July	1st to 10th	Day	8% 5% 3 6% 7% 4 6% 6% 6% 9% 41%	2520
	11th to 20th	Day	6% 5% 2 5% 6% 3 6% 7% 7% 10% 42%	2356
	21st to end	Day	10% 6% 3 6% 6% 4 6% 7% 7% 9% 37%	2547
August	1st to 10th	Day	9% 5% 2 6% 6% 5% 7% 7% 8% 9% 35%	2327
	11th to 20th	Day	11% 5% 2 5% 6% 4 5% 7% 7% 9% 39%	2276
	21st to end	Day	10% 5% 2 5% 5% 3 5% 6% 7% 9% 42%	2376
September	1st to 10th	Day	16% 5% 2 5% 4 2 5% 4 6% 8% 43%	2160
	11th to 20th	Day	12% 6% 2 5% 4 3 4 5% 6% 8% 47%	2110
	21st to end	Day	18% 6% 3 6% 5% 3 4 4 5% 6% 40%	1906
October	1st to 10th	Day	16% 5% 3 4 5% 3 4 4 6% 7% 45%	1800
	11th to 20th	Day	16% 5% 3 4 4 3 4 5% 5% 7% 46%	1800
	21st to end	Day	15% 5% 2 4 5% 3 4 4 5% 6% 47%	1980
November	1st to 10th	Day	10% 5% 2 4 4 3 5% 4 5% 6% 52%	1834
	11th to 20th	Day	15% 5% 2 5% 4 2 4 5% 6% 6% 45%	1601
	21st to end	Day	15% 5% 2 5% 4 3 4 5% 4 9% 43%	1520
December	1st to 10th	Day	16% 8% 4 4 5% 3 6% 6% 7% 7% 34%	1520
	11th to 20th	Day	15% 5% 3 6% 5% 3 6% 4 6% 7% 39%	1910
	21st to end	Day	17% 5% 2 6% 6% 3 6% 7% 7% 9% 31%	2772
January	1st to 10th	Night	11% 7% 3 6% 6% 3 6% 5% 6% 7% 41%	4232
	11th to 20th	Night	15% 8% 3 5% 5% 2 4 5% 6% 6% 41%	4120
	21st to end	Night	17% 8% 4 6% 5% 3 4 5% 5% 6% 36%	4532
February	1st to 10th	Night	13% 7% 3 7% 5% 3 4 5% 6% 8% 41%	2470
	11th to 20th	Night	15% 8% 3 6% 5% 4 5% 5% 6% 6% 38%	2403
	21st to end	Night	14% 10% 4 6% 5% 3 4 4 6% 7% 37%	1862
March	1st to 10th	Night	13% 6% 2 5% 6% 2 4 5% 5% 6% 48%	2090
	11th to 20th	Night	16% 6% 2 5% 4 3 4 4 5% 7% 43%	2090
	21st to end	Night	14% 7% 3 5% 6% 2 5% 4 4 6% 44%	2299
April	1st to 10th	Night	15% 7% 3 6% 6% 3 5% 5% 6% 7% 38%	1836
	11th to 20th	Night	13% 5% 2 6% 5% 3 5% 5% 6% 8% 42%	1775
	21st to end	Night	18% 4 2 4 4 2 5% 5% 5% 7% 43%	1763
May	1st to 10th	Night	17% 5% 2 5% 5% 3 4 5% 5% 8% 40%	1620
	11th to 20th	Night	18% 5% 3 5% 5% 3 4 5% 5% 6% 39%	1557
	21st to end	Night	12% 5% 2 5% 5% 3 5% 5% 6% 10% 41%	1550
June	1st to 10th	Night	11% 4 1 4 4 3 5% 5% 7% 10% 47%	1440
	11th to 20th	Night	11% 5% 2 5% 5% 4 5% 6% 6% 9% 42%	1440
	21st to end	Night	8% 3 2 4 5% 4 5% 6% 7% 12% 44%	1440
July	1st to 10th	Night	9% 5% 2 6% 5% 4 4 6% 6% 10% 41%	1440
	11th to 20th	Night	5% 4 3 5% 6% 3 4 5% 7% 9% 47%	1599
	21st to end	Night	8% 4 2 4 4 4 6% 7% 9% 12% 40%	1771
August	1st to 10th	Night	8% 6% 2 5% 6% 3 5% 7% 7% 7% 43%	1611
	11th to 20th	Night	10% 5% 2 5% 5% 4 5% 5% 6% 8% 44%	1606
	21st to end	Night	11% 5% 2 4 5% 3 5% 6% 6% 10% 43%	1980
September	1st to 10th	Night	16% 4 2 5% 4 2 5% 5% 5% 6% 46%	1800
	11th to 20th	Night	12% 5% 2 5% 4 2 3 3 4 7% 51%	1846
	21st to end	Night	18% 5% 2 4 5% 3 4 5% 5% 6% 44%	2052
October	1st to 10th	Night	19% 5% 2 3 4 2 3 4 5% 7% 46%	2160
	11th to 20th	Night	14% 5% 2 4 4 2 3 4 6% 6% 50%	2160
	21st to end	Night	15% 5% 2 4 3 2 4 5% 5% 6% 49%	2376
November	1st to 10th	Night	9% 6% 2 5% 4 3 4 4 5% 7% 51%	2244
	11th to 20th	Night	14% 6% 3 5% 4 3 4 3 5% 7% 46%	2574
	21st to end	Night	13% 5% 2 5% 6% 3 5% 5% 5% 8% 43%	2660
December	1st to 10th	Night	15% 7% 3 6% 6% 3 4 5% 6% 6% 39%	2660
	11th to 20th	Night	15% 6% 3 5% 5% 4 5% 5% 6% 7% 40%	3285
	21st to end	Night	15% 6% 2 6% 6% 4 6% 6% 7% 9% 32%	4884

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	41% 8% 9% 28% 12% 3	2712
	11th to 20th	Day	48% 6% 10% 19% 14% 3	2840
	21st to end	Day	48% 8% 10% 22% 11% 2	3124
February	1st to 10th	Day	46% 5% 14% 21% 12% 2	1710
	11th to 20th	Day	46% 5% 11% 21% 15% 2	1672
	21st to end	Day	46% 6% 11% 19% 14% 3	1588
March	1st to 10th	Day	38% 6% 12% 25% 18% 2	2090
	11th to 20th	Day	42% 6% 10% 20% 17% 5%	2090
	21st to end	Day	45% 6% 11% 19% 15% 3	2299
April	1st to 10th	Day	41% 8% 13% 17% 17% 4	2168
	11th to 20th	Day	38% 6% 11% 21% 19% 5%	2134
	21st to end	Day	45% 6% 10% 21% 14% 4	2212
May	1st to 10th	Day	43% 8% 13% 19% 14% 2	2340
	11th to 20th	Day	46% 9% 12% 18% 12% 3	2339
	21st to end	Day	41% 8% 15% 24% 11% 1	2725
June	1st to 10th	Day	37% 8% 13% 25% 15% 1	2520
	11th to 20th	Day	40% 10% 14% 21% 13% 2	2520
	21st to end	Day	32% 10% 16% 25% 15% 2	2520
July	1st to 10th	Day	38% 8% 13% 22% 15% 3	2450
	11th to 20th	Day	32% 10% 16% 25% 15% 2	2286
	21st to end	Day	39% 10% 13% 20% 16% 3	2405
August	1st to 10th	Day	41% 11% 14% 23% 11%	2314
	11th to 20th	Day	39% 9% 16% 21% 14% 1	2276
	21st to end	Day	36% 9% 17% 24% 12% 3	2376
September	1st to 10th	Day	39% 8% 12% 19% 19% 4	2160
	11th to 20th	Day	35% 6% 10% 27% 19% 3	2110
	21st to end	Day	43% 6% 11% 20% 17% 2	1906
October	1st to 10th	Day	38% 7% 12% 24% 16% 3	1800
	11th to 20th	Day	37% 6% 12% 25% 19% 2	1800
	21st to end	Day	37% 5% 10% 26% 19% 3	1980
November	1st to 10th	Day	33% 5% 12% 24% 22% 4	1834
	11th to 20th	Day	38% 5% 12% 21% 21% 4	1601
	21st to end	Day	39% 5% 13% 27% 15% 1	1520
December	1st to 10th	Day	46% 9% 12% 23% 9% 1	1520
	11th to 20th	Day	41% 7% 11% 29% 9% 3	1906
	21st to end	Day	44% 10% 16% 21% 9% 1	2772
January	1st to 10th	Night	37% 9% 11% 26% 14% 2	4232
	11th to 20th	Night	41% 7% 12% 24% 13% 3	4120
	21st to end	Night	46% 7% 11% 24% 10% 1	4532
February	1st to 10th	Night	41% 6% 14% 24% 13% 1	2470
	11th to 20th	Night	45% 6% 10% 21% 15% 3	2405
	21st to end	Night	45% 7% 11% 20% 12% 5%	1862
March	1st to 10th	Night	37% 6% 11% 28% 16% 2	2090
	11th to 20th	Night	40% 6% 11% 23% 18% 3	2090
	21st to end	Night	41% 6% 12% 21% 15% 4	2299
April	1st to 10th	Night	44% 7% 10% 20% 16% 4	1836
	11th to 20th	Night	37% 8% 12% 21% 17% 6%	1775
	21st to end	Night	39% 6% 13% 25% 13% 4	1763
May	1st to 10th	Night	41% 7% 13% 22% 13% 4	1620
	11th to 20th	Night	45% 8% 12% 18% 16% 2	1558
	21st to end	Night	37% 9% 17% 24% 11% 1	1551
June	1st to 10th	Night	33% 9% 12% 27% 17% 3	1440
	11th to 20th	Night	37% 10% 14% 24% 14% 2	1440
	21st to end	Night	32% 9% 15% 26% 16% 2	1440
July	1st to 10th	Night	37% 10% 15% 20% 12% 4	1400
	11th to 20th	Night	31% 9% 18% 23% 16% 3	1547
	21st to end	Night	30% 11% 18% 23% 16% 1	1672
August	1st to 10th	Night	36% 11% 15% 23% 14% 1	1600
	11th to 20th	Night	36% 8% 16% 24% 15% 1	1606
	21st to end	Night	35% 8% 15% 25% 14% 3	1980
September	1st to 10th	Night	38% 7% 13% 20% 19% 4	1800
	11th to 20th	Night	34% 5% 10% 27% 23% 2	1846
	21st to end	Night	40% 7% 11% 22% 19% 2	2052
October	1st to 10th	Night	39% 5% 12% 25% 17% 2	2160
	11th to 20th	Night	33% 7% 10% 27% 21% 2	2160
	21st to end	Night	34% 7% 10% 25% 22% 2	2376
November	1st to 10th	Night	32% 5% 11% 27% 20% 5%	2244
	11th to 20th	Night	37% 5% 8% 25% 20% 5%	2574
	21st to end	Night	38% 6% 12% 24% 17% 3	2660
December	1st to 10th	Night	44% 7% 12% 25% 11% 1	2660
	11th to 20th	Night	42% 6% 11% 24% 16% 2	3288
	21st to end	Night	41% 11% 18% 18% 12% 1	4884

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	27% 32% 25% 11% 5%	236
	11th to 20th	Day	30% 28% 27% 11% 3	270
	21st to end	Day	22% 30% 29% 14% 4	297
February	1st to 10th	Day	26% 23% 36% 10% 5%	270
	11th to 20th	Day	34% 29% 31% 5%	270
	21st to end	Day	32% 23% 27% 16% 2	364
March	1st to 10th	Day	26% 23% 27% 20% 3	460
	11th to 20th	Day	25% 24% 29% 15% 6% 1	460
	21st to end	Day	22% 24% 34% 15% 5% 1	506
April	1st to 10th	Day	23% 24% 25% 19% 8% 1	428
	11th to 20th	Day	19% 17% 29% 19% 11% 5%	412
	21st to end	Day	26% 15% 32% 16% 9% 2	420
May	1st to 10th	Day	21% 21% 27% 17% 12% 3	423
	11th to 20th	Day	21% 17% 27% 20% 12% 4	447
	21st to end	Day	21% 8% 21% 23% 20% 7%	485
June	1st to 10th	Day	16% 10% 27% 22% 19% 6%	450
	11th to 20th	Day	16% 10% 26% 25% 18% 5%	450
	21st to end	Day	12% 6% 29% 30% 18% 6%	450
July	1st to 10th	Day	11% 5% 20% 27% 23% 13%	445
	11th to 20th	Day	6% 3 20% 30% 28% 12%	444
	21st to end	Day	11% 7% 19% 31% 23% 9%	488
August	1st to 10th	Day	13% 3 21% 24% 25% 13%	446
	11th to 20th	Day	12% 3 19% 28% 26% 12%	439
	21st to end	Day	9% 4 20% 27% 25% 14%	495
September	1st to 10th	Day	20% 6% 17% 22% 25% 11%	450
	11th to 20th	Day	16% 4 18% 28% 24% 11%	450
	21st to end	Day	24% 10% 19% 25% 16% 5%	450
October	1st to 10th	Day	19% 6% 20% 28% 20% 6%	450
	11th to 20th	Day	18% 15% 26% 25% 13% 3	450
	21st to end	Day	19% 14% 34% 20% 11% 2	495
November	1st to 10th	Day	17% 14% 27% 24% 14% 3	444
	11th to 20th	Day	23% 16% 31% 20% 10% 1	261
	21st to end	Day	21% 22% 30% 20% 6% 1	180
December	1st to 10th	Day	19% 30% 31% 13% 7%	180
	11th to 20th	Day	24% 22% 27% 22% 4	180
	21st to end	Day	20% 22% 28% 21% 9%	198
January	1st to 10th	Night	19% 20% 38% 19% 4	760
	11th to 20th	Night	23% 19% 29% 25% 4	760
	21st to end	Night	19% 20% 37% 21% 2	836
February	1st to 10th	Night	17% 20% 34% 27% 2	760
	11th to 20th	Night	23% 26% 33% 17% 1	743
	21st to end	Night	24% 21% 34% 19% 3	628
March	1st to 10th	Night	20% 18% 37% 22% 4	760
	11th to 20th	Night	24% 21% 28% 21% 5% 1	760
	21st to end	Night	18% 17% 39% 20% 5% 1	836
April	1st to 10th	Night	20% 22% 32% 21% 5%	729
	11th to 20th	Night	17% 15% 32% 26% 9%	711
	21st to end	Night	20% 13% 29% 30% 6%	683
May	1st to 10th	Night	20% 9% 28% 28% 14% 1	540
	11th to 20th	Night	20% 7% 27% 33% 13% 1	496
	21st to end	Night	14% 4 22% 38% 21% 2	389
June	1st to 10th	Night	11% 4 24% 37% 24% 1	360
	11th to 20th	Night	13% 4 20% 40% 20% 3	360
	21st to end	Night	7% 3 26% 45% 18% 1	360
July	1st to 10th	Night	11% 4 15% 37% 29% 4	350
	11th to 20th	Night	6% 2 12% 40% 35% 5%	503
	21st to end	Night	9% 3 14% 42% 29% 3	591
August	1st to 10th	Night	10% 3 14% 41% 28% 3	537
	11th to 20th	Night	12% 3 12% 38% 31% 4	542
	21st to end	Night	12% 3 15% 39% 27% 4	792
September	1st to 10th	Night	17% 3 15% 32% 28% 5%	720
	11th to 20th	Night	13% 5% 18% 33% 29% 3	720
	21st to end	Night	20% 4 22% 32% 20% 2	720
October	1st to 10th	Night	20% 8% 20% 29% 21% 2	720
	11th to 20th	Night	17% 11% 25% 33% 12% 2	720
	21st to end	Night	18% 14% 31% 28% 10% 1	792
November	1st to 10th	Night	14% 11% 31% 32% 11% 1	740
	11th to 20th	Night	18% 19% 26% 29% 8% 1	760
	21st to end	Night	17% 17% 30% 27% 7%	760
December	1st to 10th	Night	20% 16% 34% 23% 6%	760
	11th to 20th	Night	18% 14% 37% 28% 4	760
	21st to end	Night	18% 18% 30% 33% 2	836

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