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 2024-11-04.

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%	101157
	Night	14% 6% 2 6% 4 4 4 4 6% 7% 41%	97792
Transparency	Day	39% 7% 12% 24% 15% 3	104801
	Night	38% 7% 13% 24% 15% 3	102288
Seeing	Day	19% 14% 26% 22% 14% 5%	14903
	Night	17% 13% 27% 29% 13% 1	24488

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% 2 7% 5% 4 5% 4 5% 7% 40%	2711
	11th to 20th	Day	12% 10% 3 8% 6% 6% 5% 4 5% 6% 34%	2840
	21st to end	Day	15% 8% 4 8% 5% 5% 5% 4 5% 6% 33%	3124
February	1st to 10th	Day	13% 10% 2 8% 5% 5% 4 4 5% 6% 37%	1710
	11th to 20th	Day	14% 8% 2 7% 5% 5% 5% 3 5% 7% 38%	1663
	21st to end	Day	16% 9% 3 6% 5% 4 4 4 5% 6% 38%	1588
March	1st to 10th	Day	19% 9% 3 7% 5% 5% 5% 3 5% 6% 34%	3435
	11th to 20th	Day	10% 6% 2 5% 3 4 4 4 6% 8% 48%	3480
	21st to end	Day	16% 9% 3 7% 4 4 4 4 5% 6% 39%	3815
April	1st to 10th	Day	11% 7% 3 6% 5% 5% 6% 5% 6% 8% 38%	2264
	11th to 20th	Day	12% 7% 2 5% 4 5% 4 4 5% 6% 44%	2254
	21st to end	Day	18% 6% 2 6% 5% 4 5% 3 5% 6% 39%	2335
May	1st to 10th	Day	21% 7% 3 7% 5% 5% 5% 4 6% 8% 29%	4082
	11th to 20th	Day	23% 7% 2 6% 4 4 4 4 6% 8% 31%	4143
	21st to end	Day	10% 4 2 6% 4 5% 5% 5% 8% 10% 41%	4823
June	1st to 10th	Day	12% 5% 2 5% 4 5% 5% 4 7% 9% 42%	2660
	11th to 20th	Day	10% 6% 2 6% 5% 6% 5% 5% 6% 10% 38%	2660
	21st to end	Day	9% 4 2 5% 5% 4 6% 4 8% 11% 43%	2660
July	1st to 10th	Day	5% 4 2 6% 6% 5% 6% 5% 7% 11% 44%	4440
	11th to 20th	Day	8% 5% 2 6% 5% 5% 5% 5% 7% 11% 42%	4136
	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 5% 5% 52%	1864
	11th to 20th	Day	15% 6% 2 5% 3 3 4 4 5% 7% 45%	1601
	21st to end	Day	15% 6% 2 5% 4 4 4 4 5% 8% 43%	1520
December	1st to 10th	Day	16% 9% 3 5% 4 5% 5% 6% 7% 7% 33%	1520
	11th to 20th	Day	15% 6% 2 7% 5% 4 5% 3 5% 7% 39%	1910
	21st to end	Day	17% 5% 2 7% 5% 5% 6% 5% 7% 9% 31%	2772
January	1st to 10th	Night	11% 7% 3 7% 5% 5% 5% 4 6% 7% 41%	4232
	11th to 20th	Night	15% 9% 3 6% 4 3 4 4 6% 6% 41%	4120
	21st to end	Night	17% 9% 3 7% 5% 4 4 4 5% 6% 36%	4532
February	1st to 10th	Night	13% 7% 3 7% 4 4 4 4 6% 7% 41%	2470
	11th to 20th	Night	14% 9% 2 7% 4 5% 5% 3 6% 5% 38%	2403
	21st to end	Night	14% 11% 3 7% 5% 3 4 3 6% 7% 37%	1862
March	1st to 10th	Night	19% 8% 3 6% 5% 4 4 4 5% 6% 38%	3418
	11th to 20th	Night	10% 5% 2 6% 5% 5% 5% 4 6% 8% 46%	3480
	21st to end	Night	12% 7% 2 6% 4 3 4 3 4 5% 49%	3817
April	1st to 10th	Night	14% 8% 2 6% 6% 4 4 4 6% 7% 38%	1915
	11th to 20th	Night	12% 5% 2 7% 4 5% 5% 4 6% 7% 42%	1875
	21st to end	Night	17% 5% 2 5% 4 3 4 4 6% 7% 43%	1860
May	1st to 10th	Night	23% 6% 3 8% 5% 3 4 4 5% 8% 30%	2813
	11th to 20th	Night	25% 8% 3 6% 4 4 4 4 5% 5% 33%	2749
	21st to end	Night	10% 5% 2 6% 4 5% 5% 5% 7% 9% 42%	2732
June	1st to 10th	Night	10% 4 2 5% 4 5% 4 4 7% 9% 46%	1520
	11th to 20th	Night	10% 5% 2 5% 5% 5% 5% 4 6% 10% 42%	1520
	21st to end	Night	8% 4 2 5% 5% 5% 4 5% 7% 11% 44%	1520
July	1st to 10th	Night	6% 5% 2 6% 5% 5% 6% 6% 7% 10% 42%	2520
	11th to 20th	Night	5% 6% 3 7% 7% 5% 6% 5% 7% 9% 40%	2805
	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%	2287
	11th to 20th	Night	14% 7% 2 5% 4 4 4 3 5% 6% 46%	2574
	21st to end	Night	13% 5% 2 6% 5% 4 5% 4 6% 8% 43%	2660
December	1st to 10th	Night	15% 7% 3 7% 5% 4 4 4 6% 6% 39%	2660
	11th to 20th	Night	15% 6% 2 6% 5% 5% 5% 4 6% 7% 39%	3285
	21st to end	Night	15% 6% 2 7% 6% 6% 6% 5% 7% 8% 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	48% 7% 11% 20% 12% 2	3435
	11th to 20th	Day	32% 7% 10% 29% 20% 3	3480
	21st to end	Day	44% 7% 12% 20% 14% 3	3815
April	1st to 10th	Day	41% 8% 13% 17% 17% 4	2264
	11th to 20th	Day	38% 6% 11% 22% 19% 5%	2254
	21st to end	Day	45% 6% 10% 22% 14% 3	2335
May	1st to 10th	Day	50% 8% 11% 20% 10% 1	4120
	11th to 20th	Day	48% 9% 12% 22% 8% 2	4147
	21st to end	Day	33% 10% 16% 24% 15% 2	4837
June	1st to 10th	Day	37% 8% 13% 25% 16% 1	2660
	11th to 20th	Day	38% 10% 15% 22% 13% 2	2660
	21st to end	Day	32% 9% 16% 25% 15% 2	2660
July	1st to 10th	Day	33% 9% 17% 26% 11% 5%	4370
	11th to 20th	Day	33% 9% 16% 30% 11% 1	4066
	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% 22% 4	1864
	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	45% 7% 11% 26% 10% 1	3418
	11th to 20th	Night	32% 8% 11% 32% 15% 2	3480
	21st to end	Night	37% 6% 11% 25% 18% 4	3817
April	1st to 10th	Night	43% 7% 10% 20% 17% 4	1915
	11th to 20th	Night	37% 8% 12% 21% 16% 5%	1875
	21st to end	Night	38% 7% 13% 26% 13% 4	1860
May	1st to 10th	Night	51% 8% 10% 18% 11% 2	2840
	11th to 20th	Night	49% 9% 12% 19% 9% 1	2750
	21st to end	Night	33% 10% 17% 26% 10% 4	2739
June	1st to 10th	Night	33% 9% 12% 27% 16% 3	1520
	11th to 20th	Night	36% 10% 14% 25% 14% 2	1520
	21st to end	Night	32% 9% 15% 26% 17% 2	1520
July	1st to 10th	Night	36% 9% 20% 18% 12% 5%	2480
	11th to 20th	Night	38% 9% 17% 22% 13% 2	2753
	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	32% 5% 11% 26% 20% 5%	2287
	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	22% 25% 26% 19% 7% 1	460
	11th to 20th	Day	19% 17% 29% 18% 10% 5%	452
	21st to end	Day	26% 16% 32% 17% 8% 2	460
May	1st to 10th	Day	21% 21% 27% 17% 11% 3	463
	11th to 20th	Day	22% 15% 26% 20% 12% 4	487
	21st to end	Day	19% 9% 22% 23% 19% 7%	529
June	1st to 10th	Day	16% 10% 27% 21% 19% 6%	490
	11th to 20th	Day	14% 10% 25% 26% 19% 6%	490
	21st to end	Day	12% 6% 29% 29% 18% 6%	490
July	1st to 10th	Day	11% 5% 21% 27% 24% 12%	485
	11th to 20th	Day	6% 4 22% 31% 27% 11%	484
	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% 27% 25% 14% 3	456
	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%	760
	11th to 20th	Night	17% 15% 33% 26% 9%	751
	21st to end	Night	19% 14% 29% 30% 7%	720
May	1st to 10th	Night	21% 8% 27% 29% 14% 1	570
	11th to 20th	Night	20% 7% 26% 32% 13% 1	524
	21st to end	Night	13% 4 22% 37% 21% 1	411
June	1st to 10th	Night	11% 4 24% 37% 23% 1	380
	11th to 20th	Night	12% 4 20% 42% 19% 2	380
	21st to end	Night	7% 3 26% 44% 19% 1	380
July	1st to 10th	Night	11% 5% 16% 37% 28% 4	370
	11th to 20th	Night	5% 2 13% 40% 35% 5%	532
	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	14% 11% 31% 33% 11% 1	755
	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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