Lookout Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Lookout Observatory clear sky chart from 2005-11-06 to 2025-05-26.

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	10% 7% 3 7% 5% 5% 5% 4 6% 8% 39%	114513
	Night	10% 8% 3 8% 5% 5% 5% 4 6% 8% 40%	114775
Transparency	Day	40% 8% 12% 20% 16% 4	118268
	Night	39% 8% 13% 20% 16% 4	119159
Seeing	Day	18% 13% 22% 22% 16% 9%	15741
	Night	16% 11% 20% 30% 19% 3	25858

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	9% 7% 3 7% 5% 4 4 3 5% 8% 46%	4056
	11th to 20th	Day	8% 6% 3 6% 5% 4 5% 4 4 5% 50%	4060
	21st to end	Day	11% 10% 3 7% 3 3 4 4 5% 6% 42%	4466
February	1st to 10th	Day	12% 8% 3 7% 5% 5% 5% 4 6% 7% 39%	1885
	11th to 20th	Day	13% 9% 4 9% 4 4 4 4 5% 6% 36%	1948
	21st to end	Day	12% 8% 2 8% 5% 4 6% 3 5% 6% 40%	1650
March	1st to 10th	Day	14% 9% 4 8% 5% 4 4 4 5% 6% 38%	4834
	11th to 20th	Day	10% 7% 3 7% 4 3 4 3 4 6% 49%	4980
	21st to end	Day	12% 8% 3 6% 4 4 4 4 5% 6% 43%	5465
April	1st to 10th	Day	10% 6% 3 5% 5% 6% 5% 4 6% 9% 41%	2200
	11th to 20th	Day	12% 8% 3 6% 5% 4 4 4 6% 8% 42%	2396
	21st to end	Day	11% 7% 3 6% 5% 4 5% 4 6% 7% 43%	2600
May	1st to 10th	Day	9% 8% 4 10% 6% 6% 6% 6% 6% 8% 32%	4212
	11th to 20th	Day	13% 8% 4 9% 6% 6% 6% 5% 7% 8% 28%	4211
	21st to end	Day	6% 5% 3 7% 5% 6% 6% 5% 7% 11% 38%	4541
June	1st to 10th	Day	9% 6% 3 7% 7% 8% 6% 6% 8% 10% 31%	2470
	11th to 20th	Day	5% 7% 3 8% 6% 8% 6% 7% 8% 10% 33%	2470
	21st to end	Day	7% 6% 3 6% 7% 6% 7% 6% 8% 9% 34%	2470
July	1st to 10th	Day	5% 6% 3 9% 7% 7% 7% 6% 7% 10% 33%	4120
	11th to 20th	Day	8% 6% 3 8% 7% 7% 7% 6% 8% 9% 31%	4120
	21st to end	Day	12% 9% 4 11% 9% 8% 8% 6% 8% 8% 17%	4466
August	1st to 10th	Day	11% 7% 3 9% 6% 6% 7% 6% 10% 11% 24%	4104
	11th to 20th	Day	7% 5% 3 7% 6% 6% 8% 5% 9% 11% 33%	4044
	21st to end	Day	5% 4 2 6% 5% 5% 6% 5% 7% 7% 47%	4197
September	1st to 10th	Day	9% 6% 4 7% 6% 5% 6% 5% 7% 8% 36%	2090
	11th to 20th	Day	14% 7% 2 8% 6% 5% 6% 5% 7% 9% 32%	2090
	21st to end	Day	12% 7% 2 8% 5% 5% 6% 4 7% 7% 37%	2090
October	1st to 10th	Day	10% 7% 3 7% 3 4 4 3 5% 6% 48%	2057
	11th to 20th	Day	8% 6% 2 6% 5% 3 5% 3 3 6% 53%	1900
	21st to end	Day	10% 7% 3 5% 4 3 4 2 4 6% 52%	2090
November	1st to 10th	Day	8% 7% 2 6% 4 3 4 3 5% 6% 54%	1862
	11th to 20th	Day	9% 7% 3 6% 5% 3 4 4 5% 6% 49%	1800
	21st to end	Day	10% 8% 4 6% 4 4 3 4 5% 7% 46%	1800
December	1st to 10th	Day	10% 6% 3 6% 4 5% 5% 4 5% 6% 47%	2930
	11th to 20th	Day	11% 7% 3 8% 5% 4 5% 5% 7% 8% 37%	3373
	21st to end	Day	11% 8% 3 8% 6% 5% 5% 3 5% 8% 37%	4466
January	1st to 10th	Night	9% 9% 3 11% 7% 5% 5% 3 5% 7% 37%	5889
	11th to 20th	Night	10% 7% 3 8% 5% 4 5% 4 6% 6% 43%	5900
	21st to end	Night	10% 10% 3 8% 4 4 4 3 5% 6% 43%	6490
February	1st to 10th	Night	10% 8% 3 8% 4 4 5% 4 7% 7% 39%	2510
	11th to 20th	Night	15% 9% 3 7% 5% 5% 4 4 5% 6% 36%	2338
	21st to end	Night	12% 8% 4 9% 5% 5% 5% 4 6% 6% 37%	1980
March	1st to 10th	Night	15% 10% 4 9% 5% 4 5% 3 5% 5% 34%	5017
	11th to 20th	Night	8% 6% 3 6% 4 3 4 4 6% 8% 48%	4980
	21st to end	Night	12% 8% 3 7% 5% 4 4 3 4 7% 42%	5467
April	1st to 10th	Night	11% 7% 3 5% 5% 5% 4 4 6% 8% 43%	2200
	11th to 20th	Night	8% 8% 3 7% 4 4 4 3 6% 8% 45%	1967
	21st to end	Night	9% 6% 3 7% 5% 4 4 3 6% 7% 46%	1800
May	1st to 10th	Night	9% 8% 4 8% 5% 5% 5% 4 7% 10% 35%	2903
	11th to 20th	Night	10% 8% 4 10% 6% 5% 5% 5% 6% 7% 32%	2902
	21st to end	Night	6% 5% 3 8% 5% 6% 5% 6% 8% 12% 38%	3130
June	1st to 10th	Night	7% 6% 2 7% 4 6% 6% 5% 9% 14% 34%	1710
	11th to 20th	Night	5% 7% 2 5% 5% 5% 5% 5% 10% 11% 40%	1710
	21st to end	Night	5% 6% 2 6% 5% 6% 5% 4 8% 12% 42%	1710
July	1st to 10th	Night	6% 6% 3 9% 6% 8% 7% 5% 8% 11% 32%	2840
	11th to 20th	Night	7% 5% 2 7% 6% 5% 7% 5% 9% 11% 36%	2840
	21st to end	Night	11% 10% 5% 12% 7% 7% 7% 5% 7% 9% 20%	3087
August	1st to 10th	Night	7% 7% 3 8% 6% 6% 7% 6% 8% 12% 31%	2825
	11th to 20th	Night	8% 7% 4 10% 6% 7% 6% 6% 7% 9% 32%	2785
	21st to end	Night	6% 5% 2 6% 4 4 5% 5% 7% 10% 47%	3422
September	1st to 10th	Night	9% 6% 2 8% 5% 6% 6% 4 6% 9% 40%	2090
	11th to 20th	Night	11% 9% 3 8% 6% 5% 5% 4 6% 7% 36%	2090
	21st to end	Night	9% 6% 3 8% 4 3 5% 5% 4 7% 46%	2090
October	1st to 10th	Night	12% 6% 3 6% 4 4 4 4 5% 6% 47%	2122
	11th to 20th	Night	8% 7% 2 6% 4 5% 4 3 6% 6% 48%	2280
	21st to end	Night	8% 7% 3 7% 3 3 4 3 5% 5% 52%	2508
November	1st to 10th	Night	10% 7% 2 5% 4 4 4 5% 5% 6% 48%	2431
	11th to 20th	Night	10% 8% 3 7% 4 4 3 4 5% 6% 47%	2600
	21st to end	Night	11% 7% 3 7% 4 4 4 4 5% 6% 45%	2600
December	1st to 10th	Night	10% 6% 3 8% 5% 4 4 4 5% 6% 44%	4250
	11th to 20th	Night	14% 10% 4 9% 5% 5% 5% 4 6% 6% 33%	4822
	21st to end	Night	12% 8% 4 9% 5% 5% 5% 4 6% 7% 36%	6490

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	34% 6% 9% 24% 25% 2	4057
	11th to 20th	Day	32% 7% 7% 14% 36% 4	4060
	21st to end	Day	39% 6% 9% 19% 21% 6%	4466
February	1st to 10th	Day	43% 6% 11% 20% 17% 3	1885
	11th to 20th	Day	46% 7% 10% 17% 17% 4	1958
	21st to end	Day	44% 6% 10% 16% 19% 6%	1650
March	1st to 10th	Day	44% 7% 7% 14% 19% 9%	4834
	11th to 20th	Day	35% 6% 6% 17% 22% 15%	4980
	21st to end	Day	40% 5% 10% 26% 16% 3	5465
April	1st to 10th	Day	36% 7% 13% 22% 17% 4	2200
	11th to 20th	Day	39% 6% 10% 20% 20% 6%	2396
	21st to end	Day	38% 7% 14% 24% 15% 2	2600
May	1st to 10th	Day	44% 11% 14% 18% 11% 1	4250
	11th to 20th	Day	47% 13% 13% 20% 7% 1	4215
	21st to end	Day	35% 12% 17% 20% 15% 2	4554
June	1st to 10th	Day	44% 12% 19% 17% 8% 1	2470
	11th to 20th	Day	41% 11% 18% 17% 12% 1	2470
	21st to end	Day	42% 10% 16% 18% 12% 2	2470
July	1st to 10th	Day	44% 11% 19% 19% 7%	4055
	11th to 20th	Day	45% 10% 22% 18% 5% 1	4051
	21st to end	Day	61% 10% 14% 11% 4	4324
August	1st to 10th	Day	49% 9% 19% 20% 3	4091
	11th to 20th	Day	40% 8% 24% 17% 9% 2	4083
	21st to end	Day	33% 8% 14% 33% 11% 2	4197
September	1st to 10th	Day	41% 10% 14% 16% 15% 3	2090
	11th to 20th	Day	45% 8% 12% 19% 14% 2	2090
	21st to end	Day	43% 7% 14% 17% 15% 4	2090
October	1st to 10th	Day	29% 6% 7% 19% 26% 12%	3421
	11th to 20th	Day	24% 5% 6% 23% 23% 20%	3140
	21st to end	Day	27% 3 8% 26% 22% 13%	3428
November	1st to 10th	Day	31% 5% 9% 21% 25% 8%	1862
	11th to 20th	Day	34% 6% 7% 22% 26% 6%	1800
	21st to end	Day	36% 5% 9% 23% 21% 5%	1800
December	1st to 10th	Day	38% 4 6% 20% 27% 5%	2930
	11th to 20th	Day	41% 5% 15% 17% 19% 3	3370
	21st to end	Day	41% 6% 14% 23% 13% 3	4466
January	1st to 10th	Night	38% 9% 12% 20% 19% 2	5889
	11th to 20th	Night	35% 8% 8% 18% 25% 6%	5900
	21st to end	Night	40% 5% 8% 19% 22% 6%	6490
February	1st to 10th	Night	41% 6% 13% 20% 17% 3	2510
	11th to 20th	Night	47% 7% 10% 17% 15% 4	2339
	21st to end	Night	44% 7% 10% 17% 18% 4	1980
March	1st to 10th	Night	45% 11% 7% 14% 18% 5%	5017
	11th to 20th	Night	31% 6% 10% 19% 22% 13%	4980
	21st to end	Night	42% 4 12% 26% 13% 3	5467
April	1st to 10th	Night	36% 7% 14% 22% 17% 3	2200
	11th to 20th	Night	35% 7% 12% 22% 20% 4	1967
	21st to end	Night	36% 8% 15% 25% 15% 1	1800
May	1st to 10th	Night	41% 12% 15% 23% 8% 1	2930
	11th to 20th	Night	42% 16% 15% 20% 6%	2903
	21st to end	Night	30% 16% 20% 24% 10% 1	3138
June	1st to 10th	Night	37% 14% 21% 20% 7% 1	1710
	11th to 20th	Night	32% 11% 26% 21% 10%	1710
	21st to end	Night	34% 11% 21% 20% 12% 2	1710
July	1st to 10th	Night	43% 13% 19% 19% 6%	2795
	11th to 20th	Night	39% 13% 25% 18% 4 1	2786
	21st to end	Night	56% 11% 17% 12% 4	2988
August	1st to 10th	Night	42% 12% 28% 15% 4	2814
	11th to 20th	Night	45% 10% 23% 15% 7%	2812
	21st to end	Night	30% 9% 18% 34% 8% 1	3422
September	1st to 10th	Night	39% 12% 13% 20% 12% 3	2090
	11th to 20th	Night	45% 10% 13% 18% 14% 1	2090
	21st to end	Night	37% 9% 12% 23% 17% 3	2090
October	1st to 10th	Night	32% 6% 9% 18% 25% 9%	3538
	11th to 20th	Night	25% 4 7% 22% 23% 18%	3779
	21st to end	Night	30% 4 6% 25% 24% 12%	4120
November	1st to 10th	Night	34% 8% 11% 20% 21% 6%	2431
	11th to 20th	Night	36% 6% 7% 21% 23% 7%	2600
	21st to end	Night	37% 5% 9% 24% 20% 5%	2600
December	1st to 10th	Night	39% 6% 10% 22% 21% 3	4250
	11th to 20th	Night	47% 6% 11% 19% 14% 2	4824
	21st to end	Night	43% 7% 14% 21% 14% 1	6490

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	34% 26% 28% 10% 2 1	300
	11th to 20th	Day	36% 23% 20% 17% 4	300
	21st to end	Day	26% 33% 26% 12% 4	330
February	1st to 10th	Day	27% 27% 29% 13% 3	300
	11th to 20th	Day	40% 28% 22% 9% 1	300
	21st to end	Day	37% 27% 25% 10% 2	249
March	1st to 10th	Day	29% 20% 24% 19% 6% 1	447
	11th to 20th	Day	28% 21% 24% 20% 5% 1	500
	21st to end	Day	28% 21% 26% 19% 6%	550
April	1st to 10th	Day	26% 19% 25% 19% 9% 2	500
	11th to 20th	Day	20% 20% 26% 20% 12% 3	492
	21st to end	Day	20% 14% 27% 19% 15% 5%	500
May	1st to 10th	Day	17% 19% 24% 22% 13% 5%	503
	11th to 20th	Day	18% 13% 20% 25% 16% 9%	527
	21st to end	Day	13% 7% 21% 26% 19% 13%	549
June	1st to 10th	Day	11% 6% 23% 27% 20% 12%	490
	11th to 20th	Day	8% 6% 22% 23% 25% 16%	490
	21st to end	Day	10% 6% 27% 23% 19% 15%	490
July	1st to 10th	Day	9% 3 13% 23% 32% 20%	485
	11th to 20th	Day	8% 4 10% 26% 27% 25%	484
	21st to end	Day	8% 3 14% 23% 30% 21%	532
August	1st to 10th	Day	16% 4 16% 23% 19% 21%	486
	11th to 20th	Day	9% 5% 13% 30% 25% 18%	479
	21st to end	Day	11% 3 12% 26% 27% 21%	539
September	1st to 10th	Day	9% 4 14% 27% 31% 15%	490
	11th to 20th	Day	16% 6% 15% 21% 29% 14%	490
	21st to end	Day	16% 6% 17% 29% 25% 7%	490
October	1st to 10th	Day	15% 9% 20% 26% 22% 8%	490
	11th to 20th	Day	13% 14% 22% 27% 20% 5%	490
	21st to end	Day	18% 14% 22% 26% 17% 3	539
November	1st to 10th	Day	17% 13% 27% 26% 14% 4	400
	11th to 20th	Day	21% 21% 32% 17% 8% 1	300
	21st to end	Day	20% 22% 30% 18% 9% 1	300
December	1st to 10th	Day	28% 26% 27% 14% 4	300
	11th to 20th	Day	23% 20% 31% 18% 7% 1	300
	21st to end	Day	25% 20% 31% 15% 6% 2	330
January	1st to 10th	Night	26% 18% 27% 23% 5%	800
	11th to 20th	Night	22% 17% 28% 27% 5%	800
	21st to end	Night	21% 20% 29% 27% 3	880
February	1st to 10th	Night	19% 16% 31% 30% 4	800
	11th to 20th	Night	27% 22% 27% 22% 1	783
	21st to end	Night	23% 22% 31% 22% 3	660
March	1st to 10th	Night	20% 19% 29% 29% 4	800
	11th to 20th	Night	22% 15% 29% 25% 8% 1	800
	21st to end	Night	19% 18% 30% 25% 8%	880
April	1st to 10th	Night	20% 18% 25% 29% 9%	800
	11th to 20th	Night	17% 13% 25% 32% 11% 1	688
	21st to end	Night	13% 10% 22% 39% 15% 1	600
May	1st to 10th	Night	14% 9% 22% 37% 17% 1	600
	11th to 20th	Night	14% 6% 19% 35% 23% 3	591
	21st to end	Night	9% 4 12% 35% 34% 6%	634
June	1st to 10th	Night	9% 2 15% 35% 33% 7%	570
	11th to 20th	Night	6% 2 11% 34% 36% 11%	570
	21st to end	Night	6% 2 9% 36% 40% 7%	570
July	1st to 10th	Night	9% 3 6% 27% 46% 8%	555
	11th to 20th	Night	9% 3 4 24% 46% 14%	552
	21st to end	Night	8% 2 3 25% 47% 14%	624
August	1st to 10th	Night	8% 1 7% 31% 43% 11%	567
	11th to 20th	Night	10% 4 7% 29% 41% 9%	564
	21st to end	Night	10% 2 4 28% 44% 11%	674
September	1st to 10th	Night	9% 4 10% 30% 40% 7%	760
	11th to 20th	Night	14% 5% 11% 28% 35% 6%	760
	21st to end	Night	14% 6% 15% 32% 29% 4	760
October	1st to 10th	Night	15% 9% 16% 28% 26% 6%	760
	11th to 20th	Night	14% 9% 21% 35% 18% 2	760
	21st to end	Night	15% 12% 19% 36% 16% 2	836
November	1st to 10th	Night	16% 8% 18% 38% 18% 3	780
	11th to 20th	Night	19% 12% 20% 36% 13% 1	800
	21st to end	Night	19% 14% 21% 33% 12% 1	800
December	1st to 10th	Night	22% 18% 24% 30% 6%	800
	11th to 20th	Night	19% 14% 27% 32% 8% 1	800
	21st to end	Night	22% 16% 25% 31% 5%	880

Page generated at 2025-05-23 09:21:02. Version ac07

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