Lewis Smith Lake Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Lewis Smith Lake Observatory clear sky chart from 2005-11-06 to 2024-12-24.

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% 2 6% 4 4 5% 4 6% 8% 42%	101408
	Night	14% 6% 2 5% 4 4 4 3 5% 7% 45%	98611
Transparency	Day	39% 6% 11% 21% 18% 5%	105204
	Night	37% 6% 10% 21% 20% 7%	102954
Seeing	Day	16% 10% 23% 23% 18% 9%	14724
	Night	15% 8% 22% 29% 21% 4	24656

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	18% 12% 3 6% 4 3 3 3 5% 6% 37%	2836
	11th to 20th	Day	20% 8% 3 6% 4 3 2 2 3 4 45%	2967
	21st to end	Day	24% 9% 2 4 2 2 3 2 4 5% 41%	3476
February	1st to 10th	Day	20% 10% 3 6% 3 3 3 4 3 6% 39%	1900
	11th to 20th	Day	23% 7% 2 6% 4 4 3 3 4 4 40%	1848
	21st to end	Day	23% 9% 3 5% 3 3 3 3 5% 5% 37%	1570
March	1st to 10th	Day	24% 9% 3 7% 4 4 4 3 5% 5% 33%	3120
	11th to 20th	Day	17% 6% 3 6% 4 4 4 3 4 7% 43%	3313
	21st to end	Day	15% 6% 3 7% 5% 4 5% 4 6% 7% 39%	4055
April	1st to 10th	Day	14% 6% 2 6% 4 3 4 4 5% 7% 46%	2280
	11th to 20th	Day	15% 6% 2 5% 4 4 4 3 5% 7% 45%	2254
	21st to end	Day	10% 5% 2 5% 5% 4 4 4 5% 9% 47%	2280
May	1st to 10th	Day	13% 5% 2 6% 4 5% 5% 4 6% 8% 43%	4015
	11th to 20th	Day	16% 7% 2 6% 4 4 5% 4 6% 8% 38%	4081
	21st to end	Day	11% 5% 2 6% 5% 5% 5% 4 6% 8% 42%	4476
June	1st to 10th	Day	11% 6% 2 6% 5% 6% 6% 5% 7% 10% 36%	2470
	11th to 20th	Day	5% 4 2 6% 5% 5% 6% 5% 8% 10% 44%	2470
	21st to end	Day	8% 5% 2 6% 5% 5% 6% 5% 7% 9% 42%	2524
July	1st to 10th	Day	6% 5% 3 8% 6% 8% 8% 6% 8% 13% 29%	4181
	11th to 20th	Day	8% 6% 2 6% 5% 7% 8% 6% 10% 13% 29%	4120
	21st to end	Day	14% 8% 4 10% 7% 7% 7% 6% 8% 8% 22%	4373
August	1st to 10th	Day	4 3 1 5% 3 4 4 4 7% 10% 54%	3785
	11th to 20th	Day	6% 4 2 6% 5% 5% 5% 4 7% 10% 46%	3729
	21st to end	Day	7% 4 1 5% 5% 5% 5% 4 5% 7% 51%	4180
September	1st to 10th	Day	11% 5% 2 6% 4 6% 5% 5% 6% 8% 41%	2250
	11th to 20th	Day	13% 6% 2 6% 4 5% 5% 4 6% 7% 42%	2090
	21st to end	Day	12% 5% 2 5% 3 3 4 4 6% 7% 50%	2090
October	1st to 10th	Day	9% 4 2 5% 4 3 4 2 5% 6% 56%	2090
	11th to 20th	Day	10% 4 2 5% 3 3 4 3 4 7% 55%	2077
	21st to end	Day	14% 4 2 5% 3 3 3 2 4 6% 54%	1952
November	1st to 10th	Day	13% 5% 1 5% 3 3 3 3 4 5% 54%	1762
	11th to 20th	Day	14% 4 2 5% 4 4 4 2 4 6% 52%	1800
	21st to end	Day	16% 6% 2 5% 4 3 4 4 5% 5% 46%	1800
December	1st to 10th	Day	22% 9% 3 4 4 3 4 4 5% 5% 38%	1800
	11th to 20th	Day	19% 8% 2 6% 4 4 4 2 3 5% 42%	2243
	21st to end	Day	19% 10% 4 6% 4 4 3 3 4 6% 35%	3151
January	1st to 10th	Night	19% 12% 2 5% 4 4 4 3 3 4 40%	4109
	11th to 20th	Night	20% 8% 4 5% 3 3 3 2 4 5% 43%	3991
	21st to end	Night	22% 13% 2 5% 3 3 3 3 3 4 39%	4180
February	1st to 10th	Night	20% 8% 3 6% 3 3 3 3 5% 5% 42%	2280
	11th to 20th	Night	22% 7% 2 6% 4 3 3 2 4 4 43%	2218
	21st to end	Night	22% 7% 2 5% 3 4 4 4 5% 6% 38%	1884
March	1st to 10th	Night	25% 8% 3 7% 3 3 3 3 4 5% 35%	3733
	11th to 20th	Night	13% 5% 2 4 4 4 4 4 5% 8% 48%	3645
	21st to end	Night	17% 8% 3 8% 4 4 4 3 4 4 41%	3604
April	1st to 10th	Night	12% 5% 1 6% 4 5% 4 4 6% 7% 47%	1900
	11th to 20th	Night	15% 6% 2 4 4 3 4 3 4 7% 49%	1875
	21st to end	Night	11% 5% 2 5% 4 4 4 3 5% 7% 51%	1900
May	1st to 10th	Night	11% 5% 2 7% 5% 4 4 4 5% 8% 44%	2876
	11th to 20th	Night	16% 7% 2 6% 4 4 5% 4 5% 7% 39%	2812
	21st to end	Night	7% 4 2 5% 4 5% 5% 5% 8% 11% 44%	3079
June	1st to 10th	Night	7% 5% 2 6% 6% 5% 6% 5% 7% 9% 42%	1710
	11th to 20th	Night	4 3 1 4 4 4 5% 4 6% 11% 52%	1710
	21st to end	Night	6% 4 2 6% 5% 4 4 4 7% 10% 46%	1623
July	1st to 10th	Night	8% 6% 3 7% 6% 5% 7% 4 6% 11% 36%	2737
	11th to 20th	Night	8% 5% 3 7% 6% 5% 6% 5% 8% 11% 37%	2840
	21st to end	Night	11% 9% 3 10% 8% 6% 6% 6% 7% 8% 25%	3175
August	1st to 10th	Night	2 1 1 3 3 4 4 4 6% 10% 60%	3144
	11th to 20th	Night	5% 4 1 5% 3 4 4 4 6% 9% 55%	3100
	21st to end	Night	6% 4 2 4 3 4 4 4 6% 7% 58%	3476
September	1st to 10th	Night	8% 5% 2 5% 4 5% 4 4 6% 7% 48%	1909
	11th to 20th	Night	12% 6% 2 6% 4 4 4 3 6% 7% 45%	2090
	21st to end	Night	8% 5% 2 6% 3 4 3 3 6% 6% 54%	2090
October	1st to 10th	Night	8% 4 1 4 4 3 3 3 3 6% 61%	2090
	11th to 20th	Night	10% 6% 2 5% 3 3 3 3 5% 5% 55%	2099
	21st to end	Night	15% 6% 2 4 3 2 3 2 3 5% 56%	2646
November	1st to 10th	Night	12% 5% 2 5% 3 3 4 3 5% 5% 54%	2536
	11th to 20th	Night	13% 6% 2 5% 3 3 3 3 5% 5% 54%	2600
	21st to end	Night	15% 6% 2 5% 4 3 5% 3 4 6% 47%	2600
December	1st to 10th	Night	20% 6% 2 6% 4 4 3 3 5% 6% 40%	2600
	11th to 20th	Night	18% 7% 3 5% 3 3 3 3 4 5% 46%	3172
	21st to end	Night	20% 8% 3 6% 5% 5% 4 4 7% 7% 32%	4578

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	49% 3 5% 18% 21% 4	2837
	11th to 20th	Day	44% 4 6% 23% 22% 2	2967
	21st to end	Day	47% 3 6% 19% 21% 4	3476
February	1st to 10th	Day	47% 3 8% 18% 19% 4	1900
	11th to 20th	Day	48% 4 5% 18% 19% 6%	1858
	21st to end	Day	49% 4 7% 14% 21% 6%	1570
March	1st to 10th	Day	52% 5% 8% 10% 15% 9%	3120
	11th to 20th	Day	42% 5% 7% 20% 21% 6%	3313
	21st to end	Day	44% 5% 10% 18% 18% 4	4055
April	1st to 10th	Day	37% 5% 10% 21% 19% 7%	2280
	11th to 20th	Day	39% 5% 7% 21% 22% 7%	2254
	21st to end	Day	34% 6% 10% 26% 21% 3	2280
May	1st to 10th	Day	37% 8% 12% 25% 15% 2	4052
	11th to 20th	Day	42% 8% 12% 26% 12% 1	4085
	21st to end	Day	37% 7% 12% 27% 16% 1	4489
June	1st to 10th	Day	41% 10% 13% 18% 16% 1	2470
	11th to 20th	Day	32% 10% 15% 25% 17% 2	2470
	21st to end	Day	35% 9% 16% 22% 13% 4	2524
July	1st to 10th	Day	41% 10% 23% 20% 5% 1	4116
	11th to 20th	Day	40% 10% 24% 20% 5% 1	4051
	21st to end	Day	55% 10% 14% 15% 5%	4234
August	1st to 10th	Day	22% 9% 12% 37% 19% 1	3773
	11th to 20th	Day	31% 8% 15% 27% 16% 3	3765
	21st to end	Day	31% 6% 14% 20% 24% 4	4179
September	1st to 10th	Day	38% 8% 14% 18% 16% 6%	2250
	11th to 20th	Day	41% 7% 11% 17% 20% 4	2090
	21st to end	Day	33% 6% 11% 22% 21% 7%	2090
October	1st to 10th	Day	21% 5% 5% 15% 33% 22%	3480
	11th to 20th	Day	19% 3 5% 17% 30% 25%	3433
	21st to end	Day	25% 3 5% 21% 34% 11%	3190
November	1st to 10th	Day	34% 4 8% 18% 27% 10%	1762
	11th to 20th	Day	35% 4 5% 20% 27% 10%	1800
	21st to end	Day	39% 5% 6% 18% 24% 7%	1800
December	1st to 10th	Day	49% 4 7% 15% 20% 5%	1800
	11th to 20th	Day	47% 3 6% 19% 21% 5%	2240
	21st to end	Day	49% 4 8% 19% 17% 2	3151
January	1st to 10th	Night	48% 3 5% 17% 25% 3	4109
	11th to 20th	Night	45% 4 5% 22% 18% 6%	3991
	21st to end	Night	50% 4 4 18% 20% 4	4180
February	1st to 10th	Night	44% 3 7% 20% 20% 6%	2280
	11th to 20th	Night	47% 3 5% 18% 19% 8%	2219
	21st to end	Night	47% 5% 6% 15% 18% 8%	1884
March	1st to 10th	Night	51% 4 8% 14% 15% 8%	3733
	11th to 20th	Night	34% 5% 9% 21% 22% 10%	3645
	21st to end	Night	46% 4 7% 20% 19% 3	3604
April	1st to 10th	Night	35% 5% 11% 22% 20% 6%	1900
	11th to 20th	Night	36% 5% 6% 20% 22% 12%	1875
	21st to end	Night	32% 5% 10% 26% 22% 5%	1900
May	1st to 10th	Night	37% 8% 12% 21% 20% 3	2904
	11th to 20th	Night	42% 9% 13% 24% 11% 2	2813
	21st to end	Night	28% 10% 15% 27% 17% 2	3087
June	1st to 10th	Night	35% 9% 14% 20% 19% 2	1710
	11th to 20th	Night	24% 9% 17% 28% 18% 3	1710
	21st to end	Night	32% 8% 16% 22% 18% 5%	1623
July	1st to 10th	Night	41% 8% 23% 20% 6% 2	2692
	11th to 20th	Night	36% 10% 25% 22% 7% 1	2786
	21st to end	Night	53% 10% 16% 14% 6% 1	3074
August	1st to 10th	Night	18% 8% 14% 39% 20% 1	3132
	11th to 20th	Night	25% 8% 18% 31% 16% 3	3130
	21st to end	Night	25% 7% 15% 22% 26% 6%	3476
September	1st to 10th	Night	33% 9% 16% 19% 16% 7%	1909
	11th to 20th	Night	38% 7% 11% 18% 21% 5%	2090
	21st to end	Night	30% 5% 11% 22% 24% 9%	2090
October	1st to 10th	Night	19% 4 5% 17% 32% 23%	3480
	11th to 20th	Night	19% 3 4 16% 30% 27%	3482
	21st to end	Night	23% 3 4 24% 31% 16%	4358
November	1st to 10th	Night	32% 4 7% 19% 25% 13%	2536
	11th to 20th	Night	33% 3 7% 18% 28% 12%	2600
	21st to end	Night	38% 3 7% 20% 24% 8%	2600
December	1st to 10th	Night	44% 4 7% 18% 22% 4	2600
	11th to 20th	Night	41% 4 6% 19% 25% 5%	3174
	21st to end	Night	48% 5% 7% 21% 17% 2	4578

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	29% 23% 32% 14% 3	270
	11th to 20th	Day	32% 19% 30% 13% 6%	270
	21st to end	Day	25% 23% 36% 13% 3	297
February	1st to 10th	Day	28% 21% 33% 13% 5%	270
	11th to 20th	Day	37% 20% 30% 11% 2 1	270
	21st to end	Day	36% 23% 32% 8% 1	225
March	1st to 10th	Day	26% 20% 34% 15% 3 1	270
	11th to 20th	Day	26% 17% 34% 17% 5% 1	358
	21st to end	Day	23% 18% 33% 19% 7%	506
April	1st to 10th	Day	23% 15% 27% 22% 11% 2	460
	11th to 20th	Day	17% 17% 30% 21% 12% 2	452
	21st to end	Day	15% 11% 32% 25% 14% 1	460
May	1st to 10th	Day	16% 8% 29% 25% 16% 5%	463
	11th to 20th	Day	12% 6% 27% 29% 17% 9%	487
	21st to end	Day	11% 6% 22% 29% 19% 14%	529
June	1st to 10th	Day	14% 4 19% 28% 24% 12%	490
	11th to 20th	Day	6% 3 17% 28% 26% 20%	490
	21st to end	Day	10% 3 15% 28% 27% 16%	544
July	1st to 10th	Day	8% 2 11% 28% 29% 23%	520
	11th to 20th	Day	8% 3 9% 21% 30% 29%	484
	21st to end	Day	10% 3 11% 19% 36% 22%	532
August	1st to 10th	Day	8% 2 12% 26% 30% 22%	486
	11th to 20th	Day	10% 2 13% 30% 32% 15%	479
	21st to end	Day	14% 3 9% 23% 32% 19%	539
September	1st to 10th	Day	11% 3 12% 31% 34% 10%	490
	11th to 20th	Day	12% 6% 13% 22% 35% 13%	490
	21st to end	Day	14% 3 17% 27% 29% 10%	490
October	1st to 10th	Day	10% 6% 22% 32% 22% 9%	490
	11th to 20th	Day	13% 9% 27% 34% 16% 2	477
	21st to end	Day	18% 11% 24% 32% 13% 2	330
November	1st to 10th	Day	22% 8% 30% 26% 11% 3	300
	11th to 20th	Day	16% 16% 37% 22% 7% 1	300
	21st to end	Day	17% 19% 32% 22% 8% 2	300
December	1st to 10th	Day	31% 16% 33% 15% 5% 1	300
	11th to 20th	Day	26% 21% 31% 15% 7% 1	300
	21st to end	Day	26% 20% 32% 17% 4 1	306
January	1st to 10th	Night	24% 14% 35% 23% 4	760
	11th to 20th	Night	24% 16% 29% 24% 7%	760
	21st to end	Night	21% 17% 35% 24% 3	836
February	1st to 10th	Night	24% 12% 32% 28% 4	760
	11th to 20th	Night	26% 14% 30% 26% 3	743
	21st to end	Night	27% 17% 31% 24% 2	628
March	1st to 10th	Night	20% 15% 32% 28% 6%	760
	11th to 20th	Night	18% 14% 33% 26% 8% 1	760
	21st to end	Night	18% 13% 32% 27% 10%	713
April	1st to 10th	Night	13% 14% 32% 27% 13%	570
	11th to 20th	Night	17% 9% 32% 27% 13% 2	563
	21st to end	Night	15% 9% 27% 33% 15% 2	570
May	1st to 10th	Night	11% 6% 25% 33% 23% 2	570
	11th to 20th	Night	11% 5% 22% 31% 27% 4	561
	21st to end	Night	8% 2 12% 37% 35% 6%	617
June	1st to 10th	Night	7% 3 12% 37% 35% 7%	570
	11th to 20th	Night	5% 1 10% 31% 39% 14%	570
	21st to end	Night	6% 2 9% 29% 45% 9%	570
July	1st to 10th	Night	7% 1 7% 28% 43% 14%	555
	11th to 20th	Night	6% 1 6% 25% 44% 18%	552
	21st to end	Night	7% 1 8% 25% 45% 14%	624
August	1st to 10th	Night	4 4 28% 50% 14%	567
	11th to 20th	Night	9% 1 8% 31% 43% 9%	564
	21st to end	Night	11% 3 6% 30% 41% 10%	627
September	1st to 10th	Night	9% 1 12% 30% 41% 8%	579
	11th to 20th	Night	13% 4 11% 30% 36% 6%	760
	21st to end	Night	10% 2 15% 34% 35% 4	760
October	1st to 10th	Night	10% 4 21% 30% 29% 6%	760
	11th to 20th	Night	12% 7% 24% 34% 21% 2	760
	21st to end	Night	17% 8% 18% 36% 19% 2	836
November	1st to 10th	Night	14% 7% 21% 35% 20% 3	780
	11th to 20th	Night	15% 10% 30% 32% 12% 1	800
	21st to end	Night	19% 9% 29% 30% 11% 1	800
December	1st to 10th	Night	23% 13% 26% 30% 8%	800
	11th to 20th	Night	24% 12% 28% 28% 9%	800
	21st to end	Night	27% 10% 30% 26% 8%	851

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