Bert and Jeanette Lynch Memorial Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Bert and Jeanette Lynch Memorial Observatory clear sky chart from 2005-11-06 to 2026-05-17.

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	11% 7% 3 6% 4 4 4 4 6% 7% 43%	131855
	Night	12% 8% 3 7% 4 4 4 3 5% 6% 44%	131207
Transparency	Day	39% 4 11% 26% 17% 3	135451
	Night	40% 4 11% 24% 18% 4	135689
Seeing	Day	18% 13% 29% 24% 12% 4	17569
	Night	18% 14% 30% 25% 12% 1	27469

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	12% 10% 4 12% 4 3 4 3 4 5% 39%	4691
	11th to 20th	Day	8% 8% 3 7% 4 4 4 3 5% 6% 49%	5292
	21st to end	Day	16% 8% 4 8% 4 3 3 3 5% 5% 41%	5812
February	1st to 10th	Day	15% 10% 4 8% 5% 4 4 3 5% 6% 37%	1890
	11th to 20th	Day	14% 9% 3 7% 4 4 4 4 6% 6% 39%	2038
	21st to end	Day	12% 11% 2 6% 5% 4 5% 4 5% 6% 39%	1730
March	1st to 10th	Day	14% 13% 3 5% 3 3 3 2 3 4 45%	4716
	11th to 20th	Day	9% 7% 2 4 3 3 3 4 5% 6% 53%	5090
	21st to end	Day	11% 9% 3 7% 4 5% 5% 4 6% 8% 39%	5854
April	1st to 10th	Day	11% 6% 3 6% 4 5% 5% 3 6% 7% 45%	2520
	11th to 20th	Day	15% 9% 3 6% 5% 3 4 3 4 6% 40%	2494
	21st to end	Day	17% 8% 3 6% 5% 4 5% 4 5% 7% 37%	2520
May	1st to 10th	Day	15% 8% 3 8% 6% 5% 5% 5% 7% 9% 30%	5525
	11th to 20th	Day	15% 7% 3 7% 5% 5% 6% 4 6% 8% 34%	6274
	21st to end	Day	15% 8% 3 7% 5% 4 4 3 5% 6% 40%	6935
June	1st to 10th	Day	9% 6% 2 6% 5% 5% 5% 4 7% 8% 44%	2800
	11th to 20th	Day	6% 4 2 5% 5% 5% 5% 5% 7% 9% 47%	2800
	21st to end	Day	5% 4 1 5% 5% 5% 6% 5% 8% 10% 47%	2800
July	1st to 10th	Day	10% 4 2 6% 4 5% 5% 4 6% 9% 45%	6360
	11th to 20th	Day	5% 4 2 6% 5% 5% 6% 5% 8% 12% 42%	6323
	21st to end	Day	7% 5% 2 6% 5% 6% 6% 5% 8% 10% 39%	6422
August	1st to 10th	Day	5% 3 1 4 4 4 5% 4 6% 10% 52%	4185
	11th to 20th	Day	10% 5% 2 6% 4 5% 5% 4 6% 8% 45%	3849
	21st to end	Day	7% 5% 2 5% 3 4 4 4 6% 9% 52%	4312
September	1st to 10th	Day	12% 5% 2 5% 4 3 4 4 6% 8% 48%	2400
	11th to 20th	Day	10% 5% 2 6% 4 4 4 4 6% 9% 47%	2385
	21st to end	Day	11% 5% 2 4 4 4 4 4 4 7% 52%	2180
October	1st to 10th	Day	8% 4 1 5% 3 4 4 3 4 6% 57%	2000
	11th to 20th	Day	12% 7% 2 5% 3 3 4 2 4 6% 54%	2000
	21st to end	Day	18% 8% 3 5% 3 3 3 2 4 5% 47%	2200
November	1st to 10th	Day	11% 6% 2 5% 4 4 3 3 4 5% 53%	1877
	11th to 20th	Day	11% 8% 3 6% 4 3 4 3 4 7% 46%	1890
	21st to end	Day	14% 11% 3 5% 4 4 3 2 4 5% 43%	1843
December	1st to 10th	Day	9% 10% 2 5% 3 3 3 3 4 6% 50%	2690
	11th to 20th	Day	12% 11% 4 7% 5% 3 4 2 5% 6% 41%	3088
	21st to end	Day	17% 17% 6% 10% 5% 4 4 3 4 5% 26%	4070
January	1st to 10th	Night	13% 10% 5% 11% 5% 4 4 3 4 4 37%	8232
	11th to 20th	Night	9% 8% 3 10% 5% 4 4 3 4 5% 45%	7680
	21st to end	Night	15% 10% 4 8% 6% 4 3 3 4 5% 38%	8402
February	1st to 10th	Night	16% 11% 3 9% 4 4 3 3 5% 6% 37%	2730
	11th to 20th	Night	15% 8% 4 8% 4 4 4 3 4 5% 42%	2467
	21st to end	Night	14% 8% 3 7% 5% 4 4 3 4 5% 42%	2076
March	1st to 10th	Night	16% 12% 3 6% 4 4 4 3 4 5% 40%	5399
	11th to 20th	Night	11% 10% 2 5% 4 4 4 3 5% 5% 48%	5090
	21st to end	Night	13% 8% 3 7% 5% 4 5% 3 5% 7% 38%	5316
April	1st to 10th	Night	10% 7% 2 5% 3 3 4 4 6% 8% 48%	2100
	11th to 20th	Night	13% 8% 2 6% 4 4 4 4 4 6% 45%	2075
	21st to end	Night	17% 7% 3 7% 4 4 3 2 5% 5% 41%	2100
May	1st to 10th	Night	17% 10% 2 7% 5% 4 4 3 5% 8% 34%	4590
	11th to 20th	Night	13% 8% 2 6% 4 3 4 4 5% 8% 41%	3748
	21st to end	Night	17% 8% 2 7% 4 4 4 3 6% 7% 37%	3942
June	1st to 10th	Night	10% 5% 1 5% 4 4 4 5% 7% 8% 47%	1600
	11th to 20th	Night	7% 4 1 4 3 3 4 4 6% 10% 55%	1600
	21st to end	Night	2 3 1 3 4 5% 4 4 7% 10% 57%	1600
July	1st to 10th	Night	10% 5% 2 5% 3 4 3 3 6% 9% 49%	3618
	11th to 20th	Night	6% 4 2 6% 4 4 4 3 6% 9% 52%	3650
	21st to end	Night	5% 5% 2 7% 5% 4 5% 4 6% 11% 46%	4449
August	1st to 10th	Night	5% 3 1 4 3 4 4 4 6% 9% 58%	2957
	11th to 20th	Night	8% 5% 3 6% 4 4 4 3 6% 8% 49%	3200
	21st to end	Night	5% 4 2 4 3 3 4 4 5% 8% 58%	3586
September	1st to 10th	Night	11% 6% 2 5% 3 3 4 3 5% 6% 52%	2000
	11th to 20th	Night	8% 5% 2 6% 3 4 4 3 6% 7% 53%	2000
	21st to end	Night	10% 4 2 4 4 4 4 3 5% 7% 53%	2210
October	1st to 10th	Night	6% 4 1 4 3 3 2 3 4 5% 64%	2400
	11th to 20th	Night	12% 5% 2 4 3 2 3 3 4 6% 56%	2400
	21st to end	Night	18% 7% 2 5% 2 2 3 3 4 5% 48%	2640
November	1st to 10th	Night	9% 6% 2 5% 3 3 2 2 4 6% 57%	2639
	11th to 20th	Night	12% 8% 2 5% 3 3 4 3 5% 6% 50%	2730
	21st to end	Night	16% 11% 3 5% 5% 2 3 2 3 5% 44%	2772
December	1st to 10th	Night	12% 9% 3 8% 4 3 3 2 4 5% 46%	4720
	11th to 20th	Night	11% 10% 3 7% 5% 3 4 3 4 6% 43%	5339
	21st to end	Night	18% 15% 5% 9% 5% 5% 4 3 5% 5% 26%	7150

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	48% 3 10% 25% 11% 2	4692
	11th to 20th	Day	37% 2 7% 29% 20% 6%	5292
	21st to end	Day	46% 2 9% 18% 16% 9%	5812
February	1st to 10th	Day	50% 2 11% 22% 13% 2	1890
	11th to 20th	Day	46% 4 11% 20% 15% 5%	2048
	21st to end	Day	44% 4 8% 25% 14% 6%	1730
March	1st to 10th	Day	45% 2 6% 21% 18% 9%	4716
	11th to 20th	Day	31% 2 8% 27% 24% 8%	5090
	21st to end	Day	43% 3 13% 29% 11% 1	5856
April	1st to 10th	Day	39% 4 12% 25% 19% 2	2520
	11th to 20th	Day	46% 2 11% 21% 16% 4	2494
	21st to end	Day	47% 3 11% 24% 14% 1	2520
May	1st to 10th	Day	50% 4 12% 21% 12% 1	5560
	11th to 20th	Day	47% 4 12% 23% 13% 1	6278
	21st to end	Day	45% 4 8% 24% 17% 2	6949
June	1st to 10th	Day	37% 4 13% 28% 16% 2	2800
	11th to 20th	Day	31% 5% 11% 28% 21% 3	2800
	21st to end	Day	30% 5% 18% 26% 16% 5%	2800
July	1st to 10th	Day	36% 4 11% 28% 19% 2	6291
	11th to 20th	Day	33% 7% 16% 31% 12% 1	6248
	21st to end	Day	35% 7% 15% 30% 12% 1	6240
August	1st to 10th	Day	26% 5% 16% 34% 19%	4171
	11th to 20th	Day	37% 4 12% 27% 17% 2	3885
	21st to end	Day	29% 4 14% 36% 14% 3	4311
September	1st to 10th	Day	33% 4 12% 29% 18% 5%	2400
	11th to 20th	Day	34% 4 8% 27% 25% 2	2385
	21st to end	Day	32% 5% 6% 28% 25% 3	2180
October	1st to 10th	Day	18% 2 5% 20% 44% 11%	3260
	11th to 20th	Day	23% 2 6% 22% 37% 10%	3240
	21st to end	Day	31% 2 6% 30% 26% 5%	3538
November	1st to 10th	Day	34% 3 10% 26% 21% 6%	1877
	11th to 20th	Day	38% 3 8% 24% 20% 7%	1890
	21st to end	Day	45% 2 8% 25% 18% 2	1843
December	1st to 10th	Day	36% 3 7% 36% 15% 1	2690
	11th to 20th	Day	45% 3 7% 24% 19% 1	3085
	21st to end	Day	62% 3 11% 14% 9% 2	4070
January	1st to 10th	Night	51% 3 11% 22% 12% 1	8233
	11th to 20th	Night	42% 3 5% 24% 23% 3	7680
	21st to end	Night	49% 3 8% 20% 17% 3	8402
February	1st to 10th	Night	49% 3 9% 23% 14% 2	2730
	11th to 20th	Night	45% 3 11% 22% 17% 3	2468
	21st to end	Night	45% 2 7% 24% 17% 5%	2076
March	1st to 10th	Night	49% 2 7% 20% 17% 5%	5399
	11th to 20th	Night	39% 3 9% 22% 19% 8%	5090
	21st to end	Night	45% 4 14% 26% 11% 1	5316
April	1st to 10th	Night	33% 4 15% 27% 19% 3	2100
	11th to 20th	Night	40% 4 10% 24% 18% 4	2075
	21st to end	Night	46% 3 9% 25% 16% 2	2100
May	1st to 10th	Night	49% 5% 15% 19% 12% 1	4620
	11th to 20th	Night	42% 5% 13% 24% 14% 2	3749
	21st to end	Night	47% 4 10% 24% 13% 3	3950
June	1st to 10th	Night	33% 5% 15% 27% 19% 2	1600
	11th to 20th	Night	24% 4 13% 33% 22% 4	1600
	21st to end	Night	21% 5% 17% 31% 19% 7%	1600
July	1st to 10th	Night	32% 5% 12% 27% 21% 2	3580
	11th to 20th	Night	29% 7% 15% 33% 15% 2	3605
	21st to end	Night	31% 7% 19% 28% 13% 1	4320
August	1st to 10th	Night	22% 5% 20% 31% 20% 1	2947
	11th to 20th	Night	33% 5% 17% 26% 17% 3	3230
	21st to end	Night	24% 5% 13% 37% 18% 3	3586
September	1st to 10th	Night	33% 4 9% 28% 20% 6%	2000
	11th to 20th	Night	30% 4 10% 28% 26% 2	2000
	21st to end	Night	32% 4 8% 24% 28% 4	2210
October	1st to 10th	Night	14% 1 7% 17% 48% 13%	3920
	11th to 20th	Night	23% 2 6% 22% 33% 14%	3899
	21st to end	Night	31% 3 7% 27% 26% 7%	4252
November	1st to 10th	Night	31% 3 10% 28% 23% 5%	2639
	11th to 20th	Night	36% 3 7% 25% 23% 6%	2730
	21st to end	Night	44% 3 7% 25% 20% 2	2772
December	1st to 10th	Night	41% 3 8% 29% 17% 1	4720
	11th to 20th	Night	43% 3 11% 22% 20% 1	5341
	21st to end	Night	60% 3 10% 17% 7% 1	7150

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% 22% 35% 11% 2	330
	11th to 20th	Day	30% 29% 34% 7%	330
	21st to end	Day	24% 29% 38% 8% 1	363
February	1st to 10th	Day	27% 25% 35% 12% 2	330
	11th to 20th	Day	35% 27% 30% 7% 1 1	330
	21st to end	Day	32% 23% 37% 7% 1	273
March	1st to 10th	Day	25% 25% 40% 9% 2	330
	11th to 20th	Day	32% 22% 37% 8% 2	330
	21st to end	Day	29% 24% 35% 10% 2	479
April	1st to 10th	Day	21% 23% 37% 15% 4	540
	11th to 20th	Day	28% 22% 29% 16% 4 1	532
	21st to end	Day	23% 18% 37% 17% 5% 1	540
May	1st to 10th	Day	21% 18% 38% 17% 5% 1	543
	11th to 20th	Day	20% 13% 32% 23% 9% 3	696
	21st to end	Day	14% 11% 34% 25% 10% 5%	789
June	1st to 10th	Day	14% 7% 26% 27% 21% 5%	730
	11th to 20th	Day	8% 5% 27% 38% 19% 4	730
	21st to end	Day	6% 6% 33% 34% 17% 4	730
July	1st to 10th	Day	11% 3 19% 31% 26% 10%	721
	11th to 20th	Day	6% 3 22% 40% 24% 5%	718
	21st to end	Day	9% 3 19% 37% 26% 6%	794
August	1st to 10th	Day	10% 2 20% 38% 24% 7%	703
	11th to 20th	Day	13% 3 22% 29% 25% 7%	519
	21st to end	Day	11% 3 16% 32% 28% 10%	583
September	1st to 10th	Day	13% 4 18% 37% 21% 6%	530
	11th to 20th	Day	10% 4 18% 33% 24% 11%	530
	21st to end	Day	15% 7% 23% 29% 18% 6%	510
October	1st to 10th	Day	11% 8% 34% 32% 11% 3	330
	11th to 20th	Day	22% 12% 38% 22% 4 1	330
	21st to end	Day	32% 20% 32% 14% 2	363
November	1st to 10th	Day	19% 15% 42% 18% 4 2	330
	11th to 20th	Day	23% 21% 35% 16% 5% 1	330
	21st to end	Day	30% 20% 30% 16% 4 1	330
December	1st to 10th	Day	24% 24% 35% 12% 4 2	330
	11th to 20th	Day	28% 22% 34% 12% 3 1	330
	21st to end	Day	31% 25% 34% 9% 1	363
January	1st to 10th	Night	22% 24% 35% 17% 2	1050
	11th to 20th	Night	24% 29% 31% 13% 3	1050
	21st to end	Night	20% 26% 37% 16% 1	1155
February	1st to 10th	Night	25% 21% 37% 16% 1	1050
	11th to 20th	Night	23% 22% 39% 14% 2	832
	21st to end	Night	25% 22% 35% 16% 1	692
March	1st to 10th	Night	27% 19% 36% 16% 2	798
	11th to 20th	Night	23% 19% 37% 19% 3	630
	21st to end	Night	27% 16% 35% 18% 4	693
April	1st to 10th	Night	16% 20% 39% 22% 3	630
	11th to 20th	Night	22% 17% 31% 24% 6%	623
	21st to end	Night	22% 13% 37% 22% 6%	630
May	1st to 10th	Night	20% 10% 36% 26% 8%	630
	11th to 20th	Night	19% 8% 33% 26% 13% 1	609
	21st to end	Night	13% 8% 29% 32% 16% 3	650
June	1st to 10th	Night	12% 5% 21% 37% 23% 2	600
	11th to 20th	Night	9% 2 23% 45% 20% 1	600
	21st to end	Night	4 3 28% 41% 23% 2	600
July	1st to 10th	Night	9% 2 11% 37% 33% 8%	585
	11th to 20th	Night	6% 2 15% 46% 27% 4	582
	21st to end	Night	8% 2 13% 43% 31% 3	657
August	1st to 10th	Night	8% 1 15% 39% 33% 4	597
	11th to 20th	Night	7% 3 15% 39% 31% 4	594
	21st to end	Night	8% 1 14% 37% 33% 7%	660
September	1st to 10th	Night	13% 2 16% 35% 30% 5%	600
	11th to 20th	Night	10% 3 15% 36% 30% 7%	600
	21st to end	Night	12% 6% 25% 27% 25% 4	795
October	1st to 10th	Night	8% 6% 27% 34% 21% 4	800
	11th to 20th	Night	17% 13% 31% 26% 12% 1	800
	21st to end	Night	25% 16% 33% 20% 6%	880
November	1st to 10th	Night	13% 14% 40% 25% 8% 1	820
	11th to 20th	Night	21% 16% 32% 24% 6%	840
	21st to end	Night	25% 20% 33% 17% 4	882
December	1st to 10th	Night	24% 17% 37% 18% 4	1050
	11th to 20th	Night	24% 16% 37% 19% 3	1050
	21st to end	Night	26% 21% 35% 15% 2	1155

Page generated at 2026-05-14 00:58:54. Version ac07

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