Larry's Backyard Observing Site
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Larry's Backyard Observing Site clear sky chart from 2005-11-06 to 2026-01-02.

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	9% 6% 2 6% 4 4 4 3 5% 7% 51%	127353
	Night	9% 5% 2 5% 3 3 4 3 5% 7% 53%	124311
Transparency	Day	33% 4 11% 27% 21% 4	131035
	Night	31% 4 11% 27% 23% 5%	128694
Seeing	Day	12% 9% 31% 30% 16% 3	17230
	Night	11% 8% 34% 36% 10% 1	27175

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	10% 9% 3 8% 4 5% 4 5% 6% 8% 39%	4086
	11th to 20th	Day	6% 10% 4 9% 6% 4 4 3 4 6% 44%	4080
	21st to end	Day	6% 11% 5% 9% 4 3 4 2 4 5% 47%	4488
February	1st to 10th	Day	8% 11% 3 7% 4 3 5% 4 5% 7% 44%	1800
	11th to 20th	Day	6% 7% 3 6% 6% 5% 5% 4 6% 7% 45%	1941
	21st to end	Day	5% 6% 2 6% 4 4 4 3 5% 7% 52%	1650
March	1st to 10th	Day	9% 7% 2 5% 3 3 3 2 3 5% 57%	4717
	11th to 20th	Day	8% 6% 2 4 3 3 4 3 4 5% 58%	4980
	21st to end	Day	11% 7% 3 6% 4 4 4 4 5% 7% 46%	5462
April	1st to 10th	Day	11% 6% 2 5% 4 3 4 3 4 7% 50%	2210
	11th to 20th	Day	10% 7% 3 7% 5% 5% 4 3 5% 7% 44%	2520
	21st to end	Day	17% 7% 3 5% 4 4 4 4 4 6% 43%	2600
May	1st to 10th	Day	13% 6% 2 5% 4 4 4 3 5% 6% 48%	5862
	11th to 20th	Day	11% 5% 2 5% 3 3 3 3 4 6% 54%	5862
	21st to end	Day	17% 6% 3 6% 5% 4 4 4 6% 7% 40%	6434
June	1st to 10th	Day	5% 4 2 4 3 4 3 3 6% 8% 57%	2600
	11th to 20th	Day	6% 4 1 4 4 4 4 4 6% 9% 53%	2600
	21st to end	Day	5% 4 2 4 4 4 4 4 6% 7% 57%	2600
July	1st to 10th	Day	7% 4 2 4 3 4 4 4 7% 9% 52%	5900
	11th to 20th	Day	7% 4 2 6% 4 5% 5% 4 7% 9% 48%	5900
	21st to end	Day	6% 4 2 5% 4 4 4 4 6% 8% 53%	6422
August	1st to 10th	Day	8% 5% 2 6% 4 4 4 3 5% 7% 52%	4234
	11th to 20th	Day	11% 4 2 6% 4 4 4 4 6% 8% 49%	4171
	21st to end	Day	4 4 2 5% 3 3 5% 3 6% 7% 59%	4581
September	1st to 10th	Day	12% 5% 1 5% 3 3 3 2 4 6% 56%	2215
	11th to 20th	Day	9% 5% 2 4 3 3 4 3 6% 7% 55%	2200
	21st to end	Day	10% 3 2 5% 3 4 4 3 4 6% 57%	2200
October	1st to 10th	Day	14% 4 2 4 2 3 3 2 5% 5% 56%	2145
	11th to 20th	Day	9% 5% 2 3 3 3 3 3 4 5% 62%	2000
	21st to end	Day	11% 6% 2 4 3 3 3 2 5% 4 56%	2162
November	1st to 10th	Day	7% 4 2 3 3 2 3 2 5% 7% 60%	1853
	11th to 20th	Day	8% 6% 2 5% 4 3 2 3 4 6% 59%	1890
	21st to end	Day	11% 8% 2 4 3 3 3 2 4 6% 55%	1890
December	1st to 10th	Day	5% 4 1 5% 3 4 4 3 5% 6% 60%	3030
	11th to 20th	Day	6% 6% 3 6% 4 3 3 2 5% 6% 57%	3481
	21st to end	Day	14% 10% 4 10% 4 4 4 3 5% 6% 37%	4587
January	1st to 10th	Night	9% 9% 4 11% 5% 4 4 3 5% 6% 40%	5902
	11th to 20th	Night	5% 6% 3 7% 4 5% 4 3 4 6% 53%	5900
	21st to end	Night	10% 8% 4 9% 4 4 3 3 4 5% 48%	6490
February	1st to 10th	Night	9% 9% 3 6% 4 5% 4 4 5% 5% 45%	2600
	11th to 20th	Night	8% 6% 2 6% 4 5% 6% 4 6% 6% 47%	2363
	21st to end	Night	7% 6% 2 5% 5% 3 3 4 4 5% 56%	1980
March	1st to 10th	Night	8% 4 1 4 2 2 3 3 4 6% 63%	5145
	11th to 20th	Night	7% 4 1 4 2 3 3 3 6% 9% 58%	4980
	21st to end	Night	11% 6% 2 5% 4 4 5% 3 6% 7% 46%	5467
April	1st to 10th	Night	9% 5% 2 5% 4 4 3 4 5% 7% 51%	2186
	11th to 20th	Night	12% 6% 1 6% 3 3 3 3 5% 6% 50%	1829
	21st to end	Night	13% 7% 2 6% 4 3 3 3 6% 6% 46%	1800
May	1st to 10th	Night	17% 6% 2 4 4 2 4 3 4 6% 49%	4053
	11th to 20th	Night	11% 4 2 4 3 3 3 3 5% 7% 55%	4053
	21st to end	Night	17% 5% 2 5% 3 3 4 3 5% 7% 45%	4443
June	1st to 10th	Night	6% 3 1 3 3 3 4 3 7% 9% 60%	1800
	11th to 20th	Night	5% 4 2 5% 3 3 3 4 7% 10% 55%	1800
	21st to end	Night	3 4 1 4 3 3 4 5% 7% 11% 55%	1800
July	1st to 10th	Night	6% 4 2 5% 4 3 4 3 6% 9% 53%	4078
	11th to 20th	Night	6% 4 2 6% 4 4 5% 4 7% 11% 49%	4077
	21st to end	Night	6% 3 2 5% 3 3 4 3 6% 8% 55%	4449
August	1st to 10th	Night	3 3 1 5% 4 4 4 4 7% 11% 54%	2926
	11th to 20th	Night	8% 5% 2 5% 4 4 5% 4 6% 8% 50%	2888
	21st to end	Night	5% 3 1 3 3 3 3 3 6% 8% 61%	3323
September	1st to 10th	Night	8% 3 1 4 3 3 3 3 5% 5% 63%	2185
	11th to 20th	Night	8% 4 2 4 3 4 4 3 4 7% 58%	2200
	21st to end	Night	6% 4 1 4 3 3 3 3 5% 5% 61%	2200
October	1st to 10th	Night	11% 4 1 4 2 3 4 3 4 6% 57%	2235
	11th to 20th	Night	9% 5% 2 4 2 3 3 2 4 5% 61%	2400
	21st to end	Night	11% 5% 2 4 3 3 3 2 3 5% 59%	2675
November	1st to 10th	Night	8% 3 1 4 2 2 3 3 4 6% 63%	2665
	11th to 20th	Night	7% 5% 1 4 3 3 3 4 4 6% 60%	2730
	21st to end	Night	11% 6% 2 4 3 3 3 3 5% 5% 55%	2730
December	1st to 10th	Night	4 5% 1 3 3 3 3 3 4 6% 65%	4380
	11th to 20th	Night	7% 5% 2 5% 4 3 4 3 6% 7% 55%	4946
	21st to end	Night	13% 10% 3 8% 5% 4 4 3 4 5% 40%	6633

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	42% 6% 11% 25% 15% 2	4087
	11th to 20th	Day	42% 4 10% 22% 20% 2	4080
	21st to end	Day	41% 3 11% 24% 18% 2	4488
February	1st to 10th	Day	40% 5% 16% 23% 11% 4	1800
	11th to 20th	Day	37% 6% 15% 25% 14% 4	1951
	21st to end	Day	32% 4 10% 26% 23% 6%	1650
March	1st to 10th	Day	32% 3 7% 21% 27% 11%	4717
	11th to 20th	Day	29% 3 7% 23% 29% 8%	4980
	21st to end	Day	38% 4 17% 32% 8% 1	5464
April	1st to 10th	Day	34% 5% 12% 24% 21% 4	2210
	11th to 20th	Day	40% 5% 10% 26% 16% 3	2520
	21st to end	Day	42% 5% 12% 25% 15% 1	2600
May	1st to 10th	Day	37% 4 10% 28% 17% 5%	5900
	11th to 20th	Day	32% 4 8% 23% 27% 6%	5866
	21st to end	Day	43% 5% 9% 27% 14% 2	6447
June	1st to 10th	Day	24% 7% 16% 34% 15% 4	2600
	11th to 20th	Day	27% 7% 15% 28% 21% 3	2600
	21st to end	Day	25% 6% 12% 29% 23% 5%	2600
July	1st to 10th	Day	30% 3 13% 28% 23% 4	5836
	11th to 20th	Day	33% 3 15% 26% 20% 3	5831
	21st to end	Day	29% 4 12% 28% 23% 4	6240
August	1st to 10th	Day	32% 5% 13% 26% 21% 4	4220
	11th to 20th	Day	35% 4 15% 26% 18% 2	4210
	21st to end	Day	26% 3 15% 29% 21% 7%	4580
September	1st to 10th	Day	31% 3 10% 25% 22% 8%	2215
	11th to 20th	Day	29% 4 11% 28% 25% 4	2200
	21st to end	Day	29% 4 9% 28% 26% 4	2200
October	1st to 10th	Day	22% 2 4 19% 45% 7%	3496
	11th to 20th	Day	19% 2 9% 33% 28% 9%	3240
	21st to end	Day	28% 2 9% 26% 31% 4	3478
November	1st to 10th	Day	25% 4 11% 31% 26% 4	1853
	11th to 20th	Day	28% 3 9% 27% 29% 3	1890
	21st to end	Day	33% 3 11% 29% 21% 3	1890
December	1st to 10th	Day	26% 3 11% 38% 21% 2	3030
	11th to 20th	Day	29% 3 14% 39% 14% 1	3479
	21st to end	Day	48% 4 8% 28% 11% 1	4587
January	1st to 10th	Night	45% 4 10% 24% 15% 1	5902
	11th to 20th	Night	33% 4 11% 26% 23% 3	5900
	21st to end	Night	40% 3 10% 21% 18% 8%	6490
February	1st to 10th	Night	41% 5% 12% 23% 15% 5%	2600
	11th to 20th	Night	37% 6% 11% 27% 17% 3	2363
	21st to end	Night	30% 4 9% 24% 23% 9%	1980
March	1st to 10th	Night	24% 3 7% 27% 30% 9%	5145
	11th to 20th	Night	24% 4 7% 20% 34% 12%	4980
	21st to end	Night	36% 4 20% 29% 9% 1	5467
April	1st to 10th	Night	32% 5% 12% 24% 24% 4	2186
	11th to 20th	Night	34% 5% 14% 24% 20% 3	1829
	21st to end	Night	37% 6% 12% 25% 17% 3	1800
May	1st to 10th	Night	37% 4 11% 24% 19% 5%	4080
	11th to 20th	Night	28% 5% 11% 23% 23% 9%	4054
	21st to end	Night	37% 6% 14% 25% 16% 1	4452
June	1st to 10th	Night	21% 7% 17% 31% 19% 4	1800
	11th to 20th	Night	23% 6% 14% 33% 19% 5%	1800
	21st to end	Night	22% 6% 15% 29% 23% 5%	1800
July	1st to 10th	Night	28% 4 14% 26% 23% 5%	4035
	11th to 20th	Night	31% 3 14% 26% 23% 3	4026
	21st to end	Night	27% 3 13% 30% 23% 4	4320
August	1st to 10th	Night	24% 4 16% 28% 21% 7%	2916
	11th to 20th	Night	33% 4 16% 26% 19% 2	2915
	21st to end	Night	22% 3 17% 29% 21% 8%	3323
September	1st to 10th	Night	24% 4 13% 26% 25% 9%	2185
	11th to 20th	Night	27% 4 10% 26% 27% 6%	2200
	21st to end	Night	25% 4 11% 23% 32% 6%	2200
October	1st to 10th	Night	20% 2 5% 18% 49% 7%	3651
	11th to 20th	Night	19% 2 7% 29% 30% 12%	3899
	21st to end	Night	24% 2 7% 28% 34% 5%	4311
November	1st to 10th	Night	23% 3 8% 31% 27% 8%	2665
	11th to 20th	Night	25% 4 9% 30% 27% 5%	2730
	21st to end	Night	31% 3 10% 28% 23% 4	2730
December	1st to 10th	Night	21% 3 14% 38% 22% 2	4380
	11th to 20th	Night	29% 3 11% 34% 20% 3	4947
	21st to end	Night	46% 3 9% 28% 13% 1	6633

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	12% 13% 36% 29% 9% 2	303
	11th to 20th	Day	18% 16% 35% 26% 6%	300
	21st to end	Day	11% 9% 50% 23% 8%	330
February	1st to 10th	Day	14% 18% 42% 22% 4	300
	11th to 20th	Day	12% 14% 44% 23% 5% 1	300
	21st to end	Day	10% 16% 53% 17% 4	249
March	1st to 10th	Day	14% 14% 44% 21% 6% 1	300
	11th to 20th	Day	20% 13% 38% 21% 7% 2	300
	21st to end	Day	18% 18% 46% 13% 5%	330
April	1st to 10th	Day	16% 17% 41% 19% 7%	310
	11th to 20th	Day	14% 15% 44% 23% 4	638
	21st to end	Day	21% 16% 35% 18% 7% 2	700
May	1st to 10th	Day	15% 11% 34% 28% 11% 1	703
	11th to 20th	Day	17% 10% 28% 27% 15% 3	725
	21st to end	Day	15% 8% 31% 30% 13% 3	789
June	1st to 10th	Day	5% 4 26% 34% 23% 7%	730
	11th to 20th	Day	7% 4 32% 36% 19% 3	730
	21st to end	Day	7% 5% 35% 35% 16% 1	730
July	1st to 10th	Day	7% 2 20% 35% 29% 6%	721
	11th to 20th	Day	5% 3 18% 42% 26% 6%	718
	21st to end	Day	8% 3 14% 39% 31% 5%	794
August	1st to 10th	Day	7% 3 17% 43% 27% 3	726
	11th to 20th	Day	7% 2 18% 42% 27% 4	715
	21st to end	Day	10% 1 19% 37% 27% 6%	748
September	1st to 10th	Day	13% 2 17% 42% 22% 3	345
	11th to 20th	Day	7% 3 28% 38% 19% 5%	330
	21st to end	Day	13% 5% 29% 27% 19% 6%	330
October	1st to 10th	Day	18% 8% 27% 27% 16% 3	330
	11th to 20th	Day	8% 11% 44% 24% 12% 1	330
	21st to end	Day	17% 14% 36% 23% 9% 2	363
November	1st to 10th	Day	12% 15% 40% 23% 9% 1	330
	11th to 20th	Day	11% 15% 36% 26% 11% 2	330
	21st to end	Day	20% 12% 34% 23% 9% 3	330
December	1st to 10th	Day	9% 9% 42% 27% 12% 1	330
	11th to 20th	Day	10% 13% 41% 28% 7% 1	330
	21st to end	Day	19% 12% 40% 23% 6% 1	363
January	1st to 10th	Night	11% 10% 34% 37% 6%	1006
	11th to 20th	Night	11% 11% 38% 34% 6% 1	1000
	21st to end	Night	10% 9% 42% 32% 6%	1100
February	1st to 10th	Night	14% 9% 41% 32% 4	1000
	11th to 20th	Night	11% 13% 42% 30% 5%	809
	21st to end	Night	11% 13% 42% 29% 4 1	660
March	1st to 10th	Night	12% 14% 41% 29% 4	701
	11th to 20th	Night	16% 10% 35% 29% 8% 1	600
	21st to end	Night	19% 11% 42% 22% 5%	660
April	1st to 10th	Night	12% 14% 44% 26% 5%	600
	11th to 20th	Night	16% 13% 39% 28% 5%	593
	21st to end	Night	18% 11% 37% 26% 8% 1	600
May	1st to 10th	Night	19% 9% 35% 30% 8%	600
	11th to 20th	Night	14% 7% 33% 34% 10% 1	591
	21st to end	Night	13% 9% 29% 36% 11% 2	650
June	1st to 10th	Night	7% 5% 30% 40% 17% 1	600
	11th to 20th	Night	5% 3 37% 41% 13% 1	600
	21st to end	Night	5% 4 40% 40% 11%	600
July	1st to 10th	Night	7% 1 24% 43% 23% 1	585
	11th to 20th	Night	4 1 23% 48% 22% 1	582
	21st to end	Night	6% 2 16% 49% 26% 1	657
August	1st to 10th	Night	5% 1 22% 51% 20%	597
	11th to 20th	Night	7% 2 19% 54% 19%	594
	21st to end	Night	8% 1 22% 46% 21% 3	660
September	1st to 10th	Night	10% 3 24% 46% 16% 2	600
	11th to 20th	Night	10% 3 32% 39% 15% 2	600
	21st to end	Night	8% 4 32% 42% 15% 1	600
October	1st to 10th	Night	14% 7% 28% 34% 16% 1	635
	11th to 20th	Night	13% 9% 36% 35% 7%	800
	21st to end	Night	15% 10% 34% 31% 10% 1	915
November	1st to 10th	Night	9% 9% 34% 38% 10%	1025
	11th to 20th	Night	11% 9% 40% 32% 8%	1050
	21st to end	Night	15% 9% 32% 34% 9% 1	1050
December	1st to 10th	Night	10% 8% 38% 37% 6%	1050
	11th to 20th	Night	13% 10% 37% 34% 6%	1050
	21st to end	Night	16% 10% 35% 32% 6%	1155

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