Fred's Backyard Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Fred's Backyard Observatory clear sky chart from 2005-11-06 to 2024-01-15.

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	15% 8% 4 7% 6% 4 5% 5% 6% 7% 33%	74670
	Night	16% 9% 4 7% 6% 3 5% 5% 5% 7% 35%	75021
Transparency	Day	49% 4 12% 22% 12% 1	74386
	Night	50% 3 11% 22% 12% 1	74839
Seeing	Day	18% 9% 28% 26% 14% 6%	13314
	Night	19% 7% 28% 30% 13% 2	22863

By Month

Percentage Cloud Cover NE (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	16% 13% 4 9% 9% 3 6% 6% 6% 6% 22%	1517
	11th to 20th	Day	16% 14% 6% 11% 7% 4 5% 5% 6% 6% 20%	1619
	21st to end	Day	13% 14% 7% 11% 9% 4 6% 4 6% 5% 21%	1782
February	1st to 10th	Day	16% 13% 4 12% 8% 5% 6% 5% 5% 6% 21%	1620
	11th to 20th	Day	13% 12% 7% 10% 9% 4 4 5% 4 5% 27%	1573
	21st to end	Day	18% 13% 4 9% 6% 3 5% 5% 4 5% 27%	1386
March	1st to 10th	Day	14% 9% 4 9% 7% 4 6% 5% 6% 7% 30%	1977
	11th to 20th	Day	14% 11% 5% 8% 7% 3 6% 4 5% 5% 32%	1980
	21st to end	Day	18% 9% 4 7% 7% 3 5% 4 4 6% 32%	2178
April	1st to 10th	Day	15% 11% 5% 8% 6% 3 5% 4 6% 6% 30%	2119
	11th to 20th	Day	18% 9% 3 6% 6% 3 4 5% 5% 6% 36%	2205
	21st to end	Day	20% 8% 3 6% 6% 4 5% 4 5% 6% 33%	2340
May	1st to 10th	Day	15% 9% 3 7% 6% 4 5% 5% 6% 8% 34%	2340
	11th to 20th	Day	14% 6% 3 5% 5% 4 5% 5% 5% 7% 40%	2450
	21st to end	Day	12% 4 2 6% 5% 3 5% 6% 6% 8% 44%	2725
June	1st to 10th	Day	13% 6% 2 5% 6% 4 5% 5% 7% 8% 40%	2520
	11th to 20th	Day	12% 4 2 5% 6% 3 5% 6% 8% 9% 40%	2520
	21st to end	Day	12% 5% 2 5% 5% 4 6% 6% 7% 9% 41%	2520
July	1st to 10th	Day	7% 3 2 5% 6% 3 6% 7% 7% 9% 45%	2520
	11th to 20th	Day	10% 5% 3 4 5% 4 6% 6% 7% 9% 41%	2520
	21st to end	Day	9% 5% 2 6% 5% 4 7% 7% 8% 10% 37%	2559
August	1st to 10th	Day	8% 5% 2 5% 6% 4 5% 7% 7% 10% 40%	2327
	11th to 20th	Day	8% 5% 3 5% 7% 4 5% 5% 6% 10% 41%	2303
	21st to end	Day	10% 5% 3 5% 5% 4 5% 6% 6% 9% 42%	2481
September	1st to 10th	Day	13% 5% 3 5% 5% 3 6% 5% 6% 9% 41%	2160
	11th to 20th	Day	11% 5% 3 5% 5% 3 5% 4 6% 9% 45%	2048
	21st to end	Day	12% 5% 3 6% 6% 3 6% 5% 5% 9% 38%	1948
October	1st to 10th	Day	19% 8% 3 5% 5% 3 5% 5% 5% 7% 35%	1800
	11th to 20th	Day	20% 9% 3 7% 7% 4 5% 5% 5% 6% 29%	1800
	21st to end	Day	24% 10% 5% 6% 7% 3 4 6% 6% 6% 23%	1980
November	1st to 10th	Day	14% 8% 4 7% 6% 4 4 5% 6% 8% 34%	1615
	11th to 20th	Day	15% 10% 4 7% 7% 4 5% 7% 5% 9% 27%	1520
	21st to end	Day	18% 13% 5% 7% 6% 4 5% 5% 5% 8% 23%	1520
December	1st to 10th	Day	17% 14% 5% 11% 8% 5% 5% 5% 5% 6% 19%	1520
	11th to 20th	Day	20% 11% 6% 8% 8% 4 5% 5% 5% 5% 23%	1906
	21st to end	Day	30% 15% 5% 9% 7% 4 4 4 4 5% 14%	2772
January	1st to 10th	Night	18% 13% 5% 10% 6% 4 5% 5% 5% 5% 23%	2649
	11th to 20th	Night	20% 14% 6% 9% 7% 3 5% 5% 4 6% 20%	2427
	21st to end	Night	13% 12% 6% 9% 8% 4 6% 4 6% 6% 27%	2574
February	1st to 10th	Night	15% 13% 6% 9% 8% 4 6% 5% 5% 6% 24%	2340
	11th to 20th	Night	15% 13% 5% 7% 8% 3 6% 5% 5% 5% 27%	2273
	21st to end	Night	15% 10% 5% 8% 7% 4 6% 5% 6% 6% 29%	1867
March	1st to 10th	Night	13% 8% 4 8% 7% 4 6% 5% 5% 6% 35%	1970
	11th to 20th	Night	16% 10% 5% 8% 7% 3 3 4 4 5% 34%	1979
	21st to end	Night	16% 9% 3 7% 5% 4 4 5% 4 5% 40%	2178
April	1st to 10th	Night	18% 8% 4 6% 6% 4 5% 5% 6% 6% 33%	1839
	11th to 20th	Night	16% 8% 2 6% 6% 4 5% 4 4 6% 38%	1719
	21st to end	Night	17% 8% 4 6% 5% 3 4 5% 6% 7% 36%	1620
May	1st to 10th	Night	16% 6% 2 6% 5% 3 5% 5% 6% 7% 39%	1620
	11th to 20th	Night	15% 4 2 4 5% 3 4 5% 5% 8% 46%	1443
	21st to end	Night	10% 4 2 4 5% 2 5% 3 6% 8% 51%	1550
June	1st to 10th	Night	12% 4 2 5% 5% 4 6% 5% 6% 9% 42%	1440
	11th to 20th	Night	10% 6% 2 5% 6% 3 4 5% 6% 9% 44%	1440
	21st to end	Night	10% 5% 2 5% 6% 3 4 5% 6% 8% 46%	1364
July	1st to 10th	Night	5% 4 2 3 5% 3 4 5% 5% 8% 58%	1440
	11th to 20th	Night	10% 3 2 4 4 3 5% 4 7% 8% 50%	1440
	21st to end	Night	8% 4 2 4 4 3 4 5% 7% 9% 51%	1756
August	1st to 10th	Night	8% 4 1 4 6% 4 5% 5% 6% 8% 49%	1611
	11th to 20th	Night	7% 4 2 5% 4 2 4 5% 6% 9% 53%	1598
	21st to end	Night	10% 5% 2 5% 5% 3 5% 5% 6% 7% 48%	1856
September	1st to 10th	Night	10% 5% 2 5% 5% 3 4 5% 7% 8% 46%	1800
	11th to 20th	Night	11% 4 1 5% 4 2 4 4 5% 7% 54%	1910
	21st to end	Night	13% 7% 2 6% 6% 3 4 4 5% 8% 43%	2011
October	1st to 10th	Night	16% 6% 3 7% 5% 3 4 5% 5% 7% 39%	2160
	11th to 20th	Night	18% 9% 3 6% 6% 3 3 5% 5% 6% 35%	2160
	21st to end	Night	22% 8% 3 7% 6% 3 5% 4 5% 6% 29%	2376
November	1st to 10th	Night	17% 7% 3 6% 5% 3 5% 4 5% 7% 39%	2459
	11th to 20th	Night	16% 9% 4 7% 6% 3 6% 6% 6% 7% 31%	2660
	21st to end	Night	20% 10% 4 7% 6% 4 5% 5% 6% 7% 27%	2660
December	1st to 10th	Night	20% 13% 6% 7% 6% 3 5% 4 5% 5% 25%	2660
	11th to 20th	Night	16% 16% 5% 9% 6% 3 5% 5% 5% 6% 23%	3288
	21st to end	Night	31% 14% 6% 8% 6% 3 4 4 4 5% 14%	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	62% 3 12% 16% 7%	1518
	11th to 20th	Day	64% 3 11% 15% 6%	1619
	21st to end	Day	65% 3 8% 13% 9% 1	1782
February	1st to 10th	Day	64% 3 12% 17% 4 1	1620
	11th to 20th	Day	60% 3 10% 16% 9% 2	1582
	21st to end	Day	59% 3 9% 20% 8% 1	1386
March	1st to 10th	Day	53% 4 12% 20% 10% 2	1980
	11th to 20th	Day	54% 3 9% 20% 12% 2	1980
	21st to end	Day	53% 2 9% 20% 13% 2	2178
April	1st to 10th	Day	54% 3 11% 19% 13% 1	2119
	11th to 20th	Day	48% 2 10% 22% 16% 1	2205
	21st to end	Day	52% 3 10% 20% 12% 3	2340
May	1st to 10th	Day	48% 3 11% 22% 15% 1	2340
	11th to 20th	Day	44% 4 11% 26% 14% 2	2454
	21st to end	Day	37% 3 17% 26% 15% 2	2725
June	1st to 10th	Day	41% 5% 12% 25% 16% 2	2520
	11th to 20th	Day	38% 4 12% 27% 17% 1	2520
	21st to end	Day	38% 4 14% 28% 16% 1	2520
July	1st to 10th	Day	32% 5% 12% 29% 19% 3	2450
	11th to 20th	Day	37% 4 14% 30% 13% 1	2445
	21st to end	Day	39% 5% 15% 25% 15% 2	2416
August	1st to 10th	Day	37% 4 17% 27% 13% 2	2314
	11th to 20th	Day	38% 3 17% 28% 13% 1	2303
	21st to end	Day	37% 5% 13% 30% 14% 1	2481
September	1st to 10th	Day	40% 5% 14% 29% 12% 1	2160
	11th to 20th	Day	35% 3 14% 32% 15% 1	2048
	21st to end	Day	43% 4 15% 28% 10%	1948
October	1st to 10th	Day	48% 3 14% 22% 12% 1	1800
	11th to 20th	Day	55% 3 11% 23% 7% 1	1800
	21st to end	Day	60% 4 12% 17% 6% 1	1980
November	1st to 10th	Day	48% 4 13% 24% 9% 1	1615
	11th to 20th	Day	54% 4 15% 16% 9% 1	1520
	21st to end	Day	60% 3 14% 18% 4	1520
December	1st to 10th	Day	66% 3 12% 14% 5%	1520
	11th to 20th	Day	63% 3 14% 13% 7%	1906
	21st to end	Day	75% 2 9% 12% 3	2772
January	1st to 10th	Night	62% 3 10% 18% 7%	2649
	11th to 20th	Night	66% 2 11% 13% 6% 1	2427
	21st to end	Night	58% 3 11% 17% 9% 1	2574
February	1st to 10th	Night	60% 3 10% 19% 6% 1	2340
	11th to 20th	Night	59% 3 9% 18% 10% 1	2275
	21st to end	Night	56% 2 12% 21% 9% 1	1867
March	1st to 10th	Night	50% 3 12% 22% 11% 2	1980
	11th to 20th	Night	53% 2 9% 21% 12% 2	1980
	21st to end	Night	48% 2 7% 23% 16% 3	2178
April	1st to 10th	Night	52% 2 9% 20% 15% 1	1839
	11th to 20th	Night	48% 3 9% 21% 16% 2	1719
	21st to end	Night	48% 4 9% 21% 16% 3	1620
May	1st to 10th	Night	45% 3 9% 23% 18% 3	1620
	11th to 20th	Night	38% 4 10% 27% 17% 4	1444
	21st to end	Night	32% 3 14% 26% 20% 4	1551
June	1st to 10th	Night	38% 4 10% 26% 18% 2	1440
	11th to 20th	Night	37% 3 11% 25% 22% 3	1440
	21st to end	Night	37% 3 14% 26% 18% 1	1364
July	1st to 10th	Night	27% 3 11% 27% 28% 4	1400
	11th to 20th	Night	33% 3 11% 32% 19% 2	1392
	21st to end	Night	31% 3 15% 29% 19% 3	1658
August	1st to 10th	Night	34% 4 16% 31% 14% 2	1600
	11th to 20th	Night	29% 3 15% 37% 14% 2	1598
	21st to end	Night	36% 3 12% 29% 18% 1	1856
September	1st to 10th	Night	35% 4 15% 30% 15% 2	1800
	11th to 20th	Night	31% 3 10% 37% 18%	1910
	21st to end	Night	41% 3 13% 30% 12% 1	2011
October	1st to 10th	Night	44% 3 11% 25% 15% 1	2160
	11th to 20th	Night	50% 3 11% 26% 9%	2160
	21st to end	Night	56% 4 10% 18% 11% 1	2376
November	1st to 10th	Night	46% 3 10% 26% 13% 1	2459
	11th to 20th	Night	51% 3 14% 20% 10% 1	2660
	21st to end	Night	55% 4 16% 20% 4	2660
December	1st to 10th	Night	61% 3 11% 18% 6%	2660
	11th to 20th	Night	62% 3 13% 16% 5%	3288
	21st to end	Night	74% 3 11% 9% 3	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	19% 16% 31% 23% 9% 1	180
	11th to 20th	Day	24% 19% 35% 11% 9% 1	235
	21st to end	Day	24% 13% 36% 20% 6%	264
February	1st to 10th	Day	26% 14% 42% 14% 3	240
	11th to 20th	Day	21% 20% 41% 17% 1 1	240
	21st to end	Day	29% 22% 34% 15%	211
March	1st to 10th	Day	21% 12% 36% 23% 8%	420
	11th to 20th	Day	21% 16% 31% 22% 8% 1	420
	21st to end	Day	26% 9% 32% 23% 8% 1	462
April	1st to 10th	Day	19% 13% 33% 26% 8% 1	420
	11th to 20th	Day	25% 12% 30% 18% 10% 4	412
	21st to end	Day	23% 10% 32% 23% 10% 3	420
May	1st to 10th	Day	21% 8% 29% 23% 12% 6%	423
	11th to 20th	Day	17% 5% 29% 27% 15% 7%	447
	21st to end	Day	14% 5% 24% 31% 18% 8%	485
June	1st to 10th	Day	12% 8% 28% 33% 14% 5%	450
	11th to 20th	Day	16% 6% 24% 29% 22% 3	450
	21st to end	Day	14% 6% 24% 30% 20% 6%	450
July	1st to 10th	Day	9% 1 19% 35% 25% 11%	445
	11th to 20th	Day	13% 3 19% 32% 22% 10%	444
	21st to end	Day	9% 2 23% 38% 19% 8%	488
August	1st to 10th	Day	11% 3 20% 28% 26% 13%	446
	11th to 20th	Day	9% 4 19% 33% 21% 15%	439
	21st to end	Day	12% 4 22% 28% 22% 12%	495
September	1st to 10th	Day	14% 4 21% 31% 18% 12%	450
	11th to 20th	Day	10% 4 22% 25% 24% 16%	450
	21st to end	Day	14% 8% 29% 23% 17% 9%	450
October	1st to 10th	Day	22% 6% 26% 22% 15% 8%	450
	11th to 20th	Day	21% 10% 34% 23% 9% 2	450
	21st to end	Day	28% 11% 27% 25% 8% 1	495
November	1st to 10th	Day	19% 17% 26% 25% 11% 2	265
	11th to 20th	Day	24% 18% 30% 20% 7% 2	180
	21st to end	Day	23% 10% 42% 19% 6% 1	180
December	1st to 10th	Day	26% 13% 35% 21% 5% 1	180
	11th to 20th	Day	28% 19% 32% 17% 4	180
	21st to end	Day	26% 16% 37% 17% 4	198
January	1st to 10th	Night	24% 13% 34% 22% 6% 1	760
	11th to 20th	Night	29% 10% 30% 24% 5%	737
	21st to end	Night	19% 8% 37% 28% 7% 1	792
February	1st to 10th	Night	23% 13% 38% 22% 3	720
	11th to 20th	Night	23% 12% 36% 24% 5%	703
	21st to end	Night	20% 12% 41% 24% 3	592
March	1st to 10th	Night	18% 12% 36% 27% 7%	720
	11th to 20th	Night	22% 9% 36% 24% 8% 1	720
	21st to end	Night	20% 10% 29% 31% 9% 1	792
April	1st to 10th	Night	22% 9% 33% 30% 6%	720
	11th to 20th	Night	19% 9% 35% 25% 10% 2	655
	21st to end	Night	21% 5% 27% 35% 11%	540
May	1st to 10th	Night	19% 3 27% 32% 17% 2	540
	11th to 20th	Night	16% 3 23% 43% 14% 1	382
	21st to end	Night	11% 2 24% 37% 25% 1	389
June	1st to 10th	Night	13% 3 23% 42% 19% 1	360
	11th to 20th	Night	10% 4 27% 39% 19% 1	360
	21st to end	Night	11% 3 19% 44% 21% 1	360
July	1st to 10th	Night	7% 1 18% 46% 26% 3	350
	11th to 20th	Night	12% 1 20% 40% 24% 3	348
	21st to end	Night	9% 2 15% 46% 25% 3	576
August	1st to 10th	Night	9% 2 11% 44% 30% 4	537
	11th to 20th	Night	7% 3 12% 43% 31% 4	534
	21st to end	Night	11% 2 18% 37% 28% 4	668
September	1st to 10th	Night	12% 5% 19% 35% 23% 6%	720
	11th to 20th	Night	13% 3 16% 32% 29% 6%	720
	21st to end	Night	17% 4 20% 31% 23% 4	720
October	1st to 10th	Night	19% 3 25% 27% 22% 3	720
	11th to 20th	Night	22% 8% 28% 30% 10% 2	720
	21st to end	Night	27% 8% 28% 31% 6%	792
November	1st to 10th	Night	20% 9% 26% 32% 12% 1	740
	11th to 20th	Night	22% 9% 33% 25% 9% 2	760
	21st to end	Night	26% 8% 32% 27% 7%	760
December	1st to 10th	Night	28% 10% 34% 22% 6%	760
	11th to 20th	Night	25% 10% 37% 23% 5%	760
	21st to end	Night	31% 9% 34% 22% 3	836

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