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 2024-11-30.

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% 9% 3 8% 5% 5% 5% 4 6% 7% 34%	101652
	Night	16% 9% 3 8% 5% 4 4 4 5% 6% 35%	97872
Transparency	Day	48% 3 13% 23% 12% 1	105257
	Night	48% 3 12% 23% 12% 1	102355
Seeing	Day	18% 9% 28% 25% 14% 5%	14499
	Night	19% 7% 28% 30% 13% 2	23944

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% 14% 3 9% 7% 4 5% 4 6% 5% 27%	2517
	11th to 20th	Day	12% 17% 6% 16% 8% 5% 5% 3 5% 6% 18%	2777
	21st to end	Day	21% 20% 6% 10% 6% 5% 4 3 4 4 17%	3124
February	1st to 10th	Day	15% 14% 4 13% 7% 6% 5% 4 5% 6% 22%	1710
	11th to 20th	Day	12% 13% 6% 11% 8% 5% 4 4 5% 6% 26%	1663
	21st to end	Day	17% 13% 4 11% 6% 5% 4 4 5% 5% 27%	1471
March	1st to 10th	Day	19% 11% 4 9% 5% 5% 4 3 5% 5% 31%	3432
	11th to 20th	Day	11% 14% 4 10% 6% 4 6% 4 6% 6% 29%	3480
	21st to end	Day	16% 9% 3 8% 5% 5% 4 3 5% 6% 37%	3815
April	1st to 10th	Day	16% 13% 4 8% 5% 5% 4 4 6% 6% 30%	2237
	11th to 20th	Day	18% 9% 3 6% 5% 4 4 3 5% 6% 36%	2330
	21st to end	Day	19% 9% 3 7% 5% 5% 4 3 5% 6% 33%	2470
May	1st to 10th	Day	17% 8% 3 8% 5% 4 4 4 6% 8% 34%	4108
	11th to 20th	Day	15% 9% 3 8% 5% 5% 5% 4 6% 8% 32%	4356
	21st to end	Day	11% 5% 2 6% 3 4 5% 4 6% 8% 47%	4837
June	1st to 10th	Day	14% 6% 2 6% 6% 5% 5% 4 7% 8% 38%	2660
	11th to 20th	Day	12% 5% 2 6% 5% 5% 5% 5% 8% 9% 39%	2660
	21st to end	Day	12% 5% 2 6% 5% 5% 5% 4 7% 9% 40%	2660
July	1st to 10th	Day	9% 4 2 5% 5% 4 6% 5% 7% 11% 42%	4440
	11th to 20th	Day	9% 4 2 5% 4 4 5% 5% 7% 9% 47%	4440
	21st to end	Day	7% 5% 2 6% 5% 5% 6% 5% 7% 9% 43%	4478
August	1st to 10th	Day	17% 7% 2 5% 4 4 4 4 6% 7% 39%	4104
	11th to 20th	Day	10% 6% 3 7% 6% 5% 5% 4 7% 9% 37%	4044
	21st to end	Day	8% 5% 2 6% 4 5% 5% 4 7% 9% 45%	4355
September	1st to 10th	Day	13% 5% 2 6% 4 5% 5% 4 6% 8% 42%	2280
	11th to 20th	Day	10% 5% 3 5% 4 4 4 3 6% 9% 47%	2161
	21st to end	Day	13% 6% 3 7% 5% 5% 6% 4 5% 9% 37%	2056
October	1st to 10th	Day	18% 8% 3 6% 5% 4 5% 4 6% 6% 36%	1900
	11th to 20th	Day	19% 9% 3 8% 6% 5% 5% 4 5% 6% 31%	1900
	21st to end	Day	23% 10% 4 7% 6% 5% 4 5% 6% 6% 23%	2090
November	1st to 10th	Day	14% 9% 4 8% 6% 5% 4 4 6% 8% 34%	1701
	11th to 20th	Day	15% 12% 3 8% 6% 5% 5% 6% 5% 9% 26%	1600
	21st to end	Day	18% 15% 4 8% 6% 6% 5% 3 6% 8% 22%	1598
December	1st to 10th	Day	17% 16% 5% 12% 7% 6% 5% 4 5% 6% 19%	1520
	11th to 20th	Day	20% 12% 5% 10% 6% 5% 5% 5% 5% 5% 23%	1906
	21st to end	Day	29% 16% 5% 10% 5% 5% 4 3 4 5% 14%	2772
January	1st to 10th	Night	18% 16% 4 10% 6% 5% 5% 4 5% 5% 23%	4429
	11th to 20th	Night	15% 14% 5% 12% 8% 6% 5% 4 4 6% 20%	4180
	21st to end	Night	22% 13% 5% 10% 7% 6% 4 3 5% 5% 20%	4532
February	1st to 10th	Night	14% 14% 5% 9% 6% 5% 6% 4 5% 6% 25%	2470
	11th to 20th	Night	14% 14% 4 9% 6% 5% 5% 4 5% 5% 28%	2403
	21st to end	Night	14% 10% 5% 10% 6% 6% 5% 4 6% 6% 29%	1980
March	1st to 10th	Night	19% 9% 3 8% 6% 6% 5% 4 5% 5% 32%	3408
	11th to 20th	Night	13% 10% 3 8% 6% 4 4 4 5% 6% 36%	3479
	21st to end	Night	17% 8% 2 6% 3 3 3 4 4 5% 45%	3817
April	1st to 10th	Night	18% 10% 3 7% 5% 5% 4 4 6% 5% 33%	1941
	11th to 20th	Night	16% 8% 2 7% 5% 5% 5% 3 5% 6% 38%	1815
	21st to end	Night	17% 8% 3 7% 5% 4 4 4 6% 7% 36%	1710
May	1st to 10th	Night	18% 7% 2 7% 4 4 4 3 5% 6% 39%	2833
	11th to 20th	Night	16% 8% 3 7% 4 5% 4 5% 5% 8% 37%	2537
	21st to end	Night	10% 4 2 5% 4 4 4 3 6% 8% 50%	2738
June	1st to 10th	Night	12% 5% 2 6% 5% 5% 5% 4 7% 8% 42%	1520
	11th to 20th	Night	9% 6% 2 6% 5% 4 4 5% 7% 9% 43%	1520
	21st to end	Night	10% 6% 3 6% 5% 3 4 5% 6% 8% 45%	1439
July	1st to 10th	Night	6% 5% 2 4 4 4 4 4 7% 8% 52%	2520
	11th to 20th	Night	12% 4 1 4 3 3 4 3 6% 9% 51%	2520
	21st to end	Night	7% 4 2 4 3 4 4 4 7% 8% 51%	3072
August	1st to 10th	Night	10% 5% 2 6% 5% 4 5% 3 6% 7% 47%	2825
	11th to 20th	Night	11% 4 2 6% 3 3 4 3 5% 8% 50%	2785
	21st to end	Night	11% 5% 2 6% 3 4 4 4 6% 8% 47%	3268
September	1st to 10th	Night	9% 5% 2 6% 4 3 5% 4 7% 8% 47%	1900
	11th to 20th	Night	10% 4 1 5% 3 3 4 3 5% 7% 55%	2017
	21st to end	Night	14% 7% 3 7% 4 4 4 3 5% 7% 41%	2123
October	1st to 10th	Night	16% 6% 3 7% 4 3 4 4 5% 6% 40%	2280
	11th to 20th	Night	18% 9% 3 8% 5% 4 4 3 4 6% 36%	2280
	21st to end	Night	20% 9% 3 8% 5% 4 5% 3 6% 6% 31%	2508
November	1st to 10th	Night	16% 8% 2 7% 5% 5% 4 3 5% 7% 38%	2593
	11th to 20th	Night	15% 11% 4 8% 5% 5% 6% 4 6% 7% 31%	2800
	21st to end	Night	20% 10% 3 8% 6% 5% 5% 4 6% 7% 26%	2798
December	1st to 10th	Night	20% 15% 5% 8% 5% 5% 4 3 6% 5% 25%	2660
	11th to 20th	Night	15% 17% 4 10% 5% 5% 5% 4 5% 6% 23%	3288
	21st to end	Night	30% 15% 6% 9% 5% 5% 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	60% 2 15% 14% 8%	2518
	11th to 20th	Day	69% 2 10% 14% 5%	2777
	21st to end	Day	72% 2 9% 11% 5% 1	3124
February	1st to 10th	Day	62% 3 12% 18% 4 1	1710
	11th to 20th	Day	61% 3 9% 17% 9% 2	1672
	21st to end	Day	58% 3 10% 19% 8% 1	1471
March	1st to 10th	Day	56% 2 11% 21% 8% 1	3435
	11th to 20th	Day	55% 2 11% 19% 11% 2	3480
	21st to end	Day	50% 1 11% 22% 14% 2	3815
April	1st to 10th	Day	54% 3 11% 19% 13% 1	2237
	11th to 20th	Day	49% 2 10% 22% 16% 1	2330
	21st to end	Day	52% 3 10% 20% 13% 3	2470
May	1st to 10th	Day	49% 2 12% 22% 15% 1	4120
	11th to 20th	Day	51% 3 14% 22% 9% 1	4360
	21st to end	Day	36% 2 14% 26% 18% 4	4837
June	1st to 10th	Day	43% 4 11% 25% 16% 2	2660
	11th to 20th	Day	39% 4 12% 27% 17% 1	2660
	21st to end	Day	39% 4 14% 27% 15% 1	2660
July	1st to 10th	Day	34% 5% 15% 28% 15% 2	4370
	11th to 20th	Day	32% 4 13% 38% 12% 1	4365
	21st to end	Day	36% 6% 15% 23% 19% 1	4335
August	1st to 10th	Day	43% 4 14% 23% 15% 1	4091
	11th to 20th	Day	42% 4 17% 26% 11% 1	4083
	21st to end	Day	34% 5% 12% 37% 11% 1	4355
September	1st to 10th	Day	39% 5% 13% 30% 12% 1	2280
	11th to 20th	Day	35% 3 13% 32% 16% 1	2161
	21st to end	Day	44% 4 14% 27% 10%	2056
October	1st to 10th	Day	37% 4 14% 29% 14% 1	3160
	11th to 20th	Day	39% 3 14% 28% 15%	3140
	21st to end	Day	53% 4 14% 22% 7%	3428
November	1st to 10th	Day	49% 4 13% 24% 10% 1	1701
	11th to 20th	Day	54% 5% 15% 17% 9% 1	1600
	21st to end	Day	60% 4 14% 18% 4	1598
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	64% 2 12% 16% 5% 1	4429
	11th to 20th	Night	66% 2 12% 16% 4 1	4180
	21st to end	Night	67% 2 12% 13% 5% 1	4532
February	1st to 10th	Night	59% 3 10% 21% 6% 1	2470
	11th to 20th	Night	58% 3 10% 19% 10% 1	2405
	21st to end	Night	55% 2 12% 21% 9% 1	1980
March	1st to 10th	Night	55% 2 10% 25% 7% 1	3418
	11th to 20th	Night	48% 1 14% 22% 13% 2	3480
	21st to end	Night	43% 1 8% 24% 21% 2	3817
April	1st to 10th	Night	52% 2 9% 20% 15% 1	1941
	11th to 20th	Night	49% 3 9% 21% 16% 2	1815
	21st to end	Night	47% 3 9% 21% 17% 2	1710
May	1st to 10th	Night	48% 2 12% 19% 17% 3	2840
	11th to 20th	Night	50% 3 14% 21% 10% 2	2538
	21st to end	Night	32% 2 13% 26% 23% 3	2739
June	1st to 10th	Night	39% 4 10% 27% 18% 2	1520
	11th to 20th	Night	37% 3 11% 25% 21% 2	1520
	21st to end	Night	38% 3 14% 26% 18% 1	1439
July	1st to 10th	Night	29% 4 15% 25% 24% 4	2480
	11th to 20th	Night	31% 3 13% 35% 17% 1	2472
	21st to end	Night	30% 5% 16% 30% 18% 2	2974
August	1st to 10th	Night	38% 3 12% 29% 15% 3	2814
	11th to 20th	Night	34% 3 15% 34% 12% 1	2812
	21st to end	Night	35% 4 13% 32% 15% 1	3268
September	1st to 10th	Night	34% 4 14% 31% 15% 2	1900
	11th to 20th	Night	31% 3 11% 37% 19%	2017
	21st to end	Night	43% 3 13% 29% 12% 1	2123
October	1st to 10th	Night	34% 3 8% 30% 24% 1	3800
	11th to 20th	Night	43% 3 12% 29% 13%	3779
	21st to end	Night	41% 4 16% 26% 11% 1	4120
November	1st to 10th	Night	46% 4 11% 26% 13% 1	2593
	11th to 20th	Night	51% 3 14% 20% 10% 1	2800
	21st to end	Night	55% 4 16% 20% 4	2798
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% 34% 13% 9% 1	253
	21st to end	Day	27% 12% 34% 21% 6%	297
February	1st to 10th	Day	24% 14% 41% 16% 5%	270
	11th to 20th	Day	20% 19% 40% 18% 2 1	270
	21st to end	Day	29% 21% 34% 15% 1	239
March	1st to 10th	Day	22% 12% 36% 23% 8%	460
	11th to 20th	Day	21% 17% 31% 23% 7% 1	460
	21st to end	Day	25% 9% 34% 23% 8% 1	506
April	1st to 10th	Day	20% 12% 34% 26% 8% 1	460
	11th to 20th	Day	27% 13% 29% 18% 9% 4	452
	21st to end	Day	24% 10% 32% 23% 9% 3	460
May	1st to 10th	Day	21% 8% 29% 25% 12% 5%	463
	11th to 20th	Day	16% 6% 28% 28% 15% 7%	487
	21st to end	Day	14% 5% 26% 30% 18% 7%	529
June	1st to 10th	Day	14% 8% 27% 32% 14% 5%	490
	11th to 20th	Day	16% 6% 25% 30% 21% 3	490
	21st to end	Day	14% 7% 26% 28% 19% 5%	490
July	1st to 10th	Day	10% 3 19% 34% 24% 10%	485
	11th to 20th	Day	13% 4 20% 31% 21% 10%	484
	21st to end	Day	9% 3 22% 38% 20% 8%	532
August	1st to 10th	Day	14% 4 19% 27% 25% 12%	486
	11th to 20th	Day	9% 5% 20% 32% 20% 15%	479
	21st to end	Day	12% 4 23% 29% 22% 11%	539
September	1st to 10th	Day	13% 5% 22% 30% 18% 12%	490
	11th to 20th	Day	9% 4 20% 25% 26% 17%	490
	21st to end	Day	15% 8% 29% 23% 17% 8%	490
October	1st to 10th	Day	21% 7% 27% 23% 15% 7%	490
	11th to 20th	Day	20% 10% 34% 23% 9% 3	490
	21st to end	Day	27% 11% 28% 25% 8% 1	539
November	1st to 10th	Day	20% 17% 26% 25% 10% 2	291
	11th to 20th	Day	24% 20% 30% 20% 7% 2	200
	21st to end	Day	24% 14% 39% 18% 6% 1	200
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% 11% 31% 25% 5%	760
	21st to end	Night	20% 8% 37% 27% 7% 1	836
February	1st to 10th	Night	23% 13% 37% 24% 3	760
	11th to 20th	Night	22% 12% 36% 25% 5%	743
	21st to end	Night	19% 12% 40% 25% 4	628
March	1st to 10th	Night	19% 11% 35% 27% 7%	760
	11th to 20th	Night	22% 10% 36% 25% 7% 1	760
	21st to end	Night	20% 10% 30% 30% 9% 1	836
April	1st to 10th	Night	22% 9% 33% 31% 6%	760
	11th to 20th	Night	20% 9% 35% 25% 10% 2	691
	21st to end	Night	21% 5% 27% 36% 11%	570
May	1st to 10th	Night	19% 3 27% 33% 16% 2	570
	11th to 20th	Night	16% 3 23% 43% 15% 1	403
	21st to end	Night	11% 2 24% 37% 25% 1	411
June	1st to 10th	Night	13% 3 23% 41% 19% 1	380
	11th to 20th	Night	9% 4 26% 41% 18% 1	380
	21st to end	Night	12% 4 20% 44% 20% 1	380
July	1st to 10th	Night	6% 1 20% 45% 25% 2	370
	11th to 20th	Night	12% 1 20% 41% 23% 3	368
	21st to end	Night	9% 2 15% 46% 26% 3	609
August	1st to 10th	Night	10% 2 12% 43% 30% 3	567
	11th to 20th	Night	8% 3 13% 42% 30% 4	564
	21st to end	Night	12% 2 17% 37% 28% 4	706
September	1st to 10th	Night	11% 6% 19% 35% 23% 6%	760
	11th to 20th	Night	13% 3 15% 32% 31% 6%	760
	21st to end	Night	18% 4 20% 31% 23% 4	760
October	1st to 10th	Night	19% 4 25% 27% 22% 3	760
	11th to 20th	Night	21% 8% 28% 30% 11% 2	760
	21st to end	Night	26% 8% 29% 31% 6%	836
November	1st to 10th	Night	21% 9% 27% 31% 11% 1	780
	11th to 20th	Night	22% 10% 33% 25% 9% 2	800
	21st to end	Night	26% 9% 33% 26% 7%	800
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

Page generated at 2024-11-27 17:30:48. Version ac07

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