Don's Backyard
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Don's Backyard clear sky chart from 2005-11-06 to 2024-01-27.

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	16% 10% 4 8% 6% 3 5% 5% 5% 7% 31%	74715
	Night	16% 11% 4 7% 6% 3 4 4 5% 7% 33%	75326
Transparency	Day	52% 6% 11% 19% 10% 1	74436
	Night	51% 6% 11% 19% 11% 2	75124
Seeing	Day	21% 11% 27% 21% 14% 6%	13557
	Night	20% 9% 27% 29% 13% 2	23538

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	17% 16% 7% 12% 9% 4 4 4 4 5% 18%	1706
	11th to 20th	Day	18% 20% 8% 12% 9% 3 3 3 3 5% 15%	1710
	21st to end	Day	17% 19% 7% 13% 9% 3 4 5% 4 5% 14%	1836
February	1st to 10th	Day	19% 17% 6% 12% 8% 3 4 3 3 4 19%	1620
	11th to 20th	Day	16% 13% 7% 12% 9% 3 5% 5% 4 5% 21%	1623
	21st to end	Day	18% 15% 5% 11% 6% 3 4 5% 4 6% 24%	1480
March	1st to 10th	Day	18% 12% 5% 9% 6% 3 4 4 5% 7% 28%	1941
	11th to 20th	Day	18% 13% 6% 8% 5% 3 4 3 4 6% 29%	1980
	21st to end	Day	22% 11% 6% 9% 6% 2 4 4 4 6% 26%	2178
April	1st to 10th	Day	14% 11% 4 7% 6% 3 5% 4 5% 7% 33%	2018
	11th to 20th	Day	18% 10% 4 7% 6% 3 4 5% 4 5% 33%	2264
	21st to end	Day	20% 9% 4 6% 6% 3 4 5% 4 6% 32%	2340
May	1st to 10th	Day	17% 10% 3 7% 7% 4 6% 5% 5% 6% 31%	2340
	11th to 20th	Day	18% 8% 4 6% 6% 3 6% 6% 5% 8% 30%	2301
	21st to end	Day	10% 5% 2 5% 7% 4 5% 5% 7% 10% 38%	2539
June	1st to 10th	Day	13% 7% 3 5% 6% 3 5% 6% 7% 9% 36%	2520
	11th to 20th	Day	13% 7% 3 6% 7% 3 5% 5% 6% 9% 35%	2520
	21st to end	Day	12% 5% 3 4 6% 3 5% 6% 7% 10% 39%	2483
July	1st to 10th	Day	11% 6% 3 6% 6% 4 5% 6% 6% 9% 39%	2340
	11th to 20th	Day	11% 6% 3 6% 5% 4 5% 6% 6% 9% 40%	2340
	21st to end	Day	11% 6% 3 6% 6% 4 6% 5% 8% 9% 36%	2547
August	1st to 10th	Day	10% 6% 2 6% 6% 4 7% 7% 8% 8% 35%	2327
	11th to 20th	Day	11% 6% 3 7% 6% 4 6% 7% 7% 9% 33%	2303
	21st to end	Day	10% 6% 2 6% 6% 4 6% 5% 7% 8% 39%	2522
September	1st to 10th	Day	14% 6% 2 5% 5% 3 4 4 5% 7% 43%	2070
	11th to 20th	Day	11% 5% 3 6% 5% 3 5% 5% 7% 9% 42%	1980
	21st to end	Day	19% 7% 2 6% 5% 2 4 4 5% 7% 38%	1980
October	1st to 10th	Day	16% 6% 3 6% 5% 4 5% 4 6% 8% 38%	1854
	11th to 20th	Day	19% 10% 3 6% 5% 4 5% 5% 4 5% 35%	1800
	21st to end	Day	22% 11% 3 8% 6% 2 5% 4 5% 7% 29%	1980
November	1st to 10th	Day	12% 9% 3 7% 5% 3 5% 4 5% 7% 40%	1688
	11th to 20th	Day	17% 13% 5% 8% 6% 3 3 4 5% 6% 31%	1694
	21st to end	Day	17% 15% 6% 8% 7% 4 4 5% 4 5% 24%	1520
December	1st to 10th	Day	20% 17% 11% 11% 5% 4 4 4 3 4 18%	1520
	11th to 20th	Day	18% 16% 6% 15% 5% 3 4 5% 5% 5% 18%	1910
	21st to end	Day	24% 23% 8% 11% 7% 3 3 3 3 5% 11%	2941
January	1st to 10th	Night	17% 16% 6% 11% 8% 4 5% 4 4 4 20%	2459
	11th to 20th	Night	18% 18% 6% 11% 8% 4 3 4 4 5% 17%	2470
	21st to end	Night	18% 18% 7% 10% 8% 3 4 4 4 4 20%	2664
February	1st to 10th	Night	22% 14% 6% 10% 7% 4 4 4 4 5% 20%	2340
	11th to 20th	Night	17% 14% 6% 9% 7% 3 5% 5% 5% 6% 22%	2222
	21st to end	Night	21% 12% 6% 10% 8% 4 4 4 4 4 24%	1776
March	1st to 10th	Night	17% 12% 6% 8% 6% 3 4 4 4 5% 33%	2018
	11th to 20th	Night	18% 9% 3 7% 5% 2 4 4 5% 8% 34%	1980
	21st to end	Night	19% 10% 5% 8% 7% 2 4 4 4 5% 32%	2178
April	1st to 10th	Night	16% 10% 6% 6% 5% 4 5% 4 5% 7% 34%	1940
	11th to 20th	Night	16% 9% 3 7% 7% 3 4 4 6% 6% 35%	1661
	21st to end	Night	15% 8% 4 7% 5% 3 5% 4 5% 8% 36%	1620
May	1st to 10th	Night	17% 9% 4 6% 6% 5% 5% 6% 5% 7% 30%	1620
	11th to 20th	Night	15% 8% 2 7% 5% 2 7% 4 5% 8% 37%	1592
	21st to end	Night	9% 5% 3 5% 5% 3 5% 5% 7% 11% 42%	1731
June	1st to 10th	Night	8% 6% 3 7% 5% 3 5% 5% 6% 8% 44%	1440
	11th to 20th	Night	11% 6% 3 6% 5% 3 5% 5% 7% 8% 42%	1440
	21st to end	Night	10% 4 3 5% 6% 3 5% 5% 6% 9% 44%	1469
July	1st to 10th	Night	8% 5% 3 6% 5% 4 5% 4 6% 8% 46%	1620
	11th to 20th	Night	8% 4 2 4 5% 3 5% 5% 7% 11% 47%	1620
	21st to end	Night	9% 5% 3 6% 6% 3 5% 5% 7% 8% 43%	1771
August	1st to 10th	Night	6% 5% 2 5% 5% 4 4 5% 6% 11% 45%	1611
	11th to 20th	Night	9% 7% 2 6% 5% 3 4 5% 6% 9% 45%	1598
	21st to end	Night	9% 4 2 5% 5% 3 5% 6% 6% 9% 47%	1814
September	1st to 10th	Night	13% 6% 2 4 4 3 4 5% 5% 7% 47%	1885
	11th to 20th	Night	10% 4 2 4 5% 3 4 4 6% 7% 52%	1980
	21st to end	Night	14% 7% 3 4 5% 3 4 5% 6% 6% 44%	1980
October	1st to 10th	Night	13% 7% 2 5% 4 3 4 5% 5% 8% 43%	2105
	11th to 20th	Night	17% 9% 4 7% 6% 3 5% 4 5% 7% 35%	2160
	21st to end	Night	23% 11% 3 6% 5% 3 5% 4 4 5% 32%	2376
November	1st to 10th	Night	14% 9% 2 5% 5% 3 4 4 5% 7% 42%	2388
	11th to 20th	Night	17% 12% 4 7% 6% 3 4 4 5% 6% 31%	2483
	21st to end	Night	19% 14% 5% 7% 6% 2 5% 4 4 7% 27%	2660
December	1st to 10th	Night	20% 16% 8% 9% 5% 3 4 4 4 5% 21%	2660
	11th to 20th	Night	17% 17% 7% 10% 6% 3 4 4 5% 6% 22%	3285
	21st to end	Night	25% 19% 7% 9% 6% 3 4 4 4 5% 13%	4710

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	70% 4 11% 10% 5%	1707
	11th to 20th	Day	75% 3 11% 8% 3	1710
	21st to end	Day	73% 5% 8% 10% 3 1	1836
February	1st to 10th	Day	70% 4 8% 12% 5% 1	1620
	11th to 20th	Day	66% 5% 8% 13% 7%	1632
	21st to end	Day	62% 4 10% 15% 7% 1	1480
March	1st to 10th	Day	57% 4 12% 20% 7%	1941
	11th to 20th	Day	58% 5% 9% 17% 10% 1	1980
	21st to end	Day	60% 4 8% 17% 10% 1	2178
April	1st to 10th	Day	51% 6% 11% 20% 11% 1	2018
	11th to 20th	Day	52% 6% 10% 17% 13% 2	2264
	21st to end	Day	53% 5% 11% 20% 10% 2	2340
May	1st to 10th	Day	53% 7% 11% 18% 10% 1	2340
	11th to 20th	Day	51% 7% 11% 16% 12% 2	2305
	21st to end	Day	40% 9% 15% 23% 12% 2	2539
June	1st to 10th	Day	43% 8% 13% 23% 12% 1	2520
	11th to 20th	Day	45% 7% 13% 21% 13% 1	2520
	21st to end	Day	39% 8% 13% 24% 15% 1	2483
July	1st to 10th	Day	41% 7% 12% 24% 14% 2	2275
	11th to 20th	Day	40% 7% 12% 25% 15% 2	2271
	21st to end	Day	40% 9% 13% 22% 14% 2	2405
August	1st to 10th	Day	42% 10% 13% 24% 11%	2314
	11th to 20th	Day	43% 9% 13% 23% 10% 2	2303
	21st to end	Day	40% 8% 13% 22% 13% 4	2522
September	1st to 10th	Day	40% 7% 12% 21% 16% 3	2070
	11th to 20th	Day	38% 8% 14% 24% 15% 2	1980
	21st to end	Day	45% 6% 11% 22% 15% 1	1980
October	1st to 10th	Day	43% 7% 11% 22% 15% 1	1854
	11th to 20th	Day	50% 5% 13% 21% 10% 1	1800
	21st to end	Day	56% 4 13% 16% 10% 1	1980
November	1st to 10th	Day	45% 5% 11% 23% 15% 2	1688
	11th to 20th	Day	55% 4 10% 19% 8% 3	1694
	21st to end	Day	62% 5% 11% 15% 6% 1	1520
December	1st to 10th	Day	71% 4 9% 12% 4	1520
	11th to 20th	Day	68% 5% 9% 15% 3	1906
	21st to end	Day	78% 5% 9% 6% 2	2941
January	1st to 10th	Night	67% 4 9% 13% 6%	2459
	11th to 20th	Night	70% 4 10% 12% 3	2470
	21st to end	Night	69% 4 9% 14% 4 1	2664
February	1st to 10th	Night	67% 5% 9% 14% 5%	2340
	11th to 20th	Night	63% 4 10% 15% 7%	2224
	21st to end	Night	64% 5% 8% 14% 8% 2	1776
March	1st to 10th	Night	54% 4 8% 22% 11% 1	2018
	11th to 20th	Night	49% 5% 12% 20% 11% 2	1980
	21st to end	Night	55% 5% 8% 19% 12% 2	2178
April	1st to 10th	Night	50% 6% 11% 18% 15% 1	1940
	11th to 20th	Night	49% 5% 12% 16% 16% 3	1661
	21st to end	Night	47% 5% 12% 18% 16% 2	1620
May	1st to 10th	Night	52% 8% 9% 16% 12% 3	1620
	11th to 20th	Night	46% 6% 11% 18% 15% 3	1593
	21st to end	Night	35% 8% 15% 26% 14% 1	1732
June	1st to 10th	Night	39% 7% 10% 25% 16% 3	1440
	11th to 20th	Night	38% 7% 14% 22% 17% 2	1440
	21st to end	Night	35% 9% 13% 24% 18% 1	1469
July	1st to 10th	Night	35% 8% 12% 23% 19% 4	1575
	11th to 20th	Night	30% 9% 14% 26% 19% 2	1566
	21st to end	Night	37% 8% 14% 23% 15% 3	1672
August	1st to 10th	Night	32% 8% 14% 28% 18% 1	1600
	11th to 20th	Night	35% 8% 16% 26% 14% 2	1598
	21st to end	Night	33% 7% 12% 26% 17% 4	1814
September	1st to 10th	Night	35% 6% 12% 24% 17% 5%	1885
	11th to 20th	Night	30% 7% 12% 26% 23% 2	1980
	21st to end	Night	40% 6% 11% 23% 19% 1	1980
October	1st to 10th	Night	38% 7% 11% 23% 19% 1	2105
	11th to 20th	Night	49% 6% 10% 21% 12% 1	2160
	21st to end	Night	54% 4 10% 20% 10% 2	2376
November	1st to 10th	Night	42% 5% 11% 23% 15% 3	2388
	11th to 20th	Night	54% 5% 11% 18% 11% 3	2483
	21st to end	Night	58% 5% 12% 18% 6% 1	2660
December	1st to 10th	Night	66% 6% 10% 12% 6%	2660
	11th to 20th	Night	64% 5% 12% 16% 3	3288
	21st to end	Night	74% 5% 10% 9% 2	4710

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	28% 20% 36% 11% 4 2	270
	11th to 20th	Day	30% 20% 38% 9% 3	270
	21st to end	Day	28% 24% 34% 11% 4 1	282
February	1st to 10th	Day	34% 17% 38% 11%	240
	11th to 20th	Day	31% 25% 32% 10% 1	240
	21st to end	Day	28% 28% 36% 8%	198
March	1st to 10th	Day	23% 20% 32% 20% 5%	381
	11th to 20th	Day	30% 13% 31% 17% 7% 1	420
	21st to end	Day	33% 12% 29% 21% 5% 1	462
April	1st to 10th	Day	22% 18% 31% 17% 10% 1	420
	11th to 20th	Day	24% 15% 29% 17% 10% 5%	412
	21st to end	Day	27% 12% 31% 19% 8% 3	420
May	1st to 10th	Day	27% 12% 27% 20% 10% 3	423
	11th to 20th	Day	20% 10% 32% 21% 12% 5%	447
	21st to end	Day	14% 5% 23% 28% 23% 8%	485
June	1st to 10th	Day	15% 4 29% 31% 17% 4	450
	11th to 20th	Day	18% 6% 23% 28% 18% 7%	450
	21st to end	Day	14% 8% 25% 28% 18% 7%	450
July	1st to 10th	Day	13% 3 18% 24% 26% 16%	445
	11th to 20th	Day	12% 5% 21% 27% 24% 12%	444
	21st to end	Day	13% 2 24% 25% 25% 11%	488
August	1st to 10th	Day	14% 3 18% 26% 26% 13%	446
	11th to 20th	Day	16% 6% 17% 25% 23% 13%	439
	21st to end	Day	12% 5% 19% 24% 25% 15%	495
September	1st to 10th	Day	16% 5% 19% 24% 23% 13%	450
	11th to 20th	Day	9% 4 16% 27% 30% 14%	450
	21st to end	Day	23% 9% 22% 22% 15% 9%	450
October	1st to 10th	Day	14% 8% 28% 23% 21% 6%	450
	11th to 20th	Day	21% 14% 34% 22% 7% 2	450
	21st to end	Day	31% 13% 29% 20% 7%	495
November	1st to 10th	Day	16% 20% 33% 18% 10% 2	286
	11th to 20th	Day	27% 21% 29% 15% 7% 2	262
	21st to end	Day	25% 16% 41% 16% 2	180
December	1st to 10th	Day	26% 20% 34% 13% 4 2	180
	11th to 20th	Day	24% 17% 43% 12% 3	180
	21st to end	Day	30% 20% 34% 13% 3	247
January	1st to 10th	Night	25% 17% 35% 18% 4	760
	11th to 20th	Night	28% 14% 34% 19% 5%	760
	21st to end	Night	26% 15% 31% 24% 4	820
February	1st to 10th	Night	30% 12% 33% 22% 3	720
	11th to 20th	Night	26% 14% 35% 23% 2	703
	21st to end	Night	28% 14% 36% 19% 3	592
March	1st to 10th	Night	23% 11% 35% 25% 6%	720
	11th to 20th	Night	23% 12% 32% 26% 7% 1	720
	21st to end	Night	24% 10% 32% 27% 7% 1	792
April	1st to 10th	Night	21% 13% 33% 27% 7%	720
	11th to 20th	Night	21% 9% 34% 24% 11% 1	597
	21st to end	Night	19% 8% 34% 31% 8% 1	540
May	1st to 10th	Night	21% 6% 27% 35% 10% 1	540
	11th to 20th	Night	19% 3 24% 38% 14% 2	531
	21st to end	Night	12% 2 18% 41% 23% 4	571
June	1st to 10th	Night	9% 3 25% 45% 17% 1	360
	11th to 20th	Night	13% 4 21% 44% 18% 1	360
	21st to end	Night	11% 3 23% 40% 23% 1	389
July	1st to 10th	Night	9% 2 13% 40% 30% 5%	525
	11th to 20th	Night	10% 2 16% 40% 28% 4	522
	21st to end	Night	10% 3 14% 42% 30% 2	591
August	1st to 10th	Night	10% 2 13% 42% 31% 3	537
	11th to 20th	Night	13% 1 13% 39% 31% 4	534
	21st to end	Night	10% 3 15% 36% 32% 5%	626
September	1st to 10th	Night	15% 3 15% 32% 29% 5%	720
	11th to 20th	Night	12% 2 12% 37% 32% 6%	720
	21st to end	Night	17% 5% 22% 31% 21% 4	720
October	1st to 10th	Night	17% 5% 22% 32% 20% 4	720
	11th to 20th	Night	21% 8% 32% 27% 10% 1	720
	21st to end	Night	28% 10% 26% 30% 7%	792
November	1st to 10th	Night	19% 8% 29% 30% 13% 1	740
	11th to 20th	Night	23% 12% 33% 23% 8% 1	760
	21st to end	Night	27% 13% 31% 22% 7%	760
December	1st to 10th	Night	30% 12% 32% 21% 5%	760
	11th to 20th	Night	25% 14% 33% 23% 4	760
	21st to end	Night	29% 13% 33% 23% 2	836

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