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 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	15% 11% 4 10% 5% 5% 5% 4 5% 7% 30%	131604
	Night	15% 12% 4 10% 5% 4 4 3 5% 6% 30%	131212
Transparency	Day	52% 6% 11% 20% 11% 1	135249
	Night	53% 5% 10% 19% 11% 1	135637
Seeing	Day	21% 12% 28% 21% 13% 5%	16570
	Night	20% 9% 27% 29% 13% 1	26531

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	15% 20% 6% 22% 6% 4 3 3 4 5% 13%	5258
	11th to 20th	Day	11% 20% 8% 19% 9% 6% 4 2 3 4 14%	5280
	21st to end	Day	18% 17% 8% 13% 10% 7% 4 3 3 4 14%	5780
February	1st to 10th	Day	17% 17% 5% 13% 6% 4 4 2 4 5% 22%	1890
	11th to 20th	Day	16% 14% 6% 14% 7% 4 5% 3 4 5% 21%	1901
	21st to end	Day	16% 15% 4 12% 5% 4 4 4 4 6% 25%	1730
March	1st to 10th	Day	17% 13% 6% 12% 5% 4 4 3 4 5% 26%	4952
	11th to 20th	Day	12% 10% 4 10% 5% 5% 5% 3 5% 6% 36%	5090
	21st to end	Day	20% 11% 4 10% 5% 4 4 3 4 6% 28%	5586
April	1st to 10th	Day	15% 13% 3 8% 6% 4 5% 3 5% 7% 31%	2355
	11th to 20th	Day	17% 11% 4 8% 5% 4 5% 4 5% 5% 33%	2647
	21st to end	Day	18% 10% 3 7% 5% 4 4 4 5% 6% 34%	2730
May	1st to 10th	Day	21% 10% 3 7% 5% 5% 5% 4 5% 6% 28%	5992
	11th to 20th	Day	18% 8% 3 7% 5% 4 5% 4 6% 8% 31%	5939
	21st to end	Day	18% 7% 2 6% 5% 5% 5% 4 7% 8% 33%	6468
June	1st to 10th	Day	13% 8% 2 6% 5% 5% 5% 4 7% 8% 36%	2800
	11th to 20th	Day	13% 8% 3 8% 6% 4 4 4 6% 9% 35%	2800
	21st to end	Day	12% 6% 2 6% 5% 4 5% 5% 8% 9% 39%	2758
July	1st to 10th	Day	9% 6% 3 7% 4 4 5% 5% 7% 9% 42%	5900
	11th to 20th	Day	11% 6% 2 7% 4 5% 6% 5% 8% 10% 36%	5900
	21st to end	Day	12% 7% 2 6% 5% 5% 5% 5% 7% 8% 39%	6424
August	1st to 10th	Day	12% 8% 3 8% 5% 6% 6% 4 7% 8% 33%	4234
	11th to 20th	Day	11% 7% 3 7% 4 5% 6% 5% 8% 10% 35%	4174
	21st to end	Day	8% 5% 2 6% 6% 6% 5% 4 8% 9% 41%	4578
September	1st to 10th	Day	13% 6% 2 5% 4 4 4 3 5% 7% 46%	2300
	11th to 20th	Day	10% 6% 2 6% 4 4 5% 4 6% 8% 44%	2200
	21st to end	Day	20% 8% 2 7% 5% 3 4 4 5% 7% 37%	2200
October	1st to 10th	Day	15% 6% 2 6% 4 4 4 3 6% 8% 41%	2060
	11th to 20th	Day	18% 10% 2 7% 4 5% 4 4 4 5% 37%	2000
	21st to end	Day	20% 11% 3 9% 5% 4 4 3 5% 7% 30%	2200
November	1st to 10th	Day	12% 9% 3 8% 4 4 4 3 6% 7% 40%	1869
	11th to 20th	Day	17% 15% 4 8% 5% 4 4 3 5% 5% 30%	1872
	21st to end	Day	16% 18% 6% 10% 6% 5% 4 4 4 5% 22%	1680
December	1st to 10th	Day	19% 14% 8% 15% 6% 4 3 3 3 4 19%	2680
	11th to 20th	Day	22% 17% 7% 12% 4 4 3 3 4 5% 19%	3070
	21st to end	Day	23% 23% 6% 13% 6% 5% 3 3 3 4 11%	4307
January	1st to 10th	Night	14% 19% 7% 21% 9% 5% 4 2 4 4 11%	7635
	11th to 20th	Night	10% 18% 10% 17% 9% 6% 5% 4 4 4 13%	7680
	21st to end	Night	17% 17% 6% 12% 7% 6% 5% 3 4 5% 20%	8401
February	1st to 10th	Night	20% 14% 5% 11% 6% 5% 4 3 4 5% 22%	2730
	11th to 20th	Night	16% 14% 6% 12% 6% 4 5% 4 4 6% 22%	2603
	21st to end	Night	19% 14% 5% 12% 7% 4 4 3 4 4 24%	2076
March	1st to 10th	Night	22% 13% 4 9% 4 4 3 2 3 5% 32%	5120
	11th to 20th	Night	14% 9% 3 7% 4 4 5% 4 6% 7% 38%	5090
	21st to end	Night	17% 11% 4 9% 5% 4 4 3 4 5% 34%	5588
April	1st to 10th	Night	18% 12% 5% 7% 4 5% 4 3 5% 6% 32%	2263
	11th to 20th	Night	16% 10% 3 9% 5% 4 5% 3 6% 6% 34%	1940
	21st to end	Night	14% 10% 3 8% 4 4 5% 4 5% 7% 37%	1890
May	1st to 10th	Night	24% 11% 3 7% 4 5% 5% 3 5% 6% 26%	4123
	11th to 20th	Night	16% 9% 3 7% 4 4 4 3 5% 8% 38%	4086
	21st to end	Night	16% 8% 2 6% 4 4 4 3 6% 10% 37%	4381
June	1st to 10th	Night	9% 7% 2 7% 5% 5% 5% 4 5% 7% 44%	1600
	11th to 20th	Night	10% 6% 2 7% 4 4 5% 4 7% 8% 42%	1600
	21st to end	Night	10% 6% 2 6% 6% 4 5% 4 7% 9% 42%	1633
July	1st to 10th	Night	10% 6% 3 7% 4 4 4 4 6% 9% 43%	4058
	11th to 20th	Night	10% 6% 2 6% 5% 5% 5% 4 7% 10% 40%	4057
	21st to end	Night	8% 6% 3 8% 5% 4 5% 4 6% 8% 45%	4427
August	1st to 10th	Night	10% 8% 3 8% 5% 5% 5% 4 6% 9% 39%	2915
	11th to 20th	Night	11% 8% 3 6% 4 4 3 3 5% 7% 45%	2875
	21st to end	Night	7% 4 2 7% 5% 5% 6% 5% 7% 8% 44%	3285
September	1st to 10th	Night	12% 5% 2 4 3 4 4 4 5% 7% 49%	2095
	11th to 20th	Night	9% 4 2 4 5% 4 3 3 6% 7% 53%	2200
	21st to end	Night	16% 8% 3 5% 5% 3 4 4 6% 6% 42%	2200
October	1st to 10th	Night	13% 6% 2 5% 4 4 4 4 5% 7% 46%	2339
	11th to 20th	Night	16% 9% 4 7% 5% 4 4 4 5% 7% 37%	2400
	21st to end	Night	21% 10% 2 7% 4 4 4 3 4 6% 34%	2640
November	1st to 10th	Night	13% 9% 2 6% 4 4 4 3 5% 6% 42%	2646
	11th to 20th	Night	17% 14% 4 8% 5% 4 4 3 5% 6% 30%	2745
	21st to end	Night	19% 15% 5% 8% 5% 4 5% 3 4 7% 25%	2940
December	1st to 10th	Night	16% 16% 7% 11% 6% 4 5% 4 5% 5% 22%	4720
	11th to 20th	Night	19% 19% 7% 12% 5% 4 3 3 4 5% 19%	5345
	21st to end	Night	23% 20% 6% 11% 5% 4 4 3 4 5% 14%	6886

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	76% 4 9% 8% 3	5259
	11th to 20th	Day	77% 2 7% 8% 6%	5280
	21st to end	Day	76% 3 6% 12% 3	5780
February	1st to 10th	Day	67% 3 8% 14% 7% 1	1890
	11th to 20th	Day	66% 4 8% 15% 7%	1910
	21st to end	Day	61% 4 10% 16% 7% 1	1730
March	1st to 10th	Day	59% 5% 9% 21% 6%	4952
	11th to 20th	Day	49% 5% 11% 25% 8% 2	5090
	21st to end	Day	57% 5% 7% 18% 13% 1	5586
April	1st to 10th	Day	53% 6% 11% 18% 11% 1	2355
	11th to 20th	Day	51% 6% 11% 18% 12% 2	2647
	21st to end	Day	51% 5% 11% 22% 10% 2	2730
May	1st to 10th	Day	55% 7% 9% 18% 9% 3	6027
	11th to 20th	Day	50% 6% 12% 21% 10% 1	5943
	21st to end	Day	46% 7% 12% 21% 12% 3	6483
June	1st to 10th	Day	43% 7% 13% 23% 12% 1	2800
	11th to 20th	Day	45% 7% 13% 22% 13% 1	2800
	21st to end	Day	38% 8% 14% 23% 15% 1	2758
July	1st to 10th	Day	37% 6% 12% 29% 14% 2	5835
	11th to 20th	Day	40% 8% 13% 24% 15% 1	5831
	21st to end	Day	39% 7% 11% 21% 19% 2	6240
August	1st to 10th	Day	45% 8% 13% 22% 11%	4221
	11th to 20th	Day	41% 8% 11% 30% 10% 1	4213
	21st to end	Day	37% 8% 13% 31% 10% 2	4578
September	1st to 10th	Day	38% 6% 11% 23% 18% 4	2300
	11th to 20th	Day	36% 7% 14% 25% 16% 2	2200
	21st to end	Day	47% 6% 10% 21% 15% 1	2200
October	1st to 10th	Day	31% 5% 12% 31% 20% 1	3359
	11th to 20th	Day	46% 4 11% 20% 15% 4	3240
	21st to end	Day	41% 4 13% 25% 16% 1	3538
November	1st to 10th	Day	45% 5% 11% 23% 15% 1	1869
	11th to 20th	Day	56% 4 10% 19% 8% 3	1872
	21st to end	Day	63% 5% 10% 15% 6%	1680
December	1st to 10th	Day	69% 4 11% 12% 4	2680
	11th to 20th	Day	69% 4 9% 14% 4	3066
	21st to end	Day	78% 4 7% 8% 2	4307
January	1st to 10th	Night	77% 4 7% 9% 3	7636
	11th to 20th	Night	75% 3 8% 9% 5%	7680
	21st to end	Night	69% 3 9% 13% 6% 1	8401
February	1st to 10th	Night	65% 5% 9% 16% 6%	2730
	11th to 20th	Night	64% 4 10% 15% 7%	2605
	21st to end	Night	63% 4 9% 14% 8% 1	2076
March	1st to 10th	Night	57% 3 10% 20% 9% 1	5120
	11th to 20th	Night	44% 5% 13% 26% 9% 2	5090
	21st to end	Night	51% 5% 9% 19% 14% 1	5588
April	1st to 10th	Night	53% 5% 10% 17% 13% 2	2263
	11th to 20th	Night	49% 6% 11% 17% 15% 2	1940
	21st to end	Night	46% 5% 11% 19% 16% 2	1890
May	1st to 10th	Night	58% 7% 8% 15% 8% 4	4150
	11th to 20th	Night	45% 7% 10% 23% 14% 1	4087
	21st to end	Night	41% 7% 13% 25% 11% 2	4388
June	1st to 10th	Night	39% 7% 10% 25% 17% 3	1600
	11th to 20th	Night	38% 7% 14% 22% 16% 3	1600
	21st to end	Night	36% 9% 13% 23% 18% 1	1633
July	1st to 10th	Night	36% 7% 11% 24% 19% 3	4015
	11th to 20th	Night	37% 8% 14% 27% 13% 1	4006
	21st to end	Night	36% 6% 11% 20% 25% 2	4297
August	1st to 10th	Night	42% 7% 12% 24% 14% 1	2904
	11th to 20th	Night	38% 6% 11% 28% 16% 2	2902
	21st to end	Night	35% 7% 12% 30% 13% 2	3285
September	1st to 10th	Night	33% 6% 12% 26% 19% 4	2095
	11th to 20th	Night	29% 7% 12% 27% 23% 2	2200
	21st to end	Night	42% 6% 11% 22% 18% 1	2200
October	1st to 10th	Night	31% 5% 8% 29% 26% 1	3820
	11th to 20th	Night	41% 5% 12% 21% 17% 5%	3899
	21st to end	Night	38% 4 10% 29% 18% 2	4252
November	1st to 10th	Night	41% 5% 11% 24% 16% 3	2646
	11th to 20th	Night	54% 5% 10% 18% 10% 3	2745
	21st to end	Night	60% 5% 12% 18% 5% 1	2940
December	1st to 10th	Night	63% 6% 11% 17% 4	4720
	11th to 20th	Night	68% 4 11% 14% 2	5348
	21st to end	Night	72% 5% 9% 11% 2	6886

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% 19% 38% 10% 3 2	330
	11th to 20th	Day	29% 26% 35% 8% 2	330
	21st to end	Day	27% 25% 34% 11% 3 1	363
February	1st to 10th	Day	28% 20% 36% 13% 1 1	330
	11th to 20th	Day	31% 24% 32% 12% 2	330
	21st to end	Day	25% 28% 39% 8%	273
March	1st to 10th	Day	24% 20% 34% 18% 5%	495
	11th to 20th	Day	30% 14% 32% 16% 6% 1	540
	21st to end	Day	31% 15% 30% 20% 4 1	594
April	1st to 10th	Day	24% 19% 33% 16% 9% 1	540
	11th to 20th	Day	24% 19% 27% 17% 8% 4	532
	21st to end	Day	25% 12% 33% 20% 8% 2	540
May	1st to 10th	Day	27% 16% 28% 19% 9% 2	543
	11th to 20th	Day	19% 11% 33% 21% 11% 5%	551
	21st to end	Day	15% 7% 24% 26% 21% 6%	573
June	1st to 10th	Day	16% 5% 30% 30% 15% 4	530
	11th to 20th	Day	17% 6% 24% 29% 17% 6%	530
	21st to end	Day	14% 8% 25% 27% 19% 7%	530
July	1st to 10th	Day	12% 3 20% 26% 25% 14%	525
	11th to 20th	Day	11% 5% 23% 27% 23% 11%	524
	21st to end	Day	13% 3 22% 26% 25% 11%	576
August	1st to 10th	Day	13% 3 19% 27% 25% 12%	526
	11th to 20th	Day	15% 6% 17% 26% 23% 12%	519
	21st to end	Day	11% 5% 20% 25% 25% 14%	583
September	1st to 10th	Day	15% 7% 19% 23% 23% 13%	530
	11th to 20th	Day	8% 3 14% 27% 31% 16%	530
	21st to end	Day	25% 9% 21% 23% 14% 8%	530
October	1st to 10th	Day	13% 8% 28% 24% 22% 5%	530
	11th to 20th	Day	19% 15% 33% 22% 8% 3	530
	21st to end	Day	28% 14% 31% 19% 7%	583
November	1st to 10th	Day	18% 19% 36% 17% 9% 1	347
	11th to 20th	Day	28% 22% 30% 14% 6% 1	320
	21st to end	Day	27% 19% 38% 15% 2	220
December	1st to 10th	Day	26% 20% 36% 12% 4 2	220
	11th to 20th	Day	25% 19% 39% 12% 4	220
	21st to end	Day	32% 20% 33% 13% 2	303
January	1st to 10th	Night	25% 17% 35% 19% 4	840
	11th to 20th	Night	27% 18% 34% 17% 4	840
	21st to end	Night	26% 16% 31% 23% 4	924
February	1st to 10th	Night	27% 13% 33% 23% 3	840
	11th to 20th	Night	24% 16% 35% 23% 2	823
	21st to end	Night	26% 15% 37% 18% 3	692
March	1st to 10th	Night	25% 12% 34% 24% 5%	840
	11th to 20th	Night	23% 14% 32% 24% 6% 1	840
	21st to end	Night	23% 11% 34% 24% 6% 1	924
April	1st to 10th	Night	23% 14% 33% 24% 6%	840
	11th to 20th	Night	22% 10% 34% 24% 10% 1	696
	21st to end	Night	19% 8% 33% 32% 8% 1	630
May	1st to 10th	Night	22% 6% 28% 34% 10% 1	630
	11th to 20th	Night	18% 5% 24% 38% 13% 2	609
	21st to end	Night	13% 3 18% 42% 21% 3	635
June	1st to 10th	Night	9% 4 26% 44% 17% 1	400
	11th to 20th	Night	12% 4 22% 45% 17% 1	400
	21st to end	Night	11% 3 23% 39% 24% 1	433
July	1st to 10th	Night	10% 2 14% 41% 29% 5%	585
	11th to 20th	Night	10% 3 16% 39% 28% 4	582
	21st to end	Night	9% 3 14% 42% 30% 2	657
August	1st to 10th	Night	10% 2 14% 43% 30% 2	597
	11th to 20th	Night	13% 1 12% 39% 30% 4	594
	21st to end	Night	10% 2 14% 37% 32% 5%	696
September	1st to 10th	Night	14% 3 16% 34% 28% 5%	800
	11th to 20th	Night	11% 2 12% 37% 33% 6%	800
	21st to end	Night	19% 5% 22% 31% 20% 4	800
October	1st to 10th	Night	17% 5% 23% 32% 21% 3	800
	11th to 20th	Night	20% 8% 31% 28% 11% 2	800
	21st to end	Night	26% 9% 27% 30% 7%	880
November	1st to 10th	Night	18% 9% 31% 30% 12% 1	820
	11th to 20th	Night	23% 13% 33% 22% 7% 1	840
	21st to end	Night	28% 13% 32% 21% 6%	840
December	1st to 10th	Night	29% 13% 32% 21% 5%	840
	11th to 20th	Night	26% 15% 33% 22% 4	840
	21st to end	Night	29% 13% 34% 22% 2	924

Page generated at 2026-05-14 01:59:01. Version ac07

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