Driftway Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Driftway Observatory clear sky chart from 2005-11-06 to 2026-10-14.

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	17% 8% 3 6% 4 4 4 3 5% 7% 39%	148850
	Night	18% 8% 3 6% 4 4 4 3 5% 6% 39%	142861
Transparency	Day	44% 5% 13% 22% 14% 2	152400
	Night	46% 4 11% 21% 15% 2	147387
Seeing	Day	20% 11% 29% 27% 11% 2	17694
	Night	22% 12% 29% 26% 10% 1	26514

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	11% 9% 3 7% 4 4 3 3 5% 6% 46%	4661
	11th to 20th	Day	16% 9% 3 5% 3 3 3 3 3 4 47%	4650
	21st to end	Day	15% 9% 2 6% 3 3 3 2 3 5% 47%	5124
February	1st to 10th	Day	18% 8% 3 6% 5% 4 4 4 5% 6% 37%	2031
	11th to 20th	Day	15% 8% 2 7% 4 5% 3 3 5% 6% 42%	2048
	21st to end	Day	19% 10% 2 6% 4 4 4 3 4 6% 38%	1730
March	1st to 10th	Day	22% 11% 3 6% 3 3 3 2 4 5% 38%	4576
	11th to 20th	Day	14% 9% 3 6% 4 4 4 3 5% 7% 42%	4902
	21st to end	Day	25% 10% 3 7% 4 3 4 3 5% 5% 32%	5853
April	1st to 10th	Day	20% 8% 3 6% 3 4 3 4 5% 6% 37%	2520
	11th to 20th	Day	19% 8% 2 5% 4 3 4 4 4 7% 40%	2505
	21st to end	Day	20% 7% 2 6% 5% 4 4 3 5% 6% 38%	2730
May	1st to 10th	Day	25% 11% 3 7% 4 4 4 3 5% 6% 27%	7639
	11th to 20th	Day	20% 10% 3 7% 4 3 4 3 4 6% 36%	7625
	21st to end	Day	22% 9% 3 7% 4 5% 4 4 5% 8% 29%	8976
June	1st to 10th	Day	18% 7% 2 6% 4 5% 4 5% 7% 8% 35%	2940
	11th to 20th	Day	16% 7% 2 5% 4 4 5% 5% 6% 8% 38%	2940
	21st to end	Day	16% 7% 2 5% 4 4 4 3 7% 8% 40%	2940
July	1st to 10th	Day	20% 8% 3 6% 4 3 4 4 6% 9% 33%	8280
	11th to 20th	Day	11% 7% 3 6% 4 4 4 4 6% 10% 39%	8280
	21st to end	Day	16% 7% 2 6% 4 4 4 4 5% 7% 41%	8917
August	1st to 10th	Day	15% 7% 3 7% 5% 5% 4 4 7% 9% 33%	6015
	11th to 20th	Day	12% 6% 2 6% 4 4 5% 4 6% 8% 42%	5860
	21st to end	Day	11% 4 2 5% 4 4 5% 4 6% 9% 46%	6080
September	1st to 10th	Day	15% 7% 2 5% 4 3 5% 4 5% 6% 45%	2495
	11th to 20th	Day	13% 6% 2 5% 3 3 3 3 5% 6% 51%	2310
	21st to end	Day	22% 5% 2 6% 5% 4 4 3 5% 5% 39%	2310
October	1st to 10th	Day	20% 5% 2 5% 3 3 3 3 4 5% 46%	2310
	11th to 20th	Day	16% 8% 2 4 3 3 3 2 4 7% 47%	2018
	21st to end	Day	18% 5% 2 5% 5% 4 3 3 4 6% 45%	1980
November	1st to 10th	Day	17% 6% 2 5% 4 3 3 3 4 5% 47%	1851
	11th to 20th	Day	16% 6% 2 5% 3 4 3 4 5% 7% 46%	1890
	21st to end	Day	18% 7% 2 5% 4 4 3 3 5% 6% 43%	1890
December	1st to 10th	Day	15% 8% 3 6% 3 3 3 2 4 7% 48%	2860
	11th to 20th	Day	19% 8% 2 5% 2 3 2 2 3 5% 49%	3066
	21st to end	Day	21% 11% 3 7% 3 4 3 2 4 5% 37%	4048
January	1st to 10th	Night	15% 6% 2 5% 3 3 3 3 4 5% 50%	8234
	11th to 20th	Night	16% 10% 3 7% 4 3 4 3 4 5% 41%	8235
	21st to end	Night	15% 9% 3 7% 4 5% 4 3 4 5% 39%	9043
February	1st to 10th	Night	16% 9% 3 7% 5% 4 3 3 4 6% 40%	2586
	11th to 20th	Night	19% 8% 3 6% 4 3 4 3 5% 6% 40%	2458
	21st to end	Night	17% 9% 2 7% 4 4 4 3 5% 5% 40%	2076
March	1st to 10th	Night	23% 9% 2 5% 3 4 4 3 4 6% 37%	5484
	11th to 20th	Night	17% 10% 4 7% 4 4 4 4 4 5% 38%	5263
	21st to end	Night	23% 11% 3 7% 4 4 4 3 4 4 33%	5318
April	1st to 10th	Night	19% 9% 3 6% 4 4 4 3 4 6% 37%	2100
	11th to 20th	Night	18% 9% 2 6% 4 3 4 4 5% 6% 38%	2062
	21st to end	Night	21% 7% 3 6% 6% 3 3 3 4 5% 39%	1890
May	1st to 10th	Night	27% 10% 3 7% 3 4 3 3 5% 6% 29%	5259
	11th to 20th	Night	19% 11% 3 7% 4 4 4 3 5% 7% 32%	5235
	21st to end	Night	22% 9% 3 8% 5% 5% 6% 4 6% 6% 25%	5240
June	1st to 10th	Night	17% 9% 3 6% 4 4 4 4 5% 8% 36%	1680
	11th to 20th	Night	19% 7% 2 6% 4 3 4 3 5% 8% 39%	1680
	21st to end	Night	18% 8% 3 6% 4 3 4 4 6% 8% 36%	1680
July	1st to 10th	Night	22% 13% 3 6% 4 4 4 3 4 7% 29%	4680
	11th to 20th	Night	17% 9% 3 6% 4 4 4 4 6% 8% 36%	4680
	21st to end	Night	17% 7% 2 6% 5% 4 5% 4 5% 7% 39%	5098
August	1st to 10th	Night	19% 8% 2 7% 5% 5% 6% 5% 7% 8% 30%	4135
	11th to 20th	Night	10% 7% 3 8% 4 5% 6% 5% 6% 9% 38%	4181
	21st to end	Night	11% 5% 2 5% 4 3 5% 4 6% 9% 47%	5045
September	1st to 10th	Night	14% 6% 3 6% 4 4 4 3 6% 7% 42%	2123
	11th to 20th	Night	15% 6% 2 5% 3 3 3 3 4 6% 51%	2310
	21st to end	Night	23% 7% 2 5% 4 4 5% 4 4 5% 38%	2310
October	1st to 10th	Night	18% 8% 2 6% 4 4 4 3 4 6% 42%	2310
	11th to 20th	Night	16% 5% 2 5% 3 3 4 3 5% 7% 47%	2444
	21st to end	Night	19% 8% 2 7% 3 3 4 3 4 6% 41%	2860
November	1st to 10th	Night	15% 6% 2 6% 4 4 4 3 5% 6% 46%	2667
	11th to 20th	Night	16% 8% 2 5% 3 3 4 3 4 5% 46%	2730
	21st to end	Night	15% 6% 2 6% 4 3 4 3 6% 7% 44%	2730
December	1st to 10th	Night	18% 7% 2 5% 4 3 3 2 3 4 48%	4537
	11th to 20th	Night	21% 9% 2 5% 3 3 3 2 4 5% 42%	5348
	21st to end	Night	19% 10% 3 7% 4 3 3 3 4 5% 40%	7150

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	39% 3 10% 24% 21% 2	4662
	11th to 20th	Day	42% 4 9% 20% 17% 8%	4680
	21st to end	Day	42% 3 10% 24% 20% 2	5124
February	1st to 10th	Day	47% 6% 13% 18% 15% 2	2031
	11th to 20th	Day	43% 4 12% 23% 15% 3	2057
	21st to end	Day	47% 4 11% 20% 15% 3	1730
March	1st to 10th	Day	49% 3 11% 13% 13% 11%	4579
	11th to 20th	Day	40% 5% 17% 26% 11% 2	4902
	21st to end	Day	54% 4 7% 14% 16% 5%	5853
April	1st to 10th	Day	45% 5% 11% 18% 18% 3	2520
	11th to 20th	Day	43% 5% 11% 20% 18% 3	2505
	21st to end	Day	45% 5% 10% 20% 17% 3	2730
May	1st to 10th	Day	57% 6% 12% 15% 9% 1	7651
	11th to 20th	Day	50% 4 15% 21% 10% 1	7629
	21st to end	Day	54% 6% 11% 18% 11% 1	8977
June	1st to 10th	Day	45% 7% 12% 22% 14% 1	2940
	11th to 20th	Day	43% 6% 13% 23% 13% 1	2940
	21st to end	Day	40% 8% 15% 23% 13% 1	2940
July	1st to 10th	Day	50% 6% 13% 19% 10% 1	8210
	11th to 20th	Day	41% 4 16% 27% 12% 1	8205
	21st to end	Day	42% 4 11% 25% 16% 1	8720
August	1st to 10th	Day	47% 5% 20% 20% 8%	6002
	11th to 20th	Day	38% 4 19% 31% 7% 1	5899
	21st to end	Day	34% 5% 15% 29% 16% 1	6080
September	1st to 10th	Day	39% 6% 10% 26% 19% 1	2495
	11th to 20th	Day	33% 6% 12% 24% 22% 3	2310
	21st to end	Day	47% 5% 8% 19% 19% 2	2310
October	1st to 10th	Day	37% 4 11% 27% 18% 4	3700
	11th to 20th	Day	32% 4 8% 27% 25% 4	3234
	21st to end	Day	31% 4 15% 30% 19% 2	3180
November	1st to 10th	Day	42% 4 13% 20% 19% 2	1851
	11th to 20th	Day	39% 5% 12% 21% 19% 3	1890
	21st to end	Day	44% 5% 11% 26% 13% 2	1890
December	1st to 10th	Day	41% 4 13% 28% 14% 1	2860
	11th to 20th	Day	42% 3 13% 17% 19% 6%	3066
	21st to end	Day	51% 4 8% 20% 13% 4	4048
January	1st to 10th	Night	38% 3 9% 30% 18% 2	8234
	11th to 20th	Night	48% 3 9% 19% 15% 5%	8280
	21st to end	Night	49% 4 7% 19% 17% 4	9043
February	1st to 10th	Night	47% 6% 11% 21% 13% 1	2586
	11th to 20th	Night	46% 4 9% 21% 16% 3	2460
	21st to end	Night	47% 4 9% 23% 14% 3	2076
March	1st to 10th	Night	50% 3 9% 17% 14% 7%	5494
	11th to 20th	Night	48% 5% 14% 18% 11% 3	5264
	21st to end	Night	55% 3 6% 10% 20% 5%	5318
April	1st to 10th	Night	49% 3 9% 19% 16% 4	2100
	11th to 20th	Night	46% 4 10% 18% 18% 3	2062
	21st to end	Night	49% 5% 9% 18% 15% 4	1890
May	1st to 10th	Night	56% 5% 13% 14% 11% 1	5265
	11th to 20th	Night	53% 4 14% 19% 8% 2	5236
	21st to end	Night	58% 6% 10% 18% 8%	5241
June	1st to 10th	Night	48% 6% 11% 20% 14% 1	1680
	11th to 20th	Night	45% 5% 11% 24% 14% 1	1680
	21st to end	Night	46% 6% 13% 21% 13% 1	1680
July	1st to 10th	Night	56% 3 10% 17% 13%	4640
	11th to 20th	Night	45% 5% 14% 23% 12% 1	4632
	21st to end	Night	45% 4 12% 23% 15% 1	4981
August	1st to 10th	Night	50% 7% 17% 15% 10% 1	4123
	11th to 20th	Night	41% 5% 18% 30% 7%	4208
	21st to end	Night	35% 5% 16% 28% 16% 1	5045
September	1st to 10th	Night	39% 7% 11% 24% 17% 1	2123
	11th to 20th	Night	37% 5% 10% 23% 22% 3	2310
	21st to end	Night	49% 5% 7% 19% 17% 2	2310
October	1st to 10th	Night	41% 4 10% 26% 16% 3	3700
	11th to 20th	Night	28% 4 9% 24% 32% 4	3954
	21st to end	Night	34% 4 12% 27% 22% 2	4610
November	1st to 10th	Night	41% 6% 11% 22% 18% 3	2667
	11th to 20th	Night	42% 5% 10% 21% 18% 4	2730
	21st to end	Night	41% 5% 13% 26% 13% 2	2730
December	1st to 10th	Night	43% 4 11% 25% 16% 1	4537
	11th to 20th	Night	48% 3 12% 16% 19% 2	5348
	21st to end	Night	48% 4 10% 20% 17% 2	7150

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	22% 20% 33% 20% 5%	220
	11th to 20th	Day	19% 23% 37% 17% 4	220
	21st to end	Day	18% 19% 40% 18% 5% 1	242
February	1st to 10th	Day	22% 20% 34% 20% 4	503
	11th to 20th	Day	23% 21% 38% 16% 1	536
	21st to end	Day	22% 17% 37% 20% 4	446
March	1st to 10th	Day	24% 17% 34% 21% 5%	540
	11th to 20th	Day	18% 20% 35% 23% 3 1	540
	21st to end	Day	23% 16% 37% 18% 6% 1	594
April	1st to 10th	Day	26% 15% 32% 21% 6% 1	540
	11th to 20th	Day	23% 12% 36% 25% 4	532
	21st to end	Day	23% 9% 34% 25% 8% 1	540
May	1st to 10th	Day	28% 13% 31% 23% 5%	543
	11th to 20th	Day	21% 8% 35% 26% 9% 1	567
	21st to end	Day	22% 6% 27% 30% 12% 2	617
June	1st to 10th	Day	19% 5% 28% 31% 15% 2	570
	11th to 20th	Day	18% 5% 29% 36% 11% 1	570
	21st to end	Day	18% 4 26% 34% 16% 2	570
July	1st to 10th	Day	19% 4 19% 36% 20% 1	565
	11th to 20th	Day	13% 3 20% 38% 23% 3	564
	21st to end	Day	15% 6% 22% 41% 14% 2	620
August	1st to 10th	Day	13% 4 21% 40% 20% 2	566
	11th to 20th	Day	16% 5% 17% 37% 20% 4	559
	21st to end	Day	13% 4 23% 35% 20% 5%	627
September	1st to 10th	Day	18% 4 22% 33% 17% 5%	570
	11th to 20th	Day	16% 5% 21% 32% 19% 7%	570
	21st to end	Day	24% 6% 26% 25% 14% 4	570
October	1st to 10th	Day	21% 9% 23% 26% 16% 5%	570
	11th to 20th	Day	20% 10% 28% 27% 14% 2	479
	21st to end	Day	22% 15% 27% 25% 10% 2	462
November	1st to 10th	Day	16% 15% 32% 24% 12% 2	425
	11th to 20th	Day	20% 17% 36% 21% 6%	430
	21st to end	Day	21% 19% 30% 20% 8% 1	430
December	1st to 10th	Day	20% 14% 36% 23% 6%	335
	11th to 20th	Day	25% 22% 34% 15% 4	220
	21st to end	Day	25% 17% 34% 15% 7% 2	242
January	1st to 10th	Night	20% 20% 38% 19% 3	840
	11th to 20th	Night	28% 21% 33% 16% 3	840
	21st to end	Night	21% 22% 36% 18% 3	924
February	1st to 10th	Night	21% 19% 36% 22% 3	840
	11th to 20th	Night	24% 20% 38% 16% 2	823
	21st to end	Night	23% 17% 36% 20% 3	692
March	1st to 10th	Night	22% 18% 33% 23% 4	840
	11th to 20th	Night	20% 14% 37% 24% 4 1	840
	21st to end	Night	25% 11% 35% 22% 6% 1	924
April	1st to 10th	Night	24% 10% 35% 26% 5%	840
	11th to 20th	Night	23% 8% 36% 26% 6%	818
	21st to end	Night	23% 7% 33% 29% 7% 1	630
May	1st to 10th	Night	28% 6% 30% 27% 9%	630
	11th to 20th	Night	23% 9% 33% 27% 7% 1	622
	21st to end	Night	26% 3 17% 38% 15% 1	503
June	1st to 10th	Night	23% 5% 21% 36% 15% 1	420
	11th to 20th	Night	21% 4 21% 41% 13%	420
	21st to end	Night	24% 2 16% 43% 15%	420
July	1st to 10th	Night	21% 3 17% 38% 19% 1	410
	11th to 20th	Night	16% 2 17% 38% 24% 3	408
	21st to end	Night	17% 3 16% 41% 21% 1	462
August	1st to 10th	Night	14% 2 20% 43% 21% 1	628
	11th to 20th	Night	13% 3 17% 45% 21% 2	659
	21st to end	Night	17% 2 15% 38% 23% 4	924
September	1st to 10th	Night	15% 3 23% 37% 20% 2	840
	11th to 20th	Night	18% 3 21% 29% 24% 4	840
	21st to end	Night	26% 5% 21% 30% 15% 2	840
October	1st to 10th	Night	23% 8% 24% 25% 16% 4	840
	11th to 20th	Night	19% 11% 29% 26% 12% 2	813
	21st to end	Night	23% 13% 31% 22% 9% 1	880
November	1st to 10th	Night	19% 11% 32% 26% 10% 2	820
	11th to 20th	Night	19% 21% 32% 20% 7%	840
	21st to end	Night	19% 22% 31% 21% 7% 1	840
December	1st to 10th	Night	25% 20% 31% 18% 5%	840
	11th to 20th	Night	24% 23% 32% 17% 3	840
	21st to end	Night	25% 20% 34% 17% 4	924

Page generated at 2026-10-11 05:06:29. Version ac07

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