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 2025-06-10.

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	18% 8% 3 6% 4 4 4 3 5% 7% 39%	119077
	Night	18% 8% 3 6% 4 4 4 3 5% 6% 39%	119129
Transparency	Day	45% 5% 12% 21% 15% 3	122670
	Night	46% 5% 11% 20% 15% 3	123684
Seeing	Day	20% 11% 29% 27% 11% 2	15899
	Night	22% 12% 29% 26% 10% 1	24864

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% 8% 3 6% 3 3 3 3 4 6% 48%	3597
	11th to 20th	Day	16% 9% 2 4 3 3 3 3 4 4 49%	3570
	21st to end	Day	18% 9% 3 7% 4 3 3 3 3 4 41%	3960
February	1st to 10th	Day	19% 8% 3 6% 4 4 5% 4 5% 6% 36%	1934
	11th to 20th	Day	15% 8% 2 6% 4 5% 3 3 5% 6% 42%	1948
	21st to end	Day	19% 10% 2 6% 4 4 4 3 4 6% 38%	1650
March	1st to 10th	Day	21% 11% 3 6% 3 3 3 2 4 5% 38%	4476
	11th to 20th	Day	14% 10% 3 6% 4 4 4 3 5% 6% 42%	4796
	21st to end	Day	25% 10% 3 7% 4 3 4 3 5% 5% 32%	5727
April	1st to 10th	Day	20% 8% 3 6% 3 4 3 4 6% 6% 37%	2400
	11th to 20th	Day	19% 8% 2 5% 4 3 5% 3 4 7% 40%	2384
	21st to end	Day	20% 7% 2 6% 5% 4 4 3 5% 6% 38%	2600
May	1st to 10th	Day	28% 11% 3 7% 4 4 4 3 5% 7% 24%	5886
	11th to 20th	Day	21% 11% 3 7% 4 4 4 3 5% 6% 32%	5861
	21st to end	Day	20% 8% 3 7% 4 4 4 4 5% 8% 32%	6893
June	1st to 10th	Day	19% 7% 2 6% 4 4 4 5% 7% 8% 33%	2786
	11th to 20th	Day	16% 7% 2 6% 4 3 4 4 6% 8% 39%	2660
	21st to end	Day	15% 7% 2 5% 5% 4 4 3 7% 8% 40%	2660
July	1st to 10th	Day	17% 9% 3 8% 5% 4 4 4 5% 8% 33%	4440
	11th to 20th	Day	15% 7% 3 6% 4 4 4 4 6% 9% 37%	4440
	21st to end	Day	19% 8% 3 7% 5% 5% 5% 4 6% 7% 32%	4766
August	1st to 10th	Day	17% 8% 3 8% 5% 5% 4 4 6% 7% 33%	4105
	11th to 20th	Day	12% 7% 2 6% 4 5% 5% 4 6% 9% 38%	3982
	21st to end	Day	8% 4 2 5% 4 4 5% 5% 6% 9% 49%	4180
September	1st to 10th	Day	15% 7% 2 5% 4 3 4 4 5% 6% 46%	2257
	11th to 20th	Day	13% 6% 2 5% 3 3 3 3 5% 6% 51%	2090
	21st to end	Day	21% 6% 2 6% 4 4 4 3 5% 5% 41%	2090
October	1st to 10th	Day	22% 6% 2 6% 4 3 3 3 4 5% 43%	2090
	11th to 20th	Day	15% 8% 2 4 3 3 3 3 5% 7% 48%	1886
	21st to end	Day	19% 5% 2 5% 5% 4 3 3 4 6% 45%	1881
November	1st to 10th	Day	17% 6% 2 6% 4 3 3 3 4 4 47%	1761
	11th to 20th	Day	17% 6% 2 5% 3 4 3 3 5% 7% 46%	1800
	21st to end	Day	18% 7% 2 5% 5% 4 3 3 5% 6% 43%	1800
December	1st to 10th	Day	15% 9% 3 6% 3 3 3 2 4 6% 48%	2775
	11th to 20th	Day	19% 8% 2 5% 2 3 2 2 3 5% 48%	2986
	21st to end	Day	21% 11% 3 7% 3 4 3 2 4 4 37%	3960
January	1st to 10th	Night	14% 5% 2 5% 3 3 3 3 4 6% 52%	6349
	11th to 20th	Night	15% 10% 4 7% 3 3 3 2 4 5% 42%	6315
	21st to end	Night	17% 10% 4 7% 4 4 4 3 5% 5% 36%	6982
February	1st to 10th	Night	17% 9% 3 7% 5% 4 3 3 4 5% 40%	2463
	11th to 20th	Night	19% 8% 3 6% 4 3 4 3 5% 6% 40%	2338
	21st to end	Night	17% 9% 2 7% 5% 4 4 3 5% 5% 40%	1980
March	1st to 10th	Night	23% 9% 2 5% 3 4 4 3 4 6% 37%	5364
	11th to 20th	Night	17% 10% 4 7% 4 4 4 4 4 5% 37%	5149
	21st to end	Night	23% 11% 3 7% 4 4 4 3 4 4 33%	5202
April	1st to 10th	Night	19% 9% 3 6% 5% 4 4 3 4 6% 37%	2000
	11th to 20th	Night	19% 9% 2 6% 4 3 3 4 5% 6% 38%	1963
	21st to end	Night	21% 7% 3 6% 6% 3 3 4 4 5% 39%	1800
May	1st to 10th	Night	28% 11% 4 8% 4 4 4 3 5% 6% 24%	4054
	11th to 20th	Night	21% 12% 4 7% 4 4 4 3 5% 6% 28%	4033
	21st to end	Night	21% 9% 3 8% 5% 5% 6% 4 6% 6% 26%	4024
June	1st to 10th	Night	18% 10% 3 6% 4 4 5% 4 5% 8% 34%	1598
	11th to 20th	Night	19% 7% 2 5% 4 3 4 3 5% 8% 40%	1520
	21st to end	Night	19% 8% 2 6% 4 3 4 4 7% 8% 35%	1520
July	1st to 10th	Night	21% 13% 3 7% 3 4 4 3 5% 7% 30%	2520
	11th to 20th	Night	18% 7% 3 6% 5% 4 4 4 7% 9% 33%	2520
	21st to end	Night	20% 9% 2 7% 5% 4 5% 4 5% 7% 30%	2753
August	1st to 10th	Night	23% 8% 3 7% 4 5% 5% 4 6% 7% 27%	2825
	11th to 20th	Night	9% 8% 3 8% 5% 6% 6% 5% 6% 8% 37%	2841
	21st to end	Night	9% 4 2 4 3 3 4 4 6% 8% 52%	3476
September	1st to 10th	Night	15% 6% 3 6% 4 4 4 3 5% 7% 43%	1921
	11th to 20th	Night	15% 6% 2 5% 3 2 3 3 4 6% 51%	2090
	21st to end	Night	22% 6% 2 5% 4 4 5% 4 4 5% 40%	2090
October	1st to 10th	Night	19% 8% 3 6% 4 4 4 3 4 6% 39%	2090
	11th to 20th	Night	15% 5% 2 5% 3 3 4 3 5% 7% 48%	2289
	21st to end	Night	19% 8% 2 7% 3 3 4 3 4 6% 40%	2717
November	1st to 10th	Night	15% 6% 2 6% 4 4 4 3 4 6% 46%	2537
	11th to 20th	Night	16% 8% 2 5% 3 3 3 3 4 5% 45%	2600
	21st to end	Night	15% 6% 2 6% 4 3 4 3 6% 7% 45%	2600
December	1st to 10th	Night	18% 6% 2 5% 4 3 3 2 3 4 48%	4402
	11th to 20th	Night	22% 9% 2 5% 3 3 3 2 4 5% 42%	5208
	21st to end	Night	19% 10% 3 7% 4 3 3 3 4 5% 40%	6996

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	37% 3 7% 26% 23% 3	3598
	11th to 20th	Day	40% 5% 10% 21% 17% 8%	3600
	21st to end	Day	48% 3 9% 21% 18% 2	3960
February	1st to 10th	Day	48% 6% 13% 18% 14% 2	1934
	11th to 20th	Day	43% 5% 12% 22% 15% 3	1957
	21st to end	Day	47% 4 11% 20% 16% 3	1650
March	1st to 10th	Day	49% 3 11% 13% 12% 11%	4479
	11th to 20th	Day	40% 5% 17% 26% 11% 2	4796
	21st to end	Day	54% 4 7% 14% 16% 5%	5727
April	1st to 10th	Day	45% 5% 11% 19% 17% 3	2400
	11th to 20th	Day	43% 6% 11% 20% 18% 3	2384
	21st to end	Day	45% 5% 10% 20% 17% 4	2600
May	1st to 10th	Day	60% 7% 13% 12% 6% 1	5897
	11th to 20th	Day	53% 5% 14% 16% 11% 1	5865
	21st to end	Day	50% 7% 10% 19% 13% 1	6893
June	1st to 10th	Day	46% 7% 12% 21% 13% 1	2786
	11th to 20th	Day	42% 7% 14% 23% 13% 1	2660
	21st to end	Day	41% 8% 14% 23% 13% 1	2660
July	1st to 10th	Day	51% 7% 9% 18% 13% 1	4370
	11th to 20th	Day	42% 6% 17% 22% 11% 1	4365
	21st to end	Day	48% 6% 13% 18% 13% 1	4614
August	1st to 10th	Day	50% 5% 18% 20% 8%	4092
	11th to 20th	Day	41% 5% 17% 28% 8% 1	4021
	21st to end	Day	30% 6% 15% 29% 19% 1	4180
September	1st to 10th	Day	38% 6% 10% 25% 20% 1	2257
	11th to 20th	Day	33% 6% 12% 23% 22% 3	2090
	21st to end	Day	46% 5% 8% 20% 20% 2	2090
October	1st to 10th	Day	38% 4 11% 26% 17% 4	3480
	11th to 20th	Day	31% 4 8% 27% 26% 4	3102
	21st to end	Day	31% 4 15% 30% 19% 2	3081
November	1st to 10th	Day	42% 5% 13% 20% 18% 2	1761
	11th to 20th	Day	39% 5% 13% 20% 20% 4	1800
	21st to end	Day	43% 5% 11% 26% 12% 2	1800
December	1st to 10th	Day	41% 4 13% 28% 13% 1	2775
	11th to 20th	Day	42% 3 14% 17% 19% 6%	2986
	21st to end	Day	51% 4 8% 19% 13% 4	3960
January	1st to 10th	Night	34% 4 8% 33% 18% 3	6349
	11th to 20th	Night	47% 3 10% 20% 15% 5%	6360
	21st to end	Night	51% 5% 9% 18% 14% 4	6982
February	1st to 10th	Night	48% 6% 12% 21% 12% 2	2463
	11th to 20th	Night	47% 5% 9% 20% 16% 4	2340
	21st to end	Night	47% 4 9% 22% 14% 3	1980
March	1st to 10th	Night	50% 4 9% 17% 14% 7%	5374
	11th to 20th	Night	49% 5% 14% 18% 11% 3	5150
	21st to end	Night	55% 3 6% 10% 21% 5%	5202
April	1st to 10th	Night	49% 3 8% 20% 16% 3	2000
	11th to 20th	Night	47% 4 10% 18% 18% 3	1963
	21st to end	Night	49% 5% 8% 18% 15% 4	1800
May	1st to 10th	Night	61% 6% 13% 11% 8% 1	4060
	11th to 20th	Night	57% 5% 13% 15% 8% 3	4034
	21st to end	Night	56% 7% 11% 16% 10%	4025
June	1st to 10th	Night	50% 6% 11% 19% 14% 1	1598
	11th to 20th	Night	44% 5% 11% 25% 14% 1	1520
	21st to end	Night	47% 6% 13% 20% 13% 1	1520
July	1st to 10th	Night	55% 5% 9% 16% 15%	2480
	11th to 20th	Night	46% 7% 16% 22% 9%	2472
	21st to end	Night	53% 5% 13% 16% 11% 2	2665
August	1st to 10th	Night	54% 7% 13% 17% 8% 1	2813
	11th to 20th	Night	42% 6% 16% 28% 8%	2868
	21st to end	Night	30% 5% 16% 28% 20% 1	3476
September	1st to 10th	Night	39% 7% 11% 23% 18% 2	1921
	11th to 20th	Night	37% 5% 10% 23% 22% 2	2090
	21st to end	Night	47% 6% 7% 20% 18% 3	2090
October	1st to 10th	Night	42% 4 10% 26% 15% 2	3480
	11th to 20th	Night	27% 4 8% 24% 33% 4	3799
	21st to end	Night	34% 4 12% 27% 22% 2	4467
November	1st to 10th	Night	41% 6% 12% 21% 18% 3	2537
	11th to 20th	Night	43% 5% 10% 20% 18% 4	2600
	21st to end	Night	40% 5% 13% 26% 12% 2	2600
December	1st to 10th	Night	42% 4 11% 26% 16% 1	4402
	11th to 20th	Night	48% 3 12% 16% 19% 2	5208
	21st to end	Night	48% 4 10% 20% 17% 2	6996

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% 21% 34% 19% 4 1	200
	11th to 20th	Day	19% 23% 36% 18% 5%	200
	21st to end	Day	18% 19% 39% 19% 5% 1	220
February	1st to 10th	Day	23% 21% 34% 20% 2	466
	11th to 20th	Day	23% 22% 39% 15% 1	496
	21st to end	Day	22% 19% 36% 20% 4	414
March	1st to 10th	Day	22% 18% 35% 20% 4	500
	11th to 20th	Day	18% 20% 36% 22% 4 1	500
	21st to end	Day	23% 16% 36% 18% 6% 1	550
April	1st to 10th	Day	26% 15% 31% 22% 6%	500
	11th to 20th	Day	24% 12% 37% 23% 3	492
	21st to end	Day	23% 9% 34% 24% 9% 1	500
May	1st to 10th	Day	29% 12% 30% 23% 5%	503
	11th to 20th	Day	21% 8% 36% 25% 9% 1	527
	21st to end	Day	21% 5% 27% 31% 13% 2	573
June	1st to 10th	Day	20% 6% 27% 32% 15% 1	526
	11th to 20th	Day	17% 5% 30% 36% 12% 1	490
	21st to end	Day	18% 4 28% 34% 14% 1	490
July	1st to 10th	Day	19% 4 20% 35% 20% 2	485
	11th to 20th	Day	13% 4 20% 38% 22% 3	484
	21st to end	Day	14% 6% 23% 41% 14% 2	532
August	1st to 10th	Day	13% 4 22% 40% 18% 2	486
	11th to 20th	Day	16% 6% 17% 37% 20% 4	479
	21st to end	Day	13% 4 22% 35% 20% 6%	539
September	1st to 10th	Day	18% 4 22% 32% 17% 6%	490
	11th to 20th	Day	17% 6% 21% 31% 17% 8%	490
	21st to end	Day	23% 6% 27% 28% 13% 4	490
October	1st to 10th	Day	23% 10% 22% 26% 15% 4	490
	11th to 20th	Day	18% 10% 29% 27% 14% 2	433
	21st to end	Day	22% 14% 28% 24% 10% 2	429
November	1st to 10th	Day	15% 15% 32% 24% 12% 2	395
	11th to 20th	Day	21% 17% 35% 22% 6%	400
	21st to end	Day	22% 20% 29% 20% 9% 1	400
December	1st to 10th	Day	20% 15% 36% 23% 6%	310
	11th to 20th	Day	26% 22% 34% 15% 4 1	200
	21st to end	Day	25% 15% 35% 15% 8% 3	220
January	1st to 10th	Night	20% 21% 37% 19% 3	800
	11th to 20th	Night	28% 20% 33% 17% 3	800
	21st to end	Night	21% 21% 36% 18% 3	880
February	1st to 10th	Night	22% 19% 36% 21% 2	800
	11th to 20th	Night	24% 21% 39% 14% 2	783
	21st to end	Night	23% 18% 37% 20% 3	660
March	1st to 10th	Night	21% 19% 34% 22% 4	800
	11th to 20th	Night	20% 14% 37% 24% 4 1	800
	21st to end	Night	24% 11% 35% 22% 6% 1	880
April	1st to 10th	Night	24% 11% 35% 26% 4	800
	11th to 20th	Night	23% 9% 37% 25% 6%	779
	21st to end	Night	23% 8% 33% 29% 7% 1	600
May	1st to 10th	Night	28% 6% 29% 27% 9% 1	600
	11th to 20th	Night	24% 9% 33% 27% 7% 1	592
	21st to end	Night	26% 3 18% 38% 15% 1	479
June	1st to 10th	Night	24% 4 22% 35% 15% 1	400
	11th to 20th	Night	21% 4 20% 41% 14% 1	380
	21st to end	Night	25% 2 17% 43% 13%	380
July	1st to 10th	Night	20% 3 18% 38% 19% 1	370
	11th to 20th	Night	17% 2 17% 40% 23% 1	368
	21st to end	Night	18% 3 17% 41% 21% 1	418
August	1st to 10th	Night	13% 2 21% 44% 20% 1	568
	11th to 20th	Night	13% 3 17% 44% 21% 2	595
	21st to end	Night	17% 2 15% 38% 24% 5%	836
September	1st to 10th	Night	16% 3 24% 35% 20% 3	760
	11th to 20th	Night	18% 4 22% 28% 23% 5%	760
	21st to end	Night	25% 5% 21% 31% 15% 3	760
October	1st to 10th	Night	25% 9% 23% 24% 16% 3	760
	11th to 20th	Night	18% 11% 30% 26% 12% 2	760
	21st to end	Night	24% 13% 31% 22% 9% 1	836
November	1st to 10th	Night	19% 10% 32% 26% 11% 2	780
	11th to 20th	Night	20% 21% 32% 20% 7% 1	800
	21st to end	Night	19% 22% 30% 21% 7% 1	800
December	1st to 10th	Night	25% 20% 30% 18% 6% 1	800
	11th to 20th	Night	25% 22% 31% 17% 4	800
	21st to end	Night	26% 19% 34% 17% 4	880

Page generated at 2025-06-07 10:28:19. Version ac07

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