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 2024-11-02.

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% 38%	101111
	Night	19% 9% 3 6% 4 4 4 3 5% 6% 38%	97864
Transparency	Day	45% 6% 12% 21% 14% 2	100871
	Night	48% 6% 11% 20% 13% 2	97724
Seeing	Day	20% 11% 29% 27% 11% 2	15171
	Night	22% 12% 29% 26% 10% 1	24055

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% 8% 3 6% 3 4 3 3 4 5% 45%	2517
	11th to 20th	Day	19% 9% 3 4 4 4 4 4 4 5% 40%	2520
	21st to end	Day	22% 12% 3 8% 4 3 3 3 4 5% 34%	2772
February	1st to 10th	Day	20% 8% 2 6% 4 4 5% 4 5% 6% 36%	1837
	11th to 20th	Day	15% 8% 2 6% 4 5% 3 3 5% 6% 42%	1848
	21st to end	Day	20% 10% 2 6% 4 4 4 3 4 6% 36%	1570
March	1st to 10th	Day	26% 13% 4 6% 4 3 2 2 4 5% 31%	3116
	11th to 20th	Day	12% 6% 2 5% 3 3 4 4 5% 8% 47%	3352
	21st to end	Day	23% 10% 3 7% 4 3 3 3 4 5% 35%	3993
April	1st to 10th	Day	20% 8% 3 6% 3 4 3 4 6% 7% 37%	2280
	11th to 20th	Day	19% 8% 2 5% 4 3 5% 4 4 6% 41%	2263
	21st to end	Day	20% 8% 2 6% 5% 4 4 3 5% 6% 37%	2470
May	1st to 10th	Day	27% 11% 4 7% 4 4 4 3 4 5% 27%	4106
	11th to 20th	Day	23% 11% 3 7% 4 3 4 3 4 6% 32%	4081
	21st to end	Day	17% 7% 2 5% 4 4 4 4 5% 8% 40%	4795
June	1st to 10th	Day	19% 7% 2 6% 4 5% 4 5% 7% 8% 34%	2660
	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	18% 6% 2 5% 4 4 3 2 4 4 47%	1681
	11th to 20th	Day	17% 6% 2 5% 3 4 3 3 5% 7% 44%	1710
	21st to end	Day	17% 7% 2 5% 5% 4 3 3 5% 6% 43%	1710
December	1st to 10th	Day	21% 8% 3 6% 4 4 3 3 4 6% 39%	1625
	11th to 20th	Day	21% 8% 2 5% 3 3 3 2 4 5% 44%	1906
	21st to end	Day	22% 11% 3 7% 4 4 3 3 5% 5% 33%	2772
January	1st to 10th	Night	18% 6% 2 5% 3 3 3 3 4 6% 48%	4429
	11th to 20th	Night	17% 10% 3 7% 3 3 3 2 4 5% 40%	4440
	21st to end	Night	22% 13% 4 8% 4 4 4 3 4 4 30%	4884
February	1st to 10th	Night	17% 9% 4 7% 5% 4 3 3 4 5% 40%	2340
	11th to 20th	Night	18% 8% 3 6% 5% 3 4 3 5% 5% 40%	2218
	21st to end	Night	18% 9% 2 7% 5% 4 4 3 5% 5% 40%	1884
March	1st to 10th	Night	28% 11% 2 5% 3 3 3 3 4 6% 32%	3724
	11th to 20th	Night	11% 8% 3 7% 5% 5% 4 5% 5% 6% 41%	3593
	21st to end	Night	24% 9% 2 7% 5% 4 4 4 4 3 33%	3636
April	1st to 10th	Night	18% 9% 3 6% 4 4 4 3 4 6% 37%	1900
	11th to 20th	Night	19% 9% 2 7% 4 3 3 4 5% 6% 38%	1864
	21st to end	Night	21% 7% 3 6% 6% 3 3 3 4 4 39%	1710
May	1st to 10th	Night	25% 12% 5% 9% 4 4 4 4 5% 5% 24%	2834
	11th to 20th	Night	24% 12% 3 7% 4 5% 4 3 6% 6% 25%	2813
	21st to end	Night	17% 8% 3 7% 5% 5% 6% 5% 7% 6% 31%	2808
June	1st to 10th	Night	18% 10% 3 6% 4 4 4 4 5% 8% 34%	1520
	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 6% 3 3 4 3 4 6% 40%	2717
November	1st to 10th	Night	15% 7% 2 6% 4 4 4 3 5% 6% 45%	2431
	11th to 20th	Night	17% 9% 2 6% 3 3 3 3 4 5% 44%	2470
	21st to end	Night	15% 6% 3 6% 4 3 4 3 6% 7% 44%	2470
December	1st to 10th	Night	21% 6% 2 6% 5% 4 4 3 4 5% 39%	2552
	11th to 20th	Night	19% 7% 2 5% 4 3 4 3 5% 6% 42%	3288
	21st to end	Night	20% 11% 4 7% 4 3 3 2 4 5% 36%	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	42% 4 10% 28% 15% 2	2518
	11th to 20th	Day	46% 7% 11% 23% 11% 3	2520
	21st to end	Day	54% 4 9% 22% 10% 1	2772
February	1st to 10th	Day	48% 6% 13% 18% 13% 2	1837
	11th to 20th	Day	43% 5% 12% 23% 15% 2	1857
	21st to end	Day	48% 4 11% 20% 14% 3	1570
March	1st to 10th	Day	56% 4 12% 14% 12% 3	3119
	11th to 20th	Day	32% 6% 16% 29% 15% 2	3352
	21st to end	Day	53% 4 7% 16% 15% 5%	3993
April	1st to 10th	Day	44% 5% 11% 19% 17% 3	2280
	11th to 20th	Day	43% 6% 11% 21% 18% 3	2263
	21st to end	Day	46% 5% 11% 19% 16% 4	2470
May	1st to 10th	Day	58% 8% 11% 12% 8% 1	4120
	11th to 20th	Day	53% 5% 15% 16% 10% 1	4085
	21st to end	Day	42% 8% 10% 20% 18% 1	4795
June	1st to 10th	Day	47% 7% 12% 21% 12% 1	2660
	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	44% 5% 10% 22% 15% 3	2090
	11th to 20th	Day	37% 6% 10% 28% 17% 2	1886
	21st to end	Day	42% 5% 14% 20% 16% 3	1881
November	1st to 10th	Day	43% 5% 13% 19% 18% 2	1681
	11th to 20th	Day	41% 5% 13% 21% 18% 3	1710
	21st to end	Day	43% 5% 11% 26% 12% 2	1710
December	1st to 10th	Day	49% 6% 12% 17% 15% 2	1625
	11th to 20th	Day	45% 4 14% 16% 16% 5%	1906
	21st to end	Day	52% 6% 11% 20% 10% 1	2772
January	1st to 10th	Night	39% 5% 10% 31% 12% 3	4429
	11th to 20th	Night	48% 5% 11% 22% 12% 2	4440
	21st to end	Night	59% 6% 7% 16% 10% 1	4884
February	1st to 10th	Night	48% 6% 12% 21% 12% 2	2340
	11th to 20th	Night	47% 5% 9% 20% 15% 4	2220
	21st to end	Night	47% 4 9% 23% 14% 4	1884
March	1st to 10th	Night	54% 4 9% 19% 11% 2	3734
	11th to 20th	Night	41% 7% 13% 20% 15% 3	3594
	21st to end	Night	55% 4 7% 11% 18% 5%	3636
April	1st to 10th	Night	49% 4 9% 20% 16% 3	1900
	11th to 20th	Night	46% 5% 10% 18% 18% 3	1864
	21st to end	Night	49% 5% 8% 19% 15% 4	1710
May	1st to 10th	Night	61% 8% 11% 12% 7% 1	2840
	11th to 20th	Night	59% 7% 12% 13% 7% 2	2814
	21st to end	Night	49% 10% 11% 18% 12% 1	2809
June	1st to 10th	Night	50% 7% 11% 19% 13% 1	1520
	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	49% 5% 9% 20% 14% 3	2090
	11th to 20th	Night	37% 6% 12% 27% 16% 1	2289
	21st to end	Night	45% 6% 12% 21% 14% 2	2717
November	1st to 10th	Night	42% 6% 12% 20% 17% 3	2431
	11th to 20th	Night	44% 5% 10% 20% 17% 3	2470
	21st to end	Night	40% 6% 13% 26% 12% 2	2470
December	1st to 10th	Night	49% 6% 12% 18% 14% 1	2552
	11th to 20th	Night	45% 5% 14% 17% 17% 2	3288
	21st to end	Night	51% 5% 11% 22% 10% 1	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	24% 21% 30% 21% 4 1	180
	11th to 20th	Day	20% 23% 35% 17% 5%	180
	21st to end	Day	19% 18% 38% 19% 6% 1	198
February	1st to 10th	Day	24% 21% 32% 19% 2	429
	11th to 20th	Day	23% 21% 40% 15% 1	456
	21st to end	Day	23% 20% 34% 19% 4	382
March	1st to 10th	Day	23% 17% 35% 20% 5%	460
	11th to 20th	Day	17% 21% 37% 21% 3	460
	21st to end	Day	22% 17% 36% 18% 6% 1	506
April	1st to 10th	Day	25% 15% 32% 22% 6%	460
	11th to 20th	Day	23% 12% 37% 24% 3	452
	21st to end	Day	24% 10% 33% 23% 8% 1	460
May	1st to 10th	Day	29% 13% 30% 21% 5%	463
	11th to 20th	Day	21% 8% 37% 25% 9% 1	487
	21st to end	Day	21% 5% 27% 31% 14% 2	529
June	1st to 10th	Day	20% 6% 27% 32% 15% 1	490
	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% 13% 28% 24% 10% 2	429
November	1st to 10th	Day	16% 15% 30% 24% 12% 2	369
	11th to 20th	Day	22% 18% 33% 21% 6%	370
	21st to end	Day	21% 20% 29% 20% 9% 1	370
December	1st to 10th	Day	21% 15% 37% 21% 6%	285
	11th to 20th	Day	26% 23% 33% 14% 4	180
	21st to end	Day	24% 16% 37% 14% 6% 3	198
January	1st to 10th	Night	21% 21% 36% 19% 3	760
	11th to 20th	Night	28% 21% 33% 16% 3	760
	21st to end	Night	22% 21% 35% 18% 3	836
February	1st to 10th	Night	21% 19% 36% 21% 3	760
	11th to 20th	Night	24% 20% 39% 15% 2	743
	21st to end	Night	23% 18% 36% 18% 4	628
March	1st to 10th	Night	22% 18% 34% 23% 4	760
	11th to 20th	Night	19% 14% 38% 23% 4	760
	21st to end	Night	24% 12% 35% 22% 6% 1	836
April	1st to 10th	Night	24% 11% 35% 26% 4	760
	11th to 20th	Night	22% 9% 36% 26% 6%	740
	21st to end	Night	23% 8% 33% 29% 8% 1	570
May	1st to 10th	Night	28% 6% 30% 26% 9% 1	570
	11th to 20th	Night	24% 9% 33% 26% 7% 1	562
	21st to end	Night	25% 3 17% 38% 15% 1	455
June	1st to 10th	Night	24% 4 21% 35% 15% 1	380
	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% 23% 9% 1	836
November	1st to 10th	Night	20% 11% 31% 25% 11% 2	748
	11th to 20th	Night	21% 21% 31% 19% 7% 1	760
	21st to end	Night	18% 22% 30% 21% 7% 1	760
December	1st to 10th	Night	26% 21% 29% 18% 6% 1	760
	11th to 20th	Night	24% 23% 32% 17% 3	760
	21st to end	Night	26% 20% 34% 16% 4	836

Page generated at 2024-10-30 13:19:23. Version ac07

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