Drakes Bay visitors center
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Drakes Bay visitors center clear sky chart from 2005-11-06 to 2026-07-16.

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	8% 11% 3 6% 4 3 3 3 5% 8% 45%	146704
	Night	11% 12% 3 6% 3 3 3 3 4 7% 47%	143493
Transparency	Day	23% 34% 6% 13% 14% 9%	141593
	Night	27% 34% 5% 11% 14% 9%	139597
Seeing	Day	12% 7% 18% 29% 26% 9%	17645
	Night	14% 5% 17% 28% 26% 9%	26915

By Month

Percentage Cloud Cover SW (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% 6% 2 6% 4 3 3 3 6% 9% 42%	5201
	11th to 20th	Day	11% 5% 3 6% 4 4 3 3 5% 6% 50%	5340
	21st to end	Day	15% 9% 3 8% 4 3 4 3 5% 8% 38%	5843
February	1st to 10th	Day	19% 6% 2 4 4 4 4 3 4 5% 44%	1890
	11th to 20th	Day	14% 9% 4 7% 4 5% 4 3 5% 9% 35%	1877
	21st to end	Day	15% 7% 2 5% 4 3 4 3 5% 6% 45%	1746
March	1st to 10th	Day	15% 9% 3 7% 5% 4 4 4 5% 7% 36%	5062
	11th to 20th	Day	14% 5% 2 5% 3 3 4 4 6% 9% 44%	5110
	21st to end	Day	16% 8% 2 6% 3 4 5% 4 7% 10% 36%	5607
April	1st to 10th	Day	12% 8% 3 6% 5% 4 4 4 6% 6% 41%	2514
	11th to 20th	Day	9% 6% 2 6% 4 3 5% 3 5% 9% 48%	2489
	21st to end	Day	7% 6% 1 4 4 3 3 4 6% 10% 51%	2520
May	1st to 10th	Day	6% 9% 3 6% 3 2 2 2 4 7% 56%	7527
	11th to 20th	Day	3 6% 3 4 3 3 3 2 4 8% 62%	7594
	21st to end	Day	3 13% 2 4 2 2 2 2 3 6% 62%	8983
June	1st to 10th	Day	7% 12% 3 5% 4 2 3 3 5% 9% 46%	2940
	11th to 20th	Day	2 10% 3 6% 4 2 2 3 5% 9% 55%	2940
	21st to end	Day	5% 14% 3 7% 4 2 3 2 4 9% 47%	2940
July	1st to 10th	Day	1 13% 3 7% 4 3 3 2 4 9% 51%	6329
	11th to 20th	Day	3 22% 7% 11% 5% 3 3 3 4 8% 32%	5946
	21st to end	Day	2 33% 5% 8% 4 4 3 3 5% 8% 25%	6402
August	1st to 10th	Day	2 18% 6% 7% 4 3 3 3 5% 9% 40%	5777
	11th to 20th	Day	3 22% 4 7% 3 3 2 2 3 6% 44%	5647
	21st to end	Day	2 19% 4 9% 5% 4 4 3 6% 8% 36%	5984
September	1st to 10th	Day	3 11% 4 7% 4 4 5% 3 6% 9% 44%	2400
	11th to 20th	Day	5% 11% 4 8% 5% 3 3 3 6% 9% 42%	2367
	21st to end	Day	5% 7% 3 6% 4 2 3 2 5% 7% 56%	2120
October	1st to 10th	Day	4 4 1 4 2 2 4 3 5% 8% 62%	3280
	11th to 20th	Day	10% 10% 3 6% 4 2 3 3 5% 8% 46%	3261
	21st to end	Day	8% 7% 3 6% 3 3 4 3 6% 8% 48%	3558
November	1st to 10th	Day	9% 6% 2 7% 4 4 4 4 6% 8% 47%	2059
	11th to 20th	Day	14% 8% 3 6% 4 3 4 4 5% 6% 42%	1871
	21st to end	Day	12% 5% 1 5% 3 3 4 3 5% 9% 50%	1680
December	1st to 10th	Day	9% 4 2 4 3 3 3 4 7% 11% 49%	2700
	11th to 20th	Day	24% 9% 2 6% 4 4 4 4 6% 8% 29%	3108
	21st to end	Day	29% 9% 2 6% 3 3 3 3 5% 7% 29%	4092
January	1st to 10th	Night	15% 7% 3 6% 4 3 4 3 6% 8% 42%	7702
	11th to 20th	Night	11% 6% 2 5% 3 3 4 3 4 9% 50%	7650
	21st to end	Night	16% 10% 4 7% 4 4 4 3 5% 8% 36%	8371
February	1st to 10th	Night	19% 6% 2 5% 3 3 3 3 4 6% 45%	2730
	11th to 20th	Night	14% 7% 3 8% 5% 4 4 4 5% 8% 39%	2629
	21st to end	Night	15% 7% 2 6% 4 3 4 3 4 7% 46%	2055
March	1st to 10th	Night	15% 10% 3 7% 5% 4 4 3 5% 7% 36%	5052
	11th to 20th	Night	15% 6% 3 6% 3 3 3 3 5% 7% 45%	5110
	21st to end	Night	15% 11% 3 6% 4 3 3 3 5% 8% 38%	5610
April	1st to 10th	Night	12% 9% 3 5% 4 3 3 3 6% 8% 46%	2105
	11th to 20th	Night	9% 8% 3 6% 3 3 3 3 4 6% 51%	2079
	21st to end	Night	7% 7% 2 4 3 2 3 4 6% 7% 55%	2100
May	1st to 10th	Night	8% 13% 2 4 2 2 1 1 3 5% 59%	5409
	11th to 20th	Night	2 7% 2 4 3 2 3 2 3 6% 65%	5295
	21st to end	Night	4 13% 2 3 1 1 2 1 2 4 67%	5185
June	1st to 10th	Night	7% 11% 4 6% 3 3 3 3 6% 8% 47%	1680
	11th to 20th	Night	2 11% 2 5% 2 2 2 2 4 7% 62%	1680
	21st to end	Night	6% 11% 3 5% 2 2 2 2 3 7% 58%	1680
July	1st to 10th	Night	2 17% 3 7% 2 1 1 1 1 5% 60%	3850
	11th to 20th	Night	5% 30% 3 5% 2 1 2 2 3 5% 42%	4145
	21st to end	Night	3 37% 4 6% 3 2 2 2 4 6% 32%	4472
August	1st to 10th	Night	3 23% 5% 5% 2 2 2 2 3 7% 48%	4035
	11th to 20th	Night	2 25% 3 6% 2 2 2 1 3 5% 50%	4208
	21st to end	Night	3 23% 3 7% 3 2 2 2 3 6% 45%	5015
September	1st to 10th	Night	2 11% 4 6% 3 3 3 3 4 9% 52%	2000
	11th to 20th	Night	6% 13% 4 6% 4 3 3 2 3 7% 48%	2030
	21st to end	Night	5% 9% 2 6% 3 1 2 3 4 7% 58%	2280
October	1st to 10th	Night	4 5% 2 5% 2 2 3 3 5% 7% 62%	3920
	11th to 20th	Night	8% 14% 4 6% 2 2 3 2 3 6% 50%	3897
	21st to end	Night	11% 6% 1 5% 3 2 3 2 4 7% 56%	4253
November	1st to 10th	Night	10% 7% 2 6% 4 4 3 3 5% 9% 48%	2460
	11th to 20th	Night	14% 7% 2 5% 4 3 3 3 5% 7% 46%	2744
	21st to end	Night	11% 4 1 5% 3 3 4 3 4 8% 52%	2940
December	1st to 10th	Night	11% 4 2 5% 3 3 3 3 6% 8% 52%	4700
	11th to 20th	Night	25% 8% 2 6% 4 4 4 4 7% 8% 28%	5316
	21st to end	Night	31% 9% 2 5% 3 4 3 3 3 5% 31%	7106

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	32% 24% 13% 18% 10% 4	5202
	11th to 20th	Day	29% 20% 7% 8% 19% 18%	5340
	21st to end	Day	37% 22% 6% 13% 10% 13%	5843
February	1st to 10th	Day	21% 59% 3 8% 8% 3	1890
	11th to 20th	Day	24% 57% 4 6% 6% 2	1886
	21st to end	Day	16% 59% 4 11% 6% 3	1746
March	1st to 10th	Day	35% 27% 6% 15% 15% 1	5062
	11th to 20th	Day	24% 28% 13% 18% 9% 8%	5110
	21st to end	Day	33% 30% 11% 19% 6% 1	5607
April	1st to 10th	Day	19% 59% 4 11% 7% 1	2514
	11th to 20th	Day	15% 59% 7% 11% 7% 1	2492
	21st to end	Day	12% 59% 7% 13% 8% 1	2520
May	1st to 10th	Day	22% 22% 7% 17% 22% 9%	7527
	11th to 20th	Day	17% 21% 4 23% 26% 8%	7597
	21st to end	Day	18% 22% 5% 22% 31% 3	8985
June	1st to 10th	Day	12% 58% 4 10% 11% 4	2940
	11th to 20th	Day	10% 59% 4 8% 15% 5%	2940
	21st to end	Day	12% 57% 3 5% 13% 10%	2940
July	1st to 10th	Day	14% 24% 5% 9% 12% 35%	6263
	11th to 20th	Day	26% 24% 6% 10% 8% 26%	5877
	21st to end	Day	37% 24% 7% 6% 11% 15%	6217
August	1st to 10th	Day	25% 35% 7% 13% 11% 9%	4206
	11th to 20th	Day	21% 35% 5% 6% 23% 10%	4082
	21st to end	Day	20% 35% 6% 11% 17% 10%	4311
September	1st to 10th	Day	14% 56% 3 7% 12% 8%	2400
	11th to 20th	Day	15% 57% 4 10% 11% 4	2367
	21st to end	Day	12% 56% 3 8% 14% 6%	2120
October	1st to 10th	Day	10% 34% 2 7% 15% 31%	3280
	11th to 20th	Day	22% 35% 5% 12% 18% 8%	3261
	21st to end	Day	21% 36% 7% 15% 16% 6%	3558
November	1st to 10th	Day	11% 53% 6% 15% 11% 4	2059
	11th to 20th	Day	21% 54% 5% 10% 8% 2	1871
	21st to end	Day	17% 54% 5% 10% 10% 5%	1680
December	1st to 10th	Day	17% 35% 5% 22% 16% 4	2700
	11th to 20th	Day	38% 32% 6% 11% 10% 3	3108
	21st to end	Day	43% 25% 9% 15% 6% 1	4092
January	1st to 10th	Night	33% 23% 15% 17% 10% 3	7702
	11th to 20th	Night	27% 20% 7% 11% 19% 16%	7650
	21st to end	Night	39% 22% 5% 10% 13% 11%	8371
February	1st to 10th	Night	21% 58% 3 6% 7% 4	2730
	11th to 20th	Night	22% 58% 4 8% 6% 2	2631
	21st to end	Night	15% 59% 5% 11% 7% 3	2055
March	1st to 10th	Night	36% 27% 8% 14% 13% 1	5052
	11th to 20th	Night	28% 28% 7% 15% 14% 7%	5110
	21st to end	Night	35% 29% 10% 18% 8% 1	5610
April	1st to 10th	Night	18% 59% 4 9% 9% 2	2105
	11th to 20th	Night	17% 59% 4 10% 7% 2	2079
	21st to end	Night	14% 59% 4 13% 9% 1	2100
May	1st to 10th	Night	26% 22% 3 13% 26% 10%	5407
	11th to 20th	Night	17% 21% 4 18% 28% 12%	5296
	21st to end	Night	19% 22% 4 20% 32% 3	5186
June	1st to 10th	Night	14% 59% 2 7% 12% 5%	1680
	11th to 20th	Night	12% 58% 2 7% 15% 6%	1680
	21st to end	Night	12% 56% 2 3 13% 14%	1680
July	1st to 10th	Night	24% 23% 2 6% 11% 33%	3808
	11th to 20th	Night	38% 23% 3 3 5% 28%	4091
	21st to end	Night	40% 24% 3 5% 10% 18%	4345
August	1st to 10th	Night	37% 35% 3 3 13% 9%	2929
	11th to 20th	Night	22% 34% 2 3 27% 12%	3046
	21st to end	Night	27% 35% 5% 6% 18% 10%	3608
September	1st to 10th	Night	16% 56% 1 5% 14% 8%	2000
	11th to 20th	Night	15% 56% 3 9% 12% 5%	2030
	21st to end	Night	13% 56% 3 9% 10% 9%	2280
October	1st to 10th	Night	11% 34% 2 6% 14% 33%	3920
	11th to 20th	Night	27% 36% 3 10% 12% 12%	3897
	21st to end	Night	20% 35% 4 14% 17% 10%	4253
November	1st to 10th	Night	15% 52% 4 13% 11% 5%	2460
	11th to 20th	Night	20% 55% 5% 11% 7% 2	2744
	21st to end	Night	17% 55% 4 10% 9% 5%	2940
December	1st to 10th	Night	20% 35% 5% 18% 16% 6%	4700
	11th to 20th	Night	40% 31% 5% 10% 9% 4	5316
	21st to end	Night	45% 25% 8% 15% 6% 2	7106

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% 7% 22% 24% 18% 5%	388
	11th to 20th	Day	17% 5% 17% 29% 24% 7%	430
	21st to end	Day	15% 5% 14% 32% 26% 8%	473
February	1st to 10th	Day	20% 5% 16% 27% 25% 6%	430
	11th to 20th	Day	17% 14% 21% 27% 18% 3	442
	21st to end	Day	13% 9% 22% 30% 21% 5%	446
March	1st to 10th	Day	19% 10% 25% 28% 15% 3	540
	11th to 20th	Day	20% 7% 19% 26% 20% 8%	540
	21st to end	Day	14% 7% 27% 32% 15% 4	594
April	1st to 10th	Day	14% 9% 25% 28% 18% 6%	540
	11th to 20th	Day	11% 9% 25% 29% 19% 7%	532
	21st to end	Day	9% 7% 27% 28% 21% 7%	540
May	1st to 10th	Day	9% 8% 22% 34% 22% 4	543
	11th to 20th	Day	11% 8% 26% 26% 23% 6%	568
	21st to end	Day	12% 9% 24% 26% 21% 9%	617
June	1st to 10th	Day	7% 6% 23% 34% 25% 5%	570
	11th to 20th	Day	6% 7% 25% 36% 22% 3	570
	21st to end	Day	7% 8% 18% 32% 26% 9%	570
July	1st to 10th	Day	10% 6% 14% 32% 32% 6%	566
	11th to 20th	Day	5% 4 14% 29% 35% 12%	545
	21st to end	Day	9% 5% 11% 28% 36% 11%	575
August	1st to 10th	Day	8% 5% 15% 34% 28% 9%	527
	11th to 20th	Day	10% 4 14% 26% 32% 13%	517
	21st to end	Day	10% 5% 10% 27% 34% 14%	583
September	1st to 10th	Day	9% 3 12% 27% 38% 12%	530
	11th to 20th	Day	8% 4 14% 29% 33% 11%	530
	21st to end	Day	7% 5% 10% 29% 36% 13%	530
October	1st to 10th	Day	8% 4 13% 28% 27% 19%	530
	11th to 20th	Day	11% 5% 13% 22% 34% 16%	530
	21st to end	Day	11% 5% 14% 29% 30% 11%	583
November	1st to 10th	Day	8% 7% 15% 25% 31% 14%	535
	11th to 20th	Day	12% 8% 14% 26% 27% 12%	329
	21st to end	Day	11% 3 17% 25% 30% 13%	220
December	1st to 10th	Day	16% 4 16% 21% 33% 11%	220
	11th to 20th	Day	21% 9% 14% 21% 22% 13%	220
	21st to end	Day	22% 10% 20% 25% 19% 3	242
January	1st to 10th	Night	20% 7% 15% 28% 23% 7%	840
	11th to 20th	Night	18% 5% 15% 27% 27% 7%	840
	21st to end	Night	19% 6% 13% 27% 29% 6%	924
February	1st to 10th	Night	20% 5% 15% 27% 26% 6%	840
	11th to 20th	Night	17% 9% 22% 25% 21% 6%	824
	21st to end	Night	16% 10% 22% 28% 20% 4	692
March	1st to 10th	Night	19% 10% 18% 27% 21% 4	840
	11th to 20th	Night	19% 6% 19% 31% 20% 5%	840
	21st to end	Night	17% 7% 25% 29% 19% 3	924
April	1st to 10th	Night	14% 7% 25% 29% 20% 5%	840
	11th to 20th	Night	12% 8% 27% 29% 19% 6%	831
	21st to end	Night	8% 8% 30% 28% 19% 7%	840
May	1st to 10th	Night	11% 6% 25% 31% 21% 6%	666
	11th to 20th	Night	8% 5% 25% 32% 22% 9%	621
	21st to end	Night	14% 8% 24% 30% 16% 7%	459
June	1st to 10th	Night	10% 5% 22% 32% 25% 5%	420
	11th to 20th	Night	7% 6% 27% 34% 23% 3	420
	21st to end	Night	8% 5% 17% 36% 27% 8%	420
July	1st to 10th	Night	10% 2 16% 28% 32% 12%	460
	11th to 20th	Night	12% 2 8% 28% 35% 17%	597
	21st to end	Night	14% 2 9% 25% 34% 15%	657
August	1st to 10th	Night	19% 3 12% 30% 25% 11%	597
	11th to 20th	Night	12% 2 14% 28% 33% 12%	659
	21st to end	Night	14% 2 10% 25% 33% 16%	880
September	1st to 10th	Night	8% 1 12% 28% 37% 15%	800
	11th to 20th	Night	12% 3 14% 25% 34% 11%	800
	21st to end	Night	8% 4 12% 25% 34% 17%	800
October	1st to 10th	Night	8% 4 16% 27% 29% 16%	800
	11th to 20th	Night	12% 3 13% 23% 34% 15%	800
	21st to end	Night	12% 4 12% 26% 32% 14%	880
November	1st to 10th	Night	10% 4 17% 23% 34% 12%	820
	11th to 20th	Night	15% 5% 13% 29% 28% 9%	840
	21st to end	Night	12% 5% 16% 29% 31% 8%	840
December	1st to 10th	Night	19% 3 15% 29% 26% 8%	840
	11th to 20th	Night	24% 5% 16% 26% 23% 5%	840
	21st to end	Night	25% 7% 22% 25% 18% 3	924

Page generated at 2026-07-13 00:14:52. Version ac07

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