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 2024-02-20.

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	11% 9% 3 6% 5% 3 4 4 5% 8% 42%	78542
	Night	13% 9% 3 5% 4 2 3 4 4 7% 44%	80686
Transparency	Day	28% 34% 5% 12% 13% 8%	78265
	Night	28% 33% 5% 12% 13% 9%	80485
Seeing	Day	12% 6% 19% 29% 26% 9%	14425
	Night	15% 5% 17% 27% 26% 9%	23843

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	17% 7% 2 6% 5% 2 4 3 5% 6% 41%	2795
	11th to 20th	Day	20% 8% 4 7% 7% 4 4 5% 4 5% 31%	2860
	21st to end	Day	23% 9% 3 8% 6% 3 4 4 5% 7% 30%	3146
February	1st to 10th	Day	19% 6% 2 4 4 3 4 4 5% 6% 44%	1710
	11th to 20th	Day	14% 7% 4 6% 5% 3 5% 4 5% 9% 39%	1684
	21st to end	Day	15% 6% 2 5% 4 2 3 4 5% 6% 48%	1493
March	1st to 10th	Day	18% 7% 3 5% 5% 3 5% 4 5% 8% 35%	1980
	11th to 20th	Day	19% 8% 2 6% 5% 3 4 5% 6% 9% 34%	1980
	21st to end	Day	15% 5% 2 5% 4 3 4 4 5% 9% 45%	2178
April	1st to 10th	Day	14% 7% 3 6% 6% 3 4 4 6% 6% 41%	2154
	11th to 20th	Day	9% 5% 2 4 4 2 5% 4 4 9% 51%	2129
	21st to end	Day	7% 5% 2 4 4 3 4 5% 5% 11% 51%	2160
May	1st to 10th	Day	8% 8% 3 5% 4 2 3 3 5% 8% 51%	2305
	11th to 20th	Day	8% 8% 4 5% 4 2 3 4 5% 10% 47%	2300
	21st to end	Day	7% 12% 4 5% 3 2 3 4 4 7% 48%	2714
June	1st to 10th	Day	9% 10% 3 5% 4 1 3 5% 4 10% 45%	2520
	11th to 20th	Day	2 9% 4 5% 4 2 3 4 4 9% 55%	2520
	21st to end	Day	5% 12% 4 6% 5% 2 3 3 4 9% 46%	2520
July	1st to 10th	Day	4 16% 5% 7% 5% 3 4 4 4 9% 40%	2471
	11th to 20th	Day	4 12% 5% 10% 7% 3 4 4 5% 9% 36%	2340
	21st to end	Day	2 15% 6% 9% 8% 4 5% 6% 6% 10% 30%	2546
August	1st to 10th	Day	4 15% 4 8% 5% 3 4 5% 5% 11% 36%	2328
	11th to 20th	Day	4 12% 4 8% 7% 4 5% 5% 5% 9% 37%	2240
	21st to end	Day	4 15% 5% 9% 6% 4 5% 4 6% 7% 34%	2376
September	1st to 10th	Day	3 9% 4 6% 4 3 5% 4 6% 9% 46%	2160
	11th to 20th	Day	5% 11% 4 7% 6% 3 3 4 5% 9% 43%	2131
	21st to end	Day	5% 6% 3 5% 5% 2 3 3 5% 8% 57%	1908
October	1st to 10th	Day	7% 5% 1 4 3 2 4 5% 5% 7% 56%	1800
	11th to 20th	Day	10% 8% 3 5% 5% 2 3 4 5% 9% 47%	1800
	21st to end	Day	11% 7% 3 6% 4 2 3 3 6% 7% 48%	1980
November	1st to 10th	Day	10% 5% 2 6% 5% 2 4 4 6% 8% 47%	1859
	11th to 20th	Day	13% 7% 3 5% 5% 2 4 5% 5% 7% 44%	1693
	21st to end	Day	12% 4 2 4 4 2 4 4 5% 9% 50%	1520
December	1st to 10th	Day	17% 7% 3 5% 4 2 4 4 5% 6% 43%	1520
	11th to 20th	Day	29% 10% 3 5% 5% 2 3 5% 4 5% 29%	1928
	21st to end	Day	25% 8% 3 6% 4 2 3 4 5% 8% 34%	2794
January	1st to 10th	Night	14% 7% 3 5% 5% 3 4 4 5% 7% 42%	4156
	11th to 20th	Night	21% 9% 3 7% 6% 4 5% 5% 4 7% 30%	4110
	21st to end	Night	25% 12% 4 6% 6% 2 4 3 4 6% 28%	4521
February	1st to 10th	Night	19% 5% 2 4 4 2 4 4 4 6% 46%	2470
	11th to 20th	Night	13% 6% 3 8% 5% 3 5% 4 6% 7% 40%	2366
	21st to end	Night	15% 5% 2 5% 4 2 4 4 4 7% 47%	1758
March	1st to 10th	Night	20% 8% 3 6% 5% 3 4 4 4 5% 37%	1980
	11th to 20th	Night	17% 7% 3 5% 6% 3 4 4 6% 8% 38%	1980
	21st to end	Night	17% 6% 3 5% 3 2 3 4 4 7% 46%	2178
April	1st to 10th	Night	12% 7% 4 5% 4 3 3 4 5% 9% 45%	1805
	11th to 20th	Night	10% 7% 3 5% 4 2 3 4 3 7% 54%	1779
	21st to end	Night	7% 6% 3 4 3 1 3 4 6% 8% 56%	1800
May	1st to 10th	Night	6% 10% 2 5% 3 2 3 3 4 6% 55%	1652
	11th to 20th	Night	5% 9% 2 6% 5% 2 3 3 5% 8% 51%	1593
	21st to end	Night	8% 13% 3 4 3 1 3 3 3 6% 53%	1556
June	1st to 10th	Night	7% 10% 4 6% 4 2 4 4 6% 10% 45%	1440
	11th to 20th	Night	2 9% 3 4 2 2 2 2 4 8% 62%	1440
	21st to end	Night	6% 9% 3 4 2 2 2 3 3 8% 57%	1440
July	1st to 10th	Night	3 16% 3 4 2 1 2 2 3 9% 53%	1484
	11th to 20th	Night	4 13% 3 7% 5% 2 3 5% 5% 10% 44%	1620
	21st to end	Night	2 22% 7% 8% 5% 2 3 3 4 8% 35%	1773
August	1st to 10th	Night	5% 21% 4 4 4 2 3 3 3 8% 42%	1611
	11th to 20th	Night	2 18% 5% 8% 5% 2 4 3 5% 9% 39%	1660
	21st to end	Night	6% 16% 4 6% 5% 2 2 3 4 9% 44%	1980
September	1st to 10th	Night	2 9% 3 4 3 2 4 4 4 10% 54%	1800
	11th to 20th	Night	6% 13% 4 5% 4 2 4 3 3 8% 48%	1826
	21st to end	Night	5% 8% 3 5% 3 1 2 4 4 7% 59%	2052
October	1st to 10th	Night	6% 7% 3 5% 3 1 2 3 4 8% 57%	2160
	11th to 20th	Night	9% 8% 3 4 3 2 3 3 4 7% 53%	2160
	21st to end	Night	11% 6% 2 6% 4 2 3 3 3 8% 53%	2376
November	1st to 10th	Night	10% 6% 2 5% 4 3 4 4 5% 9% 48%	2220
	11th to 20th	Night	13% 6% 2 5% 5% 3 4 4 5% 7% 47%	2482
	21st to end	Night	11% 4 2 5% 4 2 4 4 4 7% 53%	2660
December	1st to 10th	Night	19% 5% 3 5% 4 2 3 3 4 7% 43%	2660
	11th to 20th	Night	30% 8% 2 5% 4 2 3 4 5% 7% 30%	3276
	21st to end	Night	20% 9% 3 5% 5% 4 4 4 4 6% 36%	4862

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	34% 23% 13% 23% 6% 2	2795
	11th to 20th	Day	45% 23% 9% 10% 8% 5%	2860
	21st to end	Day	48% 21% 6% 14% 8% 3	3146
February	1st to 10th	Day	26% 33% 4 15% 15% 6%	1710
	11th to 20th	Day	28% 35% 7% 14% 11% 4	1693
	21st to end	Day	28% 35% 5% 17% 11% 4	1493
March	1st to 10th	Day	28% 36% 9% 14% 10% 2	1980
	11th to 20th	Day	32% 36% 8% 15% 7% 2	1980
	21st to end	Day	28% 36% 8% 15% 11% 2	2178
April	1st to 10th	Day	29% 35% 9% 13% 12% 3	2154
	11th to 20th	Day	23% 34% 9% 16% 13% 5%	2132
	21st to end	Day	20% 35% 7% 20% 14% 3	2160
May	1st to 10th	Day	20% 35% 5% 15% 21% 5%	2305
	11th to 20th	Day	20% 35% 5% 18% 16% 6%	2303
	21st to end	Day	22% 36% 8% 18% 15% 1	2715
June	1st to 10th	Day	24% 35% 7% 17% 14% 2	2520
	11th to 20th	Day	18% 35% 5% 10% 21% 11%	2520
	21st to end	Day	26% 35% 1 7% 18% 14%	2520
July	1st to 10th	Day	24% 37% 4 14% 22%	2404
	11th to 20th	Day	28% 38% 1 3 10% 20%	2271
	21st to end	Day	32% 38% 2 10% 18%	2405
August	1st to 10th	Day	28% 35% 1 6% 14% 15%	2312
	11th to 20th	Day	26% 34% 1 4 17% 18%	2240
	21st to end	Day	29% 36% 4 15% 16%	2376
September	1st to 10th	Day	23% 36% 3 13% 24%	2160
	11th to 20th	Day	27% 35% 2 11% 17% 8%	2131
	21st to end	Day	18% 34% 1 14% 17% 14%	1908
October	1st to 10th	Day	22% 36% 4 12% 17% 10%	1800
	11th to 20th	Day	24% 35% 3 12% 16% 10%	1800
	21st to end	Day	24% 36% 3 14% 13% 10%	1980
November	1st to 10th	Day	25% 40% 5% 15% 9% 6%	1859
	11th to 20th	Day	25% 39% 6% 15% 10% 5%	1693
	21st to end	Day	20% 38% 6% 14% 14% 8%	1520
December	1st to 10th	Day	24% 39% 5% 13% 14% 5%	1520
	11th to 20th	Day	46% 30% 6% 9% 8% 2	1928
	21st to end	Day	43% 23% 12% 15% 5% 1	2794
January	1st to 10th	Night	32% 24% 16% 21% 6% 1	4156
	11th to 20th	Night	43% 23% 10% 11% 7% 5%	4110
	21st to end	Night	51% 21% 6% 10% 9% 4	4521
February	1st to 10th	Night	24% 33% 5% 16% 13% 8%	2470
	11th to 20th	Night	29% 35% 5% 14% 13% 4	2368
	21st to end	Night	27% 35% 6% 15% 14% 3	1758
March	1st to 10th	Night	30% 36% 6% 14% 12% 3	1980
	11th to 20th	Night	31% 35% 5% 16% 10% 3	1980
	21st to end	Night	27% 36% 7% 15% 10% 4	2178
April	1st to 10th	Night	24% 36% 7% 16% 13% 3	1805
	11th to 20th	Night	22% 34% 8% 15% 15% 5%	1779
	21st to end	Night	17% 34% 4 24% 16% 6%	1800
May	1st to 10th	Night	17% 34% 3 16% 22% 7%	1652
	11th to 20th	Night	20% 35% 4 16% 20% 6%	1594
	21st to end	Night	20% 35% 7% 19% 17% 2	1556
June	1st to 10th	Night	23% 37% 5% 15% 19% 3	1440
	11th to 20th	Night	14% 34% 4 12% 22% 14%	1440
	21st to end	Night	20% 34% 1 9% 19% 17%	1440
July	1st to 10th	Night	18% 35% 2 17% 28%	1442
	11th to 20th	Night	22% 36% 1 4 11% 27%	1566
	21st to end	Night	34% 36% 1 9% 19%	1674
August	1st to 10th	Night	27% 34% 7% 13% 18%	1602
	11th to 20th	Night	26% 35% 1 4 12% 22%	1660
	21st to end	Night	21% 35% 3 17% 24%	1980
September	1st to 10th	Night	19% 35% 4 15% 27%	1800
	11th to 20th	Night	26% 35% 1 11% 17% 9%	1826
	21st to end	Night	17% 34% 1 12% 21% 15%	2052
October	1st to 10th	Night	21% 35% 4 11% 16% 12%	2160
	11th to 20th	Night	22% 35% 2 12% 15% 14%	2160
	21st to end	Night	21% 36% 2 14% 16% 12%	2376
November	1st to 10th	Night	24% 40% 6% 14% 10% 7%	2220
	11th to 20th	Night	24% 38% 6% 16% 11% 6%	2482
	21st to end	Night	19% 39% 6% 12% 16% 8%	2660
December	1st to 10th	Night	24% 38% 5% 12% 15% 7%	2660
	11th to 20th	Night	42% 31% 6% 11% 9% 1	3276
	21st to end	Night	44% 24% 12% 13% 5% 1	4862

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	26% 7% 22% 24% 16% 5%	332
	11th to 20th	Day	19% 6% 19% 29% 20% 6%	370
	21st to end	Day	17% 5% 15% 31% 25% 7%	407
February	1st to 10th	Day	19% 6% 18% 27% 26% 5%	370
	11th to 20th	Day	15% 13% 22% 27% 19% 3	375
	21st to end	Day	12% 10% 27% 28% 19% 5%	346
March	1st to 10th	Day	20% 8% 24% 29% 15% 4	420
	11th to 20th	Day	24% 6% 19% 26% 18% 7%	420
	21st to end	Day	14% 7% 28% 34% 14% 4	462
April	1st to 10th	Day	16% 8% 28% 29% 16% 4	420
	11th to 20th	Day	10% 9% 28% 31% 17% 6%	412
	21st to end	Day	8% 8% 31% 27% 18% 8%	420
May	1st to 10th	Day	10% 9% 24% 31% 21% 5%	423
	11th to 20th	Day	14% 7% 26% 28% 21% 5%	448
	21st to end	Day	13% 9% 25% 24% 21% 9%	485
June	1st to 10th	Day	8% 6% 23% 34% 24% 5%	450
	11th to 20th	Day	6% 7% 25% 37% 22% 3	450
	21st to end	Day	8% 6% 18% 33% 26% 9%	450
July	1st to 10th	Day	11% 7% 13% 35% 30% 4	446
	11th to 20th	Day	5% 4 15% 29% 37% 11%	444
	21st to end	Day	7% 4 11% 30% 36% 12%	487
August	1st to 10th	Day	10% 5% 14% 35% 28% 9%	447
	11th to 20th	Day	10% 3 12% 25% 35% 15%	437
	21st to end	Day	10% 5% 11% 28% 33% 14%	495
September	1st to 10th	Day	7% 2 12% 26% 40% 13%	450
	11th to 20th	Day	7% 2 15% 30% 35% 11%	450
	21st to end	Day	6% 6% 10% 31% 34% 14%	450
October	1st to 10th	Day	7% 4 13% 30% 26% 19%	450
	11th to 20th	Day	11% 4 13% 22% 33% 18%	450
	21st to end	Day	11% 4 14% 29% 29% 12%	495
November	1st to 10th	Day	9% 7% 15% 24% 31% 14%	455
	11th to 20th	Day	10% 6% 14% 24% 31% 15%	271
	21st to end	Day	11% 3 18% 28% 27% 13%	180
December	1st to 10th	Day	19% 4 19% 21% 28% 9%	180
	11th to 20th	Day	21% 9% 16% 23% 21% 11%	180
	21st to end	Day	19% 12% 19% 25% 22% 4	198
January	1st to 10th	Night	20% 7% 15% 28% 23% 7%	760
	11th to 20th	Night	20% 6% 16% 27% 24% 7%	760
	21st to end	Night	20% 6% 14% 26% 28% 6%	836
February	1st to 10th	Night	20% 5% 16% 25% 27% 7%	760
	11th to 20th	Night	15% 8% 21% 25% 23% 7%	742
	21st to end	Night	17% 9% 24% 27% 19% 4	592
March	1st to 10th	Night	21% 9% 17% 26% 22% 5%	720
	11th to 20th	Night	20% 6% 18% 32% 20% 5%	720
	21st to end	Night	18% 7% 25% 28% 19% 2	792
April	1st to 10th	Night	14% 7% 26% 30% 18% 5%	720
	11th to 20th	Night	11% 9% 28% 29% 17% 6%	711
	21st to end	Night	8% 9% 31% 27% 18% 8%	720
May	1st to 10th	Night	11% 6% 26% 31% 20% 6%	572
	11th to 20th	Night	8% 4 25% 34% 22% 7%	531
	21st to end	Night	14% 9% 24% 29% 16% 7%	393
June	1st to 10th	Night	11% 5% 23% 32% 26% 4	360
	11th to 20th	Night	6% 7% 25% 36% 23% 3	360
	21st to end	Night	8% 4 15% 40% 25% 8%	360
July	1st to 10th	Night	10% 2 16% 29% 32% 11%	392
	11th to 20th	Night	11% 1 9% 27% 35% 17%	522
	21st to end	Night	14% 2 9% 24% 35% 16%	591
August	1st to 10th	Night	20% 2 11% 30% 26% 11%	537
	11th to 20th	Night	11% 2 12% 26% 36% 13%	592
	21st to end	Night	14% 2 10% 24% 34% 16%	792
September	1st to 10th	Night	6% 1 12% 27% 37% 16%	720
	11th to 20th	Night	13% 3 14% 25% 35% 11%	720
	21st to end	Night	8% 4 13% 26% 34% 17%	720
October	1st to 10th	Night	9% 4 16% 26% 29% 15%	720
	11th to 20th	Night	13% 2 13% 22% 34% 15%	720
	21st to end	Night	12% 4 11% 26% 32% 15%	792
November	1st to 10th	Night	10% 4 17% 23% 33% 12%	740
	11th to 20th	Night	14% 5% 13% 29% 29% 10%	760
	21st to end	Night	11% 4 17% 28% 30% 9%	760
December	1st to 10th	Night	21% 3 16% 28% 24% 8%	760
	11th to 20th	Night	23% 6% 17% 26% 23% 5%	760
	21st to end	Night	22% 7% 22% 25% 19% 4	836

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