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 2025-03-12.

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	9% 10% 3 6% 4 3 3 3 5% 8% 45%	112875
	Night	11% 10% 3 6% 3 3 3 3 4 7% 47%	114972
Transparency	Day	20% 42% 6% 11% 11% 9%	112638
	Night	24% 41% 5% 10% 11% 9%	114799
Seeing	Day	12% 6% 18% 29% 26% 9%	15825
	Night	14% 5% 18% 28% 26% 9%	25232

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	13% 6% 2 6% 4 3 3 3 6% 9% 44%	4007
	11th to 20th	Day	14% 6% 4 8% 4 4 3 3 4 4 46%	4100
	21st to end	Day	18% 8% 3 7% 4 3 3 3 4 6% 40%	4510
February	1st to 10th	Day	20% 6% 2 4 4 4 4 3 5% 5% 43%	1800
	11th to 20th	Day	13% 8% 3 7% 4 5% 4 4 5% 10% 37%	1785
	21st to end	Day	15% 7% 2 5% 3 3 3 3 5% 6% 47%	1665
March	1st to 10th	Day	17% 10% 3 8% 6% 4 5% 4 5% 7% 31%	3552
	11th to 20th	Day	11% 6% 3 5% 4 3 4 4 6% 9% 46%	3503
	21st to end	Day	18% 7% 2 6% 4 4 4 4 6% 8% 36%	3825
April	1st to 10th	Day	13% 8% 3 7% 5% 3 4 4 6% 6% 42%	2274
	11th to 20th	Day	9% 5% 2 6% 3 3 5% 3 5% 9% 49%	2249
	21st to end	Day	6% 6% 1 4 4 3 3 4 6% 11% 51%	2280
May	1st to 10th	Day	7% 6% 2 4 2 2 2 2 3 5% 65%	4062
	11th to 20th	Day	4 9% 4 6% 4 4 4 3 5% 10% 46%	4070
	21st to end	Day	4 9% 2 3 2 2 2 2 3 6% 65%	4804
June	1st to 10th	Day	8% 10% 3 5% 4 2 3 4 5% 10% 46%	2660
	11th to 20th	Day	2 9% 3 6% 4 2 2 3 5% 9% 56%	2660
	21st to end	Day	5% 13% 3 7% 4 2 3 3 5% 9% 47%	2660
July	1st to 10th	Day	2 10% 3 6% 3 2 3 2 4 10% 56%	4332
	11th to 20th	Day	3 12% 5% 9% 5% 4 4 3 5% 10% 40%	4110
	21st to end	Day	1 24% 5% 8% 4 4 4 3 5% 9% 33%	4455
August	1st to 10th	Day	3 26% 8% 9% 5% 4 4 3 6% 10% 25%	4091
	11th to 20th	Day	4 13% 3 7% 4 3 3 3 4 8% 47%	3916
	21st to end	Day	3 14% 4 10% 5% 5% 5% 3 6% 8% 37%	4180
September	1st to 10th	Day	3 10% 3 7% 4 4 5% 3 6% 9% 45%	2280
	11th to 20th	Day	5% 11% 4 8% 5% 3 3 3 5% 9% 42%	2249
	21st to end	Day	5% 6% 3 6% 4 2 3 2 5% 8% 57%	2014
October	1st to 10th	Day	4 4 1 4 2 2 4 3 5% 8% 62%	3180
	11th to 20th	Day	10% 10% 3 6% 4 2 3 3 5% 8% 46%	3161
	21st to end	Day	8% 7% 3 6% 3 3 4 3 6% 8% 49%	3448
November	1st to 10th	Day	9% 6% 2 6% 4 4 4 4 6% 8% 48%	1959
	11th to 20th	Day	13% 8% 2 6% 4 3 4 4 5% 6% 44%	1782
	21st to end	Day	13% 5% 1 5% 3 3 4 3 5% 9% 49%	1600
December	1st to 10th	Day	10% 4 2 4 3 3 3 4 7% 11% 48%	2620
	11th to 20th	Day	24% 9% 2 6% 4 4 4 4 6% 8% 29%	3028
	21st to end	Day	28% 9% 2 6% 3 3 3 3 5% 7% 29%	4004
January	1st to 10th	Night	10% 6% 2 5% 4 4 4 4 6% 9% 45%	5939
	11th to 20th	Night	15% 7% 3 7% 4 4 4 3 4 6% 45%	5880
	21st to end	Night	19% 10% 3 6% 4 3 3 3 4 7% 39%	6468
February	1st to 10th	Night	20% 6% 2 5% 3 3 3 3 4 6% 45%	2600
	11th to 20th	Night	13% 7% 3 8% 5% 4 4 4 5% 8% 40%	2501
	21st to end	Night	14% 7% 2 6% 3 3 4 3 4 7% 48%	1960
March	1st to 10th	Night	16% 11% 3 8% 5% 5% 4 4 5% 6% 34%	3542
	11th to 20th	Night	13% 5% 3 6% 4 3 3 3 5% 7% 49%	3505
	21st to end	Night	15% 12% 4 7% 4 4 4 3 5% 7% 35%	3828
April	1st to 10th	Night	11% 9% 3 5% 4 3 3 3 5% 8% 46%	1905
	11th to 20th	Night	9% 7% 3 6% 3 3 3 3 4 6% 53%	1879
	21st to end	Night	7% 7% 2 4 3 1 3 3 6% 8% 56%	1900
May	1st to 10th	Night	6% 8% 1 4 2 2 2 2 3 4 68%	2906
	11th to 20th	Night	3 10% 4 6% 5% 3 3 2 3 6% 56%	2833
	21st to end	Night	4 8% 2 3 2 1 2 1 2 4 70%	2766
June	1st to 10th	Night	7% 10% 3 6% 3 3 4 3 6% 9% 46%	1520
	11th to 20th	Night	2 9% 2 5% 2 2 2 2 4 7% 63%	1520
	21st to end	Night	6% 9% 3 4 2 2 2 2 3 8% 58%	1520
July	1st to 10th	Night	2 11% 2 5% 2 1 1 1 2 7% 66%	2626
	11th to 20th	Night	3 20% 2 6% 3 2 2 2 4 7% 49%	2860
	21st to end	Night	1 28% 5% 7% 4 2 3 3 5% 8% 36%	3109
August	1st to 10th	Night	4 32% 6% 5% 3 2 3 2 4 8% 31%	2848
	11th to 20th	Night	2 16% 3 6% 3 2 2 2 4 6% 54%	2926
	21st to end	Night	4 16% 3 7% 3 3 2 2 3 7% 49%	3498
September	1st to 10th	Night	2 9% 4 6% 3 3 3 3 5% 9% 53%	1900
	11th to 20th	Night	6% 13% 4 6% 4 3 3 2 4 7% 48%	1928
	21st to end	Night	4 9% 2 6% 3 2 2 3 4 7% 58%	2166
October	1st to 10th	Night	4 5% 2 5% 2 2 3 3 5% 8% 62%	3800
	11th to 20th	Night	9% 14% 4 6% 2 2 3 2 3 6% 49%	3777
	21st to end	Night	11% 6% 1 5% 3 2 3 2 4 7% 56%	4121
November	1st to 10th	Night	9% 6% 2 6% 4 4 3 3 5% 9% 49%	2340
	11th to 20th	Night	13% 6% 2 5% 4 4 3 3 5% 7% 47%	2613
	21st to end	Night	12% 4 2 5% 4 3 4 3 4 7% 52%	2800
December	1st to 10th	Night	11% 4 2 5% 3 3 3 3 5% 8% 52%	4560
	11th to 20th	Night	25% 8% 2 6% 4 4 4 4 7% 8% 28%	5176
	21st to end	Night	31% 9% 2 5% 4 4 3 3 3 5% 31%	6952

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	27% 29% 14% 16% 10% 5%	4007
	11th to 20th	Day	34% 25% 8% 8% 13% 11%	4100
	21st to end	Day	35% 27% 4 10% 9% 15%	4510
February	1st to 10th	Day	20% 61% 3 7% 7% 2	1800
	11th to 20th	Day	21% 60% 4 7% 7% 2	1794
	21st to end	Day	13% 62% 4 12% 7% 3	1665
March	1st to 10th	Day	38% 38% 5% 10% 9% 1	3552
	11th to 20th	Day	17% 39% 8% 18% 8% 10%	3503
	21st to end	Day	30% 39% 9% 17% 4 1	3825
April	1st to 10th	Day	17% 65% 3 9% 6% 1	2274
	11th to 20th	Day	12% 65% 6% 10% 6%	2252
	21st to end	Day	10% 65% 6% 10% 8% 1	2280
May	1st to 10th	Day	11% 39% 9% 18% 13% 10%	4062
	11th to 20th	Day	25% 38% 2 12% 17% 5%	4073
	21st to end	Day	11% 38% 4 24% 21% 2	4805
June	1st to 10th	Day	10% 64% 3 10% 9% 4	2660
	11th to 20th	Day	7% 65% 4 7% 13% 3	2660
	21st to end	Day	10% 63% 3 3 12% 10%	2660
July	1st to 10th	Day	10% 35% 3 4 3 45%	4265
	11th to 20th	Day	15% 35% 5% 8% 6% 32%	4041
	21st to end	Day	24% 34% 5% 5% 10% 22%	4315
August	1st to 10th	Day	26% 36% 7% 13% 9% 9%	4076
	11th to 20th	Day	20% 36% 5% 6% 23% 10%	3955
	21st to end	Day	20% 36% 6% 11% 17% 10%	4179
September	1st to 10th	Day	12% 59% 3 6% 12% 9%	2280
	11th to 20th	Day	14% 60% 3 9% 10% 4	2249
	21st to end	Day	11% 59% 3 7% 14% 6%	2014
October	1st to 10th	Day	9% 35% 2 7% 15% 32%	3180
	11th to 20th	Day	22% 36% 5% 11% 17% 8%	3161
	21st to end	Day	20% 37% 7% 15% 15% 6%	3448
November	1st to 10th	Day	10% 55% 5% 15% 10% 4	1959
	11th to 20th	Day	19% 57% 5% 10% 7% 2	1782
	21st to end	Day	17% 56% 4 9% 9% 5%	1600
December	1st to 10th	Day	18% 36% 5% 22% 16% 4	2620
	11th to 20th	Day	38% 33% 6% 11% 10% 2	3028
	21st to end	Day	43% 26% 9% 15% 6% 1	4004
January	1st to 10th	Night	25% 29% 17% 16% 10% 4	5939
	11th to 20th	Night	33% 26% 8% 12% 13% 9%	5880
	21st to end	Night	37% 27% 5% 6% 13% 13%	6468
February	1st to 10th	Night	20% 61% 3 6% 6% 3	2600
	11th to 20th	Night	19% 60% 4 8% 6% 3	2503
	21st to end	Night	13% 61% 5% 12% 7% 3	1960
March	1st to 10th	Night	36% 38% 8% 11% 7% 1	3542
	11th to 20th	Night	20% 38% 5% 13% 16% 9%	3505
	21st to end	Night	34% 39% 7% 15% 5% 1	3828
April	1st to 10th	Night	15% 65% 3 8% 8% 1	1905
	11th to 20th	Night	13% 65% 4 10% 6% 2	1879
	21st to end	Night	11% 65% 4 11% 8% 1	1900
May	1st to 10th	Night	13% 39% 3 18% 17% 9%	2906
	11th to 20th	Night	21% 38% 2 6% 21% 11%	2834
	21st to end	Night	11% 38% 2 23% 23% 2	2766
June	1st to 10th	Night	12% 65% 2 7% 10% 3	1520
	11th to 20th	Night	9% 64% 3 7% 13% 5%	1520
	21st to end	Night	9% 62% 2 2 12% 14%	1520
July	1st to 10th	Night	13% 34% 2 3 4 44%	2584
	11th to 20th	Night	24% 34% 2 3 5% 32%	2806
	21st to end	Night	29% 34% 3 3 7% 25%	3011
August	1st to 10th	Night	38% 36% 3 3 11% 9%	2839
	11th to 20th	Night	21% 35% 2 3 28% 11%	2953
	21st to end	Night	27% 36% 5% 6% 17% 9%	3498
September	1st to 10th	Night	13% 58% 1 5% 14% 8%	1900
	11th to 20th	Night	14% 59% 3 8% 11% 5%	1928
	21st to end	Night	12% 58% 3 9% 10% 8%	2166
October	1st to 10th	Night	10% 35% 2 5% 14% 34%	3800
	11th to 20th	Night	27% 37% 3 9% 12% 12%	3777
	21st to end	Night	19% 36% 4 14% 16% 11%	4121
November	1st to 10th	Night	13% 55% 3 13% 10% 5%	2340
	11th to 20th	Night	18% 58% 5% 11% 7% 2	2613
	21st to end	Night	17% 57% 3 8% 9% 6%	2800
December	1st to 10th	Night	20% 36% 4 17% 16% 6%	4560
	11th to 20th	Night	40% 32% 5% 10% 9% 4	5176
	21st to end	Night	45% 25% 8% 15% 6% 2	6952

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% 6% 22% 24% 18% 5%	360
	11th to 20th	Day	18% 6% 19% 30% 22% 6%	400
	21st to end	Day	16% 5% 15% 31% 25% 8%	440
February	1st to 10th	Day	21% 5% 17% 26% 25% 6%	400
	11th to 20th	Day	15% 14% 21% 27% 19% 3	410
	21st to end	Day	12% 8% 23% 30% 21% 5%	414
March	1st to 10th	Day	19% 10% 25% 28% 14% 4	500
	11th to 20th	Day	22% 6% 20% 27% 19% 7%	465
	21st to end	Day	14% 7% 29% 33% 13% 4	506
April	1st to 10th	Day	15% 8% 27% 28% 16% 6%	460
	11th to 20th	Day	10% 9% 26% 30% 18% 7%	452
	21st to end	Day	7% 8% 31% 27% 19% 8%	460
May	1st to 10th	Day	10% 8% 24% 32% 21% 4	463
	11th to 20th	Day	13% 7% 24% 26% 24% 6%	488
	21st to end	Day	12% 9% 23% 25% 22% 9%	529
June	1st to 10th	Day	8% 6% 24% 34% 24% 5%	490
	11th to 20th	Day	5% 7% 26% 37% 22% 3	490
	21st to end	Day	8% 5% 18% 33% 27% 9%	490
July	1st to 10th	Day	10% 6% 13% 32% 33% 6%	486
	11th to 20th	Day	5% 4 14% 30% 37% 11%	484
	21st to end	Day	8% 4 11% 29% 37% 12%	531
August	1st to 10th	Day	9% 5% 14% 35% 29% 8%	487
	11th to 20th	Day	10% 3 15% 26% 33% 13%	477
	21st to end	Day	10% 4 10% 28% 33% 14%	539
September	1st to 10th	Day	7% 2 11% 27% 40% 13%	490
	11th to 20th	Day	8% 4 15% 29% 34% 11%	490
	21st to end	Day	6% 5% 9% 30% 37% 13%	490
October	1st to 10th	Day	7% 4 12% 30% 28% 20%	490
	11th to 20th	Day	11% 5% 13% 21% 34% 16%	490
	21st to end	Day	11% 4 14% 30% 30% 11%	539
November	1st to 10th	Day	8% 7% 15% 25% 31% 14%	495
	11th to 20th	Day	12% 8% 15% 23% 29% 14%	300
	21st to end	Day	13% 3 18% 27% 29% 12%	200
December	1st to 10th	Day	18% 4 18% 22% 29% 11%	200
	11th to 20th	Day	20% 10% 15% 23% 23% 11%	200
	21st to end	Day	21% 10% 20% 25% 20% 3	220
January	1st to 10th	Night	19% 6% 16% 29% 23% 7%	800
	11th to 20th	Night	19% 5% 16% 28% 25% 7%	800
	21st to end	Night	20% 6% 14% 26% 29% 7%	880
February	1st to 10th	Night	21% 6% 16% 25% 26% 6%	800
	11th to 20th	Night	16% 8% 21% 26% 22% 7%	784
	21st to end	Night	15% 9% 23% 28% 20% 4	660
March	1st to 10th	Night	20% 10% 18% 26% 21% 4	800
	11th to 20th	Night	20% 7% 18% 31% 21% 5%	766
	21st to end	Night	18% 8% 25% 28% 19% 2	836
April	1st to 10th	Night	14% 7% 26% 30% 19% 5%	760
	11th to 20th	Night	11% 8% 28% 29% 18% 6%	751
	21st to end	Night	8% 9% 31% 27% 18% 8%	760
May	1st to 10th	Night	11% 6% 27% 32% 20% 5%	603
	11th to 20th	Night	8% 4 24% 33% 22% 9%	561
	21st to end	Night	13% 9% 25% 29% 17% 7%	415
June	1st to 10th	Night	11% 5% 22% 33% 25% 4	380
	11th to 20th	Night	6% 6% 26% 36% 23% 3	380
	21st to end	Night	8% 4 16% 39% 26% 8%	380
July	1st to 10th	Night	10% 2 16% 28% 33% 12%	415
	11th to 20th	Night	11% 1 8% 28% 35% 17%	552
	21st to end	Night	14% 2 9% 25% 34% 16%	624
August	1st to 10th	Night	20% 2 11% 30% 26% 11%	567
	11th to 20th	Night	11% 2 14% 27% 34% 12%	626
	21st to end	Night	13% 2 10% 25% 33% 17%	836
September	1st to 10th	Night	6% 1 12% 28% 38% 15%	760
	11th to 20th	Night	12% 3 14% 25% 34% 11%	760
	21st to end	Night	8% 4 12% 25% 35% 17%	760
October	1st to 10th	Night	9% 3 16% 27% 29% 16%	760
	11th to 20th	Night	13% 3 13% 23% 34% 15%	760
	21st to end	Night	12% 4 11% 26% 32% 15%	836
November	1st to 10th	Night	10% 4 18% 23% 33% 12%	780
	11th to 20th	Night	14% 5% 13% 29% 28% 10%	800
	21st to end	Night	12% 5% 17% 28% 30% 8%	800
December	1st to 10th	Night	20% 3 15% 28% 25% 8%	800
	11th to 20th	Night	23% 5% 17% 27% 23% 5%	800
	21st to end	Night	24% 7% 23% 25% 18% 4	880

Page generated at 2025-03-09 05:08:49. Version ac07

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