The Peninsula Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the The Peninsula Observatory clear sky chart from 2005-11-06 to 2026-03-03.

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	12% 6% 2 6% 4 4 5% 4 6% 8% 41%	130651
	Night	13% 6% 2 6% 4 4 4 4 6% 7% 43%	130502
Transparency	Day	25% 39% 8% 14% 11% 3	134247
	Night	25% 38% 8% 14% 12% 4	134976
Seeing	Day	15% 13% 28% 25% 14% 4	17048
	Night	16% 11% 29% 30% 13% 2	26818

By Month

Percentage Cloud Cover SE (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% 5% 2 5% 3 3 3 2 4 6% 56%	5140
	11th to 20th	Day	13% 5% 2 5% 3 3 3 2 3 5% 55%	5280
	21st to end	Day	13% 11% 3 7% 5% 4 4 3 4 5% 41%	5780
February	1st to 10th	Day	15% 6% 2 7% 4 4 4 3 6% 8% 42%	1890
	11th to 20th	Day	19% 7% 2 5% 4 3 3 3 5% 5% 43%	2026
	21st to end	Day	15% 8% 2 6% 4 4 4 4 6% 6% 41%	1730
March	1st to 10th	Day	19% 8% 2 6% 4 4 4 3 4 6% 40%	4821
	11th to 20th	Day	10% 5% 2 6% 3 3 3 3 4 6% 54%	4980
	21st to end	Day	12% 5% 3 6% 4 3 4 3 5% 6% 50%	5616
April	1st to 10th	Day	12% 6% 1 5% 4 3 3 3 6% 6% 52%	2400
	11th to 20th	Day	9% 4 2 5% 4 4 4 3 5% 8% 51%	2374
	21st to end	Day	10% 5% 2 5% 5% 4 5% 4 6% 8% 46%	2400
May	1st to 10th	Day	9% 7% 3 8% 5% 6% 6% 5% 7% 9% 36%	5405
	11th to 20th	Day	17% 8% 3 8% 5% 5% 5% 4 6% 9% 30%	5840
	21st to end	Day	12% 7% 3 6% 4 4 5% 4 6% 9% 41%	6934
June	1st to 10th	Day	9% 6% 1 6% 5% 5% 5% 6% 8% 10% 39%	2800
	11th to 20th	Day	10% 5% 2 4 4 6% 5% 6% 7% 11% 40%	2800
	21st to end	Day	6% 4 2 6% 6% 5% 6% 6% 9% 11% 39%	2800
July	1st to 10th	Day	8% 6% 2 7% 6% 6% 7% 6% 10% 13% 30%	6278
	11th to 20th	Day	8% 6% 3 9% 6% 6% 7% 6% 9% 13% 28%	5900
	21st to end	Day	9% 6% 3 8% 6% 6% 6% 5% 9% 12% 31%	6424
August	1st to 10th	Day	17% 11% 4 9% 6% 7% 6% 4 7% 9% 22%	4170
	11th to 20th	Day	9% 6% 2 7% 5% 5% 7% 5% 8% 11% 35%	3850
	21st to end	Day	7% 4 2 6% 5% 6% 6% 5% 7% 10% 42%	4312
September	1st to 10th	Day	12% 6% 2 7% 4 5% 5% 4 7% 9% 39%	2400
	11th to 20th	Day	14% 8% 2 7% 4 4 4 4 6% 7% 39%	2400
	21st to end	Day	18% 8% 3 7% 4 4 4 4 6% 7% 34%	2140
October	1st to 10th	Day	13% 7% 2 6% 4 3 4 3 5% 7% 45%	2000
	11th to 20th	Day	12% 6% 2 7% 3 3 4 2 5% 5% 50%	2000
	21st to end	Day	12% 5% 2 6% 4 3 3 2 5% 5% 52%	2200
November	1st to 10th	Day	15% 6% 2 5% 3 3 4 3 5% 6% 48%	2004
	11th to 20th	Day	14% 6% 2 4 3 2 4 3 4 7% 51%	1890
	21st to end	Day	12% 5% 2 4 4 4 4 3 5% 6% 50%	1869
December	1st to 10th	Day	12% 5% 2 5% 4 3 3 3 5% 6% 52%	2680
	11th to 20th	Day	14% 7% 3 7% 4 3 4 3 5% 5% 45%	3070
	21st to end	Day	13% 5% 2 4 4 4 4 3 4 8% 49%	4048
January	1st to 10th	Night	11% 6% 2 5% 3 3 4 3 5% 7% 51%	7735
	11th to 20th	Night	11% 5% 2 5% 4 3 3 3 4 5% 55%	7680
	21st to end	Night	15% 8% 3 7% 4 4 4 3 4 6% 43%	8401
February	1st to 10th	Night	17% 7% 2 5% 4 3 4 3 5% 7% 42%	2730
	11th to 20th	Night	20% 8% 2 6% 4 3 3 3 4 6% 40%	2478
	21st to end	Night	15% 8% 2 6% 3 3 3 3 5% 6% 45%	2076
March	1st to 10th	Night	19% 8% 2 7% 4 4 3 3 4 5% 41%	5060
	11th to 20th	Night	13% 5% 2 6% 4 4 4 3 4 5% 49%	4980
	21st to end	Night	14% 6% 2 5% 4 4 4 3 4 5% 49%	5314
April	1st to 10th	Night	12% 6% 2 6% 4 5% 4 4 5% 7% 46%	2000
	11th to 20th	Night	10% 7% 3 6% 4 4 5% 3 5% 7% 48%	1975
	21st to end	Night	12% 6% 2 5% 4 4 3 3 6% 7% 47%	2000
May	1st to 10th	Night	10% 7% 3 9% 6% 5% 6% 5% 5% 9% 35%	4490
	11th to 20th	Night	18% 8% 3 7% 5% 4 4 4 6% 8% 32%	4092
	21st to end	Night	13% 7% 2 7% 5% 4 4 4 6% 9% 38%	3920
June	1st to 10th	Night	10% 6% 2 5% 5% 4 5% 5% 7% 11% 40%	1600
	11th to 20th	Night	8% 6% 2 6% 6% 5% 6% 5% 8% 11% 37%	1600
	21st to end	Night	8% 5% 2 6% 5% 4 4 5% 9% 12% 41%	1600
July	1st to 10th	Night	6% 5% 2 6% 5% 6% 7% 6% 9% 15% 33%	3661
	11th to 20th	Night	8% 6% 2 9% 6% 6% 7% 6% 9% 12% 28%	4057
	21st to end	Night	11% 7% 3 8% 5% 5% 6% 5% 8% 9% 33%	4427
August	1st to 10th	Night	15% 10% 3 8% 5% 5% 5% 5% 6% 9% 28%	2946
	11th to 20th	Night	9% 7% 3 9% 6% 6% 5% 4 7% 9% 35%	3197
	21st to end	Night	7% 5% 2 6% 5% 4 6% 5% 7% 10% 42%	3586
September	1st to 10th	Night	10% 5% 2 6% 5% 5% 4 4 6% 9% 43%	2000
	11th to 20th	Night	14% 8% 3 6% 4 4 4 3 7% 8% 40%	2000
	21st to end	Night	17% 8% 3 8% 5% 4 5% 4 7% 8% 33%	2255
October	1st to 10th	Night	12% 6% 2 6% 3 4 3 4 5% 5% 48%	2400
	11th to 20th	Night	13% 6% 2 5% 3 3 4 2 4 6% 51%	2400
	21st to end	Night	12% 4 2 5% 3 3 3 3 5% 6% 54%	2640
November	1st to 10th	Night	15% 5% 2 5% 3 3 3 3 5% 6% 49%	2510
	11th to 20th	Night	15% 6% 2 5% 4 3 4 3 5% 6% 48%	2730
	21st to end	Night	11% 6% 2 5% 3 4 3 2 6% 7% 50%	2747
December	1st to 10th	Night	10% 5% 2 6% 3 4 4 3 4 5% 52%	4720
	11th to 20th	Night	15% 6% 3 8% 5% 4 4 4 5% 6% 39%	5345
	21st to end	Night	12% 5% 2 5% 3 3 4 3 6% 8% 50%	7150

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	24% 21% 9% 20% 16% 10%	5141
	11th to 20th	Day	25% 21% 3 11% 32% 8%	5280
	21st to end	Day	38% 24% 4 16% 12% 6%	5780
February	1st to 10th	Day	18% 59% 5% 9% 7% 2	1890
	11th to 20th	Day	21% 58% 4 7% 7% 2	2035
	21st to end	Day	18% 60% 3 8% 9% 2	1730
March	1st to 10th	Day	37% 31% 5% 10% 10% 7%	4821
	11th to 20th	Day	22% 29% 4 13% 22% 11%	4980
	21st to end	Day	24% 30% 8% 17% 16% 5%	5616
April	1st to 10th	Day	14% 62% 5% 9% 9% 1	2400
	11th to 20th	Day	13% 63% 4 12% 6% 2	2374
	21st to end	Day	14% 63% 6% 10% 7% 1	2400
May	1st to 10th	Day	31% 35% 12% 13% 7% 1	5437
	11th to 20th	Day	36% 32% 11% 13% 7%	5844
	21st to end	Day	27% 31% 9% 22% 10% 1	6949
June	1st to 10th	Day	14% 65% 5% 8% 7% 1	2800
	11th to 20th	Day	10% 65% 9% 9% 6%	2800
	21st to end	Day	11% 63% 9% 12% 5%	2800
July	1st to 10th	Day	30% 34% 16% 15% 5%	6209
	11th to 20th	Day	35% 35% 21% 9% 1	5831
	21st to end	Day	33% 31% 11% 18% 7%	6240
August	1st to 10th	Day	43% 39% 10% 6% 2	4157
	11th to 20th	Day	24% 42% 12% 15% 8%	3886
	21st to end	Day	20% 39% 11% 20% 9% 1	4312
September	1st to 10th	Day	18% 60% 8% 8% 5% 1	2400
	11th to 20th	Day	20% 59% 6% 8% 6% 2	2400
	21st to end	Day	24% 58% 6% 9% 3	2140
October	1st to 10th	Day	18% 38% 8% 18% 17% 2	3260
	11th to 20th	Day	13% 36% 3 13% 23% 12%	3240
	21st to end	Day	10% 36% 4 21% 22% 7%	3538
November	1st to 10th	Day	18% 53% 5% 11% 9% 2	2004
	11th to 20th	Day	16% 54% 4 11% 11% 4	1890
	21st to end	Day	14% 56% 4 12% 11% 2	1869
December	1st to 10th	Day	22% 36% 5% 18% 16% 4	2680
	11th to 20th	Day	30% 32% 8% 15% 11% 5%	3066
	21st to end	Day	26% 28% 8% 22% 16%	4048
January	1st to 10th	Night	26% 22% 9% 17% 18% 7%	7736
	11th to 20th	Night	25% 21% 3 13% 29% 9%	7680
	21st to end	Night	35% 24% 5% 15% 16% 6%	8401
February	1st to 10th	Night	16% 59% 5% 11% 7% 2	2730
	11th to 20th	Night	22% 58% 4 7% 8% 1	2480
	21st to end	Night	18% 59% 2 10% 8% 3	2076
March	1st to 10th	Night	36% 31% 6% 10% 10% 7%	5060
	11th to 20th	Night	24% 30% 6% 8% 19% 13%	4980
	21st to end	Night	26% 30% 7% 19% 13% 6%	5314
April	1st to 10th	Night	15% 64% 5% 6% 8% 2	2000
	11th to 20th	Night	15% 63% 5% 11% 5% 2	1975
	21st to end	Night	13% 63% 5% 11% 6% 1	2000
May	1st to 10th	Night	33% 34% 13% 14% 5% 1	4520
	11th to 20th	Night	37% 32% 8% 17% 6%	4093
	21st to end	Night	31% 33% 11% 16% 9% 1	3927
June	1st to 10th	Night	15% 64% 6% 8% 6% 1	1600
	11th to 20th	Night	12% 64% 9% 10% 5%	1600
	21st to end	Night	11% 62% 7% 14% 5%	1600
July	1st to 10th	Night	27% 34% 19% 15% 4 1	3622
	11th to 20th	Night	37% 35% 19% 9% 1	4006
	21st to end	Night	35% 30% 11% 14% 10%	4297
August	1st to 10th	Night	39% 39% 12% 10% 1	2935
	11th to 20th	Night	28% 41% 10% 15% 7%	3227
	21st to end	Night	19% 39% 9% 20% 11% 1	3586
September	1st to 10th	Night	17% 60% 9% 8% 5% 1	2000
	11th to 20th	Night	20% 59% 5% 7% 7% 2	2000
	21st to end	Night	24% 59% 6% 8% 3	2255
October	1st to 10th	Night	24% 37% 6% 17% 16% 1	3920
	11th to 20th	Night	13% 36% 3 12% 21% 15%	3899
	21st to end	Night	10% 35% 5% 22% 17% 11%	4252
November	1st to 10th	Night	20% 53% 4 11% 10% 2	2510
	11th to 20th	Night	19% 54% 4 7% 11% 5%	2730
	21st to end	Night	15% 55% 4 13% 11% 2	2747
December	1st to 10th	Night	21% 35% 4 14% 19% 7%	4720
	11th to 20th	Night	33% 33% 7% 10% 12% 5%	5348
	21st to end	Night	23% 29% 12% 22% 14% 1	7150

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	17% 32% 35% 14% 1	308
	11th to 20th	Day	22% 25% 36% 15% 1	330
	21st to end	Day	21% 26% 40% 11% 2 1	363
February	1st to 10th	Day	20% 25% 39% 14% 2	330
	11th to 20th	Day	30% 23% 34% 12% 1	514
	21st to end	Day	22% 24% 33% 17% 4 1	446
March	1st to 10th	Day	17% 26% 34% 18% 5%	508
	11th to 20th	Day	20% 23% 36% 15% 4 2	500
	21st to end	Day	19% 21% 40% 14% 5%	550
April	1st to 10th	Day	18% 25% 37% 14% 5% 2	500
	11th to 20th	Day	13% 19% 37% 20% 8% 3	492
	21st to end	Day	14% 14% 33% 29% 7% 2	500
May	1st to 10th	Day	14% 16% 36% 24% 7% 2	503
	11th to 20th	Day	16% 10% 34% 27% 11% 2	527
	21st to end	Day	15% 8% 28% 27% 17% 6%	573
June	1st to 10th	Day	11% 6% 28% 31% 21% 2	530
	11th to 20th	Day	11% 8% 20% 34% 21% 7%	530
	21st to end	Day	7% 4 24% 32% 26% 8%	530
July	1st to 10th	Day	10% 4 15% 34% 29% 9%	525
	11th to 20th	Day	9% 2 12% 35% 32% 9%	524
	21st to end	Day	8% 3 20% 38% 23% 8%	576
August	1st to 10th	Day	15% 6% 17% 33% 21% 8%	526
	11th to 20th	Day	11% 3 13% 34% 26% 13%	519
	21st to end	Day	10% 3 17% 34% 24% 10%	583
September	1st to 10th	Day	13% 4 24% 27% 20% 12%	530
	11th to 20th	Day	17% 7% 22% 28% 17% 8%	530
	21st to end	Day	21% 7% 23% 26% 17% 5%	530
October	1st to 10th	Day	16% 8% 23% 28% 17% 7%	530
	11th to 20th	Day	15% 12% 28% 27% 16% 3	530
	21st to end	Day	12% 14% 31% 24% 15% 5%	583
November	1st to 10th	Day	18% 12% 33% 23% 11% 3	507
	11th to 20th	Day	17% 17% 33% 24% 8% 1	430
	21st to end	Day	14% 17% 30% 25% 12% 1	409
December	1st to 10th	Day	20% 20% 40% 14% 5% 1	220
	11th to 20th	Day	16% 22% 33% 22% 7%	220
	21st to end	Day	20% 18% 36% 17% 8% 1	242
January	1st to 10th	Night	19% 23% 32% 22% 4	840
	11th to 20th	Night	19% 18% 34% 23% 6%	840
	21st to end	Night	18% 17% 42% 21% 2	924
February	1st to 10th	Night	21% 16% 36% 25% 3	840
	11th to 20th	Night	25% 19% 37% 18% 2	823
	21st to end	Night	17% 20% 37% 21% 4	692
March	1st to 10th	Night	18% 18% 38% 22% 5%	809
	11th to 20th	Night	21% 17% 38% 19% 5% 1	800
	21st to end	Night	18% 18% 38% 22% 4	880
April	1st to 10th	Night	17% 18% 39% 22% 4	800
	11th to 20th	Night	13% 15% 40% 24% 7%	791
	21st to end	Night	13% 9% 41% 30% 6% 1	800
May	1st to 10th	Night	14% 12% 36% 31% 7%	800
	11th to 20th	Night	18% 8% 29% 35% 10%	625
	21st to end	Night	12% 3 21% 44% 20% 1	433
June	1st to 10th	Night	12% 4 27% 42% 15% 1	400
	11th to 20th	Night	8% 4 24% 43% 19% 3	400
	21st to end	Night	7% 3 20% 50% 19% 1	400
July	1st to 10th	Night	7% 2 17% 48% 24% 2	413
	11th to 20th	Night	8% 2 11% 43% 28% 7%	582
	21st to end	Night	9% 2 15% 45% 25% 3	657
August	1st to 10th	Night	14% 3 14% 39% 26% 3	615
	11th to 20th	Night	10% 3 15% 41% 28% 4	790
	21st to end	Night	12% 4 13% 36% 31% 5%	880
September	1st to 10th	Night	11% 3 17% 37% 26% 7%	800
	11th to 20th	Night	16% 6% 19% 33% 23% 3	800
	21st to end	Night	20% 6% 18% 33% 21% 2	800
October	1st to 10th	Night	14% 6% 23% 29% 24% 5%	800
	11th to 20th	Night	17% 9% 29% 28% 16% 2	800
	21st to end	Night	15% 9% 28% 29% 16% 4	880
November	1st to 10th	Night	17% 8% 28% 31% 12% 3	820
	11th to 20th	Night	19% 11% 33% 27% 10% 1	840
	21st to end	Night	13% 15% 30% 28% 13% 1	840
December	1st to 10th	Night	20% 14% 34% 26% 6%	840
	11th to 20th	Night	17% 13% 35% 27% 8%	840
	21st to end	Night	17% 15% 33% 28% 6%	924

Page generated at 2026-02-28 01:40:18. Version ac07

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