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 2024-12-02.

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	12% 6% 2 6% 5% 4 5% 4 6% 8% 41%	101137
	Night	13% 7% 2 6% 4 4 4 4 6% 7% 42%	98438
Transparency	Day	21% 48% 7% 12% 9% 2	104777
	Night	22% 48% 6% 12% 9% 3	102937
Seeing	Day	15% 13% 28% 25% 14% 4	15458
	Night	16% 11% 29% 30% 13% 2	25137

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	14% 7% 1 5% 3 3 3 2 4 5% 51%	2772
	11th to 20th	Day	12% 7% 3 6% 4 3 3 3 4 6% 49%	2840
	21st to end	Day	13% 10% 3 7% 5% 4 4 3 4 6% 40%	3124
February	1st to 10th	Day	15% 6% 2 7% 4 4 4 3 6% 8% 41%	1710
	11th to 20th	Day	19% 6% 2 5% 4 3 3 3 5% 5% 44%	1828
	21st to end	Day	15% 8% 2 6% 4 4 4 4 5% 6% 41%	1570
March	1st to 10th	Day	23% 8% 2 6% 4 3 3 3 4 6% 37%	3343
	11th to 20th	Day	11% 5% 2 5% 3 3 3 3 4 6% 55%	3480
	21st to end	Day	16% 6% 3 6% 4 3 3 3 5% 5% 47%	3924
April	1st to 10th	Day	12% 6% 1 5% 4 3 3 2 6% 6% 53%	2280
	11th to 20th	Day	9% 5% 2 5% 4 4 4 3 5% 8% 51%	2254
	21st to end	Day	10% 5% 2 5% 5% 4 4 4 6% 7% 47%	2280
May	1st to 10th	Day	9% 7% 3 7% 5% 5% 7% 5% 7% 9% 36%	3765
	11th to 20th	Day	18% 8% 3 8% 5% 5% 5% 4 6% 8% 32%	4060
	21st to end	Day	7% 6% 3 6% 4 5% 6% 4 7% 10% 42%	4823
June	1st to 10th	Day	9% 6% 1 6% 5% 5% 5% 6% 8% 10% 39%	2660
	11th to 20th	Day	10% 5% 2 4 4 5% 5% 6% 8% 11% 41%	2660
	21st to end	Day	6% 4 2 6% 6% 5% 6% 6% 9% 11% 38%	2660
July	1st to 10th	Day	8% 5% 2 6% 5% 5% 6% 5% 10% 13% 35%	4386
	11th to 20th	Day	10% 6% 4 9% 6% 6% 6% 5% 8% 11% 29%	4120
	21st to end	Day	10% 7% 3 9% 7% 6% 7% 6% 8% 10% 29%	4466
August	1st to 10th	Day	17% 11% 4 9% 6% 7% 6% 4 7% 9% 22%	4042
	11th to 20th	Day	8% 6% 2 7% 5% 5% 7% 5% 8% 11% 35%	3730
	21st to end	Day	7% 4 2 6% 5% 6% 6% 5% 7% 10% 42%	4180
September	1st to 10th	Day	12% 6% 2 7% 4 5% 5% 4 7% 9% 39%	2280
	11th to 20th	Day	13% 8% 2 7% 4 4 4 4 6% 7% 39%	2280
	21st to end	Day	18% 8% 3 7% 5% 4 4 4 6% 7% 34%	2033
October	1st to 10th	Day	14% 7% 2 6% 4 3 4 3 5% 7% 45%	1900
	11th to 20th	Day	11% 6% 2 7% 4 3 4 3 5% 5% 51%	1900
	21st to end	Day	11% 5% 2 6% 4 3 3 2 5% 6% 52%	2090
November	1st to 10th	Day	15% 6% 2 5% 3 3 4 4 5% 6% 47%	1907
	11th to 20th	Day	15% 6% 2 4 3 2 4 3 4 7% 50%	1800
	21st to end	Day	12% 5% 2 4 4 3 4 3 5% 6% 50%	1780
December	1st to 10th	Day	17% 7% 2 6% 5% 4 4 4 5% 7% 39%	1528
	11th to 20th	Day	16% 6% 1 5% 3 2 3 2 3 5% 53%	1910
	21st to end	Day	15% 7% 2 5% 4 4 4 3 4 7% 44%	2772
January	1st to 10th	Night	14% 7% 2 5% 4 4 4 3 5% 6% 46%	4169
	11th to 20th	Night	15% 7% 3 7% 5% 4 3 4 6% 6% 41%	4120
	21st to end	Night	14% 8% 3 7% 5% 4 4 3 5% 5% 42%	4532
February	1st to 10th	Night	17% 7% 2 5% 4 3 4 3 5% 7% 41%	2470
	11th to 20th	Night	19% 9% 2 6% 4 3 3 3 4 6% 41%	2237
	21st to end	Night	15% 8% 2 6% 4 3 3 3 5% 7% 45%	1884
March	1st to 10th	Night	24% 10% 2 7% 4 4 4 4 4 5% 32%	3493
	11th to 20th	Night	13% 6% 2 6% 4 3 3 3 4 5% 51%	3480
	21st to end	Night	17% 7% 2 5% 3 3 3 2 4 5% 48%	3706
April	1st to 10th	Night	12% 6% 2 6% 4 5% 4 4 5% 7% 46%	1900
	11th to 20th	Night	10% 7% 2 6% 4 4 5% 3 5% 7% 47%	1875
	21st to end	Night	12% 6% 2 4 4 4 3 3 6% 7% 47%	1900
May	1st to 10th	Night	10% 7% 2 8% 6% 5% 6% 5% 4 8% 38%	3130
	11th to 20th	Night	19% 7% 3 6% 5% 3 5% 4 5% 7% 36%	2858
	21st to end	Night	8% 5% 2 7% 4 4 4 5% 7% 11% 42%	2732
June	1st to 10th	Night	10% 7% 2 5% 5% 4 4 5% 7% 11% 40%	1520
	11th to 20th	Night	7% 5% 2 6% 6% 4 6% 5% 9% 12% 38%	1520
	21st to end	Night	8% 5% 2 6% 5% 4 4 5% 9% 11% 40%	1520
July	1st to 10th	Night	6% 5% 2 6% 5% 6% 7% 5% 8% 14% 35%	2569
	11th to 20th	Night	9% 7% 2 8% 6% 5% 6% 6% 8% 11% 32%	2840
	21st to end	Night	15% 8% 3 9% 6% 6% 5% 5% 7% 7% 29%	3087
August	1st to 10th	Night	15% 10% 3 8% 5% 5% 5% 5% 6% 10% 29%	2855
	11th to 20th	Night	9% 7% 3 9% 6% 6% 5% 4 7% 9% 36%	3097
	21st to end	Night	7% 5% 2 6% 4 4 6% 5% 7% 10% 42%	3476
September	1st to 10th	Night	10% 5% 3 6% 5% 5% 5% 4 7% 9% 42%	1900
	11th to 20th	Night	13% 7% 3 6% 4 4 4 3 7% 8% 41%	1900
	21st to end	Night	16% 8% 3 8% 5% 4 5% 4 7% 8% 33%	2142
October	1st to 10th	Night	13% 6% 2 6% 3 4 3 4 5% 5% 48%	2280
	11th to 20th	Night	12% 6% 2 5% 3 3 3 2 5% 6% 52%	2280
	21st to end	Night	11% 4 2 5% 3 3 3 3 5% 6% 54%	2508
November	1st to 10th	Night	15% 5% 2 5% 3 3 3 3 5% 6% 49%	2387
	11th to 20th	Night	16% 6% 2 5% 4 3 4 3 5% 6% 47%	2600
	21st to end	Night	12% 6% 2 5% 3 4 3 2 6% 7% 50%	2616
December	1st to 10th	Night	15% 8% 3 7% 4 5% 5% 4 4 6% 40%	2686
	11th to 20th	Night	17% 6% 2 6% 4 3 3 3 5% 7% 46%	3285
	21st to end	Night	14% 6% 2 5% 3 3 4 3 6% 7% 45%	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	22% 38% 3 20% 15% 3	2773
	11th to 20th	Day	22% 38% 4 10% 21% 5%	2840
	21st to end	Day	30% 42% 5% 9% 9% 5%	3124
February	1st to 10th	Day	15% 65% 5% 9% 6% 1	1710
	11th to 20th	Day	17% 64% 3 6% 7% 3	1837
	21st to end	Day	16% 65% 3 6% 7% 2	1570
March	1st to 10th	Day	36% 43% 6% 7% 4 4	3343
	11th to 20th	Day	16% 41% 3 10% 19% 11%	3480
	21st to end	Day	23% 40% 6% 15% 12% 4	3924
April	1st to 10th	Day	12% 65% 5% 8% 9% 1	2280
	11th to 20th	Day	12% 66% 3 10% 6% 2	2254
	21st to end	Day	13% 66% 5% 10% 6% 1	2280
May	1st to 10th	Day	27% 47% 10% 10% 5% 1	3800
	11th to 20th	Day	31% 43% 7% 14% 5%	4064
	21st to end	Day	18% 43% 8% 20% 9% 1	4837
June	1st to 10th	Day	13% 68% 5% 7% 6% 1	2660
	11th to 20th	Day	9% 67% 8% 9% 6%	2660
	21st to end	Day	11% 66% 8% 10% 5%	2660
July	1st to 10th	Day	23% 43% 15% 13% 5% 1	4317
	11th to 20th	Day	33% 41% 16% 9% 1	4051
	21st to end	Day	35% 39% 8% 10% 8% 1	4324
August	1st to 10th	Day	42% 40% 10% 6% 2	4029
	11th to 20th	Day	22% 42% 12% 16% 8%	3766
	21st to end	Day	19% 40% 11% 20% 9% 1	4180
September	1st to 10th	Day	16% 63% 8% 6% 5% 1	2280
	11th to 20th	Day	18% 62% 6% 7% 6% 2	2280
	21st to end	Day	22% 61% 5% 8% 3	2033
October	1st to 10th	Day	18% 39% 7% 17% 17% 2	3160
	11th to 20th	Day	12% 37% 3 13% 23% 12%	3140
	21st to end	Day	9% 37% 4 22% 22% 7%	3428
November	1st to 10th	Day	18% 56% 5% 11% 8% 2	1907
	11th to 20th	Day	16% 57% 3 9% 11% 4	1800
	21st to end	Day	14% 58% 3 12% 11% 2	1780
December	1st to 10th	Day	22% 60% 5% 7% 4 2	1528
	11th to 20th	Day	16% 47% 3 16% 10% 6%	1906
	21st to end	Day	26% 39% 4 17% 14% 1	2772
January	1st to 10th	Night	24% 38% 4 16% 14% 3	4169
	11th to 20th	Night	27% 38% 3 12% 16% 4	4120
	21st to end	Night	28% 42% 4 9% 12% 5%	4532
February	1st to 10th	Night	13% 65% 4 10% 5% 1	2470
	11th to 20th	Night	20% 64% 3 5% 7% 2	2239
	21st to end	Night	15% 65% 2 9% 7% 2	1884
March	1st to 10th	Night	41% 43% 5% 5% 4 1	3493
	11th to 20th	Night	18% 41% 4 8% 17% 12%	3480
	21st to end	Night	23% 41% 4 19% 11% 4	3706
April	1st to 10th	Night	14% 67% 5% 5% 8% 2	1900
	11th to 20th	Night	15% 66% 4 9% 5% 1	1875
	21st to end	Night	13% 66% 5% 11% 6% 1	1900
May	1st to 10th	Night	28% 45% 11% 12% 4 1	3160
	11th to 20th	Night	29% 43% 6% 17% 5%	2859
	21st to end	Night	19% 45% 13% 15% 7% 1	2739
June	1st to 10th	Night	14% 67% 6% 8% 5% 1	1520
	11th to 20th	Night	9% 68% 9% 10% 5%	1520
	21st to end	Night	11% 65% 7% 12% 5%	1520
July	1st to 10th	Night	24% 44% 15% 12% 5% 1	2528
	11th to 20th	Night	32% 41% 17% 10% 1	2786
	21st to end	Night	39% 39% 6% 7% 9%	2988
August	1st to 10th	Night	38% 40% 12% 10% 1	2844
	11th to 20th	Night	27% 41% 10% 15% 7%	3127
	21st to end	Night	19% 40% 9% 20% 11% 1	3476
September	1st to 10th	Night	16% 62% 9% 7% 5% 1	1900
	11th to 20th	Night	18% 62% 5% 7% 6% 2	1900
	21st to end	Night	22% 62% 5% 8% 3	2142
October	1st to 10th	Night	24% 38% 5% 16% 16% 1	3800
	11th to 20th	Night	11% 37% 3 12% 21% 15%	3779
	21st to end	Night	8% 36% 5% 22% 17% 11%	4120
November	1st to 10th	Night	20% 56% 4 11% 8% 2	2387
	11th to 20th	Night	19% 57% 4 6% 10% 5%	2600
	21st to end	Night	14% 57% 4 13% 10% 2	2616
December	1st to 10th	Night	23% 60% 3 8% 5% 1	2686
	11th to 20th	Night	19% 49% 3 12% 13% 4	3288
	21st to end	Night	23% 39% 7% 16% 13% 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	19% 34% 34% 13% 1	252
	11th to 20th	Day	23% 27% 34% 15% 1	270
	21st to end	Day	21% 28% 38% 10% 2 1	297
February	1st to 10th	Day	19% 26% 40% 13% 2	270
	11th to 20th	Day	29% 24% 34% 11% 1	436
	21st to end	Day	23% 25% 32% 16% 3 1	382
March	1st to 10th	Day	18% 25% 34% 18% 5%	460
	11th to 20th	Day	21% 24% 35% 14% 4 2	460
	21st to end	Day	21% 21% 39% 14% 6%	506
April	1st to 10th	Day	18% 26% 35% 14% 5% 2	460
	11th to 20th	Day	14% 20% 35% 20% 8% 3	452
	21st to end	Day	15% 15% 34% 27% 7% 2	460
May	1st to 10th	Day	14% 17% 36% 24% 7% 2	463
	11th to 20th	Day	15% 11% 33% 28% 11% 2	487
	21st to end	Day	14% 7% 26% 29% 18% 6%	529
June	1st to 10th	Day	11% 6% 29% 30% 22% 2	490
	11th to 20th	Day	11% 8% 20% 32% 21% 7%	490
	21st to end	Day	7% 4 26% 33% 24% 6%	490
July	1st to 10th	Day	9% 4 15% 34% 30% 9%	485
	11th to 20th	Day	9% 2 12% 36% 32% 8%	484
	21st to end	Day	9% 3 21% 39% 22% 6%	532
August	1st to 10th	Day	14% 6% 17% 33% 21% 9%	486
	11th to 20th	Day	11% 2 14% 34% 26% 13%	479
	21st to end	Day	10% 3 17% 34% 25% 11%	539
September	1st to 10th	Day	13% 4 24% 26% 20% 12%	490
	11th to 20th	Day	15% 7% 22% 29% 18% 9%	490
	21st to end	Day	21% 7% 24% 26% 16% 6%	490
October	1st to 10th	Day	17% 8% 23% 28% 17% 7%	490
	11th to 20th	Day	13% 12% 27% 28% 16% 3	490
	21st to end	Day	11% 13% 30% 24% 17% 5%	539
November	1st to 10th	Day	19% 12% 33% 23% 11% 3	470
	11th to 20th	Day	18% 16% 33% 24% 9% 1	400
	21st to end	Day	15% 18% 30% 23% 13% 1	380
December	1st to 10th	Day	19% 23% 40% 12% 5% 2	182
	11th to 20th	Day	17% 24% 33% 18% 7%	180
	21st to end	Day	23% 19% 32% 15% 10% 1	198
January	1st to 10th	Night	20% 22% 31% 23% 4	760
	11th to 20th	Night	20% 17% 33% 24% 6%	760
	21st to end	Night	18% 18% 41% 21% 2	836
February	1st to 10th	Night	21% 17% 34% 25% 3	760
	11th to 20th	Night	25% 20% 36% 17% 2	743
	21st to end	Night	17% 21% 36% 22% 4	628
March	1st to 10th	Night	18% 17% 38% 21% 5%	760
	11th to 20th	Night	21% 18% 38% 18% 5% 1	760
	21st to end	Night	19% 17% 37% 22% 5%	836
April	1st to 10th	Night	17% 18% 40% 22% 3	760
	11th to 20th	Night	14% 15% 40% 23% 8%	751
	21st to end	Night	14% 10% 42% 29% 5% 1	760
May	1st to 10th	Night	14% 12% 37% 30% 7%	760
	11th to 20th	Night	17% 8% 29% 35% 10%	594
	21st to end	Night	11% 2 20% 45% 21% 1	411
June	1st to 10th	Night	12% 4 26% 42% 15% 1	380
	11th to 20th	Night	7% 3 24% 44% 19% 3	380
	21st to end	Night	8% 3 21% 51% 17% 1	380
July	1st to 10th	Night	7% 2 17% 48% 24% 2	392
	11th to 20th	Night	9% 2 11% 43% 29% 6%	552
	21st to end	Night	9% 3 16% 46% 24% 2	624
August	1st to 10th	Night	13% 3 14% 40% 26% 4	584
	11th to 20th	Night	9% 3 15% 42% 28% 4	750
	21st to end	Night	12% 4 13% 35% 32% 5%	836
September	1st to 10th	Night	11% 3 15% 37% 26% 7%	760
	11th to 20th	Night	15% 5% 19% 34% 24% 3	760
	21st to end	Night	20% 6% 18% 32% 21% 3	760
October	1st to 10th	Night	14% 5% 23% 30% 23% 4	760
	11th to 20th	Night	16% 8% 29% 29% 17% 2	760
	21st to end	Night	14% 9% 28% 29% 17% 4	836
November	1st to 10th	Night	17% 8% 28% 31% 12% 4	780
	11th to 20th	Night	20% 11% 32% 27% 10% 1	800
	21st to end	Night	13% 15% 30% 27% 14% 1	800
December	1st to 10th	Night	20% 14% 35% 24% 7%	768
	11th to 20th	Night	18% 13% 36% 26% 8%	760
	21st to end	Night	18% 16% 33% 27% 7%	836

Page generated at 2024-11-29 10:43:08. Version ac07

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