Oakley Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Oakley Observatory clear sky chart from 2005-11-06 to 2025-02-21.

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	14% 8% 3 7% 4 4 4 4 5% 7% 38%	108431
	Night	15% 10% 3 7% 4 4 4 3 5% 6% 39%	110310
Transparency	Day	43% 6% 12% 23% 14% 2	112065
	Night	44% 5% 11% 22% 16% 2	114817
Seeing	Day	16% 9% 26% 25% 17% 7%	15932
	Night	18% 8% 27% 31% 14% 1	25902

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	15% 13% 7% 12% 5% 5% 4 3 5% 6% 25%	3617
	11th to 20th	Day	12% 13% 7% 14% 8% 7% 5% 4 4 5% 22%	3620
	21st to end	Day	23% 15% 5% 8% 5% 4 3 3 3 4 28%	4189
February	1st to 10th	Day	18% 12% 7% 7% 5% 4 3 3 5% 6% 31%	1990
	11th to 20th	Day	17% 10% 4 11% 5% 5% 4 3 5% 5% 31%	1948
	21st to end	Day	17% 13% 3 8% 5% 4 3 4 5% 5% 33%	1570
March	1st to 10th	Day	21% 16% 4 8% 4 3 3 2 3 5% 33%	3120
	11th to 20th	Day	13% 8% 3 6% 4 4 4 3 5% 7% 44%	3274
	21st to end	Day	16% 9% 4 7% 5% 5% 5% 4 6% 6% 32%	4128
April	1st to 10th	Day	16% 10% 3 6% 4 4 4 3 5% 8% 37%	2280
	11th to 20th	Day	17% 8% 2 6% 4 4 5% 4 5% 5% 40%	2254
	21st to end	Day	20% 8% 3 7% 5% 4 4 3 5% 7% 35%	2370
May	1st to 10th	Day	18% 8% 3 8% 5% 5% 5% 4 6% 9% 29%	4082
	11th to 20th	Day	19% 7% 2 6% 4 4 4 3 5% 7% 39%	4081
	21st to end	Day	10% 6% 2 7% 5% 5% 5% 4 6% 9% 41%	4720
June	1st to 10th	Day	11% 6% 2 6% 4 4 4 4 6% 9% 44%	2660
	11th to 20th	Day	8% 5% 2 5% 4 5% 5% 5% 7% 8% 47%	2660
	21st to end	Day	11% 5% 2 6% 5% 5% 4 4 6% 10% 43%	2660
July	1st to 10th	Day	13% 6% 2 6% 4 4 5% 4 6% 11% 39%	4440
	11th to 20th	Day	6% 4 2 5% 4 5% 6% 5% 8% 11% 45%	4440
	21st to end	Day	11% 6% 2 6% 5% 5% 6% 5% 8% 10% 34%	4796
August	1st to 10th	Day	7% 4 2 5% 4 3 4 4 6% 10% 50%	4073
	11th to 20th	Day	9% 5% 2 6% 4 5% 6% 5% 8% 10% 40%	3729
	21st to end	Day	5% 5% 1 5% 4 4 4 3 6% 10% 53%	4180
September	1st to 10th	Day	11% 5% 2 5% 5% 3 5% 4 6% 7% 47%	2280
	11th to 20th	Day	9% 4 2 5% 4 4 5% 5% 6% 8% 48%	2111
	21st to end	Day	12% 5% 2 5% 4 4 4 3 5% 7% 50%	2090
October	1st to 10th	Day	13% 4 2 5% 4 4 3 3 5% 6% 52%	2090
	11th to 20th	Day	15% 5% 2 5% 4 4 4 3 5% 7% 44%	1927
	21st to end	Day	19% 8% 2 7% 3 4 4 3 5% 5% 39%	1881
November	1st to 10th	Day	11% 6% 2 5% 4 3 4 3 5% 7% 51%	1762
	11th to 20th	Day	18% 9% 2 6% 4 4 3 2 5% 6% 40%	1800
	21st to end	Day	19% 11% 3 6% 4 3 4 4 4 6% 37%	1800
December	1st to 10th	Day	13% 11% 3 8% 4 3 3 2 4 4 44%	2819
	11th to 20th	Day	16% 14% 5% 9% 5% 5% 5% 3 4 5% 28%	3008
	21st to end	Day	30% 16% 6% 9% 4 4 3 3 4 4 17%	3982
January	1st to 10th	Night	17% 14% 8% 12% 4 4 4 3 5% 6% 25%	6348
	11th to 20th	Night	14% 19% 6% 10% 6% 6% 5% 3 4 4 23%	6360
	21st to end	Night	21% 12% 4 7% 6% 4 3 3 5% 5% 29%	6785
February	1st to 10th	Night	19% 12% 5% 8% 5% 5% 4 3 5% 5% 30%	2408
	11th to 20th	Night	16% 11% 4 9% 5% 4 4 3 4 5% 33%	2338
	21st to end	Night	20% 12% 3 8% 3 4 4 3 5% 4 35%	1885
March	1st to 10th	Night	21% 15% 3 6% 3 4 3 2 4 5% 34%	3733
	11th to 20th	Night	14% 7% 2 5% 4 3 3 3 4 7% 48%	3686
	21st to end	Night	18% 7% 3 7% 5% 4 4 3 5% 6% 38%	3532
April	1st to 10th	Night	14% 9% 2 6% 4 4 4 3 6% 6% 41%	1900
	11th to 20th	Night	17% 7% 2 6% 4 3 4 3 4 7% 43%	1875
	21st to end	Night	17% 7% 3 7% 4 5% 5% 4 6% 6% 36%	1810
May	1st to 10th	Night	17% 10% 3 7% 5% 4 5% 4 5% 8% 31%	2813
	11th to 20th	Night	19% 6% 2 5% 4 4 4 3 5% 7% 42%	2812
	21st to end	Night	11% 6% 2 6% 4 5% 4 4 6% 9% 42%	2839
June	1st to 10th	Night	9% 6% 2 6% 3 3 4 3 4 8% 49%	1520
	11th to 20th	Night	8% 5% 1 5% 4 4 4 4 5% 9% 51%	1520
	21st to end	Night	7% 4 2 6% 5% 3 5% 5% 7% 11% 46%	1520
July	1st to 10th	Night	11% 5% 2 5% 3 3 5% 3 6% 9% 49%	2530
	11th to 20th	Night	5% 3 2 5% 4 3 4 4 7% 8% 54%	2530
	21st to end	Night	8% 7% 3 8% 5% 5% 4 4 6% 11% 39%	2764
August	1st to 10th	Night	7% 5% 2 5% 3 4 4 3 4 9% 54%	2852
	11th to 20th	Night	6% 5% 2 7% 4 3 5% 4 6% 8% 50%	3100
	21st to end	Night	5% 4 1 5% 4 4 4 3 6% 9% 56%	3476
September	1st to 10th	Night	11% 5% 2 5% 4 4 4 3 5% 8% 50%	1900
	11th to 20th	Night	7% 4 2 4 4 4 3 3 4 8% 58%	2049
	21st to end	Night	13% 6% 2 4 4 3 4 4 5% 7% 49%	2090
October	1st to 10th	Night	8% 4 1 5% 3 4 3 3 5% 5% 57%	2090
	11th to 20th	Night	16% 5% 2 5% 3 3 3 3 4 6% 51%	2248
	21st to end	Night	17% 8% 2 5% 4 4 4 4 5% 7% 40%	2717
November	1st to 10th	Night	10% 7% 3 5% 4 4 4 3 4 5% 51%	2536
	11th to 20th	Night	13% 9% 3 6% 4 3 4 4 5% 7% 43%	2600
	21st to end	Night	20% 11% 4 6% 4 4 3 3 4 5% 36%	2600
December	1st to 10th	Night	16% 10% 3 7% 5% 3 3 3 4 4 42%	4349
	11th to 20th	Night	18% 11% 4 9% 5% 4 4 3 5% 5% 32%	5199
	21st to end	Night	29% 17% 5% 8% 5% 4 3 3 3 4 19%	6996

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	59% 4 8% 21% 7% 1	3618
	11th to 20th	Day	65% 4 10% 10% 8% 2	3620
	21st to end	Day	63% 3 7% 19% 6% 2	4189
February	1st to 10th	Day	54% 4 12% 18% 9% 2	1990
	11th to 20th	Day	55% 4 10% 21% 9% 1	1958
	21st to end	Day	52% 6% 8% 22% 9% 2	1570
March	1st to 10th	Day	56% 4 8% 18% 12% 2	3120
	11th to 20th	Day	39% 6% 13% 28% 14% 1	3274
	21st to end	Day	49% 9% 13% 23% 6% 1	4128
April	1st to 10th	Day	45% 7% 11% 22% 14% 1	2280
	11th to 20th	Day	44% 6% 10% 25% 11% 4	2254
	21st to end	Day	48% 6% 10% 22% 10% 3	2370
May	1st to 10th	Day	48% 9% 15% 22% 7%	4120
	11th to 20th	Day	44% 8% 11% 27% 10%	4085
	21st to end	Day	37% 9% 13% 19% 20% 3	4733
June	1st to 10th	Day	35% 8% 13% 24% 16% 4	2660
	11th to 20th	Day	32% 8% 12% 22% 22% 4	2660
	21st to end	Day	36% 8% 17% 19% 17% 3	2660
July	1st to 10th	Day	40% 6% 11% 27% 14% 2	4371
	11th to 20th	Day	30% 9% 14% 29% 18% 1	4365
	21st to end	Day	40% 9% 16% 20% 13% 2	4645
August	1st to 10th	Day	29% 6% 16% 32% 16% 1	4060
	11th to 20th	Day	36% 7% 14% 27% 15% 1	3765
	21st to end	Day	26% 5% 18% 31% 15% 4	4179
September	1st to 10th	Day	35% 6% 11% 25% 18% 4	2280
	11th to 20th	Day	31% 7% 13% 24% 22% 3	2111
	21st to end	Day	34% 6% 10% 24% 23% 3	2090
October	1st to 10th	Day	22% 3 8% 28% 36% 3	3480
	11th to 20th	Day	29% 4 9% 25% 32% 1	3181
	21st to end	Day	39% 3 9% 23% 25% 1	3081
November	1st to 10th	Day	34% 4 14% 27% 17% 5%	1762
	11th to 20th	Day	45% 5% 9% 21% 15% 4	1800
	21st to end	Day	50% 5% 9% 21% 14% 1	1800
December	1st to 10th	Day	45% 4 11% 28% 12%	2819
	11th to 20th	Day	59% 5% 9% 21% 6% 1	3005
	21st to end	Day	71% 5% 9% 10% 4	3982
January	1st to 10th	Night	61% 4 10% 18% 7%	6348
	11th to 20th	Night	65% 3 9% 11% 9% 3	6360
	21st to end	Night	58% 3 9% 20% 9% 1	6785
February	1st to 10th	Night	57% 4 13% 16% 8% 1	2408
	11th to 20th	Night	52% 5% 11% 18% 13% 2	2339
	21st to end	Night	52% 6% 10% 20% 11% 2	1885
March	1st to 10th	Night	54% 5% 8% 23% 10% 1	3733
	11th to 20th	Night	36% 6% 9% 25% 22% 1	3686
	21st to end	Night	45% 7% 12% 27% 8% 1	3532
April	1st to 10th	Night	42% 6% 11% 24% 15% 2	1900
	11th to 20th	Night	41% 6% 9% 21% 18% 5%	1875
	21st to end	Night	47% 8% 9% 20% 13% 4	1810
May	1st to 10th	Night	49% 10% 13% 21% 7%	2840
	11th to 20th	Night	41% 8% 13% 26% 12%	2813
	21st to end	Night	36% 10% 11% 21% 18% 4	2847
June	1st to 10th	Night	33% 6% 10% 26% 21% 4	1520
	11th to 20th	Night	30% 7% 12% 23% 22% 6%	1520
	21st to end	Night	29% 8% 15% 22% 19% 6%	1520
July	1st to 10th	Night	33% 7% 11% 25% 20% 4	2490
	11th to 20th	Night	24% 6% 14% 31% 23% 2	2482
	21st to end	Night	38% 7% 17% 21% 15% 3	2676
August	1st to 10th	Night	28% 6% 16% 35% 14% 2	2841
	11th to 20th	Night	30% 6% 15% 27% 20% 1	3130
	21st to end	Night	25% 6% 13% 29% 22% 5%	3476
September	1st to 10th	Night	32% 6% 11% 25% 20% 6%	1900
	11th to 20th	Night	25% 5% 11% 27% 27% 5%	2049
	21st to end	Night	35% 6% 10% 22% 23% 5%	2090
October	1st to 10th	Night	19% 4 7% 23% 43% 3	3480
	11th to 20th	Night	25% 3 8% 26% 36% 2	3733
	21st to end	Night	34% 4 8% 24% 25% 5%	4467
November	1st to 10th	Night	35% 4 9% 26% 21% 5%	2536
	11th to 20th	Night	41% 5% 10% 23% 18% 3	2600
	21st to end	Night	51% 4 10% 22% 13% 1	2600
December	1st to 10th	Night	46% 4 11% 25% 13%	4349
	11th to 20th	Night	54% 5% 12% 18% 11%	5201
	21st to end	Night	70% 5% 9% 11% 4 1	6996

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	25% 17% 32% 19% 7%	300
	11th to 20th	Day	25% 16% 39% 16% 4	300
	21st to end	Day	24% 20% 33% 19% 3 1	330
February	1st to 10th	Day	27% 17% 33% 18% 4	300
	11th to 20th	Day	26% 20% 36% 14% 4 1	300
	21st to end	Day	30% 14% 38% 15% 2	225
March	1st to 10th	Day	22% 21% 36% 16% 4	270
	11th to 20th	Day	26% 17% 37% 15% 5% 1	332
	21st to end	Day	27% 16% 31% 18% 6% 1	506
April	1st to 10th	Day	19% 16% 33% 23% 8% 1	460
	11th to 20th	Day	19% 15% 30% 23% 12% 2	452
	21st to end	Day	23% 15% 30% 20% 9% 2	460
May	1st to 10th	Day	22% 11% 29% 26% 10% 3	463
	11th to 20th	Day	18% 7% 29% 25% 15% 6%	487
	21st to end	Day	10% 5% 29% 27% 18% 11%	669
June	1st to 10th	Day	12% 5% 21% 32% 23% 6%	680
	11th to 20th	Day	11% 4 22% 34% 23% 7%	680
	21st to end	Day	12% 4 26% 29% 21% 7%	680
July	1st to 10th	Day	11% 3 15% 28% 23% 20%	671
	11th to 20th	Day	6% 3 18% 32% 28% 13%	668
	21st to end	Day	11% 4 13% 29% 31% 12%	734
August	1st to 10th	Day	9% 2 14% 27% 31% 16%	486
	11th to 20th	Day	14% 3 15% 32% 25% 12%	479
	21st to end	Day	5% 2 12% 30% 33% 18%	539
September	1st to 10th	Day	10% 4 16% 28% 33% 10%	490
	11th to 20th	Day	8% 2 12% 30% 31% 17%	490
	21st to end	Day	14% 6% 24% 22% 21% 13%	490
October	1st to 10th	Day	11% 4 25% 32% 19% 9%	490
	11th to 20th	Day	14% 14% 32% 22% 13% 5%	341
	21st to end	Day	23% 18% 31% 20% 8%	330
November	1st to 10th	Day	13% 14% 33% 25% 13% 3	300
	11th to 20th	Day	22% 17% 31% 19% 10% 2	300
	21st to end	Day	24% 13% 34% 22% 6%	300
December	1st to 10th	Day	16% 21% 37% 17% 7% 2	300
	11th to 20th	Day	24% 13% 37% 19% 6% 1	300
	21st to end	Day	35% 14% 31% 15% 4	330
January	1st to 10th	Night	22% 14% 32% 27% 5%	1000
	11th to 20th	Night	25% 13% 38% 21% 4	1000
	21st to end	Night	23% 12% 37% 26% 2	1100
February	1st to 10th	Night	25% 11% 37% 25% 2	808
	11th to 20th	Night	22% 14% 37% 24% 3	783
	21st to end	Night	26% 12% 36% 24% 2	629
March	1st to 10th	Night	23% 14% 35% 25% 3	760
	11th to 20th	Night	22% 11% 35% 26% 5% 1	760
	21st to end	Night	24% 7% 31% 31% 6%	667
April	1st to 10th	Night	16% 12% 32% 33% 5%	570
	11th to 20th	Night	20% 11% 28% 29% 11%	563
	21st to end	Night	20% 9% 35% 29% 6% 1	570
May	1st to 10th	Night	18% 7% 32% 32% 11% 1	570
	11th to 20th	Night	15% 5% 25% 35% 18% 2	561
	21st to end	Night	12% 3 21% 38% 24% 2	617
June	1st to 10th	Night	11% 4 18% 45% 22% 1	570
	11th to 20th	Night	10% 3 18% 46% 23% 1	570
	21st to end	Night	8% 3 21% 41% 26% 1	570
July	1st to 10th	Night	10% 2 9% 36% 36% 7%	555
	11th to 20th	Night	6% 3 14% 39% 34% 4	552
	21st to end	Night	6% 3 10% 45% 33% 3	624
August	1st to 10th	Night	8% 1 9% 43% 36% 3	567
	11th to 20th	Night	7% 1 12% 41% 35% 4	564
	21st to end	Night	8% 1 11% 33% 41% 6%	627
September	1st to 10th	Night	11% 2 13% 38% 30% 5%	570
	11th to 20th	Night	8% 2 15% 36% 35% 5%	719
	21st to end	Night	15% 5% 18% 34% 24% 4	760
October	1st to 10th	Night	10% 4 21% 39% 22% 4	760
	11th to 20th	Night	19% 9% 26% 29% 15% 1	760
	21st to end	Night	21% 10% 33% 29% 8%	836
November	1st to 10th	Night	16% 6% 33% 33% 12% 1	780
	11th to 20th	Night	18% 11% 32% 29% 8% 1	800
	21st to end	Night	25% 11% 31% 26% 7%	800
December	1st to 10th	Night	23% 10% 37% 24% 5%	860
	11th to 20th	Night	25% 9% 33% 26% 6% 1	1000
	21st to end	Night	31% 13% 31% 22% 3	1100

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