Oak Tree Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Oak Tree Observatory clear sky chart from 2005-11-06 to 2024-12-09.

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% 5% 5% 4 6% 8% 41%	101360
	Night	13% 6% 2 5% 4 3 4 3 5% 7% 47%	98513
Transparency	Day	39% 6% 12% 19% 17% 7%	105088
	Night	35% 6% 12% 21% 18% 9%	102912
Seeing	Day	12% 5% 17% 24% 26% 14%	14944
	Night	15% 5% 22% 34% 22% 2	24843

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	17% 9% 3 6% 3 3 3 4 3 5% 44%	2847
	11th to 20th	Day	18% 8% 2 5% 3 2 3 2 3 5% 50%	2850
	21st to end	Day	25% 8% 3 6% 3 3 3 3 5% 8% 35%	3135
February	1st to 10th	Day	23% 8% 3 6% 4 4 3 3 4 5% 38%	1852
	11th to 20th	Day	21% 9% 3 6% 4 4 3 3 4 4 39%	1849
	21st to end	Day	19% 7% 3 6% 4 3 3 2 5% 6% 41%	1697
March	1st to 10th	Day	17% 7% 3 8% 6% 5% 5% 4 6% 7% 32%	3436
	11th to 20th	Day	20% 8% 3 8% 6% 5% 5% 4 5% 7% 29%	3480
	21st to end	Day	18% 7% 2 5% 4 4 4 3 5% 6% 42%	3814
April	1st to 10th	Day	14% 8% 3 8% 5% 5% 5% 4 4 6% 38%	2090
	11th to 20th	Day	18% 7% 2 6% 5% 4 4 4 5% 6% 39%	2170
	21st to end	Day	12% 7% 2 7% 5% 4 3 4 5% 7% 44%	2280
May	1st to 10th	Day	17% 8% 3 8% 5% 5% 4 4 5% 8% 31%	3972
	11th to 20th	Day	14% 6% 3 7% 4 5% 5% 4 6% 7% 41%	4082
	21st to end	Day	9% 5% 2 7% 5% 6% 6% 5% 8% 11% 35%	4477
June	1st to 10th	Day	5% 4 2 6% 5% 5% 6% 5% 8% 10% 44%	2470
	11th to 20th	Day	5% 4 2 5% 5% 5% 6% 6% 8% 10% 44%	2470
	21st to end	Day	5% 6% 2 8% 8% 8% 6% 7% 9% 10% 30%	2470
July	1st to 10th	Day	9% 4 2 5% 6% 6% 6% 6% 8% 11% 39%	4120
	11th to 20th	Day	6% 7% 3 8% 6% 6% 7% 6% 9% 11% 32%	4120
	21st to end	Day	14% 8% 3 8% 5% 5% 6% 5% 7% 10% 29%	4465
August	1st to 10th	Day	2 2 1 3 4 4 4 3 6% 10% 62%	4104
	11th to 20th	Day	3 2 1 4 4 5% 7% 6% 9% 12% 46%	3955
	21st to end	Day	5% 4 2 7% 6% 6% 7% 6% 8% 10% 40%	3965
September	1st to 10th	Day	6% 5% 3 7% 5% 7% 6% 5% 8% 10% 39%	2090
	11th to 20th	Day	11% 6% 3 7% 6% 6% 5% 5% 5% 8% 38%	2090
	21st to end	Day	7% 5% 2 6% 4 4 6% 5% 6% 7% 46%	2090
October	1st to 10th	Day	6% 3 2 4 4 4 5% 4 5% 7% 56%	2014
	11th to 20th	Day	10% 5% 2 6% 4 4 4 4 4 6% 53%	1900
	21st to end	Day	12% 5% 2 6% 3 3 3 3 4 6% 54%	2090
November	1st to 10th	Day	11% 7% 2 6% 4 4 4 4 6% 8% 45%	1958
	11th to 20th	Day	11% 6% 2 5% 3 4 3 3 6% 7% 49%	2000
	21st to end	Day	16% 7% 2 6% 5% 5% 4 3 4 6% 43%	1880
December	1st to 10th	Day	17% 8% 3 7% 5% 3 4 3 5% 6% 40%	1782
	11th to 20th	Day	16% 7% 3 7% 5% 4 4 3 5% 5% 42%	2161
	21st to end	Day	16% 7% 2 6% 4 3 3 3 4 5% 46%	3135
January	1st to 10th	Night	19% 6% 2 5% 3 3 3 2 4 6% 46%	4108
	11th to 20th	Night	19% 6% 2 4 2 2 2 2 2 4 54%	4120
	21st to end	Night	23% 8% 2 5% 3 3 3 2 4 7% 39%	4532
February	1st to 10th	Night	20% 9% 3 7% 3 3 3 3 4 6% 39%	2342
	11th to 20th	Night	22% 7% 2 4 3 3 2 3 3 5% 45%	2218
	21st to end	Night	22% 8% 3 5% 3 4 3 3 4 5% 41%	1755
March	1st to 10th	Night	23% 11% 3 5% 4 3 4 3 5% 6% 32%	3419
	11th to 20th	Night	24% 9% 3 8% 4 4 4 3 5% 6% 30%	3480
	21st to end	Night	13% 7% 3 6% 5% 4 3 3 5% 5% 45%	3817
April	1st to 10th	Night	14% 7% 3 6% 4 4 4 4 6% 7% 42%	2090
	11th to 20th	Night	16% 7% 3 6% 3 4 3 3 4 7% 44%	1956
	21st to end	Night	15% 7% 2 6% 4 3 4 3 4 7% 45%	1900
May	1st to 10th	Night	21% 9% 2 7% 5% 4 4 3 5% 7% 31%	2925
	11th to 20th	Night	13% 7% 2 6% 4 4 4 4 6% 7% 44%	2823
	21st to end	Night	10% 6% 3 7% 5% 4 5% 4 7% 10% 38%	3090
June	1st to 10th	Night	6% 3 1 4 2 3 4 3 7% 10% 57%	1710
	11th to 20th	Night	4 3 1 4 4 4 3 4 6% 12% 55%	1710
	21st to end	Night	5% 4 1 4 6% 4 4 5% 9% 10% 45%	1710
July	1st to 10th	Night	5% 4 1 4 4 4 4 5% 8% 12% 50%	2850
	11th to 20th	Night	5% 5% 2 6% 4 5% 5% 4 6% 11% 47%	2850
	21st to end	Night	9% 6% 3 7% 6% 5% 5% 4 7% 11% 38%	3098
August	1st to 10th	Night	1 1 2 2 2 2 2 5% 10% 72%	2836
	11th to 20th	Night	2 2 1 3 3 3 4 4 7% 12% 60%	2879
	21st to end	Night	4 2 2 5% 4 5% 5% 5% 8% 12% 49%	3686
September	1st to 10th	Night	5% 5% 2 5% 4 5% 5% 5% 8% 8% 48%	2090
	11th to 20th	Night	10% 5% 3 6% 4 5% 4 4 5% 8% 45%	2090
	21st to end	Night	7% 4 2 5% 3 3 3 3 6% 8% 55%	2090
October	1st to 10th	Night	4 4 1 3 4 3 3 3 5% 7% 63%	2147
	11th to 20th	Night	11% 4 1 4 3 3 3 2 3 6% 60%	2280
	21st to end	Night	11% 5% 2 4 3 2 3 2 3 4 61%	2508
November	1st to 10th	Night	10% 5% 2 5% 4 4 4 3 5% 6% 53%	2340
	11th to 20th	Night	13% 6% 2 4 4 4 3 3 4 5% 52%	2400
	21st to end	Night	16% 6% 3 5% 4 3 4 3 4 6% 47%	2515
December	1st to 10th	Night	19% 8% 2 5% 3 4 3 3 4 7% 43%	2581
	11th to 20th	Night	17% 8% 3 7% 4 4 3 3 4 5% 42%	3036
	21st to end	Night	16% 7% 2 5% 3 3 3 3 4 6% 47%	4532

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	43% 4 6% 15% 21% 10%	2848
	11th to 20th	Day	38% 3 5% 19% 25% 10%	2850
	21st to end	Day	49% 3 5% 14% 18% 11%	3135
February	1st to 10th	Day	49% 3 6% 13% 17% 12%	1852
	11th to 20th	Day	48% 4 7% 13% 18% 10%	1859
	21st to end	Day	46% 4 9% 16% 15% 11%	1697
March	1st to 10th	Day	49% 6% 10% 15% 16% 4	3436
	11th to 20th	Day	53% 7% 8% 11% 15% 6%	3480
	21st to end	Day	42% 5% 8% 18% 21% 5%	3814
April	1st to 10th	Day	47% 5% 7% 17% 15% 9%	2090
	11th to 20th	Day	45% 6% 8% 14% 18% 9%	2170
	21st to end	Day	40% 5% 8% 20% 20% 7%	2280
May	1st to 10th	Day	49% 8% 11% 17% 10% 4	4008
	11th to 20th	Day	41% 7% 9% 17% 22% 3	4086
	21st to end	Day	39% 9% 12% 29% 11% 1	4489
June	1st to 10th	Day	31% 8% 16% 26% 16% 2	2470
	11th to 20th	Day	31% 11% 15% 24% 17% 2	2470
	21st to end	Day	42% 10% 17% 21% 9%	2470
July	1st to 10th	Day	36% 7% 28% 25% 4	4056
	11th to 20th	Day	41% 9% 19% 22% 9%	4051
	21st to end	Day	48% 9% 18% 21% 4	4325
August	1st to 10th	Day	18% 6% 14% 40% 21%	4090
	11th to 20th	Day	25% 8% 32% 27% 7% 1	3994
	21st to end	Day	36% 10% 18% 22% 12% 1	3964
September	1st to 10th	Day	38% 11% 14% 17% 14% 5%	2090
	11th to 20th	Day	42% 9% 11% 17% 18% 4	2090
	21st to end	Day	33% 7% 12% 18% 17% 13%	2090
October	1st to 10th	Day	18% 4 6% 14% 19% 40%	3352
	11th to 20th	Day	22% 4 7% 15% 35% 17%	3140
	21st to end	Day	27% 4 4 18% 33% 14%	3428
November	1st to 10th	Day	36% 7% 9% 17% 21% 10%	1958
	11th to 20th	Day	35% 4 10% 15% 21% 14%	2000
	21st to end	Day	44% 5% 7% 16% 18% 11%	1880
December	1st to 10th	Day	46% 4 7% 18% 18% 7%	1782
	11th to 20th	Day	44% 5% 7% 12% 23% 10%	2159
	21st to end	Day	41% 4 5% 16% 23% 11%	3135
January	1st to 10th	Night	40% 4 8% 17% 18% 14%	4108
	11th to 20th	Night	37% 3 5% 18% 27% 12%	4120
	21st to end	Night	47% 3 4 16% 20% 10%	4532
February	1st to 10th	Night	47% 3 7% 12% 17% 14%	2342
	11th to 20th	Night	43% 3 7% 16% 18% 13%	2218
	21st to end	Night	47% 4 7% 13% 15% 14%	1755
March	1st to 10th	Night	52% 5% 9% 14% 14% 5%	3419
	11th to 20th	Night	55% 5% 7% 14% 12% 6%	3480
	21st to end	Night	40% 5% 10% 14% 23% 8%	3817
April	1st to 10th	Night	40% 7% 9% 18% 15% 10%	2090
	11th to 20th	Night	41% 5% 10% 16% 15% 13%	1956
	21st to end	Night	40% 5% 8% 21% 19% 7%	1900
May	1st to 10th	Night	51% 7% 9% 18% 10% 5%	2954
	11th to 20th	Night	37% 7% 11% 19% 19% 6%	2824
	21st to end	Night	37% 9% 16% 29% 8% 1	3099
June	1st to 10th	Night	22% 7% 19% 33% 19% 1	1710
	11th to 20th	Night	22% 8% 18% 30% 18% 4	1710
	21st to end	Night	30% 9% 21% 27% 13% 1	1710
July	1st to 10th	Night	27% 8% 33% 28% 4	2805
	11th to 20th	Night	30% 8% 25% 27% 10%	2796
	21st to end	Night	40% 10% 21% 23% 4	2999
August	1st to 10th	Night	11% 4 18% 47% 19%	2826
	11th to 20th	Night	16% 6% 35% 35% 7% 1	2906
	21st to end	Night	26% 14% 22% 27% 10% 1	3686
September	1st to 10th	Night	30% 10% 21% 21% 14% 5%	2090
	11th to 20th	Night	36% 8% 15% 21% 15% 5%	2090
	21st to end	Night	25% 6% 15% 22% 19% 12%	2090
October	1st to 10th	Night	15% 4 7% 18% 17% 40%	3576
	11th to 20th	Night	16% 3 6% 23% 32% 21%	3779
	21st to end	Night	21% 2 5% 20% 35% 17%	4120
November	1st to 10th	Night	33% 5% 10% 21% 21% 10%	2340
	11th to 20th	Night	34% 4 10% 17% 20% 14%	2400
	21st to end	Night	40% 4 9% 17% 19% 12%	2515
December	1st to 10th	Night	43% 4 7% 20% 16% 9%	2581
	11th to 20th	Night	45% 4 7% 11% 19% 15%	3037
	21st to end	Night	40% 4 6% 14% 28% 9%	4532

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	23% 9% 36% 26% 6% 1	270
	11th to 20th	Day	23% 9% 28% 30% 9% 1	270
	21st to end	Day	22% 14% 31% 22% 10% 1	297
February	1st to 10th	Day	29% 15% 26% 24% 7% 1	270
	11th to 20th	Day	24% 17% 31% 22% 6%	270
	21st to end	Day	28% 13% 37% 16% 5%	225
March	1st to 10th	Day	21% 13% 34% 24% 8% 1	270
	11th to 20th	Day	17% 15% 40% 20% 6% 2	270
	21st to end	Day	21% 11% 32% 26% 9% 1	297
April	1st to 10th	Day	14% 15% 39% 22% 9%	270
	11th to 20th	Day	25% 10% 28% 25% 11% 1	368
	21st to end	Day	13% 9% 32% 30% 13% 3	460
May	1st to 10th	Day	14% 7% 33% 30% 15% 1	580
	11th to 20th	Day	14% 5% 22% 33% 23% 3	675
	21st to end	Day	8% 3 24% 36% 25% 4	734
June	1st to 10th	Day	6% 2 7% 32% 38% 15%	680
	11th to 20th	Day	6% 1 8% 32% 37% 15%	680
	21st to end	Day	8% 2 8% 23% 36% 23%	680
July	1st to 10th	Day	8% 1 4 20% 39% 28%	671
	11th to 20th	Day	4 1 4 16% 45% 31%	668
	21st to end	Day	6% 2 4 17% 45% 27%	739
August	1st to 10th	Day	4 2 14% 46% 34%	676
	11th to 20th	Day	6% 2 14% 40% 37%	618
	21st to end	Day	9% 1 1 13% 33% 42%	415
September	1st to 10th	Day	7% 2 4 16% 40% 32%	300
	11th to 20th	Day	8% 2 5% 19% 32% 33%	300
	21st to end	Day	9% 1 7% 20% 40% 24%	300
October	1st to 10th	Day	6% 1 7% 33% 40% 13%	300
	11th to 20th	Day	10% 3 15% 31% 27% 13%	300
	21st to end	Day	18% 7% 22% 19% 25% 9%	330
November	1st to 10th	Day	14% 3 17% 37% 22% 6%	300
	11th to 20th	Day	16% 4 23% 33% 21% 4	300
	21st to end	Day	18% 14% 28% 24% 13% 3	300
December	1st to 10th	Day	20% 4 32% 24% 14% 6%	294
	11th to 20th	Day	23% 13% 27% 28% 8% 2	270
	21st to end	Day	18% 6% 36% 28% 9% 3	297
January	1st to 10th	Night	23% 8% 29% 31% 9%	950
	11th to 20th	Night	20% 12% 27% 33% 7%	950
	21st to end	Night	21% 9% 31% 33% 7%	1045
February	1st to 10th	Night	25% 10% 31% 29% 5%	822
	11th to 20th	Night	26% 8% 29% 33% 5%	744
	21st to end	Night	27% 11% 36% 24% 2	499
March	1st to 10th	Night	23% 9% 35% 29% 5%	570
	11th to 20th	Night	21% 6% 34% 32% 7%	570
	21st to end	Night	17% 8% 37% 31% 6% 1	627
April	1st to 10th	Night	16% 8% 37% 35% 5%	570
	11th to 20th	Night	19% 8% 32% 32% 9%	563
	21st to end	Night	16% 8% 36% 31% 9%	570
May	1st to 10th	Night	15% 8% 33% 36% 8%	570
	11th to 20th	Night	13% 3 28% 40% 13% 2	561
	21st to end	Night	8% 2 26% 41% 21% 2	617
June	1st to 10th	Night	6% 1 9% 46% 34% 3	570
	11th to 20th	Night	4 1 11% 48% 34% 1	570
	21st to end	Night	6% 1 7% 37% 41% 8%	570
July	1st to 10th	Night	5% 1 4 35% 46% 8%	555
	11th to 20th	Night	5% 4 32% 52% 7%	552
	21st to end	Night	2 1 4 34% 51% 7%	624
August	1st to 10th	Night	2 1 1 34% 57% 7%	567
	11th to 20th	Night	4 1 2 31% 56% 7%	564
	21st to end	Night	5% 4 26% 55% 10%	627
September	1st to 10th	Night	6% 1 5% 27% 52% 9%	570
	11th to 20th	Night	9% 2 5% 31% 48% 5%	570
	21st to end	Night	9% 1 8% 32% 43% 7%	570
October	1st to 10th	Night	4 2 10% 39% 40% 5%	627
	11th to 20th	Night	12% 2 19% 41% 24% 2	760
	21st to end	Night	13% 6% 22% 36% 21% 3	836
November	1st to 10th	Night	12% 4 20% 41% 23% 1	780
	11th to 20th	Night	15% 6% 22% 39% 19% 1	800
	21st to end	Night	18% 7% 27% 33% 14% 1	915
December	1st to 10th	Night	22% 6% 25% 33% 14%	993
	11th to 20th	Night	24% 9% 27% 32% 8% 1	950
	21st to end	Night	21% 8% 29% 32% 9% 1	1045

Page generated at 2024-12-06 02:54:48. Version ac07

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