PostOak
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the PostOak clear sky chart from 2005-11-06 to 2026-07-16.

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% 6% 2 6% 4 4 4 4 6% 8% 42%	137933
	Night	14% 6% 2 6% 4 4 4 3 5% 7% 44%	134751
Transparency	Day	39% 4 11% 23% 18% 4	141534
	Night	39% 4 10% 23% 19% 5%	139222
Seeing	Day	19% 12% 27% 25% 13% 5%	17240
	Night	17% 10% 29% 29% 13% 1	26854

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% 6% 2 6% 3 3 4 3 4 6% 48%	5081
	11th to 20th	Day	14% 7% 2 5% 3 3 3 3 3 4 53%	5280
	21st to end	Day	14% 7% 2 6% 4 4 5% 3 4 5% 45%	5780
February	1st to 10th	Day	13% 7% 3 7% 4 4 5% 3 5% 6% 42%	1890
	11th to 20th	Day	20% 7% 2 6% 4 3 4 3 4 6% 41%	1890
	21st to end	Day	17% 9% 2 6% 5% 4 4 4 4 7% 37%	1739
March	1st to 10th	Day	18% 6% 2 7% 4 4 4 4 5% 7% 39%	5045
	11th to 20th	Day	10% 6% 2 6% 4 4 3 3 5% 7% 51%	5090
	21st to end	Day	16% 6% 3 6% 4 4 4 3 5% 7% 41%	5620
April	1st to 10th	Day	12% 6% 2 5% 4 4 4 3 5% 7% 48%	2520
	11th to 20th	Day	13% 6% 1 5% 4 3 4 4 5% 6% 49%	2494
	21st to end	Day	15% 7% 2 6% 4 4 4 4 5% 6% 42%	2520
May	1st to 10th	Day	15% 6% 3 6% 4 5% 5% 4 7% 9% 36%	7497
	11th to 20th	Day	18% 6% 2 5% 4 4 4 4 6% 9% 39%	7736
	21st to end	Day	21% 6% 2 5% 3 4 4 4 6% 8% 38%	9045
June	1st to 10th	Day	11% 4 2 5% 4 5% 4 4 7% 9% 44%	2940
	11th to 20th	Day	10% 4 1 5% 5% 6% 5% 4 7% 10% 42%	2940
	21st to end	Day	8% 5% 2 5% 5% 4 6% 4 7% 10% 44%	2940
July	1st to 10th	Day	8% 4 2 6% 5% 5% 5% 5% 8% 11% 42%	6485
	11th to 20th	Day	9% 6% 3 7% 5% 6% 7% 6% 10% 12% 28%	5965
	21st to end	Day	8% 5% 2 6% 5% 5% 5% 5% 7% 10% 40%	6424
August	1st to 10th	Day	15% 6% 3 6% 5% 5% 5% 5% 8% 10% 33%	4234
	11th to 20th	Day	10% 5% 2 7% 5% 6% 7% 5% 8% 9% 35%	4012
	21st to end	Day	10% 4 2 5% 3 5% 4 4 6% 10% 48%	4312
September	1st to 10th	Day	13% 5% 2 5% 4 5% 5% 4 5% 7% 46%	2400
	11th to 20th	Day	13% 5% 2 6% 3 4 5% 4 6% 8% 45%	2397
	21st to end	Day	23% 6% 2 6% 4 4 4 4 5% 7% 36%	2086
October	1st to 10th	Day	15% 5% 3 5% 3 4 3 3 5% 5% 49%	2000
	11th to 20th	Day	17% 4 1 4 4 4 3 3 4 5% 51%	2000
	21st to end	Day	14% 6% 2 5% 3 4 3 3 4 5% 52%	2200
November	1st to 10th	Day	12% 4 2 5% 4 4 3 3 4 6% 53%	2024
	11th to 20th	Day	14% 5% 1 4 3 3 4 3 5% 6% 50%	1869
	21st to end	Day	15% 5% 2 5% 3 3 3 3 5% 5% 50%	1680
December	1st to 10th	Day	12% 6% 2 5% 4 4 3 4 5% 6% 50%	2680
	11th to 20th	Day	18% 7% 2 6% 3 2 3 2 4 7% 46%	3070
	21st to end	Day	16% 7% 3 7% 4 4 4 3 6% 7% 40%	4048
January	1st to 10th	Night	13% 6% 2 4 3 4 4 4 5% 6% 49%	7797
	11th to 20th	Night	14% 6% 2 5% 3 3 3 2 3 5% 54%	7680
	21st to end	Night	14% 6% 2 6% 4 4 3 3 4 5% 49%	8401
February	1st to 10th	Night	14% 8% 2 5% 4 4 4 4 5% 7% 42%	2730
	11th to 20th	Night	20% 8% 2 5% 3 3 4 3 4 5% 43%	2616
	21st to end	Night	16% 8% 2 5% 5% 4 3 3 5% 6% 45%	2064
March	1st to 10th	Night	16% 6% 2 5% 3 3 4 3 4 6% 47%	5028
	11th to 20th	Night	11% 6% 2 5% 3 3 3 4 6% 7% 51%	5090
	21st to end	Night	12% 7% 2 6% 4 4 4 3 5% 6% 46%	5553
April	1st to 10th	Night	15% 6% 2 5% 4 4 4 3 5% 6% 46%	2100
	11th to 20th	Night	11% 5% 2 6% 4 4 4 3 6% 8% 48%	2075
	21st to end	Night	15% 5% 2 5% 4 4 4 4 6% 7% 45%	2100
May	1st to 10th	Night	16% 9% 3 7% 5% 5% 5% 4 6% 8% 35%	5399
	11th to 20th	Night	19% 6% 2 6% 4 4 4 4 6% 7% 38%	5120
	21st to end	Night	21% 8% 3 7% 4 4 4 3 5% 7% 34%	5108
June	1st to 10th	Night	10% 6% 2 7% 4 4 5% 5% 6% 8% 45%	1680
	11th to 20th	Night	11% 6% 2 6% 4 5% 5% 5% 6% 10% 40%	1680
	21st to end	Night	9% 5% 1 6% 5% 5% 5% 4 8% 10% 43%	1680
July	1st to 10th	Night	9% 6% 2 8% 4 5% 5% 5% 7% 11% 38%	3678
	11th to 20th	Night	11% 9% 3 9% 5% 6% 6% 6% 8% 9% 28%	4098
	21st to end	Night	7% 6% 2 8% 5% 6% 6% 5% 8% 9% 37%	4427
August	1st to 10th	Night	16% 7% 3 7% 5% 5% 5% 5% 6% 8% 33%	2915
	11th to 20th	Night	10% 7% 3 9% 6% 5% 5% 4 7% 8% 36%	3002
	21st to end	Night	11% 6% 3 6% 4 3 4 3 6% 9% 45%	3586
September	1st to 10th	Night	13% 6% 2 6% 4 4 4 4 5% 7% 46%	2000
	11th to 20th	Night	12% 6% 3 6% 4 3 4 3 5% 6% 48%	2000
	21st to end	Night	23% 6% 3 6% 4 3 4 3 5% 6% 37%	2311
October	1st to 10th	Night	17% 6% 2 4 3 3 3 3 4 7% 47%	2400
	11th to 20th	Night	17% 5% 2 5% 2 3 3 3 5% 5% 50%	2400
	21st to end	Night	14% 5% 1 5% 4 3 3 3 5% 5% 52%	2640
November	1st to 10th	Night	11% 6% 2 5% 4 3 4 2 4 6% 53%	2492
	11th to 20th	Night	14% 6% 2 5% 4 3 4 3 4 6% 49%	2746
	21st to end	Night	12% 4 2 5% 4 4 4 4 5% 6% 50%	2940
December	1st to 10th	Night	10% 5% 2 5% 3 3 4 3 5% 7% 54%	4720
	11th to 20th	Night	20% 7% 2 5% 4 4 4 3 4 5% 42%	5345
	21st to end	Night	16% 7% 2 5% 3 4 4 4 6% 7% 43%	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	40% 2 7% 24% 20% 7%	5082
	11th to 20th	Day	37% 2 4 19% 26% 12%	5280
	21st to end	Day	42% 2 6% 29% 17% 3	5780
February	1st to 10th	Day	43% 3 9% 27% 15% 3	1890
	11th to 20th	Day	45% 3 5% 22% 20% 4	1899
	21st to end	Day	47% 3 10% 18% 17% 4	1739
March	1st to 10th	Day	45% 2 8% 19% 17% 9%	5045
	11th to 20th	Day	33% 3 7% 25% 24% 8%	5090
	21st to end	Day	44% 3 11% 18% 17% 7%	5620
April	1st to 10th	Day	36% 4 10% 22% 21% 7%	2520
	11th to 20th	Day	35% 3 11% 22% 23% 6%	2494
	21st to end	Day	42% 4 10% 26% 14% 5%	2520
May	1st to 10th	Day	44% 4 13% 22% 16% 1	7531
	11th to 20th	Day	42% 4 11% 26% 16% 1	7740
	21st to end	Day	45% 3 12% 19% 18% 3	9061
June	1st to 10th	Day	35% 5% 14% 27% 16% 2	2940
	11th to 20th	Day	36% 6% 15% 24% 17% 2	2940
	21st to end	Day	34% 7% 14% 26% 17% 1	2940
July	1st to 10th	Day	34% 8% 20% 20% 13% 5%	6415
	11th to 20th	Day	43% 11% 18% 21% 7% 1	5896
	21st to end	Day	37% 7% 13% 23% 20%	6240
August	1st to 10th	Day	44% 7% 16% 26% 7%	4221
	11th to 20th	Day	41% 7% 15% 24% 13% 1	4051
	21st to end	Day	31% 5% 16% 28% 19% 2	4312
September	1st to 10th	Day	37% 5% 13% 19% 21% 4	2400
	11th to 20th	Day	36% 5% 8% 26% 21% 4	2397
	21st to end	Day	47% 4 9% 20% 16% 4	2086
October	1st to 10th	Day	36% 3 6% 19% 30% 5%	3260
	11th to 20th	Day	28% 2 5% 22% 32% 11%	3240
	21st to end	Day	22% 2 9% 28% 26% 13%	3538
November	1st to 10th	Day	33% 3 8% 26% 25% 4	2024
	11th to 20th	Day	35% 3 10% 23% 23% 7%	1869
	21st to end	Day	37% 3 8% 26% 23% 4	1680
December	1st to 10th	Day	35% 3 9% 28% 18% 8%	2680
	11th to 20th	Day	41% 2 13% 25% 15% 4	3066
	21st to end	Day	44% 3 11% 25% 16% 1	4048
January	1st to 10th	Night	37% 2 9% 23% 23% 6%	7798
	11th to 20th	Night	36% 2 5% 21% 26% 10%	7680
	21st to end	Night	39% 2 6% 28% 20% 5%	8401
February	1st to 10th	Night	41% 4 10% 25% 18% 3	2730
	11th to 20th	Night	44% 3 7% 21% 19% 6%	2618
	21st to end	Night	42% 2 9% 22% 18% 7%	2064
March	1st to 10th	Night	39% 2 7% 24% 20% 8%	5028
	11th to 20th	Night	33% 3 8% 22% 26% 8%	5090
	21st to end	Night	39% 3 9% 22% 21% 7%	5553
April	1st to 10th	Night	39% 4 9% 19% 23% 6%	2100
	11th to 20th	Night	35% 4 10% 24% 21% 6%	2075
	21st to end	Night	38% 4 11% 26% 16% 5%	2100
May	1st to 10th	Night	48% 4 14% 22% 10% 3	5427
	11th to 20th	Night	44% 5% 12% 25% 13% 1	5121
	21st to end	Night	51% 4 11% 18% 14% 2	5115
June	1st to 10th	Night	37% 5% 13% 26% 17% 2	1680
	11th to 20th	Night	39% 5% 17% 24% 14% 1	1680
	21st to end	Night	35% 7% 15% 22% 19% 3	1680
July	1st to 10th	Night	40% 9% 17% 17% 14% 2	3640
	11th to 20th	Night	49% 9% 20% 16% 5% 1	4048
	21st to end	Night	40% 8% 14% 22% 16%	4297
August	1st to 10th	Night	47% 6% 15% 23% 7% 1	2904
	11th to 20th	Night	43% 7% 14% 23% 13% 1	3029
	21st to end	Night	36% 4 12% 29% 17% 1	3586
September	1st to 10th	Night	37% 5% 13% 22% 16% 6%	2000
	11th to 20th	Night	37% 4 9% 26% 22% 3	2000
	21st to end	Night	49% 4 10% 19% 16% 3	2311
October	1st to 10th	Night	41% 3 9% 19% 22% 7%	3920
	11th to 20th	Night	26% 3 8% 22% 31% 11%	3899
	21st to end	Night	23% 1 6% 32% 26% 12%	4252
November	1st to 10th	Night	34% 3 8% 24% 25% 5%	2492
	11th to 20th	Night	37% 3 7% 22% 24% 7%	2746
	21st to end	Night	34% 3 9% 26% 23% 5%	2940
December	1st to 10th	Night	31% 3 10% 28% 24% 4	4720
	11th to 20th	Night	46% 3 10% 19% 16% 6%	5348
	21st to end	Night	42% 3 11% 26% 17% 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	25% 24% 30% 18% 4	297
	11th to 20th	Day	25% 30% 29% 13% 4	330
	21st to end	Day	24% 24% 37% 12% 3	363
February	1st to 10th	Day	22% 24% 38% 12% 4 1	330
	11th to 20th	Day	35% 23% 31% 10% 1	377
	21st to end	Day	28% 18% 31% 19% 3	446
March	1st to 10th	Day	22% 20% 32% 21% 4	540
	11th to 20th	Day	23% 23% 32% 16% 5% 1	540
	21st to end	Day	25% 19% 34% 19% 4 1	594
April	1st to 10th	Day	23% 20% 32% 18% 6% 1	540
	11th to 20th	Day	18% 17% 32% 21% 9% 3	532
	21st to end	Day	21% 13% 34% 21% 9% 1	540
May	1st to 10th	Day	21% 18% 30% 20% 8% 2	543
	11th to 20th	Day	22% 10% 30% 23% 12% 2	567
	21st to end	Day	21% 8% 23% 27% 18% 5%	617
June	1st to 10th	Day	12% 6% 29% 30% 19% 5%	570
	11th to 20th	Day	13% 7% 24% 33% 16% 7%	570
	21st to end	Day	10% 5% 26% 33% 21% 5%	570
July	1st to 10th	Day	11% 3 17% 33% 26% 10%	565
	11th to 20th	Day	6% 3 15% 40% 24% 12%	544
	21st to end	Day	10% 4 20% 33% 23% 10%	576
August	1st to 10th	Day	14% 3 22% 28% 25% 9%	526
	11th to 20th	Day	12% 4 18% 32% 25% 10%	519
	21st to end	Day	11% 4 17% 30% 24% 13%	583
September	1st to 10th	Day	15% 5% 20% 28% 18% 14%	530
	11th to 20th	Day	15% 4 18% 33% 20% 11%	530
	21st to end	Day	28% 9% 22% 26% 10% 4	530
October	1st to 10th	Day	19% 7% 22% 28% 18% 7%	530
	11th to 20th	Day	20% 13% 24% 26% 15% 2	530
	21st to end	Day	16% 14% 34% 23% 10% 3	583
November	1st to 10th	Day	15% 12% 33% 26% 12% 3	517
	11th to 20th	Day	20% 13% 35% 22% 9% 1	409
	21st to end	Day	22% 19% 31% 18% 10%	220
December	1st to 10th	Day	20% 20% 37% 17% 5% 1	220
	11th to 20th	Day	25% 13% 35% 19% 9%	220
	21st to end	Day	26% 20% 31% 16% 7%	242
January	1st to 10th	Night	20% 18% 41% 18% 3	840
	11th to 20th	Night	22% 16% 34% 25% 4	840
	21st to end	Night	20% 16% 39% 23% 2	924
February	1st to 10th	Night	20% 14% 38% 24% 3	840
	11th to 20th	Night	26% 17% 37% 18% 2	823
	21st to end	Night	22% 20% 34% 22% 3	692
March	1st to 10th	Night	19% 18% 34% 26% 3	840
	11th to 20th	Night	22% 16% 35% 22% 6%	840
	21st to end	Night	18% 17% 37% 23% 5%	924
April	1st to 10th	Night	18% 16% 38% 23% 5%	840
	11th to 20th	Night	14% 15% 36% 27% 8%	831
	21st to end	Night	18% 10% 33% 32% 7%	840
May	1st to 10th	Night	18% 11% 34% 25% 11%	689
	11th to 20th	Night	17% 6% 31% 31% 14% 1	585
	21st to end	Night	16% 2 20% 39% 21% 1	455
June	1st to 10th	Night	9% 5% 24% 39% 23% 1	420
	11th to 20th	Night	14% 4 20% 39% 20% 2	420
	21st to end	Night	10% 4 22% 43% 19% 2	420
July	1st to 10th	Night	11% 4 16% 38% 28% 3	410
	11th to 20th	Night	8% 1 12% 40% 32% 7%	594
	21st to end	Night	7% 2 16% 42% 29% 4	657
August	1st to 10th	Night	12% 2 12% 40% 31% 3	597
	11th to 20th	Night	13% 4 11% 41% 28% 3	669
	21st to end	Night	12% 3 13% 36% 29% 6%	880
September	1st to 10th	Night	16% 3 14% 35% 28% 5%	800
	11th to 20th	Night	14% 4 15% 36% 29% 3	800
	21st to end	Night	24% 5% 19% 32% 19% 2	800
October	1st to 10th	Night	20% 5% 21% 29% 23% 3	800
	11th to 20th	Night	18% 10% 27% 29% 15% 1	800
	21st to end	Night	17% 9% 32% 29% 12% 1	880
November	1st to 10th	Night	15% 10% 30% 31% 13% 2	820
	11th to 20th	Night	18% 11% 35% 28% 8%	840
	21st to end	Night	14% 12% 35% 29% 9% 1	840
December	1st to 10th	Night	20% 13% 36% 23% 8%	840
	11th to 20th	Night	20% 10% 37% 28% 5%	840
	21st to end	Night	20% 11% 33% 31% 4	924

Page generated at 2026-07-13 03:14:52. Version ac07

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