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 2025-03-12.

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% 43%	108811
	Night	14% 6% 2 6% 4 4 4 3 5% 7% 45%	110173
Transparency	Day	39% 4 11% 24% 18% 4	112455
	Night	39% 4 10% 24% 19% 5%	114669
Seeing	Day	18% 12% 27% 25% 14% 5%	15423
	Night	17% 10% 28% 30% 13% 1	25177

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	16% 5% 2 5% 3 3 3 3 4 5% 51%	3919
	11th to 20th	Day	14% 7% 2 5% 3 3 3 3 4 4 50%	4060
	21st to end	Day	15% 7% 2 6% 4 4 4 3 4 5% 45%	4466
February	1st to 10th	Day	14% 8% 3 8% 4 4 5% 4 5% 7% 40%	1800
	11th to 20th	Day	20% 7% 2 6% 4 4 4 3 4 6% 42%	1798
	21st to end	Day	17% 8% 2 6% 5% 4 4 4 4 7% 38%	1659
March	1st to 10th	Day	21% 7% 2 7% 4 4 4 3 4 6% 37%	3545
	11th to 20th	Day	10% 5% 2 5% 4 4 4 4 6% 7% 52%	3491
	21st to end	Day	21% 7% 2 6% 4 4 4 3 5% 6% 38%	3835
April	1st to 10th	Day	11% 6% 2 5% 4 4 4 3 5% 7% 48%	2280
	11th to 20th	Day	13% 6% 2 5% 3 3 4 4 5% 6% 49%	2254
	21st to end	Day	16% 7% 2 6% 4 5% 4 3 6% 6% 42%	2280
May	1st to 10th	Day	19% 7% 3 6% 5% 5% 5% 4 6% 9% 30%	4019
	11th to 20th	Day	23% 7% 2 6% 4 4 4 4 5% 8% 33%	4135
	21st to end	Day	10% 4 2 5% 4 4 5% 5% 7% 10% 45%	4823
June	1st to 10th	Day	12% 4 2 5% 4 5% 4 4 7% 9% 44%	2660
	11th to 20th	Day	10% 4 2 6% 5% 6% 6% 4 6% 10% 42%	2660
	21st to end	Day	8% 4 2 5% 5% 5% 6% 4 7% 10% 43%	2660
July	1st to 10th	Day	6% 4 2 6% 5% 5% 6% 5% 9% 11% 40%	4425
	11th to 20th	Day	10% 5% 2 6% 5% 5% 5% 5% 9% 12% 36%	4120
	21st to end	Day	11% 7% 3 7% 6% 5% 6% 6% 7% 8% 35%	4466
August	1st to 10th	Day	15% 6% 3 6% 5% 5% 5% 5% 8% 10% 32%	4104
	11th to 20th	Day	9% 5% 2 7% 5% 6% 7% 5% 8% 9% 36%	3887
	21st to end	Day	10% 4 2 5% 3 5% 4 4 6% 10% 48%	4180
September	1st to 10th	Day	13% 5% 2 5% 4 5% 5% 4 5% 7% 45%	2280
	11th to 20th	Day	13% 5% 2 5% 4 4 5% 4 6% 8% 45%	2277
	21st to end	Day	23% 5% 2 5% 4 4 4 4 5% 6% 37%	1982
October	1st to 10th	Day	16% 5% 3 5% 3 4 3 3 5% 5% 48%	1900
	11th to 20th	Day	17% 4 1 4 4 4 3 3 4 5% 51%	1900
	21st to end	Day	14% 6% 2 5% 3 3 3 3 4 6% 52%	2090
November	1st to 10th	Day	12% 4 2 5% 4 4 3 3 4 6% 53%	1926
	11th to 20th	Day	15% 6% 2 4 3 3 4 3 5% 6% 50%	1780
	21st to end	Day	15% 5% 2 5% 3 3 3 3 5% 5% 50%	1600
December	1st to 10th	Day	12% 5% 2 5% 4 4 3 4 5% 6% 51%	2600
	11th to 20th	Day	18% 7% 2 5% 3 2 3 2 4 7% 46%	2990
	21st to end	Day	16% 6% 3 7% 4 4 4 3 6% 7% 40%	3960
January	1st to 10th	Night	14% 6% 2 4 3 3 3 3 4 5% 54%	6011
	11th to 20th	Night	14% 6% 2 5% 3 3 3 3 3 4 55%	5900
	21st to end	Night	15% 7% 3 6% 3 3 3 3 4 5% 48%	6490
February	1st to 10th	Night	15% 8% 2 5% 4 4 4 4 5% 6% 42%	2600
	11th to 20th	Night	20% 8% 2 5% 3 3 4 3 4 5% 43%	2488
	21st to end	Night	16% 7% 2 5% 5% 4 3 3 5% 6% 45%	1969
March	1st to 10th	Night	21% 7% 2 6% 3 4 4 4 4 5% 40%	3528
	11th to 20th	Night	13% 5% 2 4 3 3 3 3 5% 7% 51%	3498
	21st to end	Night	14% 8% 2 5% 3 3 3 3 5% 5% 48%	3796
April	1st to 10th	Night	15% 7% 2 5% 4 4 4 3 5% 6% 46%	1900
	11th to 20th	Night	12% 5% 2 6% 4 4 4 3 5% 7% 49%	1875
	21st to end	Night	16% 5% 2 5% 4 4 4 4 6% 7% 44%	1900
May	1st to 10th	Night	20% 10% 2 7% 4 4 4 3 5% 8% 33%	2891
	11th to 20th	Night	24% 8% 3 6% 4 4 4 4 5% 6% 33%	2754
	21st to end	Night	10% 5% 2 6% 4 5% 5% 4 6% 10% 43%	2732
June	1st to 10th	Night	10% 5% 2 6% 4 4 5% 5% 6% 8% 45%	1520
	11th to 20th	Night	11% 6% 2 6% 4 5% 5% 5% 5% 10% 41%	1520
	21st to end	Night	9% 5% 1 5% 5% 5% 5% 4 8% 10% 44%	1520
July	1st to 10th	Night	7% 5% 2 8% 4 5% 6% 6% 7% 11% 40%	2520
	11th to 20th	Night	8% 6% 2 7% 5% 5% 6% 5% 7% 10% 37%	2836
	21st to end	Night	10% 7% 3 8% 6% 7% 6% 5% 8% 8% 32%	3087
August	1st to 10th	Night	17% 6% 3 7% 5% 5% 5% 5% 6% 9% 33%	2825
	11th to 20th	Night	10% 7% 3 9% 6% 5% 5% 4 7% 8% 35%	2908
	21st to end	Night	11% 6% 3 6% 4 3 4 3 6% 9% 46%	3476
September	1st to 10th	Night	13% 6% 2 6% 4 4 4 4 5% 8% 44%	1900
	11th to 20th	Night	12% 6% 3 5% 4 3 4 3 5% 6% 48%	1900
	21st to end	Night	22% 6% 3 6% 4 3 4 3 5% 7% 38%	2195
October	1st to 10th	Night	18% 6% 2 4 4 3 4 3 4 7% 46%	2280
	11th to 20th	Night	16% 5% 2 6% 2 3 3 3 5% 5% 50%	2280
	21st to end	Night	14% 5% 1 5% 4 3 3 3 5% 5% 52%	2508
November	1st to 10th	Night	11% 6% 2 5% 3 3 4 2 5% 6% 53%	2370
	11th to 20th	Night	14% 6% 2 5% 4 3 4 3 4 6% 49%	2615
	21st to end	Night	12% 4 1 5% 4 4 4 4 5% 6% 51%	2800
December	1st to 10th	Night	10% 5% 2 5% 3 3 4 3 5% 7% 54%	4580
	11th to 20th	Night	20% 7% 2 5% 4 4 4 3 4 5% 41%	5205
	21st to end	Night	16% 7% 2 5% 4 4 4 4 6% 7% 42%	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	37% 2 4 26% 22% 9%	3920
	11th to 20th	Day	39% 2 5% 20% 23% 11%	4060
	21st to end	Day	43% 2 7% 26% 18% 4	4466
February	1st to 10th	Day	44% 3 9% 28% 14% 2	1800
	11th to 20th	Day	45% 3 6% 22% 19% 5%	1807
	21st to end	Day	47% 3 10% 17% 18% 4	1659
March	1st to 10th	Day	49% 3 8% 20% 14% 6%	3545
	11th to 20th	Day	31% 3 6% 25% 27% 7%	3491
	21st to end	Day	48% 3 10% 19% 16% 4	3835
April	1st to 10th	Day	36% 4 10% 22% 20% 8%	2280
	11th to 20th	Day	36% 4 11% 21% 23% 6%	2254
	21st to end	Day	43% 4 10% 25% 14% 5%	2280
May	1st to 10th	Day	51% 3 13% 21% 11% 2	4056
	11th to 20th	Day	49% 4 11% 25% 10% 1	4139
	21st to end	Day	33% 4 16% 24% 21% 2	4837
June	1st to 10th	Day	36% 5% 15% 27% 15% 2	2660
	11th to 20th	Day	36% 6% 15% 24% 17% 2	2660
	21st to end	Day	34% 7% 14% 26% 17% 1	2660
July	1st to 10th	Day	34% 8% 22% 19% 11% 6%	4355
	11th to 20th	Day	37% 7% 17% 28% 9% 1	4051
	21st to end	Day	44% 8% 12% 20% 16% 1	4324
August	1st to 10th	Day	45% 7% 16% 25% 7%	4091
	11th to 20th	Day	40% 7% 15% 24% 13% 1	3926
	21st to end	Day	31% 5% 16% 28% 18% 2	4180
September	1st to 10th	Day	38% 5% 12% 19% 21% 4	2280
	11th to 20th	Day	36% 6% 7% 27% 21% 4	2277
	21st to end	Day	46% 4 8% 21% 17% 4	1982
October	1st to 10th	Day	36% 3 6% 19% 30% 5%	3160
	11th to 20th	Day	27% 2 5% 22% 32% 11%	3140
	21st to end	Day	22% 2 9% 29% 26% 13%	3428
November	1st to 10th	Day	34% 3 7% 26% 25% 4	1926
	11th to 20th	Day	35% 3 10% 22% 23% 7%	1780
	21st to end	Day	36% 3 8% 26% 23% 4	1600
December	1st to 10th	Day	35% 3 9% 28% 18% 8%	2600
	11th to 20th	Day	41% 2 13% 25% 15% 4	2986
	21st to end	Day	44% 3 11% 25% 16% 1	3960
January	1st to 10th	Night	35% 1 5% 23% 28% 8%	6011
	11th to 20th	Night	35% 2 5% 24% 25% 9%	5900
	21st to end	Night	41% 2 6% 27% 20% 4	6490
February	1st to 10th	Night	42% 4 10% 25% 17% 2	2600
	11th to 20th	Night	44% 3 7% 21% 19% 7%	2490
	21st to end	Night	41% 2 9% 22% 19% 7%	1969
March	1st to 10th	Night	46% 2 7% 25% 16% 3	3528
	11th to 20th	Night	33% 3 7% 23% 28% 6%	3498
	21st to end	Night	39% 3 8% 23% 21% 6%	3796
April	1st to 10th	Night	39% 4 9% 18% 24% 6%	1900
	11th to 20th	Night	35% 4 10% 23% 22% 6%	1875
	21st to end	Night	38% 4 11% 26% 15% 6%	1900
May	1st to 10th	Night	51% 4 13% 20% 10% 2	2919
	11th to 20th	Night	51% 5% 11% 22% 9% 1	2755
	21st to end	Night	36% 6% 17% 25% 14% 2	2739
June	1st to 10th	Night	37% 6% 13% 27% 15% 2	1520
	11th to 20th	Night	38% 6% 16% 24% 15% 1	1520
	21st to end	Night	34% 7% 15% 22% 19% 3	1520
July	1st to 10th	Night	37% 9% 21% 18% 12% 3	2480
	11th to 20th	Night	40% 7% 22% 22% 7% 1	2783
	21st to end	Night	46% 8% 14% 19% 13%	2988
August	1st to 10th	Night	48% 7% 15% 23% 7% 1	2814
	11th to 20th	Night	43% 6% 14% 22% 14% 1	2935
	21st to end	Night	36% 4 12% 30% 17% 1	3476
September	1st to 10th	Night	38% 6% 13% 21% 16% 6%	1900
	11th to 20th	Night	36% 4 9% 26% 22% 3	1900
	21st to end	Night	48% 4 9% 19% 17% 3	2195
October	1st to 10th	Night	41% 3 9% 19% 21% 7%	3800
	11th to 20th	Night	25% 3 8% 22% 31% 11%	3779
	21st to end	Night	22% 1 6% 33% 26% 12%	4120
November	1st to 10th	Night	34% 3 9% 24% 24% 6%	2370
	11th to 20th	Night	37% 3 7% 22% 24% 7%	2615
	21st to end	Night	34% 3 9% 26% 23% 5%	2800
December	1st to 10th	Night	31% 3 9% 28% 24% 4	4580
	11th to 20th	Night	46% 3 10% 19% 16% 6%	5208
	21st to end	Night	42% 3 11% 26% 17% 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% 24% 31% 16% 4	270
	11th to 20th	Day	24% 29% 30% 14% 4	300
	21st to end	Day	23% 23% 38% 13% 3	330
February	1st to 10th	Day	23% 23% 39% 12% 3 1	300
	11th to 20th	Day	35% 22% 31% 10% 1	345
	21st to end	Day	27% 20% 30% 20% 3	414
March	1st to 10th	Day	22% 20% 33% 20% 4	500
	11th to 20th	Day	21% 23% 33% 17% 5% 1	464
	21st to end	Day	25% 18% 32% 21% 4 1	506
April	1st to 10th	Day	23% 20% 31% 19% 6% 1	460
	11th to 20th	Day	18% 16% 31% 21% 10% 3	452
	21st to end	Day	22% 14% 33% 21% 8% 1	460
May	1st to 10th	Day	21% 16% 30% 22% 9% 3	463
	11th to 20th	Day	22% 9% 31% 23% 13% 3	487
	21st to end	Day	18% 5% 22% 29% 20% 5%	529
June	1st to 10th	Day	13% 6% 26% 29% 21% 5%	490
	11th to 20th	Day	12% 7% 23% 34% 17% 8%	490
	21st to end	Day	10% 5% 26% 33% 21% 5%	490
July	1st to 10th	Day	11% 3 17% 32% 26% 10%	485
	11th to 20th	Day	6% 3 16% 40% 25% 11%	484
	21st to end	Day	10% 4 21% 33% 22% 10%	532
August	1st to 10th	Day	13% 3 21% 28% 26% 9%	486
	11th to 20th	Day	11% 4 18% 32% 25% 11%	479
	21st to end	Day	11% 4 16% 29% 25% 14%	539
September	1st to 10th	Day	16% 5% 18% 27% 19% 15%	490
	11th to 20th	Day	14% 4 18% 33% 19% 12%	490
	21st to end	Day	28% 9% 22% 27% 10% 4	490
October	1st to 10th	Day	20% 7% 22% 28% 17% 7%	490
	11th to 20th	Day	18% 12% 23% 28% 17% 2	490
	21st to end	Day	15% 12% 36% 24% 10% 3	539
November	1st to 10th	Day	16% 10% 32% 27% 13% 3	479
	11th to 20th	Day	19% 12% 35% 22% 10% 1	380
	21st to end	Day	20% 19% 31% 19% 11% 1	200
December	1st to 10th	Day	19% 20% 38% 17% 5% 1	200
	11th to 20th	Day	25% 13% 34% 20% 9%	200
	21st to end	Day	27% 20% 30% 17% 7%	220
January	1st to 10th	Night	21% 18% 40% 18% 4	800
	11th to 20th	Night	22% 15% 34% 26% 4	800
	21st to end	Night	20% 16% 38% 24% 2	880
February	1st to 10th	Night	21% 14% 38% 24% 3	800
	11th to 20th	Night	27% 17% 36% 18% 2	783
	21st to end	Night	22% 19% 34% 22% 3	660
March	1st to 10th	Night	19% 18% 34% 26% 3	800
	11th to 20th	Night	22% 16% 35% 22% 6%	767
	21st to end	Night	18% 16% 36% 24% 5%	836
April	1st to 10th	Night	19% 16% 37% 23% 5%	760
	11th to 20th	Night	15% 14% 35% 28% 9%	751
	21st to end	Night	19% 10% 33% 31% 7%	760
May	1st to 10th	Night	19% 10% 32% 26% 12%	623
	11th to 20th	Night	18% 5% 30% 32% 15% 1	529
	21st to end	Night	13% 2 19% 41% 23% 1	411
June	1st to 10th	Night	8% 5% 22% 40% 23% 1	380
	11th to 20th	Night	14% 4 19% 40% 21% 2	380
	21st to end	Night	9% 4 22% 44% 19% 2	380
July	1st to 10th	Night	11% 4 16% 38% 28% 4	370
	11th to 20th	Night	8% 1 12% 41% 32% 6%	549
	21st to end	Night	8% 2 17% 42% 28% 4	624
August	1st to 10th	Night	12% 2 11% 40% 31% 3	567
	11th to 20th	Night	12% 4 12% 42% 27% 3	635
	21st to end	Night	12% 3 12% 36% 30% 7%	836
September	1st to 10th	Night	16% 3 13% 35% 29% 5%	760
	11th to 20th	Night	14% 4 15% 35% 29% 3	760
	21st to end	Night	23% 5% 19% 31% 20% 2	760
October	1st to 10th	Night	20% 4 21% 29% 22% 3	760
	11th to 20th	Night	18% 10% 26% 29% 16% 1	760
	21st to end	Night	17% 9% 32% 29% 13% 1	836
November	1st to 10th	Night	14% 10% 30% 31% 13% 2	780
	11th to 20th	Night	18% 11% 35% 28% 8%	800
	21st to end	Night	14% 12% 34% 29% 9% 1	800
December	1st to 10th	Night	20% 13% 36% 23% 8%	800
	11th to 20th	Night	20% 10% 37% 28% 6%	800
	21st to end	Night	21% 11% 32% 32% 5%	880

Page generated at 2025-03-09 08:08:42. Version ac07

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