Post Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Post Observatory clear sky chart from 2005-11-06 to 2025-09-19.

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	19% 9% 3 6% 4 4 4 3 5% 7% 37%	125775
	Night	19% 9% 3 6% 4 4 4 3 5% 6% 36%	123312
Transparency	Day	47% 4 12% 22% 13% 2	129297
	Night	49% 4 11% 22% 13% 2	127879
Seeing	Day	24% 16% 32% 21% 7% 1	17023
	Night	25% 16% 28% 23% 8% 1	25229

By Month

Percentage Cloud Cover NE (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% 10% 3 6% 5% 5% 5% 4 5% 6% 36%	3597
	11th to 20th	Day	14% 9% 3 5% 4 4 5% 3 4 6% 42%	3570
	21st to end	Day	13% 10% 3 7% 4 4 3 3 5% 6% 42%	4122
February	1st to 10th	Day	17% 12% 3 7% 4 5% 4 4 6% 7% 33%	1800
	11th to 20th	Day	14% 9% 2 7% 4 4 5% 3 6% 7% 37%	1843
	21st to end	Day	13% 10% 3 8% 5% 5% 5% 4 6% 6% 36%	1650
March	1st to 10th	Day	25% 11% 3 6% 4 5% 4 3 4 5% 30%	4558
	11th to 20th	Day	22% 10% 2 6% 4 4 4 3 4 6% 34%	4980
	21st to end	Day	29% 9% 3 6% 4 4 3 3 5% 5% 30%	5466
April	1st to 10th	Day	21% 11% 3 6% 4 4 4 2 5% 6% 34%	2350
	11th to 20th	Day	20% 9% 3 6% 4 4 3 3 5% 7% 38%	2563
	21st to end	Day	21% 9% 2 5% 3 4 4 3 4 7% 37%	2600
May	1st to 10th	Day	29% 12% 3 7% 4 3 3 3 4 6% 26%	5886
	11th to 20th	Day	19% 9% 3 8% 4 4 4 4 5% 6% 34%	5861
	21st to end	Day	21% 8% 3 7% 4 4 5% 4 6% 7% 32%	7071
June	1st to 10th	Day	23% 8% 2 7% 5% 3 5% 3 5% 8% 32%	3000
	11th to 20th	Day	18% 8% 3 5% 4 4 4 3 4 8% 39%	3000
	21st to end	Day	25% 8% 2 6% 5% 3 4 3 5% 7% 31%	3000
July	1st to 10th	Day	18% 9% 3 7% 4 4 5% 3 7% 8% 32%	6602
	11th to 20th	Day	18% 10% 3 6% 3 3 4 3 5% 8% 38%	6360
	21st to end	Day	16% 6% 2 5% 3 4 4 3 6% 8% 43%	6745
August	1st to 10th	Day	19% 8% 3 6% 4 4 4 3 4 8% 36%	4235
	11th to 20th	Day	17% 8% 2 7% 4 4 4 3 6% 7% 36%	4175
	21st to end	Day	10% 5% 2 5% 4 3 4 4 6% 9% 50%	4675
September	1st to 10th	Day	12% 5% 1 5% 4 4 4 3 5% 8% 49%	2296
	11th to 20th	Day	13% 5% 2 4 3 4 4 3 4 7% 52%	2178
	21st to end	Day	18% 7% 2 5% 3 4 3 4 4 6% 43%	2090
October	1st to 10th	Day	17% 7% 3 5% 3 4 5% 4 5% 6% 41%	2022
	11th to 20th	Day	19% 6% 2 6% 4 4 3 4 5% 6% 43%	1900
	21st to end	Day	22% 8% 3 4 4 4 4 3 4 5% 39%	1895
November	1st to 10th	Day	18% 6% 2 7% 4 3 4 3 5% 6% 42%	1761
	11th to 20th	Day	21% 5% 2 4 2 4 4 3 5% 7% 42%	1739
	21st to end	Day	19% 10% 3 6% 3 2 3 3 4 5% 40%	1600
December	1st to 10th	Day	19% 10% 2 4 3 3 4 3 5% 6% 42%	2491
	11th to 20th	Day	21% 9% 3 5% 3 3 4 3 4 5% 41%	2618
	21st to end	Day	20% 13% 4 5% 3 3 2 3 4 5% 38%	3476
January	1st to 10th	Night	13% 10% 3 8% 5% 4 4 3 4 7% 40%	6349
	11th to 20th	Night	14% 11% 4 9% 5% 4 5% 3 5% 6% 34%	6315
	21st to end	Night	12% 10% 3 7% 5% 4 4 4 5% 7% 40%	6832
February	1st to 10th	Night	18% 11% 3 7% 5% 4 5% 3 4 6% 34%	2600
	11th to 20th	Night	14% 9% 3 8% 5% 5% 4 4 5% 5% 39%	2439
	21st to end	Night	15% 10% 3 7% 5% 4 4 3 5% 6% 39%	1980
March	1st to 10th	Night	27% 11% 3 6% 5% 3 3 2 4 5% 31%	5281
	11th to 20th	Night	17% 11% 3 6% 4 5% 5% 4 6% 7% 31%	4979
	21st to end	Night	27% 9% 2 6% 4 4 4 4 6% 6% 29%	5466
April	1st to 10th	Night	23% 12% 3 5% 4 4 3 4 4 5% 33%	2048
	11th to 20th	Night	21% 8% 3 7% 5% 4 3 3 5% 6% 35%	1786
	21st to end	Night	24% 9% 3 6% 5% 3 3 3 4 6% 34%	1800
May	1st to 10th	Night	29% 11% 3 7% 4 3 4 3 4 6% 26%	4054
	11th to 20th	Night	24% 8% 2 6% 4 4 4 4 6% 7% 30%	4033
	21st to end	Night	16% 12% 4 8% 5% 4 5% 3 5% 6% 32%	3855
June	1st to 10th	Night	24% 10% 3 6% 3 4 5% 3 6% 7% 28%	1400
	11th to 20th	Night	20% 9% 3 6% 3 4 4 3 6% 7% 37%	1400
	21st to end	Night	26% 10% 2 5% 4 4 3 4 4 7% 30%	1400
July	1st to 10th	Night	18% 10% 3 8% 6% 4 5% 4 7% 8% 27%	3351
	11th to 20th	Night	21% 13% 3 6% 4 4 4 4 6% 7% 30%	3600
	21st to end	Night	14% 7% 2 6% 4 4 5% 4 6% 9% 39%	4061
August	1st to 10th	Night	23% 9% 3 7% 4 5% 5% 3 5% 7% 29%	2915
	11th to 20th	Night	19% 9% 3 7% 4 4 6% 5% 6% 8% 30%	2872
	21st to end	Night	10% 6% 2 6% 4 4 4 3 5% 9% 45%	3223
September	1st to 10th	Night	14% 5% 2 4 4 3 4 3 5% 7% 48%	2099
	11th to 20th	Night	15% 6% 2 4 3 3 3 3 4 6% 50%	2179
	21st to end	Night	18% 7% 2 5% 3 3 3 3 5% 7% 44%	2090
October	1st to 10th	Night	20% 6% 2 5% 4 3 4 3 5% 6% 41%	2156
	11th to 20th	Night	19% 6% 2 6% 4 4 4 3 4 6% 43%	2280
	21st to end	Night	23% 9% 2 5% 4 4 3 3 3 6% 38%	2701
November	1st to 10th	Night	18% 7% 2 5% 4 4 5% 3 5% 6% 42%	2537
	11th to 20th	Night	18% 7% 2 6% 4 4 4 4 5% 5% 42%	2659
	21st to end	Night	19% 9% 3 5% 3 3 4 4 5% 6% 39%	2800
December	1st to 10th	Night	18% 9% 2 5% 4 4 4 4 6% 7% 38%	4697
	11th to 20th	Night	23% 8% 2 4 3 2 2 3 4 6% 42%	5591
	21st to end	Night	23% 10% 3 5% 4 3 3 3 5% 6% 37%	7484

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	50% 3 11% 27% 8%	3598
	11th to 20th	Day	45% 4 12% 25% 11% 2	3600
	21st to end	Day	45% 2 11% 20% 16% 6%	4122
February	1st to 10th	Day	53% 3 12% 18% 12% 2	1800
	11th to 20th	Day	48% 5% 13% 20% 13% 2	1851
	21st to end	Day	48% 3 11% 22% 11% 4	1650
March	1st to 10th	Day	56% 4 9% 11% 15% 5%	4562
	11th to 20th	Day	50% 4 15% 20% 10% 1	4980
	21st to end	Day	56% 3 5% 18% 15% 3	5466
April	1st to 10th	Day	51% 4 8% 19% 15% 3	2350
	11th to 20th	Day	45% 4 10% 24% 14% 3	2563
	21st to end	Day	47% 4 9% 20% 16% 3	2600
May	1st to 10th	Day	60% 4 11% 18% 6% 1	5897
	11th to 20th	Day	51% 5% 11% 23% 7% 2	5865
	21st to end	Day	51% 5% 11% 20% 11% 1	7071
June	1st to 10th	Day	51% 5% 12% 21% 10%	3000
	11th to 20th	Day	44% 5% 12% 21% 15% 2	3000
	21st to end	Day	53% 5% 11% 18% 11% 2	3000
July	1st to 10th	Day	51% 4 15% 20% 10%	6531
	11th to 20th	Day	46% 4 14% 22% 12% 1	6285
	21st to end	Day	39% 5% 14% 27% 14% 1	6551
August	1st to 10th	Day	48% 6% 14% 25% 8%	4222
	11th to 20th	Day	47% 4 18% 21% 9%	4214
	21st to end	Day	30% 5% 14% 30% 21% 1	4675
September	1st to 10th	Day	34% 5% 13% 29% 18% 1	2296
	11th to 20th	Day	34% 5% 11% 28% 20% 2	2178
	21st to end	Day	42% 4 9% 28% 16% 1	2090
October	1st to 10th	Day	47% 5% 12% 23% 12% 1	3360
	11th to 20th	Day	32% 4 9% 35% 21%	3140
	21st to end	Day	34% 3 18% 23% 18% 3	3095
November	1st to 10th	Day	43% 4 14% 25% 13% 1	1761
	11th to 20th	Day	43% 5% 10% 23% 17% 3	1739
	21st to end	Day	48% 4 16% 24% 8% 1	1600
December	1st to 10th	Day	45% 3 16% 24% 12% 1	2491
	11th to 20th	Day	48% 3 12% 18% 13% 6%	2618
	21st to end	Day	51% 4 11% 19% 12% 3	3476
January	1st to 10th	Night	46% 3 13% 27% 10% 1	6349
	11th to 20th	Night	52% 4 12% 22% 9% 1	6360
	21st to end	Night	46% 3 10% 22% 16% 4	6832
February	1st to 10th	Night	53% 4 12% 18% 12% 2	2600
	11th to 20th	Night	47% 4 12% 20% 15% 2	2442
	21st to end	Night	48% 3 10% 22% 14% 4	1980
March	1st to 10th	Night	58% 3 7% 12% 13% 7%	5290
	11th to 20th	Night	50% 5% 21% 17% 8% 1	4980
	21st to end	Night	56% 3 5% 18% 16% 2	5466
April	1st to 10th	Night	53% 4 7% 15% 16% 3	2048
	11th to 20th	Night	50% 3 9% 21% 15% 2	1786
	21st to end	Night	52% 3 9% 18% 14% 3	1800
May	1st to 10th	Night	61% 4 11% 15% 8% 1	4060
	11th to 20th	Night	53% 4 13% 20% 8% 1	4034
	21st to end	Night	54% 4 12% 21% 9%	3856
June	1st to 10th	Night	55% 5% 10% 21% 8% 1	1400
	11th to 20th	Night	46% 6% 12% 22% 12% 2	1400
	21st to end	Night	55% 4 9% 20% 10% 2	1400
July	1st to 10th	Night	56% 5% 12% 20% 8%	3312
	11th to 20th	Night	54% 5% 12% 20% 9%	3552
	21st to end	Night	42% 5% 17% 24% 11% 1	3942
August	1st to 10th	Night	57% 5% 11% 20% 7%	2903
	11th to 20th	Night	50% 6% 13% 22% 8%	2899
	21st to end	Night	37% 5% 11% 25% 20% 1	3223
September	1st to 10th	Night	36% 4 12% 29% 18% 1	2099
	11th to 20th	Night	37% 3 11% 27% 20% 2	2179
	21st to end	Night	40% 4 9% 29% 17% 1	2090
October	1st to 10th	Night	42% 5% 13% 25% 15% 1	3598
	11th to 20th	Night	33% 3 10% 29% 24% 1	3779
	21st to end	Night	39% 3 10% 30% 15% 2	4451
November	1st to 10th	Night	43% 4 12% 26% 14% 1	2537
	11th to 20th	Night	44% 4 11% 23% 16% 2	2659
	21st to end	Night	46% 5% 14% 21% 12% 1	2800
December	1st to 10th	Night	47% 3 11% 27% 11%	4697
	11th to 20th	Night	46% 3 12% 21% 16% 2	5592
	21st to end	Night	52% 3 11% 18% 15% 1	7484

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	27% 29% 29% 13% 3	200
	11th to 20th	Day	30% 26% 29% 14% 2 1	200
	21st to end	Day	21% 27% 34% 15% 3	291
February	1st to 10th	Day	28% 27% 27% 14% 3	400
	11th to 20th	Day	27% 30% 33% 9% 1	441
	21st to end	Day	25% 22% 35% 16% 2	414
March	1st to 10th	Day	29% 21% 34% 15% 2	500
	11th to 20th	Day	24% 24% 32% 16% 3	500
	21st to end	Day	30% 23% 28% 14% 5% 1	550
April	1st to 10th	Day	31% 22% 30% 13% 3 1	500
	11th to 20th	Day	29% 18% 35% 13% 5% 1	492
	21st to end	Day	25% 15% 38% 15% 5% 1	500
May	1st to 10th	Day	28% 19% 32% 17% 3 1	503
	11th to 20th	Day	19% 17% 35% 23% 5% 2	527
	21st to end	Day	26% 11% 35% 19% 8% 1	689
June	1st to 10th	Day	25% 10% 31% 24% 8% 1	730
	11th to 20th	Day	25% 9% 31% 22% 11% 2	730
	21st to end	Day	29% 8% 28% 25% 8% 1	730
July	1st to 10th	Day	23% 9% 30% 26% 10% 1	636
	11th to 20th	Day	20% 7% 33% 29% 10% 2	524
	21st to end	Day	17% 6% 36% 29% 11% 1	576
August	1st to 10th	Day	15% 6% 33% 33% 12% 1	526
	11th to 20th	Day	20% 5% 29% 30% 13% 3	519
	21st to end	Day	15% 9% 31% 30% 12% 3	583
September	1st to 10th	Day	17% 8% 33% 30% 10% 2	530
	11th to 20th	Day	18% 9% 32% 24% 11% 6%	522
	21st to end	Day	20% 9% 34% 25% 10% 3	490
October	1st to 10th	Day	17% 16% 27% 28% 10% 2	453
	11th to 20th	Day	20% 18% 31% 22% 8% 2	390
	21st to end	Day	29% 15% 31% 18% 6%	429
November	1st to 10th	Day	23% 22% 28% 22% 5% 1	395
	11th to 20th	Day	27% 24% 30% 15% 4 1	400
	21st to end	Day	30% 25% 26% 16% 3	400
December	1st to 10th	Day	29% 19% 33% 17% 3	333
	11th to 20th	Day	33% 30% 29% 7% 2	200
	21st to end	Day	29% 26% 27% 16% 2	220
January	1st to 10th	Night	24% 27% 29% 16% 3	800
	11th to 20th	Night	28% 24% 31% 14% 4	800
	21st to end	Night	20% 29% 31% 17% 3	880
February	1st to 10th	Night	27% 24% 34% 13% 3	800
	11th to 20th	Night	24% 31% 29% 14% 2 1	783
	21st to end	Night	23% 20% 35% 18% 4	660
March	1st to 10th	Night	27% 20% 34% 18% 2	800
	11th to 20th	Night	23% 20% 31% 20% 6%	800
	21st to end	Night	25% 16% 34% 18% 6%	880
April	1st to 10th	Night	32% 15% 31% 17% 4	800
	11th to 20th	Night	27% 10% 34% 24% 4	602
	21st to end	Night	30% 10% 29% 23% 8% 1	600
May	1st to 10th	Night	27% 8% 30% 26% 8% 2	600
	11th to 20th	Night	25% 8% 31% 27% 8% 1	592
	21st to end	Night	25% 6% 27% 29% 11% 1	521
June	1st to 10th	Night	31% 7% 22% 32% 8% 1	400
	11th to 20th	Night	24% 4 26% 32% 13% 1	400
	21st to end	Night	33% 4 19% 32% 12% 1	400
July	1st to 10th	Night	23% 5% 22% 38% 12%	390
	11th to 20th	Night	25% 3 23% 33% 16% 1	388
	21st to end	Night	23% 2 22% 39% 12% 1	497
August	1st to 10th	Night	22% 3 23% 36% 15% 1	598
	11th to 20th	Night	22% 4 19% 38% 17% 1	592
	21st to end	Night	17% 5% 22% 35% 19% 2	660
September	1st to 10th	Night	17% 5% 25% 36% 15% 2	780
	11th to 20th	Night	20% 7% 25% 29% 16% 4	793
	21st to end	Night	20% 7% 27% 29% 14% 3	760
October	1st to 10th	Night	23% 10% 24% 27% 15% 1	760
	11th to 20th	Night	22% 15% 26% 24% 11% 1	760
	21st to end	Night	29% 18% 29% 19% 6%	836
November	1st to 10th	Night	23% 16% 29% 23% 8% 1	780
	11th to 20th	Night	23% 26% 26% 19% 6% 1	830
	21st to end	Night	26% 23% 30% 14% 6%	900
December	1st to 10th	Night	30% 20% 33% 15% 3	900
	11th to 20th	Night	29% 27% 27% 14% 2	900
	21st to end	Night	29% 22% 32% 14% 3	987

Page generated at 2025-09-16 01:49:09. Version ac07

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