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 2024-11-05.

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% 8% 2 6% 4 4 4 3 5% 7% 37%	101236
	Night	20% 9% 3 6% 4 4 4 3 5% 6% 36%	97838
Transparency	Day	46% 4 13% 23% 13% 1	104776
	Night	49% 4 12% 22% 12% 1	102389
Seeing	Day	23% 15% 32% 21% 7% 1	15866
	Night	25% 15% 28% 23% 8% 1	24126

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	18% 9% 3 6% 5% 4 4 3 4 6% 39%	2517
	11th to 20th	Day	18% 8% 2 4 3 3 5% 3 4 5% 44%	2520
	21st to end	Day	17% 10% 3 7% 4 4 3 3 5% 7% 36%	2878
February	1st to 10th	Day	17% 12% 3 7% 4 4 4 4 6% 7% 32%	1710
	11th to 20th	Day	14% 9% 3 8% 4 4 6% 3 6% 6% 37%	1748
	21st to end	Day	14% 10% 3 9% 5% 5% 4 4 5% 6% 35%	1570
March	1st to 10th	Day	26% 12% 3 6% 4 3 3 3 4 5% 30%	3170
	11th to 20th	Day	19% 9% 2 7% 5% 5% 5% 3 5% 7% 34%	3480
	21st to end	Day	28% 9% 3 6% 4 4 4 3 5% 5% 29%	3816
April	1st to 10th	Day	20% 11% 3 6% 4 4 4 2 5% 6% 34%	2232
	11th to 20th	Day	19% 9% 3 6% 4 4 3 3 5% 7% 39%	2433
	21st to end	Day	21% 9% 3 5% 3 4 4 3 4 7% 36%	2470
May	1st to 10th	Day	19% 10% 2 6% 5% 4 4 4 6% 7% 33%	4106
	11th to 20th	Day	18% 8% 2 7% 4 4 4 4 6% 7% 36%	4081
	21st to end	Day	18% 8% 3 6% 4 4 5% 4 6% 8% 34%	4917
June	1st to 10th	Day	23% 8% 2 7% 5% 3 5% 4 5% 8% 31%	2850
	11th to 20th	Day	19% 8% 3 5% 4 4 4 3 4 8% 39%	2850
	21st to end	Day	26% 8% 2 6% 5% 3 4 3 5% 7% 30%	2850
July	1st to 10th	Day	18% 8% 3 6% 4 5% 5% 3 6% 7% 36%	4612
	11th to 20th	Day	17% 7% 2 5% 3 3 4 3 6% 9% 41%	4440
	21st to end	Day	21% 8% 2 5% 3 4 4 3 6% 8% 37%	4689
August	1st to 10th	Day	20% 8% 3 6% 4 4 4 3 4 8% 35%	4105
	11th to 20th	Day	18% 8% 2 7% 4 4 4 3 6% 7% 35%	4045
	21st to end	Day	10% 5% 2 5% 4 3 3 4 6% 9% 50%	4532
September	1st to 10th	Day	12% 5% 1 5% 4 4 4 3 5% 8% 49%	2181
	11th to 20th	Day	13% 5% 2 4 3 4 4 3 4 7% 52%	2090
	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%	1708
	11th to 20th	Day	22% 5% 2 4 2 4 5% 3 6% 7% 41%	1653
	21st to end	Day	19% 10% 3 6% 4 2 3 3 4 5% 40%	1520
December	1st to 10th	Day	23% 10% 2 5% 4 5% 4 3 5% 5% 33%	1457
	11th to 20th	Day	24% 10% 3 6% 4 3 4 4 4 6% 33%	1668
	21st to end	Day	25% 9% 3 5% 3 3 3 3 4 6% 34%	2431
January	1st to 10th	Night	15% 9% 3 7% 5% 4 4 3 4 7% 40%	4429
	11th to 20th	Night	14% 10% 4 9% 5% 4 4 3 5% 5% 37%	4440
	21st to end	Night	17% 11% 4 8% 5% 4 4 3 5% 7% 33%	4776
February	1st to 10th	Night	18% 10% 3 7% 5% 5% 5% 3 5% 7% 33%	2470
	11th to 20th	Night	15% 9% 3 8% 4 5% 4 4 5% 5% 39%	2314
	21st to end	Night	15% 10% 3 7% 5% 4 4 3 5% 6% 38%	1884
March	1st to 10th	Night	29% 13% 3 5% 4 2 3 2 3 4 31%	3669
	11th to 20th	Night	15% 10% 4 7% 6% 6% 6% 5% 7% 7% 29%	3479
	21st to end	Night	29% 9% 2 6% 4 5% 4 4 5% 4 28%	3816
April	1st to 10th	Night	23% 12% 2 5% 4 4 3 4 4 5% 34%	1946
	11th to 20th	Night	21% 8% 3 7% 5% 4 3 3 5% 6% 35%	1696
	21st to end	Night	24% 9% 3 6% 4 3 3 3 4 6% 34%	1710
May	1st to 10th	Night	24% 8% 2 6% 4 3 4 3 5% 8% 33%	2834
	11th to 20th	Night	18% 7% 3 7% 6% 5% 5% 5% 6% 8% 30%	2813
	21st to end	Night	15% 11% 4 8% 5% 4 5% 3 5% 6% 35%	2689
June	1st to 10th	Night	25% 10% 3 6% 3 4 5% 3 6% 7% 28%	1330
	11th to 20th	Night	20% 9% 2 6% 3 4 4 3 6% 7% 36%	1330
	21st to end	Night	27% 11% 2 6% 4 4 3 4 4 6% 29%	1330
July	1st to 10th	Night	19% 10% 3 7% 5% 4 5% 5% 7% 8% 27%	2349
	11th to 20th	Night	20% 8% 2 6% 4 4 4 4 7% 8% 34%	2520
	21st to end	Night	18% 8% 2 7% 4 4 4 4 6% 8% 34%	2832
August	1st to 10th	Night	24% 10% 3 7% 4 4 4 3 5% 7% 28%	2825
	11th to 20th	Night	19% 9% 3 7% 4 4 6% 5% 6% 8% 30%	2782
	21st to end	Night	10% 6% 2 6% 5% 4 4 3 5% 9% 45%	3124
September	1st to 10th	Night	14% 6% 2 4 4 3 4 3 5% 7% 48%	1994
	11th to 20th	Night	16% 6% 2 4 3 3 3 3 4 7% 49%	2090
	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%	2470
	11th to 20th	Night	18% 7% 2 5% 4 4 4 4 5% 5% 41%	2526
	21st to end	Night	18% 9% 3 5% 3 3 4 4 5% 6% 39%	2660
December	1st to 10th	Night	25% 9% 2 6% 4 4 4 3 5% 6% 31%	2721
	11th to 20th	Night	24% 9% 2 5% 4 3 3 3 4 7% 37%	3531
	21st to end	Night	25% 9% 3 5% 4 3 3 3 5% 6% 36%	5232

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	49% 3 15% 25% 7% 1	2518
	11th to 20th	Day	45% 4 12% 26% 11% 2	2520
	21st to end	Day	50% 2 15% 21% 11%	2878
February	1st to 10th	Day	53% 3 13% 18% 11% 2	1710
	11th to 20th	Day	48% 5% 12% 20% 13% 1	1756
	21st to end	Day	49% 4 11% 22% 10% 5%	1570
March	1st to 10th	Day	58% 3 9% 15% 13% 3	3174
	11th to 20th	Day	47% 4 13% 21% 12% 2	3480
	21st to end	Day	57% 3 6% 16% 15% 4	3816
April	1st to 10th	Day	50% 4 8% 19% 15% 3	2232
	11th to 20th	Day	44% 4 10% 24% 14% 3	2433
	21st to end	Day	48% 4 9% 19% 17% 3	2470
May	1st to 10th	Day	50% 5% 14% 23% 8% 1	4120
	11th to 20th	Day	46% 5% 15% 25% 9% 1	4085
	21st to end	Day	48% 6% 12% 20% 13% 1	4917
June	1st to 10th	Day	52% 5% 13% 21% 9%	2850
	11th to 20th	Day	45% 5% 12% 20% 15% 2	2850
	21st to end	Day	54% 5% 12% 17% 10% 2	2850
July	1st to 10th	Day	49% 5% 13% 22% 12%	4541
	11th to 20th	Day	42% 4 17% 25% 11%	4365
	21st to end	Day	46% 5% 13% 22% 13% 1	4540
August	1st to 10th	Day	49% 6% 14% 25% 7%	4092
	11th to 20th	Day	48% 5% 18% 21% 9%	4084
	21st to end	Day	30% 5% 14% 29% 21% 1	4532
September	1st to 10th	Day	34% 5% 13% 28% 18% 2	2181
	11th to 20th	Day	34% 5% 11% 28% 20% 3	2090
	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	44% 4 13% 25% 13% 1	1708
	11th to 20th	Day	43% 5% 10% 22% 17% 3	1653
	21st to end	Day	47% 4 16% 23% 8% 1	1520
December	1st to 10th	Day	54% 4 13% 20% 8% 1	1457
	11th to 20th	Day	55% 3 9% 22% 9% 1	1668
	21st to end	Day	54% 4 13% 19% 8% 2	2431
January	1st to 10th	Night	47% 4 15% 26% 8% 1	4429
	11th to 20th	Night	50% 5% 12% 22% 10% 1	4440
	21st to end	Night	53% 3 12% 22% 9%	4776
February	1st to 10th	Night	53% 4 12% 18% 11% 2	2470
	11th to 20th	Night	47% 4 11% 21% 15% 2	2317
	21st to end	Night	48% 3 10% 21% 14% 4	1884
March	1st to 10th	Night	59% 2 8% 15% 12% 3	3678
	11th to 20th	Night	49% 6% 17% 17% 10% 1	3480
	21st to end	Night	59% 3 5% 17% 14% 2	3816
April	1st to 10th	Night	53% 4 7% 16% 16% 4	1946
	11th to 20th	Night	50% 3 8% 21% 16% 2	1696
	21st to end	Night	52% 4 9% 18% 14% 3	1710
May	1st to 10th	Night	51% 5% 16% 20% 8% 1	2840
	11th to 20th	Night	51% 6% 16% 19% 6% 1	2814
	21st to end	Night	50% 5% 13% 20% 11% 1	2690
June	1st to 10th	Night	55% 5% 11% 20% 7% 1	1330
	11th to 20th	Night	47% 6% 12% 21% 11% 2	1330
	21st to end	Night	57% 4 9% 19% 9% 2	1330
July	1st to 10th	Night	55% 5% 11% 18% 10% 1	2310
	11th to 20th	Night	48% 6% 13% 24% 9%	2472
	21st to end	Night	49% 5% 13% 20% 12% 1	2742
August	1st to 10th	Night	58% 5% 11% 20% 7%	2813
	11th to 20th	Night	51% 6% 13% 22% 8%	2809
	21st to end	Night	37% 5% 11% 25% 21% 1	3124
September	1st to 10th	Night	36% 4 12% 29% 17% 1	1994
	11th to 20th	Night	37% 4 11% 27% 19% 2	2090
	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	44% 4 12% 25% 14% 1	2470
	11th to 20th	Night	45% 4 12% 23% 15% 2	2526
	21st to end	Night	46% 6% 14% 21% 13% 1	2660
December	1st to 10th	Night	56% 3 12% 19% 10%	2721
	11th to 20th	Night	49% 4 11% 22% 13% 1	3532
	21st to end	Night	53% 4 13% 18% 11% 1	5232

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% 29% 29% 13% 3	180
	11th to 20th	Day	30% 26% 30% 12% 2 1	180
	21st to end	Day	19% 25% 37% 16% 4	265
February	1st to 10th	Day	27% 26% 28% 15% 4	370
	11th to 20th	Day	25% 30% 34% 9% 1	406
	21st to end	Day	25% 21% 35% 17% 2	382
March	1st to 10th	Day	27% 21% 35% 15% 2	460
	11th to 20th	Day	23% 24% 33% 17% 3	460
	21st to end	Day	29% 22% 29% 15% 5% 1	506
April	1st to 10th	Day	29% 21% 31% 13% 3 1	460
	11th to 20th	Day	27% 17% 35% 15% 5% 1	452
	21st to end	Day	25% 15% 38% 16% 6% 1	460
May	1st to 10th	Day	26% 19% 33% 18% 3 2	463
	11th to 20th	Day	17% 16% 36% 24% 5% 2	487
	21st to end	Day	26% 10% 35% 19% 9% 1	639
June	1st to 10th	Day	25% 10% 31% 24% 9% 1	680
	11th to 20th	Day	24% 8% 31% 24% 11% 2	680
	21st to end	Day	30% 8% 28% 25% 9% 1	680
July	1st to 10th	Day	23% 9% 29% 27% 11% 1	591
	11th to 20th	Day	19% 7% 33% 30% 10% 2	484
	21st to end	Day	18% 5% 34% 29% 12% 2	532
August	1st to 10th	Day	16% 6% 32% 33% 12% 1	486
	11th to 20th	Day	21% 5% 30% 30% 12% 3	479
	21st to end	Day	16% 8% 29% 31% 13% 3	539
September	1st to 10th	Day	17% 7% 33% 30% 11% 2	490
	11th to 20th	Day	19% 8% 32% 24% 11% 6%	490
	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	22% 21% 28% 22% 5% 1	378
	11th to 20th	Day	27% 22% 30% 16% 4 1	370
	21st to end	Day	28% 25% 27% 16% 4	370
December	1st to 10th	Day	29% 19% 34% 16% 3	307
	11th to 20th	Day	33% 29% 29% 7% 2	180
	21st to end	Day	30% 27% 26% 15% 3	198
January	1st to 10th	Night	24% 27% 30% 16% 3	760
	11th to 20th	Night	28% 25% 31% 13% 3	760
	21st to end	Night	20% 29% 31% 17% 3	836
February	1st to 10th	Night	25% 24% 34% 13% 3	760
	11th to 20th	Night	23% 31% 29% 14% 2 1	743
	21st to end	Night	23% 20% 35% 18% 4	628
March	1st to 10th	Night	26% 20% 35% 18% 2	760
	11th to 20th	Night	23% 21% 30% 20% 6%	760
	21st to end	Night	25% 16% 35% 18% 6%	836
April	1st to 10th	Night	31% 14% 33% 18% 4	760
	11th to 20th	Night	27% 10% 34% 24% 5%	572
	21st to end	Night	29% 10% 29% 23% 8% 1	570
May	1st to 10th	Night	26% 7% 30% 26% 8% 2	570
	11th to 20th	Night	24% 8% 32% 27% 8% 1	562
	21st to end	Night	25% 6% 26% 30% 12% 1	495
June	1st to 10th	Night	31% 7% 21% 31% 8% 1	380
	11th to 20th	Night	24% 4 26% 32% 14% 1	380
	21st to end	Night	33% 4 18% 32% 12% 1	380
July	1st to 10th	Night	23% 5% 21% 38% 12%	370
	11th to 20th	Night	24% 2 23% 33% 17% 1	368
	21st to end	Night	24% 1 20% 40% 13% 1	472
August	1st to 10th	Night	23% 3 22% 36% 15% 1	568
	11th to 20th	Night	22% 4 19% 38% 17% 1	562
	21st to end	Night	17% 4 22% 35% 20% 2	627
September	1st to 10th	Night	17% 4 26% 36% 15% 2	741
	11th to 20th	Night	20% 7% 25% 29% 15% 4	760
	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	24% 16% 28% 23% 8% 1	760
	11th to 20th	Night	24% 24% 26% 19% 6% 1	787
	21st to end	Night	25% 23% 31% 15% 6%	850
December	1st to 10th	Night	31% 20% 33% 14% 3	850
	11th to 20th	Night	28% 27% 28% 15% 2	850
	21st to end	Night	29% 23% 31% 14% 3	933

Page generated at 2024-11-03 02:11:20. Version ac07

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