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 2024-01-16.

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	19% 7% 3 5% 5% 3 4 4 5% 7% 38%	74599
	Night	20% 8% 3 5% 5% 3 4 4 5% 7% 37%	75193
Transparency	Day	46% 5% 12% 23% 13% 2	74308
	Night	49% 4 11% 21% 13% 1	75020
Seeing	Day	24% 15% 32% 22% 7% 1	14722
	Night	25% 15% 28% 23% 8% 1	23156

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	15% 10% 4 6% 5% 3 5% 4 5% 7% 37%	1517
	11th to 20th	Day	21% 10% 3 6% 4 3 4 4 3 6% 35%	1480
	21st to end	Day	14% 9% 3 6% 5% 4 4 3 5% 8% 39%	1648
February	1st to 10th	Day	18% 11% 3 6% 4 3 5% 5% 5% 7% 32%	1620
	11th to 20th	Day	15% 8% 3 7% 5% 3 6% 5% 6% 7% 36%	1654
	21st to end	Day	13% 9% 4 8% 6% 3 6% 4 5% 7% 35%	1480
March	1st to 10th	Day	16% 10% 3 6% 5% 3 4 5% 5% 7% 35%	1823
	11th to 20th	Day	19% 6% 2 5% 5% 3 5% 4 5% 7% 40%	1980
	21st to end	Day	19% 9% 2 6% 5% 3 4 4 4 6% 38%	2178
April	1st to 10th	Day	20% 10% 4 5% 5% 3 4 3 4 6% 34%	2114
	11th to 20th	Day	19% 8% 3 6% 4 3 4 3 5% 7% 38%	2303
	21st to end	Day	22% 9% 3 5% 4 3 4 5% 4 7% 35%	2340
May	1st to 10th	Day	22% 10% 3 6% 5% 3 4 4 5% 6% 33%	2340
	11th to 20th	Day	21% 7% 2 5% 4 2 3 4 4 6% 40%	2301
	21st to end	Day	20% 7% 3 6% 5% 3 5% 4 5% 8% 35%	2763
June	1st to 10th	Day	23% 8% 3 6% 6% 2 4 5% 5% 8% 30%	2700
	11th to 20th	Day	19% 7% 3 5% 4 3 4 4 4 8% 38%	2700
	21st to end	Day	26% 7% 3 5% 5% 3 4 4 5% 7% 30%	2700
July	1st to 10th	Day	19% 6% 3 4 4 3 4 5% 5% 8% 39%	2622
	11th to 20th	Day	16% 6% 3 5% 4 2 5% 4 6% 8% 40%	2520
	21st to end	Day	15% 6% 3 5% 4 3 5% 5% 6% 10% 38%	2672
August	1st to 10th	Day	16% 6% 2 4 5% 3 4 4 6% 9% 42%	2327
	11th to 20th	Day	15% 7% 2 6% 6% 4 5% 4 6% 9% 38%	2304
	21st to end	Day	14% 5% 3 4 5% 2 4 5% 5% 8% 46%	2574
September	1st to 10th	Day	12% 5% 2 4 4 3 5% 4 4 8% 48%	2067
	11th to 20th	Day	14% 5% 2 4 4 3 4 4 4 6% 52%	1980
	21st to end	Day	18% 6% 2 4 5% 3 3 5% 4 6% 44%	1980
October	1st to 10th	Day	17% 7% 3 4 4 3 5% 5% 5% 6% 42%	1916
	11th to 20th	Day	19% 6% 2 5% 5% 3 3 4 5% 6% 42%	1800
	21st to end	Day	23% 8% 3 4 5% 3 4 4 4 5% 38%	1796
November	1st to 10th	Day	19% 6% 2 6% 5% 3 4 3 5% 6% 42%	1671
	11th to 20th	Day	22% 5% 2 3 3 3 5% 4 5% 7% 41%	1653
	21st to end	Day	19% 9% 4 5% 4 2 3 3 4 5% 41%	1520
December	1st to 10th	Day	24% 9% 2 5% 4 4 5% 4 5% 5% 33%	1457
	11th to 20th	Day	24% 9% 3 5% 4 2 4 5% 4 6% 33%	1668
	21st to end	Day	25% 9% 3 5% 4 2 3 4 4 6% 35%	2431
January	1st to 10th	Night	17% 9% 3 6% 5% 3 4 4 4 8% 36%	2649
	11th to 20th	Night	18% 12% 4 7% 5% 3 5% 5% 4 6% 31%	2591
	21st to end	Night	13% 9% 3 6% 6% 3 4 4 5% 9% 36%	2706
February	1st to 10th	Night	19% 9% 3 6% 5% 3 5% 5% 4 7% 33%	2340
	11th to 20th	Night	15% 9% 3 7% 5% 3 4 4 5% 6% 38%	2188
	21st to end	Night	15% 9% 4 6% 6% 3 4 4 5% 6% 39%	1776
March	1st to 10th	Night	16% 10% 4 6% 6% 3 4 3 4 6% 38%	2123
	11th to 20th	Night	18% 9% 4 6% 5% 3 4 4 6% 6% 35%	1979
	21st to end	Night	19% 9% 3 5% 4 3 4 4 4 6% 40%	2178
April	1st to 10th	Night	23% 11% 3 5% 4 3 4 5% 4 5% 33%	1844
	11th to 20th	Night	21% 7% 4 6% 6% 3 3 4 5% 7% 35%	1606
	21st to end	Night	25% 9% 3 6% 5% 2 3 4 4 7% 32%	1620
May	1st to 10th	Night	23% 8% 3 6% 5% 3 5% 5% 5% 8% 29%	1620
	11th to 20th	Night	24% 7% 3 6% 5% 3 4 5% 5% 7% 32%	1593
	21st to end	Night	20% 10% 4 6% 5% 3 5% 3 5% 6% 34%	1523
June	1st to 10th	Night	26% 9% 3 6% 4 3 5% 4 5% 7% 28%	1260
	11th to 20th	Night	21% 8% 3 5% 3 2 5% 4 6% 6% 36%	1260
	21st to end	Night	26% 10% 3 5% 4 4 4 4 4 6% 30%	1260
July	1st to 10th	Night	21% 7% 3 6% 5% 3 5% 4 6% 8% 32%	1334
	11th to 20th	Night	20% 9% 3 5% 5% 3 5% 5% 6% 8% 33%	1440
	21st to end	Night	21% 7% 3 6% 5% 3 4 5% 6% 7% 33%	1627
August	1st to 10th	Night	17% 7% 2 5% 4 3 5% 5% 7% 8% 36%	1612
	11th to 20th	Night	18% 7% 4 6% 4 3 5% 5% 6% 8% 32%	1596
	21st to end	Night	16% 5% 2 4 5% 3 3 4 5% 9% 43%	1782
September	1st to 10th	Night	15% 5% 2 4 4 2 4 4 5% 7% 48%	1889
	11th to 20th	Night	16% 6% 2 4 3 2 4 3 4 7% 48%	1980
	21st to end	Night	18% 6% 2 4 3 2 4 4 4 7% 45%	1980
October	1st to 10th	Night	20% 7% 2 4 5% 3 4 4 5% 6% 40%	2042
	11th to 20th	Night	19% 6% 2 5% 5% 2 4 4 4 6% 41%	2160
	21st to end	Night	25% 8% 3 5% 5% 3 3 4 4 6% 37%	2558
November	1st to 10th	Night	19% 6% 2 4 4 3 5% 5% 5% 6% 42%	2407
	11th to 20th	Night	19% 6% 2 5% 4 3 5% 4 5% 5% 41%	2526
	21st to end	Night	18% 8% 3 5% 4 2 4 5% 5% 6% 39%	2660
December	1st to 10th	Night	25% 9% 3 6% 4 3 4 4 5% 6% 31%	2721
	11th to 20th	Night	24% 8% 2 5% 4 2 3 4 4 7% 37%	3531
	21st to end	Night	25% 9% 3 5% 4 2 3 3 4 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	51% 4 11% 22% 11% 1	1518
	11th to 20th	Day	53% 5% 13% 19% 8% 2	1480
	21st to end	Day	47% 3 12% 24% 13% 1	1648
February	1st to 10th	Day	53% 4 13% 18% 11% 2	1620
	11th to 20th	Day	49% 5% 13% 19% 12% 1	1662
	21st to end	Day	49% 4 11% 22% 10% 4	1480
March	1st to 10th	Day	49% 4 13% 18% 14% 2	1827
	11th to 20th	Day	43% 4 11% 22% 17% 3	1980
	21st to end	Day	47% 4 9% 19% 16% 5%	2178
April	1st to 10th	Day	50% 4 8% 19% 15% 4	2114
	11th to 20th	Day	45% 4 11% 24% 14% 3	2303
	21st to end	Day	50% 4 9% 19% 16% 3	2340
May	1st to 10th	Day	51% 4 8% 22% 12% 2	2340
	11th to 20th	Day	45% 4 9% 24% 15% 2	2305
	21st to end	Day	49% 6% 11% 22% 11% 1	2763
June	1st to 10th	Day	52% 5% 12% 21% 9%	2700
	11th to 20th	Day	46% 5% 11% 20% 15% 3	2700
	21st to end	Day	54% 5% 12% 17% 10% 2	2700
July	1st to 10th	Day	44% 6% 14% 23% 12%	2551
	11th to 20th	Day	43% 5% 15% 24% 11% 1	2445
	21st to end	Day	41% 7% 16% 26% 10% 1	2523
August	1st to 10th	Day	39% 6% 17% 28% 9%	2314
	11th to 20th	Day	44% 5% 14% 24% 12%	2304
	21st to end	Day	35% 5% 14% 27% 18% 1	2574
September	1st to 10th	Day	34% 5% 13% 28% 18% 2	2067
	11th to 20th	Day	35% 5% 11% 27% 20% 3	1980
	21st to end	Day	42% 4 9% 28% 16% 1	1980
October	1st to 10th	Day	42% 4 11% 26% 15% 2	1916
	11th to 20th	Day	43% 4 13% 30% 10%	1800
	21st to end	Day	48% 4 11% 22% 13% 1	1796
November	1st to 10th	Day	44% 4 14% 24% 13% 1	1671
	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	49% 4 14% 22% 9% 1	2649
	11th to 20th	Night	56% 6% 11% 18% 8% 1	2591
	21st to end	Night	47% 4 13% 24% 12% 1	2706
February	1st to 10th	Night	54% 4 12% 17% 10% 2	2340
	11th to 20th	Night	48% 5% 11% 20% 14% 2	2191
	21st to end	Night	48% 3 10% 22% 14% 3	1776
March	1st to 10th	Night	49% 3 9% 20% 16% 3	2132
	11th to 20th	Night	50% 5% 11% 17% 16% 2	1980
	21st to end	Night	47% 3 8% 21% 18% 3	2178
April	1st to 10th	Night	53% 5% 7% 15% 16% 4	1844
	11th to 20th	Night	50% 3 8% 21% 16% 2	1606
	21st to end	Night	54% 4 10% 17% 13% 3	1620
May	1st to 10th	Night	53% 6% 8% 19% 14% 1	1620
	11th to 20th	Night	52% 6% 8% 20% 11% 2	1594
	21st to end	Night	53% 5% 9% 19% 13% 1	1524
June	1st to 10th	Night	57% 5% 10% 20% 7% 1	1260
	11th to 20th	Night	48% 6% 11% 22% 11% 2	1260
	21st to end	Night	55% 4 9% 20% 10% 2	1260
July	1st to 10th	Night	50% 6% 12% 20% 11%	1295
	11th to 20th	Night	49% 6% 13% 21% 10%	1392
	21st to end	Night	49% 5% 15% 20% 9% 2	1537
August	1st to 10th	Night	45% 7% 13% 26% 9%	1600
	11th to 20th	Night	48% 7% 13% 21% 10%	1596
	21st to end	Night	41% 5% 11% 24% 19% 1	1782
September	1st to 10th	Night	37% 4 13% 29% 17% 1	1889
	11th to 20th	Night	38% 4 11% 26% 19% 2	1980
	21st to end	Night	40% 4 9% 29% 17% 1	1980
October	1st to 10th	Night	44% 4 11% 24% 14% 2	2042
	11th to 20th	Night	43% 4 12% 28% 13% 1	2160
	21st to end	Night	50% 4 11% 22% 13% 1	2558
November	1st to 10th	Night	45% 4 12% 25% 14% 1	2407
	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	29% 25% 31% 12% 2 1	170
	21st to end	Day	18% 26% 36% 17% 3	240
February	1st to 10th	Day	28% 27% 28% 15% 2	340
	11th to 20th	Day	26% 30% 34% 9% 2	372
	21st to end	Day	24% 21% 36% 16% 2	346
March	1st to 10th	Day	25% 21% 37% 15% 2	420
	11th to 20th	Day	23% 23% 33% 17% 4	420
	21st to end	Day	27% 22% 29% 16% 6% 1	462
April	1st to 10th	Day	29% 22% 31% 13% 4 1	420
	11th to 20th	Day	27% 16% 35% 15% 6% 1	412
	21st to end	Day	26% 14% 36% 17% 6% 1	420
May	1st to 10th	Day	27% 18% 34% 17% 3 1	423
	11th to 20th	Day	18% 18% 36% 22% 4 2	447
	21st to end	Day	26% 11% 34% 20% 9% 1	589
June	1st to 10th	Day	26% 9% 31% 24% 9% 1	630
	11th to 20th	Day	25% 8% 31% 23% 11% 2	630
	21st to end	Day	30% 5% 28% 27% 10% 2	630
July	1st to 10th	Day	23% 7% 30% 27% 11% 1	546
	11th to 20th	Day	19% 5% 32% 31% 11% 2	444
	21st to end	Day	16% 4 35% 31% 13% 2	488
August	1st to 10th	Day	15% 6% 31% 34% 13% 1	446
	11th to 20th	Day	21% 4 31% 30% 13% 3	439
	21st to end	Day	17% 8% 27% 33% 13% 3	495
September	1st to 10th	Day	17% 5% 34% 32% 11% 2	450
	11th to 20th	Day	20% 9% 32% 24% 11% 4	450
	21st to end	Day	20% 9% 35% 24% 10% 3	450
October	1st to 10th	Day	17% 16% 27% 27% 11% 2	417
	11th to 20th	Day	19% 17% 32% 22% 8% 2	360
	21st to end	Day	30% 15% 31% 18% 6% 1	396
November	1st to 10th	Day	22% 20% 28% 23% 5% 1	365
	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% 24% 31% 13% 4	741
	21st to end	Night	20% 30% 30% 17% 3	792
February	1st to 10th	Night	26% 25% 34% 13% 3	720
	11th to 20th	Night	24% 30% 29% 15% 2 1	703
	21st to end	Night	22% 21% 35% 18% 4	592
March	1st to 10th	Night	24% 20% 35% 19% 2	720
	11th to 20th	Night	23% 20% 30% 20% 6%	720
	21st to end	Night	23% 16% 35% 19% 6% 1	792
April	1st to 10th	Night	31% 14% 33% 17% 4	720
	11th to 20th	Night	27% 9% 34% 25% 4	542
	21st to end	Night	31% 9% 28% 23% 9% 1	540
May	1st to 10th	Night	26% 7% 30% 26% 9% 2	540
	11th to 20th	Night	25% 8% 33% 26% 8%	532
	21st to end	Night	26% 6% 25% 30% 12% 1	469
June	1st to 10th	Night	32% 8% 20% 30% 8% 1	360
	11th to 20th	Night	25% 4 25% 32% 13% 1	360
	21st to end	Night	33% 3 18% 34% 12% 1	360
July	1st to 10th	Night	23% 5% 21% 37% 13%	350
	11th to 20th	Night	24% 2 21% 33% 17% 1	348
	21st to end	Night	24% 1 19% 41% 13% 2	447
August	1st to 10th	Night	23% 2 22% 37% 15% 1	538
	11th to 20th	Night	22% 3 19% 39% 17% 1	532
	21st to end	Night	18% 4 22% 35% 20% 2	594
September	1st to 10th	Night	17% 3 26% 36% 15% 2	702
	11th to 20th	Night	21% 7% 25% 28% 15% 4	720
	21st to end	Night	20% 8% 28% 28% 14% 3	720
October	1st to 10th	Night	23% 10% 24% 27% 14% 2	720
	11th to 20th	Night	22% 14% 27% 25% 11% 1	720
	21st to end	Night	29% 17% 29% 18% 6%	792
November	1st to 10th	Night	24% 15% 28% 24% 8% 1	740
	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 2024-01-13 01:38:14. Version ac07

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