Orion Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Orion Observatory clear sky chart from 2005-11-06 to 2024-01-22.

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	18% 13% 5% 8% 7% 4 5% 4 5% 6% 26%	74608
	Night	17% 13% 5% 9% 7% 4 5% 4 5% 5% 26%	75299
Transparency	Day	59% 6% 10% 16% 8% 1	74313
	Night	61% 6% 10% 15% 7% 1	75129
Seeing	Day	28% 20% 28% 16% 6% 2	14017
	Night	26% 18% 27% 21% 8% 1	22844

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	12% 18% 9% 15% 11% 4 6% 3 3 5% 12%	1517
	11th to 20th	Day	13% 16% 8% 13% 13% 4 4 5% 4 4 16%	1520
	21st to end	Day	8% 13% 8% 14% 11% 5% 6% 5% 5% 5% 19%	1592
February	1st to 10th	Day	11% 18% 7% 13% 13% 6% 6% 4 4 3 15%	1492
	11th to 20th	Day	11% 18% 8% 15% 9% 5% 5% 5% 3 5% 15%	1748
	21st to end	Day	10% 18% 6% 13% 8% 6% 7% 4 4 5% 19%	1480
March	1st to 10th	Day	13% 16% 7% 11% 9% 5% 4 4 4 4 23%	1797
	11th to 20th	Day	14% 15% 7% 9% 8% 4 4 4 4 5% 26%	1904
	21st to end	Day	16% 18% 6% 10% 7% 4 4 3 3 3 26%	2285
April	1st to 10th	Day	20% 17% 6% 10% 7% 3 4 4 4 4 23%	2160
	11th to 20th	Day	17% 15% 4 7% 5% 3 4 3 4 6% 33%	2217
	21st to end	Day	23% 15% 4 7% 5% 3 4 3 4 5% 26%	2340
May	1st to 10th	Day	22% 13% 4 7% 5% 4 4 3 4 5% 30%	2340
	11th to 20th	Day	21% 10% 3 6% 5% 3 4 4 5% 6% 32%	2455
	21st to end	Day	19% 8% 3 8% 6% 4 4 5% 6% 8% 27%	2725
June	1st to 10th	Day	29% 8% 3 5% 5% 3 4 5% 6% 7% 23%	2614
	11th to 20th	Day	19% 8% 3 6% 5% 3 6% 5% 5% 8% 31%	2700
	21st to end	Day	22% 9% 4 7% 6% 4 5% 5% 6% 6% 25%	2609
July	1st to 10th	Day	18% 7% 4 6% 6% 4 5% 6% 6% 9% 31%	2520
	11th to 20th	Day	18% 7% 3 6% 6% 4 6% 5% 7% 8% 29%	2520
	21st to end	Day	20% 7% 3 7% 6% 4 6% 6% 5% 8% 27%	2744
August	1st to 10th	Day	20% 6% 2 7% 7% 4 5% 6% 7% 9% 26%	2440
	11th to 20th	Day	18% 7% 3 7% 6% 3 5% 5% 7% 8% 31%	2304
	21st to end	Day	18% 8% 3 6% 5% 3 6% 5% 6% 6% 34%	2460
September	1st to 10th	Day	19% 7% 3 5% 5% 3 5% 4 6% 8% 36%	2159
	11th to 20th	Day	19% 8% 2 5% 4 2 4 4 5% 6% 41%	1980
	21st to end	Day	19% 9% 2 6% 5% 2 4 5% 5% 9% 34%	1980
October	1st to 10th	Day	21% 12% 3 4 5% 3 4 5% 6% 7% 29%	1980
	11th to 20th	Day	16% 15% 4 8% 7% 4 6% 4 6% 6% 25%	1682
	21st to end	Day	24% 21% 4 7% 5% 3 4 4 4 5% 20%	1782
November	1st to 10th	Day	16% 18% 6% 8% 6% 3 4 4 4 4 27%	1671
	11th to 20th	Day	16% 16% 9% 11% 7% 3 4 5% 4 5% 21%	1710
	21st to end	Day	16% 16% 8% 13% 8% 3 5% 5% 4 4 19%	1521
December	1st to 10th	Day	18% 22% 10% 13% 8% 3 4 3 2 3 14%	1330
	11th to 20th	Day	18% 16% 9% 11% 7% 4 5% 4 3 4 20%	1668
	21st to end	Day	21% 21% 6% 12% 10% 3 3 3 3 4 15%	2662
January	1st to 10th	Night	11% 17% 10% 14% 12% 4 6% 5% 4 4 11%	2649
	11th to 20th	Night	12% 16% 7% 15% 11% 6% 8% 5% 4 3 12%	2660
	21st to end	Night	8% 12% 7% 14% 11% 7% 9% 7% 5% 5% 17%	2798
February	1st to 10th	Night	10% 14% 7% 12% 11% 6% 7% 5% 4 4 18%	2462
	11th to 20th	Night	10% 16% 7% 14% 11% 5% 7% 4 3 5% 17%	2098
	21st to end	Night	11% 17% 6% 12% 9% 5% 6% 4 3 3 24%	1776
March	1st to 10th	Night	13% 17% 8% 11% 8% 3 5% 4 4 4 24%	2150
	11th to 20th	Night	14% 14% 6% 10% 7% 3 5% 4 5% 5% 28%	2054
	21st to end	Night	16% 14% 7% 10% 6% 3 4 4 4 5% 28%	2070
April	1st to 10th	Night	20% 15% 5% 9% 5% 3 4 4 4 5% 26%	1800
	11th to 20th	Night	19% 12% 3 6% 6% 3 4 4 4 6% 33%	1692
	21st to end	Night	21% 14% 4 7% 5% 2 4 4 5% 6% 28%	1620
May	1st to 10th	Night	21% 12% 4 6% 4 2 5% 5% 5% 6% 29%	1620
	11th to 20th	Night	17% 10% 3 7% 6% 4 6% 5% 4 5% 33%	1452
	21st to end	Night	21% 7% 3 5% 5% 3 5% 5% 6% 9% 32%	1550
June	1st to 10th	Night	27% 8% 3 6% 6% 3 4 4 4 7% 27%	1341
	11th to 20th	Night	20% 7% 3 5% 5% 2 5% 5% 5% 7% 35%	1260
	21st to end	Night	19% 7% 4 6% 6% 4 5% 6% 6% 6% 31%	1342
July	1st to 10th	Night	15% 7% 3 5% 6% 3 6% 5% 6% 8% 35%	1440
	11th to 20th	Night	14% 6% 3 4 6% 5% 6% 7% 7% 10% 33%	1440
	21st to end	Night	16% 7% 2 6% 6% 3 5% 6% 6% 8% 35%	1574
August	1st to 10th	Night	16% 6% 2 5% 4 3 5% 6% 7% 9% 38%	1480
	11th to 20th	Night	14% 8% 4 6% 5% 4 5% 6% 6% 8% 35%	1596
	21st to end	Night	16% 7% 2 6% 5% 2 5% 4 4 8% 41%	1894
September	1st to 10th	Night	16% 7% 2 6% 5% 3 5% 4 5% 7% 41%	1800
	11th to 20th	Night	18% 7% 2 4 4 2 4 5% 5% 5% 43%	1980
	21st to end	Night	19% 8% 3 4 4 2 4 4 4 7% 42%	1980
October	1st to 10th	Night	18% 10% 3 5% 5% 3 4 4 5% 7% 35%	1980
	11th to 20th	Night	21% 13% 3 7% 6% 4 4 5% 5% 5% 28%	2276
	21st to end	Night	23% 17% 5% 7% 4 3 4 4 4 5% 23%	2574
November	1st to 10th	Night	19% 15% 5% 7% 7% 4 5% 4 4 5% 25%	2407
	11th to 20th	Night	16% 14% 8% 11% 7% 3 4 4 5% 5% 25%	2470
	21st to end	Night	15% 15% 8% 12% 9% 3 5% 3 4 5% 20%	2653
December	1st to 10th	Night	17% 19% 9% 13% 9% 4 5% 4 5% 3 12%	2850
	11th to 20th	Night	17% 17% 9% 15% 10% 4 5% 4 4 4 13%	3531
	21st to end	Night	21% 18% 7% 10% 7% 3 3 3 3 4 20%	4980

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	77% 4 7% 8% 4	1518
	11th to 20th	Day	72% 4 6% 12% 5% 1	1520
	21st to end	Day	65% 5% 7% 12% 9% 2	1592
February	1st to 10th	Day	76% 3 5% 8% 7% 1	1492
	11th to 20th	Day	74% 3 7% 9% 6% 1	1757
	21st to end	Day	68% 4 7% 12% 7% 2	1480
March	1st to 10th	Day	67% 4 8% 13% 6% 3	1800
	11th to 20th	Day	62% 4 9% 12% 11% 2	1904
	21st to end	Day	65% 4 6% 13% 9% 2	2285
April	1st to 10th	Day	66% 3 8% 12% 8% 2	2160
	11th to 20th	Day	55% 3 8% 20% 12% 1	2217
	21st to end	Day	61% 5% 8% 14% 9% 3	2340
May	1st to 10th	Day	58% 4 7% 18% 12% 2	2340
	11th to 20th	Day	54% 6% 10% 18% 10% 1	2459
	21st to end	Day	54% 7% 12% 17% 9% 1	2725
June	1st to 10th	Day	59% 7% 9% 19% 6%	2614
	11th to 20th	Day	50% 7% 11% 21% 9% 1	2700
	21st to end	Day	58% 8% 10% 17% 6%	2609
July	1st to 10th	Day	49% 8% 14% 21% 7%	2450
	11th to 20th	Day	51% 8% 12% 22% 6%	2445
	21st to end	Day	55% 8% 12% 19% 5%	2591
August	1st to 10th	Day	53% 9% 14% 20% 4	2426
	11th to 20th	Day	50% 9% 15% 18% 8%	2304
	21st to end	Day	48% 8% 12% 21% 11%	2460
September	1st to 10th	Day	45% 8% 13% 23% 11%	2159
	11th to 20th	Day	45% 5% 11% 26% 13% 1	1980
	21st to end	Day	48% 6% 12% 23% 10% 1	1980
October	1st to 10th	Day	53% 5% 12% 20% 9%	1980
	11th to 20th	Day	60% 6% 13% 16% 5%	1682
	21st to end	Day	68% 4 8% 12% 6% 1	1782
November	1st to 10th	Day	61% 5% 10% 15% 8% 1	1671
	11th to 20th	Day	67% 5% 8% 13% 6% 2	1710
	21st to end	Day	67% 6% 10% 13% 3	1521
December	1st to 10th	Day	78% 3 6% 7% 5%	1330
	11th to 20th	Day	69% 4 6% 10% 10% 1	1668
	21st to end	Day	76% 4 5% 9% 6% 1	2662
January	1st to 10th	Night	78% 5% 6% 7% 4	2649
	11th to 20th	Night	77% 5% 6% 8% 4 1	2660
	21st to end	Night	69% 6% 8% 10% 5% 2	2798
February	1st to 10th	Night	70% 6% 7% 10% 6% 1	2462
	11th to 20th	Night	72% 4 8% 9% 6% 1	2100
	21st to end	Night	66% 5% 6% 13% 9% 1	1776
March	1st to 10th	Night	65% 4 7% 15% 7% 1	2160
	11th to 20th	Night	60% 7% 9% 12% 11% 2	2055
	21st to end	Night	60% 5% 6% 18% 9% 3	2070
April	1st to 10th	Night	62% 4 8% 13% 11% 1	1800
	11th to 20th	Night	54% 6% 8% 19% 12% 1	1692
	21st to end	Night	57% 7% 9% 16% 9% 2	1620
May	1st to 10th	Night	55% 6% 8% 18% 12% 1	1620
	11th to 20th	Night	54% 7% 9% 19% 10% 1	1453
	21st to end	Night	50% 7% 13% 20% 10% 1	1551
June	1st to 10th	Night	57% 6% 11% 20% 7%	1341
	11th to 20th	Night	48% 7% 11% 22% 10% 1	1260
	21st to end	Night	52% 9% 12% 20% 8%	1342
July	1st to 10th	Night	46% 8% 16% 21% 9%	1400
	11th to 20th	Night	43% 10% 16% 23% 7%	1392
	21st to end	Night	47% 9% 15% 21% 7%	1486
August	1st to 10th	Night	42% 9% 21% 24% 4	1470
	11th to 20th	Night	44% 11% 16% 22% 7%	1596
	21st to end	Night	43% 7% 14% 25% 10%	1894
September	1st to 10th	Night	44% 7% 13% 26% 10%	1800
	11th to 20th	Night	43% 6% 14% 27% 10% 1	1980
	21st to end	Night	43% 8% 11% 25% 11% 1	1980
October	1st to 10th	Night	49% 7% 10% 21% 12% 1	1980
	11th to 20th	Night	58% 7% 11% 18% 5%	2276
	21st to end	Night	64% 6% 8% 13% 7% 1	2574
November	1st to 10th	Night	62% 5% 10% 15% 7% 1	2407
	11th to 20th	Night	63% 5% 10% 13% 8% 1	2470
	21st to end	Night	69% 5% 10% 11% 4	2653
December	1st to 10th	Night	76% 5% 8% 8% 3	2850
	11th to 20th	Night	77% 4 6% 8% 6%	3532
	21st to end	Night	70% 5% 8% 11% 6%	4980

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	32% 28% 31% 9% 1	180
	11th to 20th	Day	36% 24% 26% 12% 2 1	180
	21st to end	Day	25% 24% 37% 12% 2 1	178
February	1st to 10th	Day	36% 28% 26% 8% 2	212
	11th to 20th	Day	31% 31% 29% 8% 1	416
	21st to end	Day	35% 26% 24% 14% 2	346
March	1st to 10th	Day	29% 27% 30% 11% 4	420
	11th to 20th	Day	31% 26% 24% 14% 4	420
	21st to end	Day	32% 21% 25% 14% 7%	462
April	1st to 10th	Day	37% 22% 24% 11% 5%	420
	11th to 20th	Day	33% 22% 25% 12% 5% 2	412
	21st to end	Day	37% 21% 25% 11% 4 2	420
May	1st to 10th	Day	36% 18% 30% 12% 3 2	423
	11th to 20th	Day	26% 21% 29% 17% 5% 1	447
	21st to end	Day	21% 24% 33% 16% 5% 1	485
June	1st to 10th	Day	35% 17% 28% 14% 6%	544
	11th to 20th	Day	24% 16% 27% 23% 9% 1	630
	21st to end	Day	24% 14% 32% 19% 9% 1	539
July	1st to 10th	Day	20% 15% 33% 23% 7% 1	445
	11th to 20th	Day	20% 16% 36% 18% 8% 2	444
	21st to end	Day	20% 13% 36% 25% 5% 2	488
August	1st to 10th	Day	20% 15% 30% 23% 11% 1	446
	11th to 20th	Day	19% 13% 30% 26% 10% 3	439
	21st to end	Day	24% 18% 24% 18% 12% 4	495
September	1st to 10th	Day	22% 16% 28% 20% 11% 4	450
	11th to 20th	Day	24% 12% 26% 19% 12% 7%	450
	21st to end	Day	22% 15% 26% 21% 12% 4	450
October	1st to 10th	Day	27% 16% 25% 16% 11% 4	450
	11th to 20th	Day	28% 20% 27% 15% 8% 2	372
	21st to end	Day	37% 22% 21% 16% 4	396
November	1st to 10th	Day	33% 22% 26% 16% 3	365
	11th to 20th	Day	29% 22% 29% 12% 6% 2	370
	21st to end	Day	33% 25% 26% 12% 3 1	265
December	1st to 10th	Day	28% 29% 29% 12% 2	180
	11th to 20th	Day	37% 28% 26% 5% 4 1	180
	21st to end	Day	35% 33% 22% 8% 2	198
January	1st to 10th	Night	26% 28% 31% 13% 2	760
	11th to 20th	Night	25% 27% 29% 15% 3	760
	21st to end	Night	17% 27% 35% 16% 4 1	800
February	1st to 10th	Night	22% 29% 32% 15% 2	720
	11th to 20th	Night	25% 25% 32% 16% 2	703
	21st to end	Night	26% 22% 33% 16% 2	592
March	1st to 10th	Night	26% 22% 32% 18% 3	720
	11th to 20th	Night	27% 24% 27% 17% 5% 1	720
	21st to end	Night	26% 21% 25% 21% 7% 1	792
April	1st to 10th	Night	33% 19% 26% 17% 5%	720
	11th to 20th	Night	27% 17% 26% 23% 6%	628
	21st to end	Night	30% 19% 26% 19% 4 1	540
May	1st to 10th	Night	29% 14% 29% 19% 8% 1	540
	11th to 20th	Night	22% 14% 31% 26% 6% 1	391
	21st to end	Night	23% 13% 30% 26% 8% 1	389
June	1st to 10th	Night	31% 7% 30% 24% 8% 1	360
	11th to 20th	Night	23% 7% 23% 33% 14%	360
	21st to end	Night	21% 8% 27% 33% 11%	360
July	1st to 10th	Night	17% 7% 30% 34% 11% 1	350
	11th to 20th	Night	16% 8% 27% 34% 14% 1	348
	21st to end	Night	20% 7% 20% 38% 15%	394
August	1st to 10th	Night	19% 4 19% 38% 19% 1	406
	11th to 20th	Night	17% 7% 23% 33% 18% 1	532
	21st to end	Night	19% 9% 24% 30% 16% 2	706
September	1st to 10th	Night	19% 9% 25% 32% 14% 1	720
	11th to 20th	Night	23% 8% 21% 27% 18% 2	720
	21st to end	Night	24% 10% 27% 23% 14% 2	720
October	1st to 10th	Night	24% 13% 22% 28% 12% 1	720
	11th to 20th	Night	30% 16% 24% 20% 10% 1	720
	21st to end	Night	35% 18% 26% 17% 4	792
November	1st to 10th	Night	33% 18% 25% 18% 6%	740
	11th to 20th	Night	27% 21% 28% 15% 8% 1	760
	21st to end	Night	27% 24% 27% 16% 5% 1	798
December	1st to 10th	Night	30% 24% 29% 13% 4	850
	11th to 20th	Night	30% 25% 25% 14% 5% 1	850
	21st to end	Night	35% 23% 25% 14% 2	863

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