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 2025-01-22.

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	17% 13% 4 10% 6% 5% 5% 4 5% 6% 26%	104632
	Night	16% 14% 5% 11% 7% 5% 5% 3 4 5% 24%	104734
Transparency	Day	58% 6% 10% 17% 8% 1	108118
	Night	61% 6% 10% 15% 7% 1	109325
Seeing	Day	28% 20% 28% 16% 6% 2	15304
	Night	26% 18% 27% 21% 8% 1	24115

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	11% 19% 9% 16% 11% 6% 6% 3 4 4 11%	2597
	11th to 20th	Day	9% 15% 6% 23% 14% 6% 3 2 3 3 15%	2600
	21st to end	Day	10% 17% 9% 17% 7% 7% 5% 3 4 4 18%	2780
February	1st to 10th	Day	11% 18% 7% 15% 11% 7% 4 3 4 3 16%	1575
	11th to 20th	Day	11% 19% 8% 16% 8% 6% 5% 4 3 4 15%	1848
	21st to end	Day	10% 19% 4 14% 7% 8% 5% 3 4 5% 19%	1570
March	1st to 10th	Day	20% 16% 4 11% 6% 5% 3 2 4 4 23%	3116
	11th to 20th	Day	15% 23% 7% 15% 6% 4 4 2 3 3 19%	3348
	21st to end	Day	16% 18% 5% 12% 5% 4 3 2 3 4 28%	4005
April	1st to 10th	Day	19% 18% 5% 10% 6% 4 3 3 4 4 24%	2280
	11th to 20th	Day	16% 16% 3 8% 5% 4 4 2 4 6% 32%	2342
	21st to end	Day	22% 16% 3 8% 4 4 3 3 4 5% 27%	2470
May	1st to 10th	Day	22% 12% 3 8% 6% 6% 4 4 5% 5% 26%	4106
	11th to 20th	Day	20% 9% 3 8% 5% 6% 5% 4 5% 7% 28%	4361
	21st to end	Day	15% 8% 3 8% 6% 5% 5% 5% 7% 9% 29%	4837
June	1st to 10th	Day	28% 9% 3 6% 4 4 4 4 7% 7% 24%	2760
	11th to 20th	Day	19% 8% 3 7% 4 5% 6% 4 6% 8% 31%	2850
	21st to end	Day	22% 10% 3 8% 5% 5% 5% 4 6% 6% 25%	2753
July	1st to 10th	Day	14% 7% 3 8% 5% 5% 6% 5% 7% 10% 29%	4440
	11th to 20th	Day	14% 8% 3 7% 5% 5% 6% 5% 7% 8% 30%	4440
	21st to end	Day	16% 8% 3 7% 5% 5% 4 4 5% 7% 37%	4814
August	1st to 10th	Day	18% 7% 3 8% 6% 6% 6% 5% 7% 9% 25%	4302
	11th to 20th	Day	18% 9% 3 9% 6% 6% 6% 4 7% 8% 24%	4045
	21st to end	Day	18% 9% 3 7% 4 4 5% 4 6% 7% 35%	4320
September	1st to 10th	Day	18% 7% 2 6% 4 4 4 4 6% 8% 37%	2278
	11th to 20th	Day	18% 8% 2 6% 3 3 4 4 5% 6% 41%	2090
	21st to end	Day	18% 9% 2 7% 4 4 4 4 5% 9% 33%	2090
October	1st to 10th	Day	20% 13% 3 6% 4 4 3 4 6% 7% 29%	2090
	11th to 20th	Day	15% 17% 4 9% 6% 5% 5% 3 6% 5% 25%	1774
	21st to end	Day	22% 21% 4 8% 4 4 4 3 4 5% 22%	1881
November	1st to 10th	Day	15% 19% 6% 8% 5% 5% 3 4 4 4 27%	1761
	11th to 20th	Day	15% 19% 7% 11% 6% 3 5% 3 4 5% 22%	1800
	21st to end	Day	16% 17% 8% 14% 6% 4 4 4 4 4 18%	1600
December	1st to 10th	Day	12% 21% 8% 20% 11% 6% 3 2 2 3 11%	2280
	11th to 20th	Day	15% 16% 9% 13% 6% 5% 4 3 4 4 21%	2618
	21st to end	Day	17% 20% 5% 14% 7% 4 5% 3 4 5% 17%	3811
January	1st to 10th	Night	12% 16% 10% 20% 12% 6% 5% 3 4 3 10%	4569
	11th to 20th	Night	10% 17% 7% 20% 14% 9% 5% 3 3 3 10%	4580
	21st to end	Night	10% 17% 8% 17% 7% 8% 6% 4 4 4 14%	4899
February	1st to 10th	Night	10% 15% 7% 14% 10% 8% 6% 4 4 4 19%	2599
	11th to 20th	Night	9% 17% 7% 17% 9% 8% 5% 3 3 4 16%	2218
	21st to end	Night	11% 17% 5% 12% 8% 6% 6% 3 4 4 25%	1884
March	1st to 10th	Night	20% 19% 5% 9% 5% 3 5% 2 3 4 24%	3724
	11th to 20th	Night	13% 21% 8% 13% 6% 4 4 2 4 4 22%	3610
	21st to end	Night	19% 17% 5% 11% 6% 3 3 2 3 4 26%	3612
April	1st to 10th	Night	20% 16% 5% 9% 5% 4 4 3 4 5% 27%	1900
	11th to 20th	Night	19% 13% 3 7% 5% 3 4 3 4 6% 32%	1787
	21st to end	Night	20% 14% 4 8% 4 3 4 3 5% 6% 29%	1710
May	1st to 10th	Night	24% 11% 3 6% 4 4 5% 4 6% 7% 26%	2834
	11th to 20th	Night	18% 11% 2 8% 5% 5% 4 3 5% 5% 32%	2560
	21st to end	Night	18% 7% 2 5% 4 4 4 4 6% 10% 34%	2738
June	1st to 10th	Night	27% 9% 2 6% 6% 4 4 3 4 7% 27%	1415
	11th to 20th	Night	19% 8% 3 6% 4 4 4 4 6% 8% 34%	1330
	21st to end	Night	19% 9% 3 7% 5% 5% 5% 5% 6% 6% 30%	1417
July	1st to 10th	Night	12% 8% 4 9% 6% 6% 6% 5% 6% 8% 31%	2520
	11th to 20th	Night	17% 7% 4 7% 5% 5% 5% 5% 6% 7% 32%	2520
	21st to end	Night	13% 9% 3 7% 4 5% 5% 4 6% 7% 39%	2739
August	1st to 10th	Night	21% 8% 3 8% 4 4 5% 4 5% 7% 30%	2596
	11th to 20th	Night	13% 8% 4 9% 6% 6% 6% 5% 7% 7% 29%	2782
	21st to end	Night	16% 8% 2 7% 4 4 5% 4 5% 6% 39%	3334
September	1st to 10th	Night	15% 8% 2 7% 4 4 4 3 5% 6% 41%	1901
	11th to 20th	Night	17% 7% 2 6% 4 3 3 4 5% 5% 44%	2090
	21st to end	Night	18% 8% 3 5% 4 3 4 3 4 7% 41%	2090
October	1st to 10th	Night	17% 10% 3 7% 4 5% 4 3 5% 7% 35%	2090
	11th to 20th	Night	20% 14% 3 8% 5% 4 4 3 5% 5% 28%	2403
	21st to end	Night	22% 17% 4 8% 3 4 5% 3 4 5% 24%	2717
November	1st to 10th	Night	19% 16% 5% 9% 5% 4 5% 3 4 5% 25%	2537
	11th to 20th	Night	15% 15% 7% 11% 6% 4 3 3 5% 5% 25%	2600
	21st to end	Night	15% 17% 8% 14% 8% 5% 4 3 4 5% 19%	2794
December	1st to 10th	Night	12% 18% 8% 22% 12% 5% 4 3 4 3 9%	4910
	11th to 20th	Night	17% 18% 7% 17% 9% 5% 4 3 3 4 13%	5591
	21st to end	Night	17% 18% 7% 11% 6% 4 4 3 4 6% 21%	7134

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	76% 5% 8% 8% 2	2598
	11th to 20th	Day	76% 3 5% 12% 4 1	2600
	21st to end	Day	69% 6% 8% 8% 8% 2	2780
February	1st to 10th	Day	74% 3 5% 9% 8% 1	1575
	11th to 20th	Day	74% 3 7% 9% 6% 1	1857
	21st to end	Day	67% 4 8% 13% 6% 2	1570
March	1st to 10th	Day	66% 4 7% 12% 9% 2	3119
	11th to 20th	Day	71% 4 7% 10% 7% 1	3348
	21st to end	Day	61% 4 6% 14% 14% 2	4005
April	1st to 10th	Day	65% 3 8% 13% 8% 2	2280
	11th to 20th	Day	55% 4 8% 20% 12% 1	2342
	21st to end	Day	61% 4 8% 14% 10% 3	2470
May	1st to 10th	Day	59% 6% 10% 16% 8% 1	4120
	11th to 20th	Day	55% 8% 15% 15% 6% 1	4365
	21st to end	Day	48% 9% 12% 21% 10%	4837
June	1st to 10th	Day	58% 7% 9% 20% 5%	2760
	11th to 20th	Day	50% 7% 12% 21% 9% 1	2850
	21st to end	Day	58% 8% 10% 18% 6%	2753
July	1st to 10th	Day	46% 9% 15% 23% 7%	4370
	11th to 20th	Day	47% 9% 14% 26% 4	4365
	21st to end	Day	47% 7% 12% 27% 6%	4661
August	1st to 10th	Day	53% 8% 15% 19% 4	4288
	11th to 20th	Day	54% 9% 16% 16% 5%	4084
	21st to end	Day	46% 8% 14% 19% 12% 2	4320
September	1st to 10th	Day	44% 8% 13% 24% 11%	2278
	11th to 20th	Day	44% 5% 11% 26% 13% 1	2090
	21st to end	Day	48% 7% 12% 22% 10% 1	2090
October	1st to 10th	Day	52% 6% 14% 21% 7%	3480
	11th to 20th	Day	55% 5% 11% 23% 6%	2912
	21st to end	Day	54% 5% 14% 19% 8%	3081
November	1st to 10th	Day	61% 5% 9% 16% 8% 1	1761
	11th to 20th	Day	66% 5% 8% 13% 7% 2	1800
	21st to end	Day	69% 6% 9% 13% 3	1600
December	1st to 10th	Day	80% 4 6% 6% 4	2280
	11th to 20th	Day	66% 5% 7% 11% 11% 1	2618
	21st to end	Day	70% 4 6% 9% 9% 2	3811
January	1st to 10th	Night	79% 6% 6% 6% 2	4569
	11th to 20th	Night	81% 5% 5% 7% 2	4580
	21st to end	Night	73% 5% 7% 7% 5% 1	4899
February	1st to 10th	Night	69% 6% 7% 11% 7% 1	2599
	11th to 20th	Night	73% 4 8% 9% 5% 1	2220
	21st to end	Night	65% 5% 6% 14% 9% 1	1884
March	1st to 10th	Night	65% 4 8% 14% 6% 2	3734
	11th to 20th	Night	68% 6% 7% 11% 7% 1	3611
	21st to end	Night	64% 4 5% 14% 12% 2	3612
April	1st to 10th	Night	61% 4 7% 14% 12% 1	1900
	11th to 20th	Night	54% 6% 8% 19% 12% 1	1787
	21st to end	Night	57% 7% 9% 16% 10% 2	1710
May	1st to 10th	Night	55% 9% 14% 16% 7% 1	2840
	11th to 20th	Night	52% 8% 12% 21% 6% 1	2561
	21st to end	Night	45% 8% 16% 19% 12%	2739
June	1st to 10th	Night	57% 6% 11% 20% 6%	1415
	11th to 20th	Night	47% 7% 12% 22% 10% 1	1330
	21st to end	Night	53% 9% 12% 19% 8%	1417
July	1st to 10th	Night	48% 9% 16% 19% 8%	2480
	11th to 20th	Night	46% 11% 15% 23% 5%	2472
	21st to end	Night	44% 8% 16% 26% 5%	2651
August	1st to 10th	Night	53% 8% 15% 22% 3	2586
	11th to 20th	Night	48% 12% 17% 16% 5%	2809
	21st to end	Night	44% 8% 13% 21% 14% 1	3334
September	1st to 10th	Night	43% 7% 13% 26% 10%	1901
	11th to 20th	Night	42% 5% 14% 27% 11% 1	2090
	21st to end	Night	44% 8% 11% 25% 11% 1	2090
October	1st to 10th	Night	47% 8% 12% 22% 11%	3480
	11th to 20th	Night	55% 6% 10% 22% 7%	3991
	21st to end	Night	52% 7% 13% 18% 7% 2	4467
November	1st to 10th	Night	62% 5% 11% 15% 6% 1	2537
	11th to 20th	Night	62% 5% 10% 13% 8% 1	2600
	21st to end	Night	70% 6% 9% 11% 4	2794
December	1st to 10th	Night	77% 8% 6% 6% 2	4910
	11th to 20th	Night	75% 4 5% 10% 6%	5592
	21st to end	Night	66% 5% 7% 12% 9% 1	7134

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% 29% 31% 8% 1	200
	11th to 20th	Day	34% 24% 27% 14% 2 1	200
	21st to end	Day	25% 23% 36% 14% 2 1	200
February	1st to 10th	Day	34% 29% 27% 8% 2	235
	11th to 20th	Day	31% 31% 29% 7% 2	456
	21st to end	Day	34% 26% 25% 14% 2	382
March	1st to 10th	Day	31% 25% 29% 11% 4	460
	11th to 20th	Day	32% 27% 24% 13% 4	460
	21st to end	Day	34% 21% 25% 13% 7%	506
April	1st to 10th	Day	37% 22% 24% 11% 5%	460
	11th to 20th	Day	33% 24% 24% 12% 5% 2	452
	21st to end	Day	36% 21% 26% 11% 4 2	460
May	1st to 10th	Day	36% 17% 29% 13% 3 2	463
	11th to 20th	Day	25% 21% 29% 18% 7% 1	487
	21st to end	Day	21% 24% 34% 16% 4 1	529
June	1st to 10th	Day	34% 16% 28% 15% 6% 1	590
	11th to 20th	Day	23% 16% 27% 24% 9% 1	680
	21st to end	Day	26% 16% 31% 17% 8% 1	583
July	1st to 10th	Day	19% 15% 35% 23% 7% 1	485
	11th to 20th	Day	20% 16% 37% 18% 7% 2	484
	21st to end	Day	20% 13% 36% 25% 5% 2	532
August	1st to 10th	Day	19% 15% 31% 23% 11% 1	486
	11th to 20th	Day	19% 13% 29% 25% 10% 4	479
	21st to end	Day	23% 17% 24% 20% 12% 4	539
September	1st to 10th	Day	22% 15% 28% 20% 10% 3	490
	11th to 20th	Day	22% 11% 25% 21% 14% 8%	490
	21st to end	Day	21% 16% 25% 21% 12% 4	490
October	1st to 10th	Day	26% 16% 27% 16% 11% 4	490
	11th to 20th	Day	29% 19% 27% 15% 8% 2	403
	21st to end	Day	36% 22% 22% 15% 5%	429
November	1st to 10th	Day	33% 22% 25% 16% 3	395
	11th to 20th	Day	29% 23% 30% 12% 5% 2	400
	21st to end	Day	35% 25% 26% 11% 2 1	289
December	1st to 10th	Day	27% 28% 31% 13% 2	200
	11th to 20th	Day	38% 28% 26% 6% 4 1	200
	21st to end	Day	34% 31% 21% 9% 5%	220
January	1st to 10th	Night	26% 28% 31% 13% 2	800
	11th to 20th	Night	24% 27% 30% 16% 3	800
	21st to end	Night	18% 27% 34% 16% 4 1	841
February	1st to 10th	Night	21% 28% 33% 15% 2	760
	11th to 20th	Night	25% 26% 32% 15% 2	743
	21st to end	Night	25% 22% 34% 16% 2	628
March	1st to 10th	Night	27% 21% 32% 18% 3	760
	11th to 20th	Night	27% 23% 28% 17% 4 1	760
	21st to end	Night	27% 21% 24% 21% 6% 1	836
April	1st to 10th	Night	33% 19% 26% 17% 5%	760
	11th to 20th	Night	27% 18% 26% 23% 6%	663
	21st to end	Night	30% 20% 27% 19% 4 1	570
May	1st to 10th	Night	29% 14% 29% 20% 7% 1	570
	11th to 20th	Night	21% 14% 31% 27% 7% 1	413
	21st to end	Night	23% 12% 30% 26% 8% 1	411
June	1st to 10th	Night	31% 7% 29% 24% 8% 1	380
	11th to 20th	Night	22% 7% 23% 33% 15%	380
	21st to end	Night	21% 9% 27% 32% 11%	380
July	1st to 10th	Night	17% 7% 29% 34% 11% 1	370
	11th to 20th	Night	17% 8% 25% 34% 14% 1	368
	21st to end	Night	19% 6% 19% 38% 16%	416
August	1st to 10th	Night	20% 4 19% 37% 19% 1	429
	11th to 20th	Night	17% 8% 23% 33% 18% 1	562
	21st to end	Night	19% 9% 24% 29% 17% 2	746
September	1st to 10th	Night	19% 9% 25% 32% 14% 1	760
	11th to 20th	Night	22% 8% 20% 28% 20% 2	760
	21st to end	Night	23% 10% 26% 23% 16% 2	760
October	1st to 10th	Night	23% 13% 23% 29% 12% 1	760
	11th to 20th	Night	30% 15% 23% 21% 9% 1	760
	21st to end	Night	33% 18% 27% 17% 4	836
November	1st to 10th	Night	33% 18% 24% 18% 5%	780
	11th to 20th	Night	27% 21% 28% 16% 8% 1	800
	21st to end	Night	28% 25% 27% 15% 4 1	843
December	1st to 10th	Night	29% 24% 29% 14% 4	900
	11th to 20th	Night	31% 25% 25% 14% 5% 1	900
	21st to end	Night	34% 22% 26% 14% 3	910

Page generated at 2025-01-19 03:06:50. Version ac07

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