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 2026-05-17.

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	16% 13% 4 11% 6% 5% 5% 4 5% 6% 25%	131169
	Night	15% 13% 5% 12% 7% 6% 5% 3 4 5% 23%	131655
Transparency	Day	59% 6% 10% 17% 7% 1	134637
	Night	61% 7% 10% 15% 7% 1	136262
Seeing	Day	28% 20% 28% 16% 6% 2	17011
	Night	25% 18% 27% 21% 8% 1	25807

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	10% 15% 9% 20% 10% 8% 8% 3 4 4 10%	4661
	11th to 20th	Day	7% 16% 8% 25% 15% 8% 3 2 2 3 10%	4650
	21st to end	Day	6% 13% 6% 15% 9% 14% 9% 4 5% 5% 14%	5124
February	1st to 10th	Day	10% 17% 7% 16% 12% 7% 4 3 4 4 17%	1742
	11th to 20th	Day	10% 18% 8% 17% 8% 6% 5% 3 3 4 16%	2048
	21st to end	Day	9% 18% 5% 15% 8% 8% 5% 3 4 5% 20%	1730
March	1st to 10th	Day	17% 16% 5% 14% 8% 5% 5% 2 4 4 20%	4576
	11th to 20th	Day	14% 17% 5% 12% 5% 5% 4 3 4 6% 25%	4898
	21st to end	Day	18% 17% 5% 12% 5% 5% 4 3 4 4 24%	5866
April	1st to 10th	Day	19% 19% 5% 10% 6% 3 3 3 4 4 24%	2520
	11th to 20th	Day	17% 17% 4 8% 5% 4 3 2 4 5% 31%	2592
	21st to end	Day	22% 15% 3 8% 4 4 4 3 4 5% 27%	2730
May	1st to 10th	Day	26% 13% 4 9% 6% 5% 4 3 5% 5% 20%	6016
	11th to 20th	Day	20% 9% 3 8% 6% 6% 5% 3 5% 6% 28%	6350
	21st to end	Day	20% 9% 3 7% 5% 5% 5% 4 6% 8% 27%	6949
June	1st to 10th	Day	27% 9% 3 7% 4 4 4 4 7% 7% 24%	2905
	11th to 20th	Day	18% 8% 3 8% 5% 5% 6% 4 6% 8% 31%	3000
	21st to end	Day	22% 10% 3 8% 5% 5% 5% 4 6% 6% 25%	2898
July	1st to 10th	Day	16% 8% 3 8% 6% 6% 6% 5% 8% 9% 24%	6360
	11th to 20th	Day	13% 8% 3 7% 5% 5% 6% 5% 7% 8% 33%	6360
	21st to end	Day	14% 7% 3 6% 4 4 5% 4 6% 9% 38%	6926
August	1st to 10th	Day	17% 7% 3 8% 6% 6% 6% 5% 7% 9% 26%	4438
	11th to 20th	Day	18% 9% 3 9% 6% 6% 6% 4 7% 8% 25%	4175
	21st to end	Day	18% 9% 3 7% 4 4 5% 4 6% 7% 34%	4456
September	1st to 10th	Day	17% 7% 3 6% 4 4 5% 4 6% 8% 38%	2398
	11th to 20th	Day	17% 8% 2 6% 3 3 4 4 5% 6% 42%	2200
	21st to end	Day	18% 9% 2 7% 4 4 4 4 5% 9% 34%	2200
October	1st to 10th	Day	20% 12% 3 6% 4 4 3 4 6% 7% 32%	2200
	11th to 20th	Day	15% 17% 4 9% 6% 5% 5% 3 6% 6% 26%	1867
	21st to end	Day	22% 21% 4 8% 4 4 4 3 4 5% 21%	1980
November	1st to 10th	Day	15% 20% 6% 8% 5% 5% 3 4 4 4 26%	1851
	11th to 20th	Day	15% 19% 8% 12% 6% 3 4 3 4 5% 21%	1890
	21st to end	Day	16% 18% 9% 14% 6% 5% 4 3 4 4 17%	1679
December	1st to 10th	Day	12% 21% 8% 20% 11% 6% 3 2 2 3 12%	2350
	11th to 20th	Day	15% 16% 9% 13% 6% 5% 4 3 4 4 21%	2688
	21st to end	Day	17% 19% 5% 15% 7% 4 5% 3 4 5% 17%	3896
January	1st to 10th	Night	8% 14% 8% 24% 12% 8% 7% 3 4 3 10%	8234
	11th to 20th	Night	8% 17% 9% 23% 14% 8% 5% 3 3 3 8%	8235
	21st to end	Night	6% 12% 6% 16% 9% 12% 9% 5% 6% 5% 14%	9057
February	1st to 10th	Night	9% 14% 7% 15% 10% 8% 6% 4 4 4 19%	2872
	11th to 20th	Night	9% 17% 7% 17% 9% 8% 5% 3 3 4 17%	2458
	21st to end	Night	10% 17% 5% 12% 8% 6% 6% 3 4 4 25%	2076
March	1st to 10th	Night	18% 17% 5% 14% 6% 5% 7% 2 3 4 19%	5484
	11th to 20th	Night	13% 17% 6% 12% 6% 5% 4 2 4 6% 24%	5280
	21st to end	Night	17% 15% 5% 11% 7% 4 4 3 4 5% 25%	5293
April	1st to 10th	Night	20% 16% 5% 9% 5% 4 4 3 4 5% 27%	2100
	11th to 20th	Night	19% 15% 4 7% 5% 3 4 3 4 5% 31%	1977
	21st to end	Night	20% 14% 4 8% 4 3 4 4 5% 6% 29%	1890
May	1st to 10th	Night	25% 12% 4 8% 5% 4 5% 3 5% 6% 22%	4144
	11th to 20th	Night	21% 11% 2 7% 5% 5% 4 3 5% 6% 30%	3718
	21st to end	Night	20% 9% 3 6% 4 4 5% 4 5% 8% 32%	3926
June	1st to 10th	Night	26% 10% 3 7% 6% 4 4 3 4 7% 27%	1489
	11th to 20th	Night	19% 8% 3 6% 5% 4 4 4 6% 8% 33%	1400
	21st to end	Night	19% 8% 3 7% 5% 5% 5% 5% 6% 6% 30%	1492
July	1st to 10th	Night	13% 10% 4 10% 6% 6% 5% 4 5% 8% 29%	3600
	11th to 20th	Night	16% 7% 3 7% 5% 5% 5% 5% 5% 7% 36%	3600
	21st to end	Night	10% 8% 2 7% 4 5% 6% 5% 7% 8% 38%	3926
August	1st to 10th	Night	20% 8% 3 8% 4 4 5% 4 5% 7% 32%	2679
	11th to 20th	Night	13% 8% 4 9% 6% 6% 6% 5% 7% 7% 30%	2872
	21st to end	Night	16% 8% 2 7% 4 4 5% 4 5% 6% 39%	3439
September	1st to 10th	Night	15% 8% 2 7% 4 4 4 3 5% 6% 42%	2001
	11th to 20th	Night	17% 7% 2 5% 4 3 4 4 5% 5% 45%	2200
	21st to end	Night	18% 8% 3 5% 4 3 4 3 4 7% 41%	2200
October	1st to 10th	Night	17% 10% 3 6% 4 4 4 3 5% 7% 37%	2200
	11th to 20th	Night	19% 14% 3 8% 5% 4 4 3 5% 5% 29%	2530
	21st to end	Night	22% 17% 4 8% 4 4 5% 3 4 5% 24%	2860
November	1st to 10th	Night	19% 17% 6% 9% 5% 4 5% 3 4 5% 24%	2667
	11th to 20th	Night	15% 16% 7% 13% 6% 4 3 3 4 5% 24%	2730
	21st to end	Night	14% 17% 8% 15% 8% 5% 4 3 4 5% 18%	2934
December	1st to 10th	Night	12% 18% 8% 22% 12% 5% 4 3 4 3 9%	5060
	11th to 20th	Night	16% 17% 7% 17% 9% 6% 4 3 3 4 13%	5741
	21st to end	Night	16% 18% 7% 11% 6% 4 4 3 4 6% 21%	7291

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% 5% 7% 7% 2 1	4662
	11th to 20th	Day	79% 5% 3 8% 3 1	4680
	21st to end	Day	70% 6% 6% 8% 9% 1	5124
February	1st to 10th	Day	73% 3 4 10% 8% 1	1742
	11th to 20th	Day	73% 3 7% 11% 6% 1	2057
	21st to end	Day	67% 4 7% 13% 7% 2	1730
March	1st to 10th	Day	69% 4 6% 9% 8% 4	4579
	11th to 20th	Day	61% 5% 12% 13% 9% 1	4898
	21st to end	Day	63% 5% 5% 13% 12% 1	5866
April	1st to 10th	Day	65% 3 8% 13% 9% 2	2520
	11th to 20th	Day	57% 3 7% 19% 12% 1	2592
	21st to end	Day	59% 4 8% 16% 10% 2	2730
May	1st to 10th	Day	66% 6% 8% 14% 5% 1	6027
	11th to 20th	Day	56% 8% 13% 16% 5% 2	6354
	21st to end	Day	52% 8% 11% 19% 9% 1	6949
June	1st to 10th	Day	58% 7% 9% 20% 6%	2905
	11th to 20th	Day	49% 7% 12% 22% 9% 1	3000
	21st to end	Day	57% 8% 10% 18% 6%	2898
July	1st to 10th	Day	51% 9% 16% 18% 6%	6290
	11th to 20th	Day	44% 9% 14% 28% 5%	6285
	21st to end	Day	43% 7% 14% 29% 7%	6728
August	1st to 10th	Day	52% 8% 15% 20% 4	4424
	11th to 20th	Day	53% 9% 16% 17% 5%	4214
	21st to end	Day	46% 8% 14% 19% 11% 2	4456
September	1st to 10th	Day	43% 8% 13% 26% 10%	2398
	11th to 20th	Day	43% 5% 11% 27% 14% 1	2200
	21st to end	Day	48% 6% 12% 23% 11% 1	2200
October	1st to 10th	Day	50% 6% 13% 22% 8%	3590
	11th to 20th	Day	55% 5% 11% 23% 6%	3005
	21st to end	Day	54% 5% 13% 18% 8%	3180
November	1st to 10th	Day	62% 5% 9% 15% 7% 1	1851
	11th to 20th	Day	67% 5% 8% 12% 6% 2	1890
	21st to end	Day	70% 6% 9% 12% 3	1679
December	1st to 10th	Day	79% 4 6% 7% 4	2350
	11th to 20th	Day	66% 5% 7% 11% 11% 1	2688
	21st to end	Day	70% 4 6% 9% 9% 2	3896
January	1st to 10th	Night	78% 7% 6% 6% 2	8234
	11th to 20th	Night	82% 5% 4 6% 3	8280
	21st to end	Night	67% 7% 7% 10% 8% 1	9057
February	1st to 10th	Night	68% 6% 7% 12% 7% 1	2872
	11th to 20th	Night	71% 4 8% 10% 6% 1	2460
	21st to end	Night	64% 5% 6% 15% 9% 1	2076
March	1st to 10th	Night	70% 5% 7% 11% 6% 2	5494
	11th to 20th	Night	63% 5% 13% 12% 6% 1	5281
	21st to end	Night	62% 6% 5% 14% 13% 1	5293
April	1st to 10th	Night	61% 4 7% 15% 11% 1	2100
	11th to 20th	Night	56% 6% 8% 18% 11% 1	1977
	21st to end	Night	56% 7% 9% 17% 10% 2	1890
May	1st to 10th	Night	61% 8% 12% 13% 6%	4150
	11th to 20th	Night	54% 7% 11% 22% 5% 1	3719
	21st to end	Night	49% 7% 14% 17% 12%	3927
June	1st to 10th	Night	57% 5% 11% 20% 7%	1489
	11th to 20th	Night	47% 8% 12% 22% 11% 1	1400
	21st to end	Night	52% 9% 12% 19% 8%	1492
July	1st to 10th	Night	50% 9% 16% 19% 5%	3560
	11th to 20th	Night	44% 10% 15% 26% 6%	3552
	21st to end	Night	41% 9% 17% 27% 6%	3809
August	1st to 10th	Night	51% 7% 15% 23% 4	2669
	11th to 20th	Night	47% 12% 17% 17% 6%	2899
	21st to end	Night	44% 8% 13% 21% 13% 1	3439
September	1st to 10th	Night	42% 7% 13% 27% 10%	2001
	11th to 20th	Night	41% 5% 14% 28% 11% 1	2200
	21st to end	Night	44% 8% 11% 25% 11% 1	2200
October	1st to 10th	Night	46% 7% 12% 22% 11% 1	3590
	11th to 20th	Night	54% 6% 11% 22% 7%	4118
	21st to end	Night	53% 7% 13% 18% 7% 2	4610
November	1st to 10th	Night	63% 5% 10% 15% 6% 1	2667
	11th to 20th	Night	64% 5% 10% 12% 8% 1	2730
	21st to end	Night	70% 6% 9% 11% 4	2934
December	1st to 10th	Night	77% 8% 6% 6% 2	5060
	11th to 20th	Night	75% 4 5% 10% 6%	5742
	21st to end	Night	66% 5% 7% 12% 9% 1	7291

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	31% 30% 30% 9%	220
	11th to 20th	Day	36% 23% 26% 13% 2	220
	21st to end	Day	29% 22% 34% 13% 2	242
February	1st to 10th	Day	30% 29% 27% 10% 3 1	282
	11th to 20th	Day	30% 29% 31% 8% 2	536
	21st to end	Day	33% 25% 28% 13% 2	446
March	1st to 10th	Day	30% 27% 29% 10% 4	540
	11th to 20th	Day	32% 28% 24% 13% 4	540
	21st to end	Day	36% 21% 25% 12% 6%	594
April	1st to 10th	Day	38% 23% 24% 10% 4	540
	11th to 20th	Day	34% 24% 25% 11% 4 1	532
	21st to end	Day	35% 21% 26% 12% 4 2	540
May	1st to 10th	Day	38% 17% 29% 12% 3 1	543
	11th to 20th	Day	26% 22% 28% 17% 7% 1	551
	21st to end	Day	22% 23% 34% 16% 4 1	573
June	1st to 10th	Day	33% 17% 28% 16% 6%	635
	11th to 20th	Day	23% 17% 27% 23% 9% 1	730
	21st to end	Day	25% 16% 32% 18% 8% 1	628
July	1st to 10th	Day	19% 15% 36% 22% 7% 1	525
	11th to 20th	Day	19% 17% 37% 19% 7% 2	524
	21st to end	Day	19% 13% 37% 24% 5% 1	576
August	1st to 10th	Day	18% 14% 31% 24% 12% 2	526
	11th to 20th	Day	18% 12% 29% 24% 12% 4	519
	21st to end	Day	22% 17% 25% 20% 11% 4	583
September	1st to 10th	Day	21% 16% 28% 20% 12% 4	530
	11th to 20th	Day	21% 10% 25% 20% 14% 8%	530
	21st to end	Day	22% 16% 25% 22% 12% 4	530
October	1st to 10th	Day	24% 16% 26% 17% 11% 5%	530
	11th to 20th	Day	29% 18% 27% 16% 9% 2	434
	21st to end	Day	35% 22% 23% 16% 5%	462
November	1st to 10th	Day	34% 22% 25% 15% 3	425
	11th to 20th	Day	28% 23% 30% 12% 5% 1	430
	21st to end	Day	34% 25% 27% 11% 2 1	313
December	1st to 10th	Day	26% 28% 32% 12% 2	220
	11th to 20th	Day	37% 28% 26% 5% 3	220
	21st to end	Day	34% 30% 22% 9% 5%	242
January	1st to 10th	Night	25% 28% 31% 13% 2	840
	11th to 20th	Night	25% 27% 30% 15% 3	840
	21st to end	Night	18% 27% 35% 16% 4 1	924
February	1st to 10th	Night	21% 28% 33% 16% 3	840
	11th to 20th	Night	25% 25% 32% 16% 2	823
	21st to end	Night	25% 22% 34% 17% 2	692
March	1st to 10th	Night	27% 22% 32% 17% 2	840
	11th to 20th	Night	28% 23% 28% 17% 4	840
	21st to end	Night	27% 21% 26% 21% 6% 1	924
April	1st to 10th	Night	34% 19% 26% 16% 4	840
	11th to 20th	Night	28% 19% 25% 22% 5%	733
	21st to end	Night	30% 19% 28% 19% 4 1	630
May	1st to 10th	Night	28% 15% 29% 21% 7% 1	630
	11th to 20th	Night	22% 13% 31% 26% 7% 1	449
	21st to end	Night	23% 12% 29% 27% 8% 1	433
June	1st to 10th	Night	30% 8% 29% 26% 8% 1	400
	11th to 20th	Night	22% 8% 23% 33% 14%	400
	21st to end	Night	21% 9% 28% 32% 10%	400
July	1st to 10th	Night	16% 8% 29% 35% 11% 1	390
	11th to 20th	Night	18% 9% 25% 35% 13% 1	388
	21st to end	Night	19% 6% 20% 39% 15%	438
August	1st to 10th	Night	19% 4 18% 38% 20% 1	452
	11th to 20th	Night	16% 8% 23% 33% 19% 1	592
	21st to end	Night	18% 9% 25% 30% 17% 2	785
September	1st to 10th	Night	19% 9% 24% 32% 15% 1	800
	11th to 20th	Night	21% 8% 20% 28% 21% 3	800
	21st to end	Night	23% 10% 26% 24% 16% 2	800
October	1st to 10th	Night	22% 13% 23% 29% 13% 1	800
	11th to 20th	Night	30% 15% 23% 22% 9% 1	800
	21st to end	Night	33% 18% 27% 18% 4	880
November	1st to 10th	Night	34% 19% 24% 18% 5%	820
	11th to 20th	Night	27% 21% 28% 16% 8% 1	840
	21st to end	Night	28% 26% 26% 15% 4 1	887
December	1st to 10th	Night	28% 25% 29% 15% 4	950
	11th to 20th	Night	30% 26% 25% 13% 5% 1	950
	21st to end	Night	33% 23% 26% 15% 3	957

Page generated at 2026-05-14 01:58:59. Version ac07

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