ONU Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the ONU 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	13% 10% 4 9% 5% 5% 5% 4 6% 7% 32%	131935
	Night	14% 11% 4 9% 5% 5% 4 4 5% 6% 33%	130972
Transparency	Day	49% 6% 11% 21% 12% 1	135547
	Night	49% 5% 11% 20% 13% 1	135430
Seeing	Day	18% 11% 27% 23% 14% 6%	17509
	Night	19% 10% 29% 28% 13% 1	27341

By Month

Percentage Cloud Cover SE (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% 17% 9% 16% 6% 5% 4 3 5% 5% 16%	4691
	11th to 20th	Day	11% 16% 7% 15% 9% 7% 6% 4 4 3 19%	4914
	21st to end	Day	17% 14% 6% 14% 8% 6% 6% 3 4 5% 15%	5780
February	1st to 10th	Day	17% 15% 6% 13% 6% 4 4 3 4 6% 21%	1909
	11th to 20th	Day	14% 12% 7% 15% 8% 5% 4 3 4 5% 22%	2048
	21st to end	Day	15% 14% 5% 10% 5% 4 4 3 6% 7% 26%	1730
March	1st to 10th	Day	17% 14% 7% 13% 6% 4 4 3 3 5% 25%	4669
	11th to 20th	Day	11% 11% 3 7% 5% 4 4 4 5% 7% 38%	5090
	21st to end	Day	17% 14% 5% 10% 6% 5% 5% 4 4 5% 25%	5586
April	1st to 10th	Day	14% 15% 3 8% 5% 4 5% 4 6% 7% 29%	2449
	11th to 20th	Day	15% 10% 3 8% 5% 4 4 4 5% 7% 34%	2613
	21st to end	Day	18% 8% 3 8% 5% 4 5% 4 6% 7% 33%	2730
May	1st to 10th	Day	21% 10% 3 9% 5% 4 4 4 6% 6% 28%	5992
	11th to 20th	Day	16% 8% 3 6% 4 5% 5% 4 6% 7% 36%	5939
	21st to end	Day	15% 6% 3 7% 4 5% 5% 4 6% 8% 37%	6433
June	1st to 10th	Day	12% 6% 3 7% 5% 5% 5% 4 6% 8% 39%	2677
	11th to 20th	Day	10% 5% 3 6% 5% 5% 6% 5% 7% 8% 40%	2800
	21st to end	Day	10% 5% 2 6% 5% 5% 5% 5% 9% 9% 39%	2800
July	1st to 10th	Day	9% 6% 2 7% 5% 5% 5% 4 6% 10% 40%	6360
	11th to 20th	Day	8% 6% 2 6% 5% 5% 6% 5% 8% 11% 40%	6273
	21st to end	Day	9% 6% 2 7% 5% 5% 6% 5% 8% 9% 38%	6424
August	1st to 10th	Day	8% 5% 2 6% 4 4 5% 5% 7% 10% 43%	4234
	11th to 20th	Day	7% 5% 2 7% 4 5% 6% 5% 8% 10% 39%	4174
	21st to end	Day	7% 5% 2 6% 4 5% 5% 4 7% 10% 44%	4493
September	1st to 10th	Day	11% 7% 2 6% 4 4 4 3 5% 8% 47%	2262
	11th to 20th	Day	8% 5% 2 6% 4 5% 4 4 6% 9% 47%	2200
	21st to end	Day	16% 8% 2 6% 3 4 4 3 5% 7% 42%	2200
October	1st to 10th	Day	11% 7% 3 6% 5% 4 4 3 5% 7% 45%	2183
	11th to 20th	Day	15% 7% 2 6% 5% 5% 5% 4 5% 7% 40%	2000
	21st to end	Day	17% 10% 3 7% 5% 6% 4 4 5% 7% 32%	2052
November	1st to 10th	Day	11% 10% 2 6% 4 4 4 4 5% 5% 47%	1852
	11th to 20th	Day	16% 14% 4 7% 5% 5% 4 3 4 6% 33%	1890
	21st to end	Day	18% 16% 5% 9% 5% 3 3 3 4 6% 27%	1890
December	1st to 10th	Day	13% 15% 7% 13% 6% 5% 4 4 4 5% 25%	3020
	11th to 20th	Day	16% 18% 7% 12% 5% 5% 4 3 4 5% 20%	3463
	21st to end	Day	23% 20% 8% 13% 5% 5% 4 2 3 4 13%	4115
January	1st to 10th	Night	13% 17% 7% 16% 7% 5% 4 3 4 4 19%	8232
	11th to 20th	Night	9% 16% 8% 18% 8% 6% 6% 5% 4 4 16%	8059
	21st to end	Night	18% 13% 5% 9% 6% 5% 5% 4 5% 6% 23%	8401
February	1st to 10th	Night	17% 14% 5% 12% 6% 5% 5% 3 4 5% 25%	2709
	11th to 20th	Night	13% 13% 5% 11% 7% 5% 4 3 6% 5% 28%	2458
	21st to end	Night	19% 14% 4 11% 6% 4 4 4 4 5% 26%	2076
March	1st to 10th	Night	17% 14% 4 8% 4 4 3 3 4 5% 34%	5404
	11th to 20th	Night	10% 8% 3 6% 5% 5% 4 4 6% 8% 41%	5090
	21st to end	Night	16% 10% 4 9% 5% 4 4 3 5% 6% 36%	5588
April	1st to 10th	Night	16% 12% 3 7% 4 4 3 4 5% 6% 34%	2189
	11th to 20th	Night	16% 8% 3 7% 5% 5% 5% 4 6% 6% 35%	1955
	21st to end	Night	17% 9% 3 8% 5% 4 4 3 5% 6% 36%	1890
May	1st to 10th	Night	20% 10% 4 9% 5% 5% 4 4 5% 5% 29%	4123
	11th to 20th	Night	16% 7% 3 7% 5% 4 4 3 5% 9% 38%	4086
	21st to end	Night	13% 7% 2 6% 4 4 4 4 6% 9% 41%	4421
June	1st to 10th	Night	10% 7% 2 6% 4 4 5% 4 5% 7% 47%	1717
	11th to 20th	Night	9% 6% 2 6% 4 4 5% 5% 6% 10% 45%	1600
	21st to end	Night	9% 5% 2 6% 5% 4 4 5% 6% 10% 45%	1600
July	1st to 10th	Night	9% 5% 2 6% 4 4 5% 4 7% 9% 47%	3618
	11th to 20th	Night	7% 5% 2 6% 5% 4 5% 4 7% 10% 45%	3697
	21st to end	Night	9% 6% 2 7% 4 5% 5% 4 6% 8% 44%	4427
August	1st to 10th	Night	10% 8% 2 7% 3 4 4 4 6% 10% 41%	2915
	11th to 20th	Night	8% 7% 3 6% 4 4 5% 4 6% 7% 45%	2875
	21st to end	Night	4 5% 2 7% 5% 4 5% 4 6% 8% 49%	3388
September	1st to 10th	Night	11% 5% 2 4 3 4 3 3 5% 7% 52%	2116
	11th to 20th	Night	9% 5% 2 4 2 3 4 3 6% 7% 55%	2200
	21st to end	Night	12% 7% 2 7% 4 4 3 4 5% 5% 46%	2200
October	1st to 10th	Night	10% 6% 2 5% 4 4 4 3 5% 6% 51%	2215
	11th to 20th	Night	16% 8% 3 6% 4 4 3 3 5% 6% 40%	2400
	21st to end	Night	20% 8% 3 6% 4 4 4 3 5% 7% 37%	2785
November	1st to 10th	Night	12% 8% 3 6% 5% 4 4 3 5% 6% 44%	2666
	11th to 20th	Night	15% 11% 4 9% 5% 4 4 3 4 7% 35%	2730
	21st to end	Night	18% 14% 4 10% 6% 4 4 3 5% 6% 27%	2730
December	1st to 10th	Night	13% 12% 5% 10% 6% 6% 5% 4 6% 7% 25%	4380
	11th to 20th	Night	17% 16% 8% 12% 5% 4 4 4 4 5% 21%	4952
	21st to end	Night	22% 20% 7% 11% 5% 5% 4 3 4 5% 14%	7080

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	71% 4 10% 12% 3	4692
	11th to 20th	Day	71% 3 7% 7% 11% 1	4914
	21st to end	Day	72% 3 8% 11% 6%	5780
February	1st to 10th	Day	65% 5% 11% 13% 6% 1	1909
	11th to 20th	Day	64% 5% 8% 16% 7%	2058
	21st to end	Day	56% 5% 14% 16% 7% 2	1730
March	1st to 10th	Day	62% 5% 7% 19% 7%	4669
	11th to 20th	Day	44% 6% 11% 27% 10% 1	5090
	21st to end	Day	58% 6% 12% 17% 6% 1	5586
April	1st to 10th	Day	52% 7% 12% 16% 12%	2449
	11th to 20th	Day	48% 6% 12% 20% 12% 2	2613
	21st to end	Day	48% 7% 13% 20% 11% 2	2730
May	1st to 10th	Day	55% 7% 10% 17% 11% 1	6027
	11th to 20th	Day	45% 8% 14% 22% 11% 1	5943
	21st to end	Day	41% 8% 12% 22% 14% 2	6447
June	1st to 10th	Day	41% 8% 11% 23% 14% 2	2677
	11th to 20th	Day	38% 9% 14% 23% 15% 2	2800
	21st to end	Day	37% 10% 16% 19% 14% 4	2800
July	1st to 10th	Day	38% 5% 9% 28% 17% 2	6291
	11th to 20th	Day	38% 5% 17% 25% 16% 1	6198
	21st to end	Day	40% 5% 14% 22% 17% 3	6240
August	1st to 10th	Day	34% 6% 17% 27% 15% 1	4221
	11th to 20th	Day	36% 7% 12% 31% 13% 1	4213
	21st to end	Day	34% 6% 13% 32% 13% 2	4492
September	1st to 10th	Day	36% 6% 12% 24% 19% 4	2262
	11th to 20th	Day	33% 7% 10% 26% 20% 3	2200
	21st to end	Day	41% 5% 11% 22% 20% 1	2200
October	1st to 10th	Day	27% 4 12% 34% 22% 1	3560
	11th to 20th	Day	36% 4 10% 26% 22% 1	3240
	21st to end	Day	42% 4 11% 23% 19% 1	3289
November	1st to 10th	Day	38% 5% 12% 25% 18% 1	1852
	11th to 20th	Day	53% 4 10% 20% 11% 2	1890
	21st to end	Day	59% 4 11% 18% 8% 1	1890
December	1st to 10th	Day	62% 4 13% 18% 3	3020
	11th to 20th	Day	65% 5% 12% 14% 4	3460
	21st to end	Day	76% 4 6% 9% 4	4115
January	1st to 10th	Night	69% 4 7% 16% 4	8233
	11th to 20th	Night	70% 4 7% 10% 8% 1	8059
	21st to end	Night	61% 3 12% 15% 7% 1	8401
February	1st to 10th	Night	61% 6% 10% 15% 7% 1	2709
	11th to 20th	Night	57% 5% 12% 17% 8% 1	2459
	21st to end	Night	59% 6% 10% 14% 9% 1	2076
March	1st to 10th	Night	53% 4 9% 24% 9% 1	5404
	11th to 20th	Night	39% 5% 14% 28% 12% 1	5090
	21st to end	Night	48% 6% 10% 22% 13% 1	5588
April	1st to 10th	Night	49% 6% 12% 20% 13% 1	2189
	11th to 20th	Night	46% 7% 13% 15% 16% 3	1955
	21st to end	Night	47% 6% 12% 20% 13% 2	1890
May	1st to 10th	Night	55% 7% 10% 14% 11% 3	4150
	11th to 20th	Night	42% 8% 12% 23% 14% 1	4087
	21st to end	Night	38% 8% 14% 25% 14% 2	4429
June	1st to 10th	Night	35% 7% 13% 23% 19% 3	1717
	11th to 20th	Night	33% 9% 15% 25% 16% 3	1600
	21st to end	Night	32% 7% 18% 23% 17% 3	1600
July	1st to 10th	Night	35% 5% 12% 26% 18% 4	3580
	11th to 20th	Night	35% 5% 16% 27% 17% 1	3652
	21st to end	Night	37% 5% 15% 20% 20% 4	4297
August	1st to 10th	Night	39% 6% 15% 24% 15% 1	2904
	11th to 20th	Night	37% 4 11% 31% 16% 1	2902
	21st to end	Night	31% 7% 14% 29% 16% 3	3388
September	1st to 10th	Night	31% 7% 11% 27% 18% 6%	2116
	11th to 20th	Night	28% 6% 10% 27% 26% 3	2200
	21st to end	Night	39% 6% 12% 22% 20% 3	2200
October	1st to 10th	Night	24% 4 8% 30% 33% 1	3618
	11th to 20th	Night	35% 4 10% 27% 23% 1	3899
	21st to end	Night	35% 4 8% 31% 19% 3	4498
November	1st to 10th	Night	40% 4 11% 24% 18% 2	2666
	11th to 20th	Night	49% 5% 10% 21% 13% 2	2730
	21st to end	Night	58% 5% 11% 18% 7%	2730
December	1st to 10th	Night	58% 4 12% 20% 6% 1	4380
	11th to 20th	Night	65% 5% 13% 11% 5%	4954
	21st to end	Night	72% 5% 9% 10% 3	7080

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% 18% 35% 18% 3 1	330
	11th to 20th	Day	22% 21% 42% 12% 3 1	330
	21st to end	Day	25% 22% 32% 17% 4 1	363
February	1st to 10th	Day	27% 18% 34% 15% 4 1	330
	11th to 20th	Day	25% 18% 35% 18% 4	330
	21st to end	Day	22% 21% 44% 11% 2	273
March	1st to 10th	Day	25% 18% 37% 16% 4	367
	11th to 20th	Day	26% 16% 34% 17% 6% 1	540
	21st to end	Day	27% 17% 34% 18% 4	594
April	1st to 10th	Day	24% 16% 32% 19% 6% 2	540
	11th to 20th	Day	21% 18% 30% 18% 10% 3	532
	21st to end	Day	22% 15% 28% 25% 9% 2	540
May	1st to 10th	Day	25% 11% 32% 22% 7% 2	543
	11th to 20th	Day	22% 9% 26% 25% 13% 5%	551
	21st to end	Day	11% 9% 26% 27% 19% 8%	573
June	1st to 10th	Day	12% 8% 28% 29% 18% 6%	607
	11th to 20th	Day	14% 6% 25% 28% 21% 5%	730
	21st to end	Day	13% 7% 25% 31% 18% 6%	730
July	1st to 10th	Day	13% 4 18% 29% 21% 15%	721
	11th to 20th	Day	8% 5% 23% 30% 23% 12%	683
	21st to end	Day	11% 2 17% 27% 28% 15%	576
August	1st to 10th	Day	11% 3 13% 28% 28% 17%	526
	11th to 20th	Day	12% 4 16% 30% 27% 12%	519
	21st to end	Day	8% 4 16% 28% 28% 17%	583
September	1st to 10th	Day	13% 4 17% 26% 26% 14%	530
	11th to 20th	Day	9% 3 15% 21% 33% 19%	530
	21st to end	Day	19% 10% 19% 24% 19% 10%	530
October	1st to 10th	Day	11% 7% 23% 28% 21% 10%	530
	11th to 20th	Day	17% 14% 29% 24% 14% 2	530
	21st to end	Day	22% 13% 38% 20% 5% 1	435
November	1st to 10th	Day	17% 14% 35% 22% 8% 3	330
	11th to 20th	Day	24% 20% 31% 15% 9% 1	330
	21st to end	Day	28% 17% 36% 17% 2	330
December	1st to 10th	Day	21% 20% 36% 15% 6% 1	330
	11th to 20th	Day	24% 18% 37% 18% 3	330
	21st to end	Day	27% 21% 35% 14% 3	363
January	1st to 10th	Night	22% 17% 36% 20% 4	1050
	11th to 20th	Night	25% 18% 38% 15% 3 1	972
	21st to end	Night	23% 18% 36% 20% 3	924
February	1st to 10th	Night	25% 12% 36% 23% 3	840
	11th to 20th	Night	22% 14% 37% 24% 2	823
	21st to end	Night	25% 14% 40% 19% 2	692
March	1st to 10th	Night	21% 13% 35% 25% 6%	840
	11th to 20th	Night	18% 15% 37% 24% 5% 1	840
	21st to end	Night	24% 12% 32% 25% 7% 1	924
April	1st to 10th	Night	21% 14% 35% 26% 4 1	719
	11th to 20th	Night	19% 15% 33% 24% 8% 1	623
	21st to end	Night	20% 9% 37% 25% 9% 1	630
May	1st to 10th	Night	23% 8% 32% 29% 8% 1	630
	11th to 20th	Night	22% 5% 30% 30% 13% 1	609
	21st to end	Night	12% 4 24% 37% 21% 3	650
June	1st to 10th	Night	11% 3 26% 42% 17%	600
	11th to 20th	Night	11% 4 22% 43% 20% 1	600
	21st to end	Night	10% 4 23% 38% 24% 2	600
July	1st to 10th	Night	11% 2 14% 42% 28% 4	585
	11th to 20th	Night	8% 2 18% 39% 29% 4	582
	21st to end	Night	10% 2 14% 42% 30% 2	657
August	1st to 10th	Night	9% 2 13% 42% 30% 4	597
	11th to 20th	Night	10% 2 13% 40% 32% 3	594
	21st to end	Night	6% 3 13% 39% 35% 4	660
September	1st to 10th	Night	11% 4 16% 35% 29% 4	716
	11th to 20th	Night	9% 3 14% 36% 33% 5%	800
	21st to end	Night	16% 5% 21% 33% 22% 2	800
October	1st to 10th	Night	13% 5% 23% 35% 20% 4	800
	11th to 20th	Night	20% 12% 28% 29% 11% 1	800
	21st to end	Night	24% 12% 31% 26% 7%	880
November	1st to 10th	Night	16% 11% 36% 25% 12% 1	820
	11th to 20th	Night	21% 16% 31% 24% 7% 1	840
	21st to end	Night	24% 15% 31% 24% 6%	840
December	1st to 10th	Night	22% 15% 36% 21% 5%	840
	11th to 20th	Night	25% 13% 37% 19% 4	840
	21st to end	Night	30% 16% 33% 19% 2	1124

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

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