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 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	14% 10% 4 8% 5% 5% 5% 4 6% 7% 33%	105355
	Night	15% 11% 4 8% 5% 4 4 4 5% 6% 34%	103988
Transparency	Day	48% 7% 12% 21% 12% 1	109011
	Night	48% 6% 11% 20% 12% 2	108471
Seeing	Day	18% 11% 27% 23% 15% 6%	15729
	Night	19% 10% 29% 28% 13% 1	25591

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	16% 18% 9% 14% 5% 4 4 3 5% 5% 17%	2607
	11th to 20th	Day	14% 20% 7% 13% 6% 5% 8% 5% 4 3 15%	2714
	21st to end	Day	25% 20% 8% 14% 6% 4 4 2 2 4 11%	3133
February	1st to 10th	Day	18% 16% 6% 12% 7% 4 4 3 4 6% 21%	1727
	11th to 20th	Day	15% 12% 6% 15% 8% 5% 4 3 4 5% 23%	1848
	21st to end	Day	17% 14% 5% 10% 5% 4 4 3 6% 7% 26%	1570
March	1st to 10th	Day	20% 18% 6% 11% 6% 3 3 3 3 5% 21%	3179
	11th to 20th	Day	13% 11% 4 8% 5% 4 5% 4 5% 6% 35%	3480
	21st to end	Day	19% 13% 5% 10% 6% 5% 4 3 4 5% 27%	3815
April	1st to 10th	Day	13% 15% 4 8% 5% 4 5% 4 5% 7% 30%	2215
	11th to 20th	Day	16% 10% 3 7% 5% 4 4 4 5% 7% 35%	2361
	21st to end	Day	19% 8% 3 8% 5% 4 4 4 6% 7% 33%	2470
May	1st to 10th	Day	18% 11% 3 8% 5% 4 5% 4 6% 7% 29%	4082
	11th to 20th	Day	17% 7% 3 6% 4 4 4 4 6% 7% 36%	4081
	21st to end	Day	9% 6% 3 7% 5% 5% 6% 4 7% 9% 40%	4476
June	1st to 10th	Day	12% 6% 3 8% 6% 5% 5% 4 6% 8% 39%	2543
	11th to 20th	Day	10% 5% 3 6% 5% 5% 6% 4 7% 8% 41%	2660
	21st to end	Day	10% 6% 2 6% 5% 5% 5% 5% 9% 9% 38%	2660
July	1st to 10th	Day	11% 7% 3 8% 5% 5% 5% 4 6% 11% 36%	4440
	11th to 20th	Day	7% 5% 2 5% 4 4 5% 4 7% 10% 46%	4381
	21st to end	Day	7% 6% 2 6% 6% 5% 6% 5% 8% 9% 40%	4466
August	1st to 10th	Day	9% 5% 2 7% 4 4 5% 5% 7% 10% 43%	4104
	11th to 20th	Day	8% 5% 2 7% 4 5% 6% 5% 8% 10% 39%	4044
	21st to end	Day	7% 5% 2 6% 4 5% 5% 4 7% 10% 44%	4355
September	1st to 10th	Day	12% 7% 2 6% 4 4 4 3 5% 7% 46%	2149
	11th to 20th	Day	8% 5% 2 6% 4 6% 4 4 6% 9% 45%	2090
	21st to end	Day	16% 8% 2 6% 3 4 4 3 5% 7% 42%	2090
October	1st to 10th	Day	12% 7% 3 6% 5% 4 4 3 5% 7% 44%	2074
	11th to 20th	Day	15% 8% 2 6% 5% 5% 5% 4 5% 7% 39%	1900
	21st to end	Day	18% 9% 3 8% 5% 6% 4 4 5% 7% 31%	1950
November	1st to 10th	Day	11% 9% 2 6% 4 4 4 4 4 5% 48%	1762
	11th to 20th	Day	16% 14% 4 7% 5% 5% 4 3 5% 6% 32%	1800
	21st to end	Day	19% 16% 4 9% 4 3 3 3 5% 6% 28%	1800
December	1st to 10th	Day	14% 15% 7% 13% 6% 5% 3 4 4 5% 26%	2930
	11th to 20th	Day	16% 18% 7% 12% 5% 5% 4 3 4 5% 20%	3373
	21st to end	Day	23% 20% 8% 12% 5% 5% 4 2 3 4 13%	4026
January	1st to 10th	Night	15% 18% 8% 14% 6% 5% 4 3 4 4 20%	4568
	11th to 20th	Night	13% 19% 8% 18% 7% 5% 6% 5% 4 3 13%	4467
	21st to end	Night	26% 20% 7% 10% 5% 4 3 2 3 4 15%	4546
February	1st to 10th	Night	19% 14% 5% 11% 6% 5% 4 3 4 5% 24%	2451
	11th to 20th	Night	13% 14% 5% 10% 7% 5% 4 3 6% 6% 28%	2218
	21st to end	Night	20% 14% 4 10% 6% 4 4 4 4 5% 26%	1884
March	1st to 10th	Night	18% 15% 4 7% 4 4 3 3 3 6% 33%	3674
	11th to 20th	Night	11% 8% 3 5% 5% 5% 4 4 6% 8% 43%	3480
	21st to end	Night	17% 10% 4 8% 5% 4 4 3 5% 5% 35%	3817
April	1st to 10th	Night	15% 11% 3 7% 4 4 3 4 6% 6% 35%	1981
	11th to 20th	Night	17% 8% 3 7% 5% 5% 4 4 6% 6% 35%	1767
	21st to end	Night	17% 9% 3 7% 5% 4 4 3 6% 6% 36%	1710
May	1st to 10th	Night	19% 10% 3 8% 5% 5% 5% 4 6% 6% 30%	2813
	11th to 20th	Night	18% 7% 3 6% 4 4 4 3 5% 8% 38%	2812
	21st to end	Night	9% 5% 2 5% 4 4 5% 4 7% 10% 45%	3079
June	1st to 10th	Night	10% 6% 2 5% 4 4 5% 4 6% 7% 47%	1631
	11th to 20th	Night	8% 6% 2 6% 4 3 5% 4 6% 10% 46%	1520
	21st to end	Night	9% 5% 2 6% 5% 4 4 5% 6% 10% 44%	1520
July	1st to 10th	Night	10% 5% 2 6% 4 4 5% 3 6% 8% 47%	2530
	11th to 20th	Night	7% 4 1 4 3 4 5% 4 7% 10% 51%	2584
	21st to end	Night	8% 6% 2 7% 5% 4 5% 4 6% 8% 44%	3087
August	1st to 10th	Night	11% 8% 2 7% 3 4 4 4 6% 10% 40%	2825
	11th to 20th	Night	8% 7% 3 6% 4 4 5% 4 6% 7% 45%	2785
	21st to end	Night	4 5% 2 7% 5% 4 5% 4 6% 8% 49%	3284
September	1st to 10th	Night	11% 6% 2 4 3 4 4 3 5% 7% 51%	2010
	11th to 20th	Night	10% 5% 2 4 2 3 4 3 6% 7% 54%	2090
	21st to end	Night	12% 7% 2 6% 5% 4 3 4 5% 5% 46%	2090
October	1st to 10th	Night	10% 6% 2 5% 4 4 4 3 5% 6% 51%	2104
	11th to 20th	Night	16% 8% 3 6% 4 4 3 3 6% 6% 40%	2280
	21st to end	Night	20% 8% 3 6% 4 4 4 3 5% 7% 37%	2646
November	1st to 10th	Night	12% 8% 3 6% 5% 4 4 3 5% 6% 44%	2536
	11th to 20th	Night	15% 11% 4 9% 4 4 4 3 4 7% 35%	2600
	21st to end	Night	17% 14% 4 10% 6% 4 4 3 5% 6% 27%	2600
December	1st to 10th	Night	13% 12% 6% 10% 6% 6% 5% 4 6% 7% 25%	4250
	11th to 20th	Night	17% 16% 8% 12% 5% 4 4 4 4 5% 21%	4822
	21st to end	Night	22% 20% 7% 10% 5% 5% 4 3 4 5% 14%	6927

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	68% 7% 8% 12% 6%	2608
	11th to 20th	Day	74% 4 8% 8% 5%	2714
	21st to end	Day	79% 4 7% 6% 3	3133
February	1st to 10th	Day	65% 5% 11% 12% 6% 1	1727
	11th to 20th	Day	63% 5% 8% 16% 7%	1858
	21st to end	Day	57% 6% 13% 15% 7% 2	1570
March	1st to 10th	Day	65% 6% 9% 15% 5%	3179
	11th to 20th	Day	46% 8% 12% 24% 9% 1	3480
	21st to end	Day	56% 7% 14% 17% 5%	3815
April	1st to 10th	Day	51% 7% 12% 17% 12% 1	2215
	11th to 20th	Day	48% 6% 12% 20% 12% 2	2361
	21st to end	Day	49% 7% 13% 19% 10% 2	2470
May	1st to 10th	Day	51% 9% 11% 21% 8% 1	4120
	11th to 20th	Day	44% 10% 14% 23% 9% 1	4085
	21st to end	Day	36% 12% 14% 19% 16% 3	4489
June	1st to 10th	Day	41% 8% 11% 24% 13% 2	2543
	11th to 20th	Day	37% 9% 13% 23% 16% 2	2660
	21st to end	Day	37% 10% 16% 18% 14% 4	2660
July	1st to 10th	Day	42% 6% 11% 22% 16% 2	4371
	11th to 20th	Day	31% 5% 15% 28% 19% 1	4306
	21st to end	Day	37% 7% 15% 19% 18% 3	4324
August	1st to 10th	Day	35% 6% 16% 26% 16% 1	4091
	11th to 20th	Day	36% 7% 12% 31% 13% 1	4083
	21st to end	Day	33% 6% 13% 32% 12% 2	4354
September	1st to 10th	Day	37% 7% 12% 22% 19% 4	2149
	11th to 20th	Day	34% 7% 11% 25% 20% 3	2090
	21st to end	Day	41% 6% 11% 22% 19% 1	2090
October	1st to 10th	Day	28% 4 12% 34% 21% 1	3451
	11th to 20th	Day	36% 5% 10% 25% 22% 1	3140
	21st to end	Day	42% 4 11% 23% 19% 1	3187
November	1st to 10th	Day	38% 5% 12% 25% 18% 1	1762
	11th to 20th	Day	53% 5% 10% 20% 11% 2	1800
	21st to end	Day	58% 5% 11% 18% 8% 1	1800
December	1st to 10th	Day	62% 4 13% 19% 3	2930
	11th to 20th	Day	65% 5% 12% 14% 4	3370
	21st to end	Day	76% 4 6% 9% 4	4026
January	1st to 10th	Night	66% 7% 7% 14% 5%	4568
	11th to 20th	Night	74% 6% 8% 10% 3	4467
	21st to end	Night	73% 5% 8% 9% 4	4546
February	1st to 10th	Night	62% 7% 10% 14% 7% 1	2451
	11th to 20th	Night	56% 6% 12% 17% 9% 1	2219
	21st to end	Night	60% 6% 9% 14% 10% 2	1884
March	1st to 10th	Night	53% 5% 11% 22% 9% 1	3674
	11th to 20th	Night	37% 7% 15% 26% 13% 1	3480
	21st to end	Night	48% 8% 11% 24% 9% 1	3817
April	1st to 10th	Night	47% 7% 12% 20% 13% 1	1981
	11th to 20th	Night	46% 7% 13% 15% 16% 3	1767
	21st to end	Night	46% 7% 12% 18% 14% 3	1710
May	1st to 10th	Night	52% 9% 11% 17% 10% 2	2840
	11th to 20th	Night	41% 10% 14% 24% 11% 1	2813
	21st to end	Night	30% 11% 16% 25% 17% 2	3087
June	1st to 10th	Night	35% 7% 14% 23% 19% 3	1631
	11th to 20th	Night	32% 10% 15% 25% 16% 3	1520
	21st to end	Night	33% 7% 18% 23% 17% 3	1520
July	1st to 10th	Night	35% 6% 11% 27% 15% 6%	2490
	11th to 20th	Night	27% 5% 16% 30% 21% 1	2536
	21st to end	Night	37% 6% 16% 19% 17% 5%	2988
August	1st to 10th	Night	40% 6% 15% 23% 15% 1	2814
	11th to 20th	Night	37% 4 12% 30% 16% 1	2812
	21st to end	Night	31% 7% 15% 29% 16% 3	3284
September	1st to 10th	Night	32% 8% 12% 26% 17% 6%	2010
	11th to 20th	Night	28% 7% 11% 25% 26% 3	2090
	21st to end	Night	38% 6% 12% 22% 19% 2	2090
October	1st to 10th	Night	24% 4 8% 30% 32% 1	3507
	11th to 20th	Night	35% 4 10% 27% 23% 1	3779
	21st to end	Night	35% 4 8% 31% 19% 3	4359
November	1st to 10th	Night	40% 5% 11% 24% 18% 2	2536
	11th to 20th	Night	49% 5% 10% 20% 13% 3	2600
	21st to end	Night	58% 6% 11% 18% 7%	2600
December	1st to 10th	Night	57% 4 12% 20% 6% 1	4250
	11th to 20th	Night	65% 5% 13% 11% 5%	4824
	21st to end	Night	72% 5% 9% 10% 3	6927

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	24% 19% 34% 18% 3 1	300
	11th to 20th	Day	22% 20% 42% 12% 3 1	300
	21st to end	Day	28% 21% 32% 16% 4 1	300
February	1st to 10th	Day	30% 20% 31% 16% 3 1	270
	11th to 20th	Day	26% 19% 35% 17% 4	270
	21st to end	Day	24% 21% 44% 9% 2	225
March	1st to 10th	Day	25% 20% 34% 17% 4	303
	11th to 20th	Day	25% 16% 35% 18% 5% 2	460
	21st to end	Day	29% 16% 32% 18% 5%	506
April	1st to 10th	Day	22% 16% 31% 21% 7% 2	460
	11th to 20th	Day	22% 17% 28% 18% 10% 4	452
	21st to end	Day	23% 15% 29% 23% 8% 2	460
May	1st to 10th	Day	25% 11% 31% 22% 8% 3	463
	11th to 20th	Day	22% 9% 26% 24% 13% 6%	487
	21st to end	Day	9% 8% 25% 28% 20% 9%	529
June	1st to 10th	Day	11% 9% 28% 28% 19% 6%	563
	11th to 20th	Day	14% 6% 26% 28% 22% 6%	680
	21st to end	Day	14% 8% 26% 30% 17% 6%	680
July	1st to 10th	Day	13% 4 19% 28% 20% 15%	671
	11th to 20th	Day	8% 5% 23% 30% 22% 13%	635
	21st to end	Day	10% 2 17% 28% 27% 14%	532
August	1st to 10th	Day	12% 3 14% 28% 27% 16%	486
	11th to 20th	Day	12% 4 17% 30% 26% 11%	479
	21st to end	Day	8% 4 15% 28% 28% 17%	539
September	1st to 10th	Day	14% 3 16% 27% 26% 13%	490
	11th to 20th	Day	9% 3 16% 22% 32% 18%	490
	21st to end	Day	19% 10% 20% 24% 17% 10%	490
October	1st to 10th	Day	11% 8% 24% 29% 20% 9%	490
	11th to 20th	Day	17% 15% 30% 22% 13% 3	490
	21st to end	Day	22% 14% 38% 20% 6% 1	399
November	1st to 10th	Day	16% 14% 35% 22% 9% 3	300
	11th to 20th	Day	24% 19% 31% 16% 10% 1	300
	21st to end	Day	27% 17% 36% 18% 2	300
December	1st to 10th	Day	22% 20% 36% 14% 7% 1	300
	11th to 20th	Day	24% 18% 37% 18% 3	300
	21st to end	Day	27% 22% 34% 14% 3	330
January	1st to 10th	Night	22% 18% 37% 20% 4	1000
	11th to 20th	Night	26% 17% 38% 16% 3 1	926
	21st to end	Night	23% 17% 35% 20% 3	841
February	1st to 10th	Night	27% 12% 36% 22% 3	760
	11th to 20th	Night	22% 15% 37% 24% 3	743
	21st to end	Night	27% 14% 37% 19% 2	628
March	1st to 10th	Night	21% 13% 34% 25% 7% 1	760
	11th to 20th	Night	18% 15% 37% 25% 5% 1	760
	21st to end	Night	24% 12% 30% 26% 8% 1	836
April	1st to 10th	Night	20% 14% 35% 27% 4 1	651
	11th to 20th	Night	20% 14% 33% 24% 8% 1	563
	21st to end	Night	20% 9% 37% 23% 9% 1	570
May	1st to 10th	Night	22% 9% 30% 30% 9% 1	570
	11th to 20th	Night	23% 4 29% 30% 14%	561
	21st to end	Night	11% 4 24% 36% 22% 3	617
June	1st to 10th	Night	10% 4 25% 43% 18%	570
	11th to 20th	Night	11% 4 22% 42% 20% 1	570
	21st to end	Night	9% 4 23% 39% 23% 2	570
July	1st to 10th	Night	11% 2 14% 42% 27% 4	555
	11th to 20th	Night	8% 2 18% 39% 29% 4	552
	21st to end	Night	9% 2 15% 43% 30% 1	624
August	1st to 10th	Night	9% 2 14% 42% 29% 5%	567
	11th to 20th	Night	10% 2 13% 40% 31% 4	564
	21st to end	Night	6% 3 14% 38% 35% 5%	627
September	1st to 10th	Night	12% 4 15% 36% 29% 4	680
	11th to 20th	Night	9% 3 14% 36% 32% 5%	760
	21st to end	Night	16% 6% 21% 33% 22% 2	760
October	1st to 10th	Night	13% 5% 24% 35% 18% 4	760
	11th to 20th	Night	20% 12% 28% 27% 11% 1	760
	21st to end	Night	25% 12% 31% 25% 7%	836
November	1st to 10th	Night	16% 12% 34% 25% 12% 1	780
	11th to 20th	Night	22% 15% 31% 24% 7% 1	800
	21st to end	Night	24% 14% 32% 24% 6%	800
December	1st to 10th	Night	22% 16% 35% 21% 5%	800
	11th to 20th	Night	26% 13% 37% 19% 5%	800
	21st to end	Night	30% 16% 33% 19% 2	1070

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