Two Rivers Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Two Rivers Observatory clear sky chart from 2005-11-06 to 2025-11-18.

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	20% 13% 4 9% 5% 5% 4 3 5% 6% 25%	127331
	Night	20% 12% 4 10% 5% 5% 4 4 5% 6% 25%	123273
Transparency	Day	58% 6% 12% 17% 7%	130792
	Night	57% 6% 12% 17% 7% 1	127909
Seeing	Day	27% 16% 30% 19% 7% 1	19125
	Night	27% 13% 30% 23% 7% 1	26431

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% 21% 7% 18% 8% 6% 4 4 3 5% 13%	3616
	11th to 20th	Day	13% 17% 10% 18% 7% 5% 4 3 4 5% 15%	3591
	21st to end	Day	12% 11% 8% 14% 8% 8% 5% 4 4 5% 21%	4043
February	1st to 10th	Day	19% 12% 4 14% 10% 7% 4 4 6% 5% 15%	1800
	11th to 20th	Day	16% 14% 6% 11% 7% 7% 5% 4 5% 5% 19%	1771
	21st to end	Day	13% 16% 6% 10% 8% 5% 3 3 4 5% 29%	1650
March	1st to 10th	Day	22% 13% 5% 11% 6% 6% 3 3 3 4 24%	4474
	11th to 20th	Day	13% 23% 8% 11% 6% 4 4 3 4 4 20%	4904
	21st to end	Day	27% 16% 6% 11% 5% 5% 4 3 4 5% 15%	5824
April	1st to 10th	Day	23% 15% 5% 9% 6% 4 4 3 5% 4 22%	2400
	11th to 20th	Day	22% 14% 4 8% 3 4 3 3 4 5% 30%	2374
	21st to end	Day	25% 16% 4 7% 4 4 3 3 4 5% 26%	2566
May	1st to 10th	Day	26% 20% 3 7% 4 3 3 3 4 5% 22%	6345
	11th to 20th	Day	21% 15% 3 7% 4 4 3 3 5% 6% 29%	6318
	21st to end	Day	22% 13% 3 7% 4 4 3 3 5% 7% 30%	6948
June	1st to 10th	Day	30% 11% 3 7% 5% 4 4 4 5% 7% 21%	2800
	11th to 20th	Day	21% 10% 3 6% 5% 4 4 4 5% 7% 31%	2800
	21st to end	Day	30% 12% 3 7% 4 3 4 4 4 6% 22%	2800
July	1st to 10th	Day	21% 10% 3 8% 5% 5% 5% 4 6% 8% 24%	6360
	11th to 20th	Day	18% 8% 2 8% 5% 5% 4 4 6% 8% 33%	6360
	21st to end	Day	16% 7% 2 6% 4 4 5% 5% 7% 8% 36%	6927
August	1st to 10th	Day	21% 10% 4 8% 5% 5% 5% 4 6% 7% 27%	4564
	11th to 20th	Day	21% 8% 3 8% 5% 5% 5% 4 6% 8% 28%	4124
	21st to end	Day	15% 7% 2 6% 5% 5% 4 3 5% 8% 39%	4312
September	1st to 10th	Day	20% 8% 3 6% 4 5% 4 3 5% 7% 35%	2400
	11th to 20th	Day	20% 9% 3 7% 5% 4 5% 3 6% 6% 33%	2381
	21st to end	Day	21% 6% 3 7% 5% 4 3 3 5% 6% 37%	2165
October	1st to 10th	Day	18% 9% 3 8% 5% 4 4 4 6% 8% 32%	2000
	11th to 20th	Day	21% 9% 4 9% 5% 6% 5% 3 6% 6% 26%	2000
	21st to end	Day	24% 13% 4 10% 6% 5% 4 3 5% 6% 20%	2004
November	1st to 10th	Day	23% 15% 3 8% 6% 5% 4 4 6% 6% 21%	1850
	11th to 20th	Day	25% 18% 5% 10% 6% 4 4 3 4 5% 15%	1672
	21st to end	Day	18% 17% 6% 11% 6% 5% 6% 5% 6% 5% 17%	1600
December	1st to 10th	Day	16% 14% 8% 14% 9% 6% 5% 4 4 5% 17%	2610
	11th to 20th	Day	29% 14% 5% 13% 6% 5% 4 3 3 4 14%	2996
	21st to end	Day	20% 15% 7% 13% 8% 5% 5% 4 4 5% 13%	3982
January	1st to 10th	Night	15% 18% 6% 16% 6% 5% 5% 4 5% 5% 16%	6349
	11th to 20th	Night	17% 14% 7% 20% 7% 6% 5% 4 4 4 13%	6314
	21st to end	Night	10% 12% 5% 14% 10% 9% 7% 4 6% 6% 17%	6927
February	1st to 10th	Night	19% 12% 4 11% 7% 6% 6% 5% 6% 5% 18%	2600
	11th to 20th	Night	17% 13% 5% 10% 9% 6% 5% 4 5% 5% 21%	2513
	21st to end	Night	11% 14% 5% 13% 7% 6% 4 4 5% 6% 25%	1980
March	1st to 10th	Night	23% 13% 4 9% 6% 5% 4 3 4 4 24%	5366
	11th to 20th	Night	19% 15% 5% 10% 4 4 4 3 4 6% 26%	5053
	21st to end	Night	29% 14% 6% 10% 6% 4 3 2 3 3 19%	5107
April	1st to 10th	Night	25% 12% 4 7% 5% 5% 4 4 5% 6% 26%	2000
	11th to 20th	Night	20% 11% 3 8% 6% 4 4 3 4 7% 30%	1976
	21st to end	Night	25% 14% 2 6% 4 3 3 2 4 5% 31%	1830
May	1st to 10th	Night	27% 14% 3 6% 3 3 4 3 5% 6% 26%	3615
	11th to 20th	Night	18% 16% 4 6% 4 4 4 3 5% 6% 31%	3596
	21st to end	Night	17% 13% 3 8% 5% 4 4 3 5% 7% 31%	3948
June	1st to 10th	Night	31% 13% 3 7% 4 3 4 3 4 5% 24%	1600
	11th to 20th	Night	24% 10% 2 5% 4 4 3 3 4 7% 35%	1600
	21st to end	Night	29% 13% 3 8% 4 3 3 4 6% 6% 21%	1600
July	1st to 10th	Night	23% 11% 4 8% 6% 5% 6% 4 6% 7% 22%	3619
	11th to 20th	Night	20% 12% 2 7% 3 4 4 3 5% 6% 34%	3620
	21st to end	Night	19% 9% 3 7% 5% 4 4 4 6% 7% 32%	3948
August	1st to 10th	Night	21% 9% 3 7% 5% 4 5% 3 5% 8% 29%	2596
	11th to 20th	Night	20% 10% 3 6% 4 4 5% 4 6% 9% 29%	2907
	21st to end	Night	18% 6% 2 5% 3 3 4 3 5% 8% 42%	3586
September	1st to 10th	Night	19% 6% 2 6% 5% 5% 5% 3 5% 7% 39%	2000
	11th to 20th	Night	21% 7% 2 7% 4 3 4 3 5% 6% 38%	2018
	21st to end	Night	20% 9% 2 6% 4 4 4 3 5% 7% 37%	2233
October	1st to 10th	Night	18% 7% 2 7% 4 4 4 4 6% 7% 36%	2400
	11th to 20th	Night	21% 8% 3 6% 5% 4 3 3 6% 7% 33%	2400
	21st to end	Night	24% 12% 3 7% 5% 4 5% 4 5% 7% 24%	2835
November	1st to 10th	Night	21% 11% 4 8% 5% 5% 4 5% 5% 7% 25%	2668
	11th to 20th	Night	25% 15% 4 8% 6% 5% 5% 4 5% 5% 19%	2880
	21st to end	Night	22% 14% 4 9% 6% 5% 6% 4 6% 6% 19%	2800
December	1st to 10th	Night	18% 11% 5% 10% 6% 5% 4 4 6% 6% 25%	4580
	11th to 20th	Night	27% 11% 4 11% 6% 5% 5% 3 4 4 18%	5213
	21st to end	Night	23% 14% 6% 12% 5% 4 5% 3 4 5% 19%	6996

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	69% 6% 12% 11% 2	3617
	11th to 20th	Day	65% 6% 15% 12% 2	3620
	21st to end	Day	58% 7% 10% 16% 9% 1	4043
February	1st to 10th	Day	63% 6% 11% 13% 6%	1800
	11th to 20th	Day	59% 6% 14% 14% 4 1	1779
	21st to end	Day	57% 4 11% 16% 11% 3	1650
March	1st to 10th	Day	62% 5% 7% 14% 11% 1	4478
	11th to 20th	Day	65% 5% 13% 12% 5%	4904
	21st to end	Day	68% 5% 8% 13% 6% 1	5824
April	1st to 10th	Day	62% 5% 9% 15% 8% 2	2400
	11th to 20th	Day	56% 4 10% 17% 11% 1	2374
	21st to end	Day	60% 5% 10% 14% 10% 1	2566
May	1st to 10th	Day	66% 4 9% 15% 6% 1	6357
	11th to 20th	Day	56% 7% 13% 16% 8%	6322
	21st to end	Day	54% 7% 12% 20% 7%	6948
June	1st to 10th	Day	62% 8% 8% 16% 5% 1	2800
	11th to 20th	Day	52% 8% 12% 20% 8% 1	2800
	21st to end	Day	64% 8% 8% 13% 7%	2800
July	1st to 10th	Day	59% 6% 15% 12% 7%	6290
	11th to 20th	Day	50% 5% 15% 22% 6%	6284
	21st to end	Day	44% 6% 20% 24% 6%	6730
August	1st to 10th	Day	56% 8% 13% 17% 6%	4550
	11th to 20th	Day	51% 6% 15% 23% 4	4163
	21st to end	Day	43% 5% 12% 29% 11%	4312
September	1st to 10th	Day	48% 6% 13% 23% 9% 1	2400
	11th to 20th	Day	50% 6% 12% 23% 8% 1	2381
	21st to end	Day	47% 6% 11% 26% 10% 1	2165
October	1st to 10th	Day	55% 5% 13% 19% 8%	3260
	11th to 20th	Day	46% 6% 12% 26% 10%	3240
	21st to end	Day	55% 6% 13% 19% 6%	3226
November	1st to 10th	Day	58% 7% 14% 16% 4	1850
	11th to 20th	Day	66% 5% 14% 12% 3	1672
	21st to end	Day	61% 8% 16% 10% 4	1600
December	1st to 10th	Day	62% 6% 10% 16% 5%	2610
	11th to 20th	Day	68% 5% 12% 11% 4	2995
	21st to end	Day	68% 7% 13% 10% 3	3982
January	1st to 10th	Night	64% 7% 16% 11% 2	6349
	11th to 20th	Night	69% 6% 12% 10% 2	6360
	21st to end	Night	56% 9% 13% 15% 7%	6927
February	1st to 10th	Night	60% 8% 12% 12% 7% 1	2600
	11th to 20th	Night	59% 6% 13% 15% 5% 1	2516
	21st to end	Night	54% 7% 12% 15% 10% 1	1980
March	1st to 10th	Night	60% 5% 6% 16% 12% 2	5375
	11th to 20th	Night	58% 5% 14% 14% 8% 1	5054
	21st to end	Night	70% 3 6% 13% 6% 1	5107
April	1st to 10th	Night	57% 6% 11% 16% 9% 1	2000
	11th to 20th	Night	52% 6% 11% 19% 11% 1	1976
	21st to end	Night	57% 4 9% 18% 11% 2	1830
May	1st to 10th	Night	59% 6% 11% 15% 7%	3620
	11th to 20th	Night	53% 6% 13% 20% 7% 1	3597
	21st to end	Night	53% 8% 11% 21% 6%	3949
June	1st to 10th	Night	64% 6% 8% 15% 6% 1	1600
	11th to 20th	Night	51% 6% 10% 20% 12% 1	1600
	21st to end	Night	66% 8% 8% 12% 6% 1	1600
July	1st to 10th	Night	63% 7% 11% 14% 6%	3581
	11th to 20th	Night	53% 4 12% 20% 10% 1	3572
	21st to end	Night	52% 5% 14% 23% 5%	3831
August	1st to 10th	Night	54% 6% 14% 20% 5%	2585
	11th to 20th	Night	50% 7% 19% 19% 4	2933
	21st to end	Night	40% 6% 16% 31% 8%	3585
September	1st to 10th	Night	44% 8% 13% 23% 12%	2000
	11th to 20th	Night	46% 6% 13% 24% 10% 1	2018
	21st to end	Night	47% 5% 11% 26% 11%	2233
October	1st to 10th	Night	48% 6% 13% 21% 12%	3920
	11th to 20th	Night	42% 5% 12% 31% 9%	3899
	21st to end	Night	51% 4 16% 21% 8%	4573
November	1st to 10th	Night	54% 9% 16% 16% 5%	2668
	11th to 20th	Night	61% 7% 15% 13% 5%	2880
	21st to end	Night	59% 8% 16% 12% 4	2800
December	1st to 10th	Night	54% 6% 12% 21% 7%	4580
	11th to 20th	Night	62% 6% 10% 16% 5%	5215
	21st to end	Night	63% 6% 13% 14% 4	6996

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	28% 25% 31% 13% 3	400
	11th to 20th	Day	28% 21% 29% 17% 5% 1	400
	21st to end	Day	23% 19% 39% 16% 3	440
February	1st to 10th	Day	31% 24% 27% 16% 2	400
	11th to 20th	Day	26% 30% 28% 14% 2 1	396
	21st to end	Day	22% 21% 35% 17% 5%	331
March	1st to 10th	Day	24% 25% 33% 14% 5%	400
	11th to 20th	Day	21% 24% 33% 15% 6% 1	483
	21st to end	Day	31% 26% 26% 12% 4 1	550
April	1st to 10th	Day	34% 25% 29% 8% 4	500
	11th to 20th	Day	30% 23% 29% 14% 3 1	492
	21st to end	Day	32% 17% 30% 17% 3 1	594
May	1st to 10th	Day	35% 17% 28% 14% 4 2	703
	11th to 20th	Day	27% 13% 28% 20% 9% 2	724
	21st to end	Day	26% 14% 32% 21% 6% 1	790
June	1st to 10th	Day	36% 10% 25% 22% 6% 1	730
	11th to 20th	Day	25% 13% 34% 20% 6% 2	730
	21st to end	Day	37% 12% 23% 19% 8% 1	730
July	1st to 10th	Day	25% 9% 31% 25% 8% 1	720
	11th to 20th	Day	24% 8% 29% 27% 11% 1	718
	21st to end	Day	26% 10% 29% 25% 9% 1	795
August	1st to 10th	Day	21% 6% 32% 27% 12% 2	725
	11th to 20th	Day	25% 9% 28% 25% 11% 2	534
	21st to end	Day	22% 11% 30% 25% 10% 2	583
September	1st to 10th	Day	20% 12% 32% 26% 7% 2	530
	11th to 20th	Day	24% 15% 30% 20% 9% 2	519
	21st to end	Day	25% 14% 28% 21% 10% 2	420
October	1st to 10th	Day	22% 12% 32% 24% 8% 3	420
	11th to 20th	Day	25% 13% 30% 19% 10% 3	420
	21st to end	Day	29% 19% 30% 15% 7%	462
November	1st to 10th	Day	30% 19% 29% 14% 7% 1	425
	11th to 20th	Day	34% 20% 26% 16% 4 1	421
	21st to end	Day	30% 20% 30% 16% 4	400
December	1st to 10th	Day	33% 16% 32% 16% 4	400
	11th to 20th	Day	32% 23% 31% 12% 3	400
	21st to end	Day	23% 25% 32% 14% 5% 1	440
January	1st to 10th	Night	27% 19% 33% 17% 4	900
	11th to 20th	Night	27% 18% 33% 17% 5%	900
	21st to end	Night	23% 17% 37% 18% 4	975
February	1st to 10th	Night	27% 17% 31% 22% 3	800
	11th to 20th	Night	23% 22% 35% 17% 3	783
	21st to end	Night	19% 16% 35% 23% 6%	660
March	1st to 10th	Night	25% 16% 35% 21% 4 1	800
	11th to 20th	Night	23% 14% 32% 23% 7% 1	800
	21st to end	Night	29% 14% 30% 22% 5% 1	728
April	1st to 10th	Night	31% 14% 31% 20% 4	600
	11th to 20th	Night	25% 10% 36% 23% 5%	594
	21st to end	Night	32% 10% 31% 20% 7% 1	600
May	1st to 10th	Night	35% 7% 30% 19% 7% 2	600
	11th to 20th	Night	29% 8% 28% 24% 10% 1	592
	21st to end	Night	25% 8% 35% 24% 8%	649
June	1st to 10th	Night	37% 9% 24% 24% 6%	600
	11th to 20th	Night	28% 9% 28% 27% 8%	600
	21st to end	Night	36% 9% 24% 25% 6% 1	600
July	1st to 10th	Night	27% 5% 28% 30% 9% 1	586
	11th to 20th	Night	27% 9% 23% 33% 8%	582
	21st to end	Night	28% 9% 24% 31% 8% 1	656
August	1st to 10th	Night	24% 5% 21% 37% 13%	598
	11th to 20th	Night	25% 7% 19% 34% 13% 2	592
	21st to end	Night	22% 7% 22% 36% 15%	660
September	1st to 10th	Night	21% 6% 25% 35% 11% 2	600
	11th to 20th	Night	25% 9% 25% 30% 11% 1	600
	21st to end	Night	24% 9% 31% 27% 8% 1	633
October	1st to 10th	Night	23% 7% 30% 29% 11% 1	800
	11th to 20th	Night	25% 8% 30% 27% 9% 2	800
	21st to end	Night	29% 13% 32% 21% 5%	987
November	1st to 10th	Night	29% 13% 31% 20% 6% 1	930
	11th to 20th	Night	33% 17% 26% 18% 6% 1	936
	21st to end	Night	30% 16% 32% 17% 4 1	900
December	1st to 10th	Night	31% 15% 30% 18% 5%	900
	11th to 20th	Night	30% 18% 32% 16% 4	900
	21st to end	Night	27% 19% 32% 18% 3 1	990

Page generated at 2025-11-15 01:56:00. Version ac07

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