Refractor Heaven
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Refractor Heaven clear sky chart from 2005-11-06 to 2024-08-30.

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%	95410
	Night	15% 10% 4 8% 5% 4 4 4 5% 7% 34%	93594
Transparency	Day	49% 5% 12% 20% 12% 1	95174
	Night	49% 5% 11% 21% 12% 1	93435
Seeing	Day	18% 9% 28% 25% 14% 6%	15204
	Night	19% 8% 27% 30% 14% 2	25068

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	18% 18% 7% 12% 7% 5% 4 3 4 5% 18%	2527
	11th to 20th	Day	15% 20% 6% 13% 7% 6% 9% 4 4 4 12%	2622
	21st to end	Day	27% 20% 9% 11% 5% 3 4 3 3 4 11%	3124
February	1st to 10th	Day	15% 15% 5% 12% 8% 6% 4 4 5% 6% 21%	1710
	11th to 20th	Day	15% 10% 5% 12% 7% 7% 6% 4 4 5% 24%	1814
	21st to end	Day	18% 12% 4 11% 5% 5% 4 4 6% 6% 25%	1570
March	1st to 10th	Day	20% 16% 4 10% 5% 4 4 3 5% 5% 25%	3209
	11th to 20th	Day	13% 11% 4 8% 5% 4 5% 3 5% 7% 34%	3480
	21st to end	Day	19% 12% 5% 9% 5% 5% 5% 4 5% 5% 26%	3815
April	1st to 10th	Day	14% 11% 4 8% 6% 6% 5% 5% 6% 6% 30%	2192
	11th to 20th	Day	17% 8% 3 7% 5% 5% 5% 4 6% 7% 34%	2396
	21st to end	Day	18% 9% 2 7% 5% 4 4 4 5% 7% 35%	2470
May	1st to 10th	Day	18% 8% 3 8% 5% 5% 5% 4 5% 7% 32%	4082
	11th to 20th	Day	17% 8% 3 6% 5% 5% 5% 4 6% 8% 34%	4081
	21st to end	Day	9% 6% 2 6% 5% 5% 5% 5% 6% 8% 42%	4476
June	1st to 10th	Day	11% 7% 3 6% 5% 5% 5% 5% 7% 9% 38%	2560
	11th to 20th	Day	10% 6% 2 6% 4 5% 5% 4 6% 9% 43%	2660
	21st to end	Day	10% 6% 3 7% 5% 5% 6% 4 8% 9% 38%	2660
July	1st to 10th	Day	11% 5% 3 7% 5% 5% 5% 4 6% 10% 38%	4440
	11th to 20th	Day	6% 4 2 5% 5% 5% 6% 4 7% 10% 46%	4390
	21st to end	Day	7% 6% 2 7% 6% 6% 6% 5% 7% 9% 41%	4466
August	1st to 10th	Day	8% 5% 2 6% 4 5% 5% 5% 7% 10% 44%	2457
	11th to 20th	Day	8% 5% 2 6% 5% 5% 6% 5% 8% 11% 39%	2433
	21st to end	Day	9% 5% 2 6% 5% 4 5% 4 7% 10% 43%	2651
September	1st to 10th	Day	12% 6% 3 6% 5% 4 5% 4 5% 8% 43%	2024
	11th to 20th	Day	9% 5% 3 6% 3 4 5% 4 6% 9% 45%	1980
	21st to end	Day	16% 6% 3 6% 3 3 4 5% 5% 7% 43%	1980
October	1st to 10th	Day	12% 7% 2 7% 5% 5% 4 3 6% 7% 42%	1936
	11th to 20th	Day	18% 8% 2 7% 4 3 4 4 5% 7% 37%	1800
	21st to end	Day	19% 9% 3 8% 5% 5% 5% 4 6% 7% 29%	1859
November	1st to 10th	Day	12% 8% 4 6% 5% 4 4 3 6% 7% 42%	1672
	11th to 20th	Day	16% 13% 3 8% 5% 4 4 2 4 6% 34%	1710
	21st to end	Day	18% 12% 5% 8% 5% 5% 5% 3 6% 7% 26%	1710
December	1st to 10th	Day	15% 17% 7% 12% 6% 5% 4 4 4 5% 20%	1570
	11th to 20th	Day	14% 14% 6% 12% 6% 5% 6% 4 5% 6% 23%	2101
	21st to end	Day	23% 20% 8% 13% 6% 4 4 3 3 5% 12%	2783
January	1st to 10th	Night	15% 17% 6% 11% 6% 5% 4 4 4 5% 23%	4428
	11th to 20th	Night	15% 18% 8% 14% 6% 5% 6% 5% 3 5% 14%	4341
	21st to end	Night	26% 18% 8% 12% 5% 4 3 2 3 4 16%	4532
February	1st to 10th	Night	16% 14% 6% 10% 6% 5% 5% 4 5% 6% 24%	2470
	11th to 20th	Night	14% 14% 4 10% 6% 6% 4 4 5% 6% 27%	2249
	21st to end	Night	19% 12% 4 8% 6% 5% 3 4 4 6% 28%	1884
March	1st to 10th	Night	18% 14% 4 7% 5% 4 3 2 3 5% 35%	3643
	11th to 20th	Night	10% 8% 2 6% 5% 5% 5% 3 6% 7% 42%	3480
	21st to end	Night	16% 10% 3 8% 5% 4 4 3 4 7% 35%	3817
April	1st to 10th	Night	15% 11% 4 7% 5% 4 4 4 6% 6% 34%	2005
	11th to 20th	Night	18% 7% 2 6% 6% 4 5% 3 6% 6% 36%	1731
	21st to end	Night	16% 8% 3 6% 5% 5% 4 3 6% 8% 37%	1710
May	1st to 10th	Night	17% 9% 3 8% 5% 5% 5% 4 6% 7% 31%	2813
	11th to 20th	Night	17% 7% 3 6% 5% 3 4 4 5% 7% 39%	2812
	21st to end	Night	9% 5% 2 6% 5% 4 4 4 6% 9% 46%	3079
June	1st to 10th	Night	11% 6% 2 6% 5% 4 5% 5% 7% 7% 44%	1614
	11th to 20th	Night	9% 6% 3 6% 4 4 4 4 7% 9% 44%	1520
	21st to end	Night	9% 5% 2 5% 4 5% 4 5% 8% 9% 44%	1520
July	1st to 10th	Night	9% 3 2 5% 4 4 4 4 6% 10% 49%	2530
	11th to 20th	Night	7% 4 2 5% 4 4 5% 4 7% 10% 48%	2577
	21st to end	Night	6% 5% 2 6% 5% 5% 6% 5% 7% 9% 45%	3087
August	1st to 10th	Night	7% 4 2 6% 4 4 5% 4 6% 10% 49%	1701
	11th to 20th	Night	9% 4 2 4 3 4 5% 5% 7% 9% 47%	1688
	21st to end	Night	8% 4 2 6% 5% 5% 5% 4 5% 8% 49%	1910
September	1st to 10th	Night	10% 6% 3 5% 4 4 4 3 5% 6% 47%	1916
	11th to 20th	Night	9% 6% 2 5% 4 4 3 2 5% 8% 51%	1980
	21st to end	Night	12% 6% 3 7% 5% 4 4 3 5% 6% 45%	1980
October	1st to 10th	Night	13% 6% 2 6% 4 4 4 3 5% 7% 45%	2022
	11th to 20th	Night	18% 8% 3 7% 5% 4 4 3 5% 6% 38%	2160
	21st to end	Night	21% 9% 3 7% 5% 3 4 3 5% 6% 34%	2493
November	1st to 10th	Night	13% 9% 3 7% 4 4 4 4 4 7% 42%	2406
	11th to 20th	Night	15% 11% 3 7% 5% 4 5% 4 6% 7% 33%	2470
	21st to end	Night	20% 13% 4 8% 5% 5% 5% 3 4 6% 27%	2470
December	1st to 10th	Night	16% 14% 5% 10% 7% 5% 5% 3 5% 6% 24%	2596
	11th to 20th	Night	16% 15% 6% 10% 5% 4 4 4 5% 6% 26%	3076
	21st to end	Night	26% 19% 6% 11% 5% 4 5% 3 4 5% 12%	4884

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% 4 10% 12% 4	2528
	11th to 20th	Day	75% 4 9% 8% 4	2622
	21st to end	Day	79% 3 7% 7% 3	3124
February	1st to 10th	Day	65% 4 11% 14% 6%	1710
	11th to 20th	Day	61% 5% 9% 17% 7% 1	1824
	21st to end	Day	57% 4 14% 16% 6% 2	1570
March	1st to 10th	Day	61% 5% 11% 18% 5%	3209
	11th to 20th	Day	48% 6% 14% 22% 9% 1	3480
	21st to end	Day	58% 5% 12% 17% 8% 1	3815
April	1st to 10th	Day	52% 5% 12% 18% 13%	2192
	11th to 20th	Day	48% 5% 12% 20% 13% 2	2396
	21st to end	Day	48% 4 12% 22% 11% 2	2470
May	1st to 10th	Day	50% 6% 11% 23% 10% 1	4120
	11th to 20th	Day	46% 7% 12% 23% 10% 1	4085
	21st to end	Day	38% 6% 11% 20% 20% 5%	4489
June	1st to 10th	Day	40% 6% 12% 27% 13% 3	2560
	11th to 20th	Day	37% 6% 15% 23% 17% 3	2660
	21st to end	Day	40% 6% 14% 21% 16% 4	2660
July	1st to 10th	Day	40% 6% 11% 23% 18% 2	4371
	11th to 20th	Day	30% 6% 13% 31% 18% 2	4315
	21st to end	Day	37% 8% 13% 18% 21% 3	4324
August	1st to 10th	Day	33% 6% 15% 29% 16%	2444
	11th to 20th	Day	35% 8% 15% 26% 13% 2	2433
	21st to end	Day	35% 7% 13% 25% 18% 3	2651
September	1st to 10th	Day	39% 6% 13% 22% 17% 3	2024
	11th to 20th	Day	34% 5% 12% 30% 18% 1	1980
	21st to end	Day	40% 5% 13% 25% 16% 1	1980
October	1st to 10th	Day	41% 5% 11% 27% 14% 2	1936
	11th to 20th	Day	47% 4 11% 24% 14% 1	1800
	21st to end	Day	52% 6% 12% 19% 11% 1	1859
November	1st to 10th	Day	41% 5% 12% 24% 16% 1	1672
	11th to 20th	Day	52% 4 11% 20% 12% 1	1710
	21st to end	Day	56% 5% 13% 18% 8%	1710
December	1st to 10th	Day	67% 5% 10% 14% 4	1570
	11th to 20th	Day	61% 5% 12% 17% 5%	2098
	21st to end	Day	76% 5% 8% 8% 3	2783
January	1st to 10th	Night	63% 5% 9% 16% 7%	4428
	11th to 20th	Night	73% 4 9% 10% 4	4341
	21st to end	Night	74% 3 10% 10% 4	4532
February	1st to 10th	Night	61% 5% 11% 17% 6% 1	2470
	11th to 20th	Night	57% 4 9% 20% 8% 1	2250
	21st to end	Night	58% 4 10% 18% 9% 1	1884
March	1st to 10th	Night	53% 3 11% 24% 8% 1	3643
	11th to 20th	Night	39% 6% 16% 21% 17% 1	3480
	21st to end	Night	49% 5% 11% 25% 9% 1	3817
April	1st to 10th	Night	48% 6% 11% 19% 14% 2	2005
	11th to 20th	Night	47% 5% 10% 20% 17% 2	1731
	21st to end	Night	45% 5% 10% 22% 15% 3	1710
May	1st to 10th	Night	51% 6% 10% 19% 13% 2	2840
	11th to 20th	Night	44% 6% 10% 26% 12% 1	2813
	21st to end	Night	34% 6% 11% 26% 21% 3	3087
June	1st to 10th	Night	37% 6% 10% 27% 17% 2	1614
	11th to 20th	Night	36% 6% 12% 27% 16% 3	1520
	21st to end	Night	33% 7% 13% 26% 19% 2	1520
July	1st to 10th	Night	31% 5% 12% 29% 17% 6%	2490
	11th to 20th	Night	28% 8% 16% 28% 18% 2	2529
	21st to end	Night	31% 8% 17% 21% 19% 4	2988
August	1st to 10th	Night	30% 7% 13% 31% 17% 2	1690
	11th to 20th	Night	31% 8% 14% 29% 16% 2	1688
	21st to end	Night	33% 5% 13% 25% 22% 3	1910
September	1st to 10th	Night	37% 5% 11% 26% 17% 4	1916
	11th to 20th	Night	32% 5% 10% 28% 24% 1	1980
	21st to end	Night	39% 5% 12% 24% 19% 2	1980
October	1st to 10th	Night	38% 5% 11% 24% 20% 1	2022
	11th to 20th	Night	47% 5% 10% 22% 16% 1	2160
	21st to end	Night	51% 5% 9% 24% 11% 1	2493
November	1st to 10th	Night	42% 4 10% 25% 17% 1	2406
	11th to 20th	Night	49% 6% 12% 22% 11% 1	2470
	21st to end	Night	58% 4 11% 20% 7%	2470
December	1st to 10th	Night	61% 5% 12% 16% 7%	2596
	11th to 20th	Night	58% 5% 14% 16% 7% 1	3078
	21st to end	Night	74% 5% 11% 8% 3	4884

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% 32% 19% 6% 1	270
	11th to 20th	Day	24% 14% 42% 13% 6%	270
	21st to end	Day	30% 13% 29% 22% 6% 1	297
February	1st to 10th	Day	24% 17% 39% 17% 3 1	270
	11th to 20th	Day	25% 17% 37% 17% 3	270
	21st to end	Day	25% 19% 43% 10% 4	225
March	1st to 10th	Day	25% 13% 36% 21% 6%	320
	11th to 20th	Day	22% 12% 37% 21% 7% 1	460
	21st to end	Day	29% 13% 32% 18% 7% 1	506
April	1st to 10th	Day	21% 14% 32% 20% 12% 1	460
	11th to 20th	Day	21% 18% 29% 21% 8% 4	452
	21st to end	Day	27% 9% 33% 20% 9% 2	460
May	1st to 10th	Day	24% 8% 32% 21% 13% 3	463
	11th to 20th	Day	19% 10% 30% 21% 13% 7%	487
	21st to end	Day	12% 5% 28% 26% 20% 9%	529
June	1st to 10th	Day	12% 7% 27% 32% 19% 4	580
	11th to 20th	Day	12% 6% 29% 31% 17% 5%	680
	21st to end	Day	11% 6% 28% 33% 16% 6%	680
July	1st to 10th	Day	10% 4 19% 34% 19% 14%	671
	11th to 20th	Day	9% 4 24% 31% 20% 11%	644
	21st to end	Day	9% 3 21% 29% 24% 13%	532
August	1st to 10th	Day	12% 2 16% 30% 28% 13%	486
	11th to 20th	Day	11% 4 19% 32% 20% 14%	479
	21st to end	Day	9% 2 18% 27% 29% 14%	532
September	1st to 10th	Day	14% 4 17% 29% 23% 12%	450
	11th to 20th	Day	10% 3 18% 27% 26% 16%	450
	21st to end	Day	18% 8% 22% 26% 18% 10%	450
October	1st to 10th	Day	14% 5% 25% 27% 20% 9%	450
	11th to 20th	Day	19% 16% 30% 25% 7% 2	450
	21st to end	Day	24% 13% 34% 23% 6% 1	374
November	1st to 10th	Day	16% 13% 32% 23% 12% 4	270
	11th to 20th	Day	24% 16% 32% 20% 7% 1	270
	21st to end	Day	25% 10% 38% 23% 4	270
December	1st to 10th	Day	24% 16% 34% 16% 9%	203
	11th to 20th	Day	18% 18% 42% 17% 5%	247
	21st to end	Day	28% 15% 36% 18% 3	297
January	1st to 10th	Night	24% 12% 35% 22% 6%	950
	11th to 20th	Night	27% 12% 34% 21% 6%	890
	21st to end	Night	25% 11% 35% 25% 4	836
February	1st to 10th	Night	25% 11% 34% 25% 4	760
	11th to 20th	Night	23% 12% 33% 29% 3	743
	21st to end	Night	25% 13% 38% 20% 4	628
March	1st to 10th	Night	21% 11% 33% 27% 8%	760
	11th to 20th	Night	18% 15% 34% 25% 7% 1	760
	21st to end	Night	23% 10% 29% 29% 8%	836
April	1st to 10th	Night	20% 11% 35% 28% 5%	675
	11th to 20th	Night	19% 10% 33% 28% 8% 2	563
	21st to end	Night	22% 4 34% 30% 9% 1	570
May	1st to 10th	Night	21% 5% 27% 34% 11% 2	570
	11th to 20th	Night	21% 3 25% 34% 16% 2	561
	21st to end	Night	11% 3 22% 39% 22% 3	617
June	1st to 10th	Night	13% 3 24% 44% 17% 1	570
	11th to 20th	Night	12% 4 20% 42% 21% 1	570
	21st to end	Night	12% 2 22% 40% 22% 2	570
July	1st to 10th	Night	10% 2 14% 42% 29% 4	555
	11th to 20th	Night	9% 3 17% 40% 26% 5%	552
	21st to end	Night	8% 1 13% 46% 30% 3	624
August	1st to 10th	Night	9% 1 13% 42% 31% 4	567
	11th to 20th	Night	10% 2 15% 33% 36% 5%	564
	21st to end	Night	9% 3 14% 35% 34% 5%	624
September	1st to 10th	Night	13% 4 15% 32% 30% 6%	656
	11th to 20th	Night	12% 3 13% 33% 34% 4	720
	21st to end	Night	15% 5% 20% 32% 23% 5%	720
October	1st to 10th	Night	17% 4 24% 32% 20% 4	720
	11th to 20th	Night	21% 10% 29% 28% 10% 1	720
	21st to end	Night	25% 10% 31% 28% 6% 1	792
November	1st to 10th	Night	18% 11% 28% 29% 12% 2	740
	11th to 20th	Night	22% 13% 32% 24% 8% 1	760
	21st to end	Night	26% 11% 31% 25% 7%	760
December	1st to 10th	Night	24% 13% 35% 21% 6% 1	760
	11th to 20th	Night	24% 11% 35% 24% 4	760
	21st to end	Night	30% 12% 32% 23% 3	1045

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