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 2026-06-01.

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 9% 5% 5% 5% 4 6% 7% 32%	131803
	Night	14% 10% 4 9% 5% 4 5% 4 5% 7% 34%	131386
Transparency	Day	48% 5% 12% 21% 12% 1	135402
	Night	48% 5% 11% 21% 13% 1	135857
Seeing	Day	18% 10% 29% 24% 14% 6%	17519
	Night	19% 9% 28% 30% 13% 2	27468

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	14% 17% 7% 16% 7% 5% 4 3 5% 5% 16%	4691
	11th to 20th	Day	11% 17% 6% 15% 9% 6% 7% 3 4 4 17%	4880
	21st to end	Day	18% 15% 7% 13% 7% 6% 6% 4 4 5% 15%	5780
February	1st to 10th	Day	15% 15% 5% 12% 8% 6% 4 4 5% 6% 21%	1890
	11th to 20th	Day	15% 11% 5% 12% 7% 7% 6% 4 4 5% 23%	2011
	21st to end	Day	16% 12% 5% 11% 5% 5% 4 4 6% 7% 25%	1730
March	1st to 10th	Day	17% 13% 5% 11% 6% 4 4 3 5% 5% 28%	4714
	11th to 20th	Day	11% 11% 3 8% 5% 4 5% 3 5% 7% 37%	5090
	21st to end	Day	17% 14% 5% 10% 5% 5% 5% 4 5% 6% 25%	5586
April	1st to 10th	Day	15% 11% 4 8% 6% 5% 5% 5% 6% 6% 29%	2423
	11th to 20th	Day	16% 8% 3 8% 6% 5% 4 4 6% 7% 34%	2652
	21st to end	Day	18% 10% 2 7% 5% 4 4 4 5% 7% 35%	2730
May	1st to 10th	Day	21% 9% 3 7% 5% 5% 4 4 5% 6% 31%	5992
	11th to 20th	Day	16% 8% 3 7% 5% 5% 6% 5% 6% 7% 34%	5991
	21st to end	Day	15% 6% 2 6% 4 4 5% 4 6% 8% 39%	6576
June	1st to 10th	Day	12% 6% 2 6% 5% 5% 5% 5% 6% 9% 39%	2695
	11th to 20th	Day	10% 6% 2 6% 4 5% 5% 4 6% 9% 42%	2800
	21st to end	Day	9% 6% 3 7% 5% 5% 5% 4 8% 10% 38%	2800
July	1st to 10th	Day	9% 5% 2 7% 5% 5% 6% 4 6% 10% 40%	6360
	11th to 20th	Day	7% 5% 2 6% 5% 6% 6% 5% 7% 11% 42%	6296
	21st to end	Day	9% 6% 2 7% 5% 5% 5% 5% 6% 10% 41%	6424
August	1st to 10th	Day	10% 5% 2 6% 4 5% 4 4 7% 9% 42%	4234
	11th to 20th	Day	7% 6% 2 6% 5% 5% 6% 6% 9% 11% 37%	4174
	21st to end	Day	7% 5% 2 7% 5% 4 5% 4 7% 10% 42%	4591
September	1st to 10th	Day	11% 6% 3 5% 4 4 5% 4 5% 8% 45%	2248
	11th to 20th	Day	9% 5% 3 6% 3 4 4 4 6% 9% 48%	2200
	21st to end	Day	16% 7% 3 6% 3 3 4 4 5% 6% 41%	2200
October	1st to 10th	Day	11% 7% 2 7% 5% 5% 4 3 6% 7% 45%	2150
	11th to 20th	Day	18% 8% 2 7% 4 3 4 3 5% 7% 39%	2000
	21st to end	Day	18% 9% 3 8% 5% 5% 5% 4 6% 8% 31%	2065
November	1st to 10th	Day	12% 8% 4 6% 5% 4 4 3 6% 7% 41%	1852
	11th to 20th	Day	16% 13% 3 8% 6% 5% 4 3 4 6% 34%	1890
	21st to end	Day	18% 12% 5% 9% 5% 5% 5% 3 5% 7% 25%	1890
December	1st to 10th	Day	13% 14% 6% 11% 7% 6% 5% 4 5% 6% 22%	2760
	11th to 20th	Day	18% 18% 6% 11% 5% 4 5% 4 5% 5% 19%	3368
	21st to end	Day	24% 19% 7% 13% 6% 4 3 2 3 4 14%	4070
January	1st to 10th	Night	14% 15% 6% 15% 7% 6% 5% 3 4 4 21%	8232
	11th to 20th	Night	9% 16% 7% 14% 7% 6% 6% 4 4 6% 20%	8083
	21st to end	Night	17% 14% 6% 10% 5% 4 4 4 5% 6% 24%	8401
February	1st to 10th	Night	15% 14% 5% 10% 6% 5% 5% 4 5% 5% 24%	2730
	11th to 20th	Night	14% 13% 4 10% 6% 5% 5% 4 5% 6% 26%	2491
	21st to end	Night	18% 13% 4 9% 6% 5% 3 3 4 6% 28%	2076
March	1st to 10th	Night	17% 12% 3 7% 4 4 4 3 4 5% 36%	5358
	11th to 20th	Night	10% 8% 2 6% 5% 5% 5% 4 6% 7% 42%	5090
	21st to end	Night	15% 10% 4 8% 5% 4 4 3 4 6% 36%	5588
April	1st to 10th	Night	16% 10% 4 7% 6% 4 4 4 6% 6% 33%	2215
	11th to 20th	Night	16% 8% 2 7% 6% 4 5% 3 6% 6% 36%	1915
	21st to end	Night	16% 9% 3 5% 4 5% 4 3 6% 8% 37%	1890
May	1st to 10th	Night	20% 10% 4 9% 5% 5% 5% 3 5% 6% 29%	4123
	11th to 20th	Night	16% 8% 3 7% 5% 4 4 3 5% 7% 37%	4122
	21st to end	Night	13% 6% 2 6% 5% 4 5% 4 5% 8% 42%	4520
June	1st to 10th	Night	11% 6% 2 6% 5% 4 5% 5% 6% 7% 44%	1699
	11th to 20th	Night	9% 6% 3 7% 4 4 4 4 7% 9% 44%	1600
	21st to end	Night	9% 5% 2 5% 4 4 4 5% 7% 9% 46%	1600
July	1st to 10th	Night	9% 4 2 6% 4 5% 5% 5% 6% 10% 46%	3618
	11th to 20th	Night	8% 5% 2 6% 5% 4 6% 4 7% 9% 43%	3677
	21st to end	Night	8% 5% 2 6% 4 4 5% 4 6% 9% 47%	4427
August	1st to 10th	Night	12% 6% 2 7% 4 4 5% 3 6% 9% 43%	2915
	11th to 20th	Night	8% 6% 3 5% 4 4 5% 4 6% 8% 48%	2875
	21st to end	Night	7% 5% 2 7% 5% 5% 5% 4 6% 9% 47%	3303
September	1st to 10th	Night	10% 6% 3 5% 4 4 4 3 5% 6% 50%	2130
	11th to 20th	Night	8% 6% 2 5% 4 3 3 2 5% 8% 54%	2200
	21st to end	Night	13% 7% 3 7% 5% 4 4 3 5% 6% 44%	2200
October	1st to 10th	Night	12% 5% 2 6% 4 4 4 3 5% 7% 48%	2248
	11th to 20th	Night	17% 8% 3 7% 5% 4 4 3 5% 6% 39%	2400
	21st to end	Night	20% 8% 3 6% 5% 3 4 4 5% 6% 36%	2771
November	1st to 10th	Night	12% 9% 3 7% 4 4 4 3 4 7% 43%	2666
	11th to 20th	Night	15% 10% 3 8% 5% 4 5% 4 6% 7% 33%	2730
	21st to end	Night	20% 14% 3 8% 5% 5% 5% 3 4 6% 26%	2730
December	1st to 10th	Night	12% 13% 6% 11% 7% 6% 5% 3 5% 6% 26%	4611
	11th to 20th	Night	15% 17% 7% 12% 5% 5% 5% 3 5% 5% 22%	5002
	21st to end	Night	25% 18% 6% 11% 5% 4 5% 3 4 5% 14%	7150

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% 5% 11% 13% 3	4692
	11th to 20th	Day	69% 4 8% 8% 10%	4880
	21st to end	Day	70% 4 8% 11% 6%	5780
February	1st to 10th	Day	64% 5% 10% 15% 6%	1890
	11th to 20th	Day	61% 5% 9% 17% 7% 1	2021
	21st to end	Day	57% 4 14% 17% 7% 1	1730
March	1st to 10th	Day	58% 4 11% 18% 8%	4714
	11th to 20th	Day	45% 5% 15% 24% 10% 1	5090
	21st to end	Day	58% 5% 10% 17% 9% 1	5586
April	1st to 10th	Day	53% 5% 12% 18% 12%	2423
	11th to 20th	Day	48% 5% 12% 20% 12% 2	2652
	21st to end	Day	48% 4 12% 22% 11% 2	2730
May	1st to 10th	Day	53% 6% 9% 18% 13% 1	6027
	11th to 20th	Day	46% 7% 12% 21% 12% 2	5995
	21st to end	Day	42% 5% 10% 22% 17% 4	6590
June	1st to 10th	Day	39% 6% 12% 27% 14% 3	2695
	11th to 20th	Day	37% 6% 15% 23% 16% 3	2800
	21st to end	Day	39% 6% 14% 21% 16% 4	2800
July	1st to 10th	Day	37% 6% 11% 27% 17% 2	6291
	11th to 20th	Day	33% 7% 15% 27% 17% 1	6221
	21st to end	Day	37% 7% 12% 22% 20% 2	6240
August	1st to 10th	Day	34% 7% 15% 27% 16% 1	4221
	11th to 20th	Day	35% 9% 14% 30% 11% 1	4213
	21st to end	Day	35% 7% 13% 32% 13% 2	4590
September	1st to 10th	Day	37% 6% 13% 24% 18% 2	2248
	11th to 20th	Day	32% 5% 12% 31% 19% 1	2200
	21st to end	Day	42% 5% 12% 24% 17% 1	2200
October	1st to 10th	Day	28% 4 13% 33% 21% 1	3501
	11th to 20th	Day	39% 4 9% 28% 19% 1	3240
	21st to end	Day	42% 4 11% 24% 18% 1	3315
November	1st to 10th	Day	41% 5% 11% 25% 16% 1	1852
	11th to 20th	Day	52% 4 10% 20% 12% 1	1890
	21st to end	Day	57% 5% 12% 19% 8%	1890
December	1st to 10th	Day	61% 6% 15% 14% 4	2760
	11th to 20th	Day	66% 5% 10% 15% 4	3365
	21st to end	Day	76% 4 7% 9% 4	4070
January	1st to 10th	Night	65% 5% 9% 16% 6%	8233
	11th to 20th	Night	65% 4 9% 12% 9% 1	8083
	21st to end	Night	59% 4 13% 15% 9%	8401
February	1st to 10th	Night	61% 5% 10% 17% 6% 1	2730
	11th to 20th	Night	57% 5% 9% 20% 8% 1	2492
	21st to end	Night	57% 4 11% 18% 9% 1	2076
March	1st to 10th	Night	50% 4 11% 24% 9% 1	5358
	11th to 20th	Night	39% 5% 14% 26% 15% 1	5090
	21st to end	Night	49% 5% 10% 21% 14% 2	5588
April	1st to 10th	Night	50% 5% 11% 19% 13% 2	2215
	11th to 20th	Night	47% 5% 10% 19% 17% 2	1915
	21st to end	Night	44% 5% 10% 23% 14% 2	1890
May	1st to 10th	Night	55% 6% 8% 15% 13% 2	4150
	11th to 20th	Night	46% 6% 11% 25% 12% 2	4123
	21st to end	Night	39% 6% 11% 25% 17% 2	4528
June	1st to 10th	Night	37% 6% 10% 27% 18% 2	1699
	11th to 20th	Night	36% 6% 12% 27% 16% 3	1600
	21st to end	Night	32% 7% 14% 25% 19% 2	1600
July	1st to 10th	Night	32% 7% 13% 26% 17% 4	3580
	11th to 20th	Night	34% 8% 15% 26% 15% 2	3632
	21st to end	Night	32% 7% 14% 22% 21% 3	4297
August	1st to 10th	Night	38% 7% 11% 26% 15% 3	2904
	11th to 20th	Night	32% 7% 11% 30% 17% 1	2902
	21st to end	Night	33% 7% 12% 30% 17% 2	3303
September	1st to 10th	Night	35% 5% 11% 28% 18% 4	2130
	11th to 20th	Night	29% 5% 10% 30% 25% 2	2200
	21st to end	Night	41% 5% 11% 22% 19% 2	2200
October	1st to 10th	Night	25% 4 8% 32% 30% 1	3677
	11th to 20th	Night	37% 4 9% 29% 21% 1	3899
	21st to end	Night	37% 4 8% 33% 16% 2	4471
November	1st to 10th	Night	42% 4 10% 26% 17% 1	2666
	11th to 20th	Night	48% 6% 12% 22% 11% 1	2730
	21st to end	Night	59% 5% 11% 19% 6%	2730
December	1st to 10th	Night	58% 5% 13% 18% 6%	4611
	11th to 20th	Night	62% 5% 12% 14% 6% 1	5004
	21st to end	Night	71% 5% 10% 9% 5%	7150

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% 17% 36% 17% 5% 1	330
	11th to 20th	Day	24% 18% 42% 11% 5%	330
	21st to end	Day	26% 17% 29% 21% 6% 1	363
February	1st to 10th	Day	23% 16% 40% 17% 4 1	330
	11th to 20th	Day	26% 17% 35% 19% 3	330
	21st to end	Day	22% 21% 44% 11% 3	273
March	1st to 10th	Day	24% 14% 39% 18% 5%	386
	11th to 20th	Day	24% 13% 35% 21% 6% 1	540
	21st to end	Day	27% 15% 34% 17% 7% 1	594
April	1st to 10th	Day	24% 16% 31% 18% 10% 1	540
	11th to 20th	Day	21% 19% 29% 20% 8% 3	532
	21st to end	Day	25% 12% 32% 20% 9% 3	540
May	1st to 10th	Day	24% 10% 31% 20% 11% 3	543
	11th to 20th	Day	19% 10% 31% 21% 12% 6%	567
	21st to end	Day	14% 6% 29% 25% 18% 7%	617
June	1st to 10th	Day	13% 7% 27% 31% 18% 3	625
	11th to 20th	Day	13% 6% 30% 31% 16% 4	730
	21st to end	Day	10% 6% 27% 34% 16% 6%	730
July	1st to 10th	Day	10% 4 19% 35% 19% 13%	721
	11th to 20th	Day	9% 4 24% 31% 21% 11%	693
	21st to end	Day	10% 4 20% 29% 24% 13%	576
August	1st to 10th	Day	11% 2 16% 30% 28% 13%	526
	11th to 20th	Day	11% 3 18% 32% 20% 15%	519
	21st to end	Day	9% 2 19% 28% 29% 13%	583
September	1st to 10th	Day	13% 5% 18% 28% 22% 14%	530
	11th to 20th	Day	8% 3 16% 27% 26% 18%	530
	21st to end	Day	19% 8% 21% 25% 17% 10%	530
October	1st to 10th	Day	12% 5% 25% 27% 21% 10%	530
	11th to 20th	Day	18% 14% 31% 25% 8% 2	530
	21st to end	Day	23% 13% 34% 23% 6% 1	448
November	1st to 10th	Day	17% 15% 32% 22% 11% 3	330
	11th to 20th	Day	25% 18% 33% 18% 5% 1	330
	21st to end	Day	25% 14% 37% 21% 3	330
December	1st to 10th	Day	24% 15% 34% 18% 9%	247
	11th to 20th	Day	22% 20% 38% 15% 5%	303
	21st to end	Day	29% 16% 34% 18% 3	363
January	1st to 10th	Night	23% 13% 35% 23% 6%	1050
	11th to 20th	Night	26% 14% 35% 19% 5%	983
	21st to end	Night	24% 13% 35% 24% 4	924
February	1st to 10th	Night	24% 11% 34% 26% 4	840
	11th to 20th	Night	23% 12% 34% 28% 3	823
	21st to end	Night	25% 14% 39% 19% 3	692
March	1st to 10th	Night	22% 11% 34% 26% 7%	840
	11th to 20th	Night	19% 16% 34% 24% 7% 1	840
	21st to end	Night	23% 11% 30% 27% 8%	924
April	1st to 10th	Night	22% 12% 34% 26% 5%	745
	11th to 20th	Night	18% 12% 33% 27% 8% 2	623
	21st to end	Night	21% 5% 34% 30% 9% 1	630
May	1st to 10th	Night	21% 6% 28% 33% 10% 2	630
	11th to 20th	Night	21% 4 26% 33% 15% 1	621
	21st to end	Night	12% 3 24% 39% 20% 3	683
June	1st to 10th	Night	13% 3 24% 43% 16% 1	600
	11th to 20th	Night	12% 4 20% 43% 20% 1	600
	21st to end	Night	11% 2 22% 41% 23% 2	600
July	1st to 10th	Night	10% 2 14% 42% 29% 3	585
	11th to 20th	Night	9% 3 17% 40% 27% 4	582
	21st to end	Night	8% 1 13% 45% 30% 3	657
August	1st to 10th	Night	8% 1 13% 42% 31% 4	597
	11th to 20th	Night	10% 2 15% 33% 36% 5%	594
	21st to end	Night	9% 3 14% 36% 34% 4	660
September	1st to 10th	Night	12% 4 17% 32% 30% 5%	730
	11th to 20th	Night	11% 3 12% 33% 37% 5%	800
	21st to end	Night	17% 5% 19% 32% 22% 5%	800
October	1st to 10th	Night	16% 5% 25% 32% 20% 4	800
	11th to 20th	Night	20% 11% 28% 28% 11% 2	800
	21st to end	Night	23% 9% 31% 29% 6% 1	880
November	1st to 10th	Night	17% 11% 30% 28% 11% 2	820
	11th to 20th	Night	21% 13% 33% 24% 8% 1	840
	21st to end	Night	28% 11% 31% 23% 6%	840
December	1st to 10th	Night	24% 14% 34% 22% 5%	840
	11th to 20th	Night	24% 12% 36% 24% 4	840
	21st to end	Night	31% 12% 32% 22% 4	1155

Page generated at 2026-05-29 01:53:32. Version ac07

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