Whisper Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Whisper Observatory 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 8% 5% 5% 5% 4 5% 7% 33%	131957
	Night	15% 11% 4 8% 5% 4 4 3 5% 6% 34%	131302
Transparency	Day	48% 7% 12% 21% 12% 1	135543
	Night	48% 6% 11% 20% 13% 1	135774
Seeing	Day	18% 13% 29% 23% 12% 5%	17677
	Night	19% 11% 30% 27% 12% 1	27542

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% 7% 17% 6% 5% 4 3 5% 5% 16%	4691
	11th to 20th	Day	13% 16% 6% 15% 9% 6% 7% 4 3 4 18%	4724
	21st to end	Day	18% 17% 7% 13% 6% 5% 7% 4 4 4 15%	5780
February	1st to 10th	Day	14% 15% 5% 12% 8% 6% 5% 4 5% 5% 21%	1890
	11th to 20th	Day	14% 13% 5% 13% 7% 6% 5% 3 5% 5% 24%	2034
	21st to end	Day	17% 13% 4 10% 6% 4 4 4 5% 6% 28%	1730
March	1st to 10th	Day	17% 13% 6% 11% 5% 4 4 3 4 5% 28%	4669
	11th to 20th	Day	12% 12% 3 8% 5% 4 4 4 5% 6% 38%	5090
	21st to end	Day	18% 14% 4 9% 5% 5% 5% 3 5% 5% 26%	5586
April	1st to 10th	Day	16% 11% 3 8% 6% 5% 5% 4 5% 6% 30%	2453
	11th to 20th	Day	17% 9% 2 8% 5% 5% 4 4 6% 7% 34%	2634
	21st to end	Day	18% 9% 3 7% 5% 4 4 3 5% 7% 34%	2730
May	1st to 10th	Day	21% 9% 3 7% 5% 4 4 4 5% 6% 31%	5992
	11th to 20th	Day	17% 8% 3 8% 5% 5% 4 4 6% 8% 34%	5991
	21st to end	Day	16% 7% 2 6% 4 4 5% 4 6% 7% 40%	6576
June	1st to 10th	Day	11% 7% 1 6% 5% 5% 6% 4 7% 8% 40%	2745
	11th to 20th	Day	10% 6% 3 6% 5% 5% 5% 4 6% 8% 43%	2800
	21st to end	Day	10% 6% 3 5% 6% 5% 6% 5% 6% 10% 39%	2800
July	1st to 10th	Day	9% 5% 2 6% 5% 5% 5% 4 6% 10% 43%	6360
	11th to 20th	Day	7% 5% 2 6% 4 5% 5% 5% 7% 11% 42%	6360
	21st to end	Day	8% 6% 2 6% 5% 5% 5% 4 7% 10% 41%	6456
August	1st to 10th	Day	10% 5% 1 6% 4 5% 5% 4 7% 9% 44%	4234
	11th to 20th	Day	9% 6% 2 6% 5% 6% 7% 5% 7% 9% 39%	4174
	21st to end	Day	8% 4 2 7% 5% 4 5% 5% 7% 10% 43%	4550
September	1st to 10th	Day	12% 5% 2 7% 5% 3 4 4 6% 8% 45%	2266
	11th to 20th	Day	8% 5% 2 6% 4 4 5% 4 6% 9% 48%	2200
	21st to end	Day	15% 9% 2 5% 3 3 5% 4 5% 7% 41%	2200
October	1st to 10th	Day	11% 6% 2 5% 5% 4 5% 4 6% 6% 46%	2170
	11th to 20th	Day	19% 7% 3 7% 4 4 4 3 5% 6% 38%	2000
	21st to end	Day	17% 10% 3 8% 5% 5% 5% 4 6% 7% 32%	2038
November	1st to 10th	Day	11% 9% 3 6% 4 4 4 4 6% 7% 41%	1852
	11th to 20th	Day	17% 14% 3 9% 5% 4 4 3 4 5% 33%	1890
	21st to end	Day	20% 14% 4 8% 5% 4 4 4 5% 5% 27%	1890
December	1st to 10th	Day	14% 14% 7% 13% 7% 6% 4 3 5% 5% 22%	3020
	11th to 20th	Day	19% 19% 6% 9% 7% 5% 5% 3 4 5% 18%	3312
	21st to end	Day	27% 20% 7% 11% 5% 3 3 3 3 4 13%	4070
January	1st to 10th	Night	15% 16% 6% 16% 6% 5% 4 3 4 4 21%	8232
	11th to 20th	Night	11% 17% 8% 14% 7% 6% 5% 4 4 5% 19%	8248
	21st to end	Night	17% 14% 6% 10% 5% 4 5% 4 4 6% 24%	8401
February	1st to 10th	Night	16% 15% 4 11% 7% 5% 4 3 4 6% 25%	2730
	11th to 20th	Night	14% 13% 5% 9% 6% 5% 4 3 5% 6% 28%	2468
	21st to end	Night	20% 12% 4 9% 5% 5% 4 3 5% 5% 29%	2076
March	1st to 10th	Night	17% 12% 3 7% 4 3 4 2 4 6% 37%	5404
	11th to 20th	Night	12% 7% 3 6% 5% 4 4 4 6% 7% 43%	5090
	21st to end	Night	16% 10% 4 8% 4 4 4 3 4 6% 36%	5588
April	1st to 10th	Night	16% 11% 3 6% 5% 4 4 4 5% 7% 34%	2184
	11th to 20th	Night	17% 8% 2 7% 5% 5% 5% 3 5% 6% 37%	1935
	21st to end	Night	16% 10% 3 6% 4 4 4 3 5% 7% 38%	1890
May	1st to 10th	Night	21% 10% 3 8% 6% 4 5% 3 4 6% 29%	4123
	11th to 20th	Night	18% 7% 3 6% 5% 3 4 3 5% 7% 38%	4122
	21st to end	Night	14% 6% 2 6% 4 4 4 4 6% 7% 43%	4520
June	1st to 10th	Night	12% 4 2 6% 5% 4 4 4 7% 8% 45%	1650
	11th to 20th	Night	10% 6% 2 7% 4 4 4 4 7% 8% 44%	1600
	21st to end	Night	10% 4 2 5% 4 4 5% 5% 7% 11% 44%	1600
July	1st to 10th	Night	8% 3 2 5% 4 4 5% 4 6% 11% 48%	3618
	11th to 20th	Night	9% 5% 2 7% 4 4 5% 5% 6% 9% 45%	3617
	21st to end	Night	9% 5% 2 6% 4 4 4 4 6% 9% 46%	4383
August	1st to 10th	Night	12% 6% 3 6% 4 4 3 3 6% 8% 44%	2915
	11th to 20th	Night	9% 5% 2 5% 3 4 4 4 6% 7% 49%	2875
	21st to end	Night	7% 5% 2 7% 4 4 4 4 6% 8% 49%	3345
September	1st to 10th	Night	10% 5% 2 6% 4 4 4 4 5% 7% 51%	2113
	11th to 20th	Night	8% 5% 2 5% 4 3 3 2 5% 7% 56%	2200
	21st to end	Night	13% 7% 3 7% 4 4 4 3 5% 6% 44%	2200
October	1st to 10th	Night	12% 7% 2 5% 5% 4 4 3 5% 7% 48%	2230
	11th to 20th	Night	16% 9% 3 7% 4 4 4 3 4 6% 39%	2400
	21st to end	Night	20% 8% 3 7% 4 4 4 3 5% 5% 37%	2800
November	1st to 10th	Night	12% 8% 3 6% 4 4 4 3 5% 6% 45%	2666
	11th to 20th	Night	16% 11% 3 7% 5% 4 4 4 7% 6% 33%	2730
	21st to end	Night	20% 14% 4 8% 5% 5% 4 3 5% 6% 26%	2730
December	1st to 10th	Night	13% 14% 6% 11% 6% 6% 4 3 5% 6% 25%	4380
	11th to 20th	Night	16% 18% 6% 11% 5% 5% 4 4 4 5% 22%	5089
	21st to end	Night	28% 19% 6% 9% 5% 4 4 2 4 5% 15%	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% 6% 11% 11% 3	4692
	11th to 20th	Day	69% 5% 8% 7% 10%	4724
	21st to end	Day	71% 5% 8% 10% 6%	5780
February	1st to 10th	Day	63% 6% 11% 14% 6%	1890
	11th to 20th	Day	60% 6% 9% 16% 8%	2044
	21st to end	Day	55% 6% 14% 17% 6% 2	1730
March	1st to 10th	Day	58% 5% 11% 18% 8% 1	4669
	11th to 20th	Day	46% 4 16% 24% 9% 1	5090
	21st to end	Day	58% 6% 11% 16% 8% 1	5586
April	1st to 10th	Day	53% 7% 12% 16% 12%	2453
	11th to 20th	Day	48% 7% 12% 20% 12% 2	2634
	21st to end	Day	48% 6% 12% 21% 11% 2	2730
May	1st to 10th	Day	51% 7% 9% 18% 13% 1	6027
	11th to 20th	Day	45% 8% 13% 21% 12% 2	5995
	21st to end	Day	41% 7% 10% 21% 17% 4	6590
June	1st to 10th	Day	39% 7% 12% 25% 14% 3	2745
	11th to 20th	Day	37% 8% 14% 23% 16% 3	2800
	21st to end	Day	38% 9% 13% 21% 15% 4	2800
July	1st to 10th	Day	35% 7% 11% 28% 17% 2	6291
	11th to 20th	Day	32% 8% 16% 27% 16% 1	6285
	21st to end	Day	36% 7% 13% 23% 19% 2	6272
August	1st to 10th	Day	34% 8% 15% 27% 14% 1	4221
	11th to 20th	Day	36% 9% 12% 30% 12% 1	4213
	21st to end	Day	34% 8% 13% 33% 11% 2	4549
September	1st to 10th	Day	37% 7% 14% 22% 17% 2	2266
	11th to 20th	Day	32% 7% 11% 31% 19% 1	2200
	21st to end	Day	41% 7% 12% 24% 16% 1	2200
October	1st to 10th	Day	28% 6% 12% 32% 22% 1	3534
	11th to 20th	Day	39% 5% 10% 27% 19% 1	3240
	21st to end	Day	42% 5% 12% 25% 16% 1	3262
November	1st to 10th	Day	42% 6% 12% 24% 16% 1	1852
	11th to 20th	Day	54% 5% 10% 18% 11% 2	1890
	21st to end	Day	58% 6% 12% 18% 7%	1890
December	1st to 10th	Day	63% 6% 15% 12% 4	3020
	11th to 20th	Day	66% 7% 10% 14% 3	3309
	21st to end	Day	76% 5% 7% 9% 4	4070
January	1st to 10th	Night	65% 5% 9% 15% 5%	8233
	11th to 20th	Night	66% 6% 10% 11% 8%	8248
	21st to end	Night	60% 5% 13% 13% 8%	8401
February	1st to 10th	Night	60% 6% 12% 15% 6% 1	2730
	11th to 20th	Night	56% 6% 11% 19% 8% 1	2469
	21st to end	Night	57% 6% 11% 17% 8% 1	2076
March	1st to 10th	Night	49% 4 12% 24% 10% 1	5404
	11th to 20th	Night	38% 6% 15% 27% 13% 1	5090
	21st to end	Night	48% 6% 10% 21% 13% 1	5588
April	1st to 10th	Night	48% 7% 11% 19% 13% 2	2184
	11th to 20th	Night	46% 6% 10% 18% 17% 2	1935
	21st to end	Night	45% 6% 11% 21% 14% 2	1890
May	1st to 10th	Night	56% 7% 8% 14% 13% 3	4150
	11th to 20th	Night	45% 8% 11% 23% 12% 1	4123
	21st to end	Night	38% 7% 12% 24% 17% 2	4528
June	1st to 10th	Night	34% 8% 12% 25% 18% 2	1650
	11th to 20th	Night	35% 9% 12% 26% 15% 3	1600
	21st to end	Night	31% 9% 15% 24% 19% 1	1600
July	1st to 10th	Night	30% 8% 13% 26% 18% 5%	3580
	11th to 20th	Night	34% 8% 15% 26% 16% 2	3572
	21st to end	Night	33% 7% 14% 22% 21% 3	4254
August	1st to 10th	Night	36% 8% 13% 25% 16% 2	2904
	11th to 20th	Night	31% 7% 13% 30% 17% 1	2902
	21st to end	Night	33% 7% 12% 30% 17% 2	3345
September	1st to 10th	Night	33% 6% 12% 27% 18% 3	2113
	11th to 20th	Night	29% 5% 11% 29% 25% 2	2200
	21st to end	Night	40% 6% 11% 24% 18% 2	2200
October	1st to 10th	Night	25% 4 9% 30% 31% 1	3646
	11th to 20th	Night	37% 4 10% 26% 21%	3899
	21st to end	Night	37% 4 8% 33% 17% 2	4513
November	1st to 10th	Night	41% 5% 11% 26% 16% 1	2666
	11th to 20th	Night	48% 7% 13% 21% 10% 2	2730
	21st to end	Night	58% 6% 12% 19% 6%	2730
December	1st to 10th	Night	59% 6% 13% 17% 6%	4380
	11th to 20th	Night	64% 6% 13% 12% 5% 1	5091
	21st to end	Night	73% 5% 9% 8% 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	23% 22% 33% 18% 2 1	330
	11th to 20th	Day	27% 24% 38% 8% 3	330
	21st to end	Day	31% 18% 28% 20% 3	363
February	1st to 10th	Day	26% 20% 38% 13% 3	330
	11th to 20th	Day	27% 21% 35% 16% 2	330
	21st to end	Day	25% 25% 38% 10% 2	273
March	1st to 10th	Day	26% 20% 36% 14% 4	367
	11th to 20th	Day	28% 15% 35% 16% 5% 1	540
	21st to end	Day	29% 19% 31% 15% 6%	594
April	1st to 10th	Day	24% 18% 32% 18% 7% 1	540
	11th to 20th	Day	23% 22% 27% 17% 8% 2	532
	21st to end	Day	25% 14% 34% 18% 9% 2	540
May	1st to 10th	Day	23% 17% 32% 17% 9% 3	543
	11th to 20th	Day	18% 14% 33% 19% 11% 4	567
	21st to end	Day	14% 11% 28% 24% 17% 7%	617
June	1st to 10th	Day	13% 9% 31% 30% 14% 4	675
	11th to 20th	Day	11% 12% 30% 31% 13% 4	730
	21st to end	Day	12% 8% 31% 31% 14% 5%	730
July	1st to 10th	Day	10% 7% 22% 32% 19% 11%	721
	11th to 20th	Day	11% 7% 25% 30% 17% 10%	718
	21st to end	Day	8% 5% 24% 31% 22% 9%	595
August	1st to 10th	Day	10% 5% 18% 29% 26% 12%	526
	11th to 20th	Day	10% 5% 23% 29% 22% 10%	519
	21st to end	Day	9% 4 21% 31% 23% 11%	583
September	1st to 10th	Day	14% 7% 19% 28% 20% 12%	530
	11th to 20th	Day	9% 3 18% 28% 28% 14%	530
	21st to end	Day	18% 11% 22% 23% 18% 7%	530
October	1st to 10th	Day	14% 7% 27% 26% 19% 7%	530
	11th to 20th	Day	18% 17% 33% 21% 9% 2	530
	21st to end	Day	25% 17% 34% 18% 6%	421
November	1st to 10th	Day	16% 19% 34% 20% 7% 3	330
	11th to 20th	Day	29% 22% 26% 17% 5%	330
	21st to end	Day	28% 21% 33% 16% 2	330
December	1st to 10th	Day	25% 24% 29% 15% 6%	330
	11th to 20th	Day	24% 25% 33% 15% 3	330
	21st to end	Day	32% 20% 33% 13% 3	363
January	1st to 10th	Night	24% 18% 34% 20% 4	1050
	11th to 20th	Night	27% 17% 35% 16% 4 1	1044
	21st to end	Night	24% 16% 36% 20% 4	924
February	1st to 10th	Night	25% 14% 35% 23% 4	840
	11th to 20th	Night	23% 14% 36% 24% 3	823
	21st to end	Night	26% 17% 38% 17% 2	692
March	1st to 10th	Night	21% 13% 34% 25% 6% 1	840
	11th to 20th	Night	18% 19% 35% 22% 5%	840
	21st to end	Night	23% 14% 31% 24% 7%	924
April	1st to 10th	Night	22% 16% 34% 24% 4	714
	11th to 20th	Night	21% 13% 33% 24% 8% 1	623
	21st to end	Night	21% 9% 36% 26% 8%	630
May	1st to 10th	Night	21% 9% 31% 30% 9% 1	630
	11th to 20th	Night	19% 8% 29% 29% 13% 1	621
	21st to end	Night	11% 4 28% 37% 18% 2	683
June	1st to 10th	Night	13% 4 28% 41% 14%	600
	11th to 20th	Night	12% 5% 26% 40% 17% 1	600
	21st to end	Night	12% 3 26% 36% 22% 2	600
July	1st to 10th	Night	9% 3 19% 41% 25% 3	585
	11th to 20th	Night	10% 3 22% 39% 22% 4	582
	21st to end	Night	7% 2 17% 43% 29% 2	657
August	1st to 10th	Night	8% 2 16% 43% 28% 3	597
	11th to 20th	Night	10% 4 17% 35% 31% 4	594
	21st to end	Night	8% 2 17% 38% 31% 4	660
September	1st to 10th	Night	12% 5% 19% 34% 26% 4	713
	11th to 20th	Night	9% 4 18% 33% 33% 4	800
	21st to end	Night	16% 7% 22% 30% 22% 3	800
October	1st to 10th	Night	13% 8% 26% 31% 19% 3	800
	11th to 20th	Night	20% 14% 29% 25% 10% 2	800
	21st to end	Night	24% 15% 28% 26% 8%	880
November	1st to 10th	Night	17% 13% 34% 25% 9% 1	820
	11th to 20th	Night	22% 15% 34% 21% 7%	840
	21st to end	Night	28% 12% 34% 20% 5%	840
December	1st to 10th	Night	23% 17% 34% 21% 4	840
	11th to 20th	Night	26% 15% 35% 21% 4	901
	21st to end	Night	30% 14% 34% 19% 3	1155

Page generated at 2026-05-29 01:53:26. Version ac07

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