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 2025-02-06.

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%	108639
	Night	15% 11% 4 8% 5% 4 4 3 5% 6% 34%	109697
Transparency	Day	48% 7% 12% 21% 12% 1	112269
	Night	48% 6% 11% 20% 12% 1	114194
Seeing	Day	18% 13% 29% 23% 13% 5%	15876
	Night	19% 10% 29% 28% 12% 1	25800

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% 16% 8% 18% 6% 5% 4 3 4 4 18%	3617
	11th to 20th	Day	14% 17% 5% 15% 9% 6% 8% 4 3 4 15%	3634
	21st to end	Day	21% 19% 8% 13% 5% 4 4 3 3 4 16%	4466
February	1st to 10th	Day	15% 16% 4 12% 8% 6% 5% 4 5% 5% 21%	1755
	11th to 20th	Day	14% 12% 4 13% 7% 6% 5% 3 5% 5% 25%	1834
	21st to end	Day	18% 14% 3 10% 6% 4 4 4 5% 5% 28%	1570
March	1st to 10th	Day	20% 17% 4 10% 6% 4 3 3 4 6% 25%	3179
	11th to 20th	Day	13% 13% 3 8% 5% 4 4 4 4 6% 34%	3480
	21st to end	Day	20% 13% 5% 8% 5% 5% 5% 3 5% 5% 27%	3815
April	1st to 10th	Day	15% 11% 3 9% 6% 5% 5% 4 5% 6% 30%	2219
	11th to 20th	Day	18% 9% 2 7% 5% 5% 4 3 6% 6% 34%	2380
	21st to end	Day	19% 9% 3 7% 5% 4 4 3 5% 8% 34%	2470
May	1st to 10th	Day	18% 8% 3 7% 5% 5% 5% 4 6% 7% 33%	4082
	11th to 20th	Day	17% 8% 2 7% 5% 5% 5% 3 6% 8% 34%	4081
	21st to end	Day	10% 6% 3 6% 5% 4 5% 5% 6% 7% 43%	4476
June	1st to 10th	Day	11% 7% 1 6% 5% 5% 6% 4 7% 8% 40%	2608
	11th to 20th	Day	10% 6% 2 6% 5% 5% 5% 3 6% 8% 43%	2660
	21st to end	Day	10% 6% 3 5% 6% 5% 6% 5% 6% 10% 38%	2660
July	1st to 10th	Day	11% 6% 3 7% 5% 5% 4 4 7% 9% 40%	4440
	11th to 20th	Day	7% 4 2 5% 4 5% 5% 5% 7% 11% 46%	4440
	21st to end	Day	6% 5% 2 6% 6% 6% 6% 5% 7% 9% 41%	4484
August	1st to 10th	Day	10% 5% 1 6% 4 5% 5% 5% 7% 9% 43%	4104
	11th to 20th	Day	9% 6% 2 6% 5% 6% 7% 5% 7% 9% 39%	4044
	21st to end	Day	8% 4 2 7% 5% 4 5% 4 7% 10% 42%	4411
September	1st to 10th	Day	12% 5% 2 7% 5% 3 4 4 6% 8% 44%	2153
	11th to 20th	Day	8% 5% 2 6% 4 5% 5% 4 6% 9% 46%	2090
	21st to end	Day	15% 9% 2 5% 3 3 5% 4 5% 7% 40%	2090
October	1st to 10th	Day	11% 6% 3 6% 5% 4 4 4 6% 6% 45%	2062
	11th to 20th	Day	19% 8% 3 7% 4 4 4 3 5% 6% 37%	1900
	21st to end	Day	18% 10% 3 8% 5% 5% 5% 4 6% 7% 31%	1936
November	1st to 10th	Day	11% 9% 3 6% 4 5% 4 3 6% 7% 41%	1762
	11th to 20th	Day	17% 14% 3 9% 5% 4 4 3 5% 5% 32%	1800
	21st to end	Day	20% 13% 4 8% 5% 3 4 4 5% 5% 27%	1800
December	1st to 10th	Day	14% 14% 7% 13% 7% 6% 4 3 5% 5% 22%	2930
	11th to 20th	Day	20% 19% 6% 8% 7% 5% 5% 3 4 5% 18%	3225
	21st to end	Day	27% 20% 7% 11% 5% 3 3 3 3 4 13%	3982
January	1st to 10th	Night	15% 16% 6% 16% 6% 4 4 3 4 5% 21%	6348
	11th to 20th	Night	13% 18% 8% 14% 7% 6% 5% 4 4 4 16%	6342
	21st to end	Night	20% 16% 7% 11% 5% 4 4 3 4 5% 23%	6490
February	1st to 10th	Night	18% 15% 4 11% 7% 5% 4 3 4 5% 24%	2536
	11th to 20th	Night	14% 14% 5% 9% 6% 5% 4 3 5% 6% 28%	2228
	21st to end	Night	21% 12% 4 9% 5% 5% 4 3 5% 5% 28%	1884
March	1st to 10th	Night	18% 14% 4 7% 4 3 4 2 4 5% 35%	3674
	11th to 20th	Night	12% 7% 3 6% 5% 4 4 4 6% 7% 43%	3480
	21st to end	Night	17% 10% 4 8% 5% 4 4 4 5% 6% 35%	3817
April	1st to 10th	Night	15% 11% 3 6% 5% 4 4 4 5% 7% 36%	1976
	11th to 20th	Night	18% 8% 2 7% 6% 5% 5% 3 5% 6% 37%	1749
	21st to end	Night	16% 10% 3 6% 4 4 4 3 5% 7% 37%	1710
May	1st to 10th	Night	18% 8% 3 7% 6% 5% 5% 3 5% 8% 32%	2813
	11th to 20th	Night	18% 7% 2 6% 4 3 4 3 5% 6% 39%	2812
	21st to end	Night	10% 5% 2 6% 4 4 4 4 6% 8% 46%	3079
June	1st to 10th	Night	11% 4 2 6% 5% 4 4 4 7% 8% 45%	1568
	11th to 20th	Night	10% 6% 2 7% 3 4 4 4 8% 7% 44%	1520
	21st to end	Night	11% 4 2 5% 4 4 5% 5% 7% 11% 43%	1520
July	1st to 10th	Night	9% 4 2 5% 3 4 4 4 6% 11% 48%	2530
	11th to 20th	Night	8% 3 2 5% 3 4 4 4 6% 9% 51%	2530
	21st to end	Night	6% 5% 2 6% 5% 5% 5% 5% 7% 9% 45%	3056
August	1st to 10th	Night	12% 7% 3 6% 4 4 3 3 6% 8% 43%	2825
	11th to 20th	Night	9% 5% 2 5% 3 4 4 5% 6% 7% 49%	2785
	21st to end	Night	8% 5% 2 7% 4 4 4 4 5% 8% 49%	3242
September	1st to 10th	Night	10% 5% 2 6% 4 4 4 4 5% 7% 50%	2007
	11th to 20th	Night	9% 5% 2 5% 4 4 3 2 5% 7% 54%	2090
	21st to end	Night	13% 7% 3 7% 4 4 4 3 5% 6% 44%	2090
October	1st to 10th	Night	12% 7% 2 5% 5% 4 3 3 5% 7% 47%	2118
	11th to 20th	Night	17% 9% 3 6% 4 4 4 3 4 6% 39%	2280
	21st to end	Night	20% 8% 3 7% 4 4 4 3 5% 5% 36%	2660
November	1st to 10th	Night	13% 8% 3 6% 4 4 4 3 5% 6% 44%	2536
	11th to 20th	Night	16% 11% 3 7% 5% 4 4 4 7% 6% 32%	2600
	21st to end	Night	20% 14% 4 8% 5% 5% 4 3 5% 6% 26%	2600
December	1st to 10th	Night	13% 13% 6% 11% 6% 6% 4 3 5% 6% 25%	4250
	11th to 20th	Night	16% 18% 6% 11% 5% 5% 4 3 4 5% 22%	4956
	21st to end	Night	28% 19% 6% 9% 5% 4 4 3 4 5% 15%	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	68% 6% 9% 13% 4	3618
	11th to 20th	Day	72% 5% 9% 6% 7%	3634
	21st to end	Day	73% 5% 8% 9% 6%	4466
February	1st to 10th	Day	63% 6% 12% 13% 5%	1755
	11th to 20th	Day	59% 6% 9% 16% 8%	1844
	21st to end	Day	55% 6% 14% 17% 6% 2	1570
March	1st to 10th	Day	61% 6% 10% 17% 5%	3179
	11th to 20th	Day	48% 5% 16% 21% 8% 1	3480
	21st to end	Day	57% 7% 13% 16% 6%	3815
April	1st to 10th	Day	52% 7% 12% 16% 13%	2219
	11th to 20th	Day	48% 7% 12% 20% 12% 2	2380
	21st to end	Day	48% 6% 11% 20% 11% 2	2470
May	1st to 10th	Day	48% 8% 11% 23% 10% 1	4120
	11th to 20th	Day	44% 9% 13% 23% 10% 1	4085
	21st to end	Day	37% 8% 11% 19% 20% 5%	4489
June	1st to 10th	Day	39% 8% 12% 25% 13% 3	2608
	11th to 20th	Day	37% 8% 14% 22% 17% 3	2660
	21st to end	Day	38% 9% 13% 20% 15% 4	2660
July	1st to 10th	Day	40% 6% 12% 22% 16% 2	4371
	11th to 20th	Day	29% 8% 14% 31% 17% 1	4365
	21st to end	Day	36% 8% 14% 19% 20% 2	4342
August	1st to 10th	Day	35% 8% 15% 27% 14% 1	4091
	11th to 20th	Day	36% 9% 12% 30% 12% 1	4083
	21st to end	Day	34% 7% 13% 32% 11% 2	4410
September	1st to 10th	Day	37% 8% 14% 21% 18% 2	2153
	11th to 20th	Day	33% 7% 11% 29% 20% 1	2090
	21st to end	Day	41% 7% 12% 24% 15% 1	2090
October	1st to 10th	Day	28% 6% 12% 32% 21% 1	3426
	11th to 20th	Day	39% 5% 10% 26% 19%	3140
	21st to end	Day	42% 5% 12% 25% 15% 1	3160
November	1st to 10th	Day	42% 6% 12% 24% 15% 1	1762
	11th to 20th	Day	55% 5% 10% 17% 11% 1	1800
	21st to end	Day	57% 6% 12% 18% 7%	1800
December	1st to 10th	Day	62% 7% 15% 12% 3	2930
	11th to 20th	Day	66% 7% 10% 14% 3	3222
	21st to end	Day	76% 5% 7% 9% 4	3982
January	1st to 10th	Night	65% 6% 8% 16% 5%	6348
	11th to 20th	Night	70% 6% 10% 8% 7%	6342
	21st to end	Night	64% 5% 13% 9% 8%	6490
February	1st to 10th	Night	62% 6% 12% 15% 5% 1	2536
	11th to 20th	Night	55% 6% 11% 19% 8% 1	2229
	21st to end	Night	57% 6% 10% 17% 8% 1	1884
March	1st to 10th	Night	52% 5% 11% 24% 8% 1	3674
	11th to 20th	Night	37% 8% 16% 22% 16% 1	3480
	21st to end	Night	48% 7% 12% 24% 8% 1	3817
April	1st to 10th	Night	46% 7% 12% 19% 14% 2	1976
	11th to 20th	Night	47% 7% 10% 19% 16% 2	1749
	21st to end	Night	45% 6% 11% 20% 15% 2	1710
May	1st to 10th	Night	50% 8% 9% 18% 13% 1	2840
	11th to 20th	Night	43% 8% 11% 25% 12% 1	2813
	21st to end	Night	32% 8% 12% 25% 20% 3	3087
June	1st to 10th	Night	34% 8% 13% 26% 17% 3	1568
	11th to 20th	Night	34% 9% 12% 26% 16% 3	1520
	21st to end	Night	32% 9% 14% 24% 19% 1	1520
July	1st to 10th	Night	30% 7% 12% 27% 18% 6%	2490
	11th to 20th	Night	28% 8% 15% 28% 19% 2	2482
	21st to end	Night	32% 9% 17% 20% 19% 4	2958
August	1st to 10th	Night	37% 8% 13% 24% 15% 2	2814
	11th to 20th	Night	31% 7% 12% 30% 18% 1	2812
	21st to end	Night	33% 7% 12% 29% 17% 2	3242
September	1st to 10th	Night	34% 6% 13% 25% 19% 3	2007
	11th to 20th	Night	30% 5% 11% 27% 25% 2	2090
	21st to end	Night	40% 6% 11% 24% 17% 2	2090
October	1st to 10th	Night	25% 4 9% 30% 31% 1	3534
	11th to 20th	Night	37% 4 10% 26% 21% 1	3779
	21st to end	Night	37% 4 8% 33% 16% 2	4373
November	1st to 10th	Night	41% 5% 11% 25% 16% 1	2536
	11th to 20th	Night	48% 7% 13% 20% 11% 2	2600
	21st to end	Night	58% 6% 12% 19% 6%	2600
December	1st to 10th	Night	58% 6% 13% 17% 5%	4250
	11th to 20th	Night	64% 6% 13% 12% 5% 1	4958
	21st to end	Night	72% 5% 10% 8% 5%	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	22% 23% 32% 20% 2 1	300
	11th to 20th	Day	25% 22% 40% 9% 4	300
	21st to end	Day	30% 18% 30% 19% 3	330
February	1st to 10th	Day	28% 19% 37% 13% 3	285
	11th to 20th	Day	26% 19% 35% 17% 2	270
	21st to end	Day	27% 23% 39% 10% 2	225
March	1st to 10th	Day	25% 18% 35% 17% 4	303
	11th to 20th	Day	24% 13% 38% 18% 6% 1	460
	21st to end	Day	31% 16% 30% 16% 6%	506
April	1st to 10th	Day	19% 17% 35% 19% 8% 1	460
	11th to 20th	Day	22% 22% 27% 18% 8% 3	452
	21st to end	Day	25% 14% 35% 17% 8% 2	460
May	1st to 10th	Day	22% 15% 32% 18% 10% 3	463
	11th to 20th	Day	18% 13% 30% 21% 13% 5%	487
	21st to end	Day	12% 9% 28% 25% 19% 8%	529
June	1st to 10th	Day	12% 8% 30% 31% 14% 4	628
	11th to 20th	Day	11% 11% 28% 32% 14% 4	680
	21st to end	Day	13% 8% 30% 30% 14% 5%	680
July	1st to 10th	Day	9% 7% 21% 32% 20% 11%	671
	11th to 20th	Day	11% 6% 24% 30% 18% 10%	668
	21st to end	Day	8% 5% 25% 31% 22% 9%	550
August	1st to 10th	Day	11% 5% 17% 29% 25% 13%	486
	11th to 20th	Day	10% 5% 24% 29% 22% 10%	479
	21st to end	Day	9% 4 20% 30% 24% 12%	539
September	1st to 10th	Day	14% 7% 19% 30% 20% 11%	490
	11th to 20th	Day	10% 3 20% 27% 27% 14%	490
	21st to end	Day	19% 12% 23% 22% 17% 7%	490
October	1st to 10th	Day	14% 7% 27% 27% 18% 7%	490
	11th to 20th	Day	19% 19% 31% 21% 9% 2	490
	21st to end	Day	25% 15% 34% 19% 6% 1	385
November	1st to 10th	Day	14% 19% 36% 20% 8% 4	300
	11th to 20th	Day	27% 21% 26% 19% 6%	300
	21st to end	Day	28% 19% 34% 17% 2	300
December	1st to 10th	Day	24% 25% 29% 15% 7%	300
	11th to 20th	Day	23% 25% 34% 15% 3	300
	21st to end	Day	31% 20% 32% 14% 3	330
January	1st to 10th	Night	23% 18% 34% 20% 5%	1000
	11th to 20th	Night	27% 15% 36% 17% 5% 1	995
	21st to end	Night	24% 15% 36% 21% 4	880
February	1st to 10th	Night	26% 14% 34% 23% 3	781
	11th to 20th	Night	23% 13% 36% 24% 3	743
	21st to end	Night	26% 17% 37% 18% 2	628
March	1st to 10th	Night	21% 12% 33% 27% 6% 1	760
	11th to 20th	Night	17% 18% 35% 24% 6% 1	760
	21st to end	Night	23% 13% 31% 25% 8%	836
April	1st to 10th	Night	20% 15% 35% 26% 4	646
	11th to 20th	Night	21% 12% 34% 25% 8% 1	563
	21st to end	Night	21% 9% 36% 25% 8% 1	570
May	1st to 10th	Night	21% 8% 30% 31% 9% 1	570
	11th to 20th	Night	19% 6% 29% 30% 14% 1	561
	21st to end	Night	10% 3 27% 37% 20% 3	617
June	1st to 10th	Night	12% 4 28% 41% 15%	570
	11th to 20th	Night	11% 5% 26% 40% 18% 1	570
	21st to end	Night	12% 4 26% 35% 21% 2	570
July	1st to 10th	Night	9% 3 19% 40% 26% 3	555
	11th to 20th	Night	9% 3 22% 39% 22% 4	552
	21st to end	Night	7% 2 17% 44% 29% 2	624
August	1st to 10th	Night	8% 2 16% 43% 28% 3	567
	11th to 20th	Night	10% 3 17% 35% 30% 4	564
	21st to end	Night	9% 2 16% 37% 32% 4	627
September	1st to 10th	Night	12% 5% 19% 34% 26% 4	677
	11th to 20th	Night	9% 4 19% 33% 31% 4	760
	21st to end	Night	16% 7% 22% 30% 21% 3	760
October	1st to 10th	Night	13% 8% 26% 31% 19% 3	760
	11th to 20th	Night	20% 15% 29% 24% 10% 2	760
	21st to end	Night	24% 15% 28% 26% 8%	836
November	1st to 10th	Night	17% 14% 33% 26% 10% 1	780
	11th to 20th	Night	22% 14% 35% 21% 7% 1	800
	21st to end	Night	27% 12% 34% 21% 6%	800
December	1st to 10th	Night	24% 17% 35% 21% 4	800
	11th to 20th	Night	27% 14% 34% 21% 4	858
	21st to end	Night	30% 13% 34% 20% 3	1100

Page generated at 2025-02-03 03:05:51. Version ac07

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