WhistleStop Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the WhistleStop Observatory clear sky chart from 2005-11-06 to 2025-01-22.

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% 9% 3 8% 5% 4 4 4 6% 7% 36%	105270
	Night	16% 10% 3 8% 5% 5% 5% 4 5% 6% 33%	104099
Transparency	Day	45% 7% 13% 22% 11% 1	108856
	Night	48% 7% 12% 20% 11% 1	108601
Seeing	Day	18% 11% 28% 26% 13% 4	14651
	Night	19% 10% 29% 28% 13% 1	24061

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	16% 14% 6% 11% 5% 5% 5% 4 5% 5% 24%	2597
	11th to 20th	Day	11% 21% 7% 15% 10% 7% 5% 4 5% 4 13%	2671
	21st to end	Day	19% 19% 6% 10% 7% 5% 5% 3 4 4 19%	3133
February	1st to 10th	Day	13% 15% 5% 13% 7% 6% 5% 4 6% 5% 21%	1710
	11th to 20th	Day	14% 10% 4 14% 7% 5% 4 3 5% 6% 27%	1663
	21st to end	Day	19% 12% 4 9% 5% 5% 5% 4 5% 7% 26%	1495
March	1st to 10th	Day	20% 8% 3 8% 5% 5% 4 4 5% 6% 31%	3357
	11th to 20th	Day	9% 13% 4 10% 6% 5% 5% 4 5% 7% 32%	3480
	21st to end	Day	15% 8% 3 6% 4 4 3 3 4 7% 44%	3867
April	1st to 10th	Day	17% 10% 3 8% 5% 4 5% 3 5% 7% 33%	2280
	11th to 20th	Day	19% 7% 3 7% 5% 3 4 3 4 6% 39%	2254
	21st to end	Day	19% 8% 2 7% 5% 4 5% 4 6% 5% 35%	2456
May	1st to 10th	Day	17% 7% 3 8% 5% 4 5% 4 6% 7% 34%	4254
	11th to 20th	Day	15% 9% 3 8% 4 4 5% 6% 7% 8% 32%	4398
	21st to end	Day	11% 5% 2 5% 5% 4 4 4 6% 9% 46%	4837
June	1st to 10th	Day	16% 6% 2 6% 5% 5% 4 5% 6% 8% 36%	2660
	11th to 20th	Day	11% 5% 1 6% 4 4 4 4 6% 10% 43%	2660
	21st to end	Day	13% 6% 2 6% 5% 4 4 5% 6% 7% 43%	2660
July	1st to 10th	Day	9% 4 2 5% 5% 5% 5% 4 6% 10% 46%	4440
	11th to 20th	Day	10% 4 1 4 4 4 4 4 6% 9% 49%	4440
	21st to end	Day	6% 4 2 6% 5% 5% 5% 5% 8% 10% 45%	4703
August	1st to 10th	Day	16% 7% 2 6% 4 4 4 3 5% 7% 42%	4104
	11th to 20th	Day	12% 5% 2 6% 4 4 5% 4 6% 10% 41%	4044
	21st to end	Day	9% 5% 2 5% 4 4 5% 4 6% 9% 45%	4252
September	1st to 10th	Day	12% 6% 1 5% 4 4 4 4 6% 7% 46%	2280
	11th to 20th	Day	10% 5% 2 4 3 3 3 3 7% 7% 51%	2267
	21st to end	Day	14% 6% 3 6% 4 5% 4 4 5% 7% 43%	1990
October	1st to 10th	Day	17% 8% 2 8% 5% 5% 4 4 4 7% 36%	1900
	11th to 20th	Day	18% 8% 3 7% 5% 4 5% 4 5% 7% 33%	1900
	21st to end	Day	24% 10% 3 8% 4 4 4 3 6% 6% 28%	2081
November	1st to 10th	Day	14% 8% 3 7% 5% 4 4 4 5% 8% 36%	1691
	11th to 20th	Day	14% 11% 3 8% 6% 5% 5% 5% 6% 7% 29%	1600
	21st to end	Day	20% 12% 3 9% 6% 5% 4 3 5% 7% 25%	1600
December	1st to 10th	Day	18% 16% 5% 9% 7% 5% 6% 4 5% 6% 18%	2600
	11th to 20th	Day	17% 16% 5% 10% 5% 4 4 3 5% 5% 25%	2986
	21st to end	Day	26% 13% 5% 11% 5% 5% 5% 4 4 4 18%	3960
January	1st to 10th	Night	19% 14% 5% 10% 7% 5% 5% 3 4 6% 22%	4569
	11th to 20th	Night	15% 14% 5% 15% 9% 7% 6% 5% 7% 4 13%	4506
	21st to end	Night	20% 16% 5% 10% 6% 5% 5% 4 4 4 20%	4546
February	1st to 10th	Night	16% 16% 5% 11% 6% 5% 5% 4 5% 6% 22%	2470
	11th to 20th	Night	14% 14% 6% 8% 6% 5% 5% 5% 5% 6% 27%	2403
	21st to end	Night	16% 11% 3 8% 7% 5% 5% 5% 5% 5% 30%	1958
March	1st to 10th	Night	19% 10% 3 8% 5% 5% 4 3 5% 5% 32%	3479
	11th to 20th	Night	16% 11% 4 8% 5% 4 4 3 6% 6% 33%	3479
	21st to end	Night	17% 6% 2 5% 3 3 3 3 4 6% 47%	3764
April	1st to 10th	Night	20% 9% 3 6% 5% 4 5% 3 5% 7% 34%	1900
	11th to 20th	Night	17% 8% 3 7% 6% 4 4 4 5% 5% 38%	1875
	21st to end	Night	19% 8% 2 7% 3 4 5% 4 6% 6% 36%	1739
May	1st to 10th	Night	21% 8% 3 7% 4 4 5% 3 5% 5% 34%	2687
	11th to 20th	Night	15% 7% 3 7% 5% 5% 6% 4 7% 6% 34%	2495
	21st to end	Night	11% 5% 2 6% 4 4 4 4 5% 9% 45%	2738
June	1st to 10th	Night	13% 7% 3 7% 6% 4 5% 4 6% 7% 39%	1520
	11th to 20th	Night	9% 5% 3 6% 5% 5% 5% 5% 7% 9% 42%	1520
	21st to end	Night	13% 7% 2 7% 5% 4 4 4 5% 7% 42%	1520
July	1st to 10th	Night	7% 5% 2 6% 5% 5% 4 5% 7% 9% 45%	2520
	11th to 20th	Night	10% 6% 2 5% 4 4 4 4 5% 8% 48%	2520
	21st to end	Night	9% 6% 2 5% 3 3 4 3 6% 8% 51%	2846
August	1st to 10th	Night	10% 5% 2 6% 4 4 4 4 6% 8% 47%	2825
	11th to 20th	Night	12% 6% 2 6% 3 4 4 3 5% 7% 48%	2785
	21st to end	Night	9% 5% 1 6% 3 4 5% 4 6% 9% 49%	3371
September	1st to 10th	Night	11% 5% 2 6% 4 3 5% 4 5% 9% 46%	1900
	11th to 20th	Night	11% 5% 2 5% 3 3 4 3 5% 6% 53%	1909
	21st to end	Night	17% 5% 2 7% 4 4 4 2 3 6% 45%	2190
October	1st to 10th	Night	16% 7% 3 7% 5% 5% 4 4 5% 7% 39%	2280
	11th to 20th	Night	20% 8% 3 7% 4 5% 5% 4 6% 7% 32%	2280
	21st to end	Night	21% 9% 3 7% 4 4 5% 4 6% 6% 30%	2517
November	1st to 10th	Night	18% 7% 3 6% 5% 5% 5% 4 5% 6% 38%	2604
	11th to 20th	Night	17% 9% 3 7% 6% 4 4 4 6% 8% 31%	2800
	21st to end	Night	22% 11% 3 7% 5% 5% 5% 4 6% 7% 25%	2800
December	1st to 10th	Night	18% 17% 4 8% 6% 6% 5% 4 6% 6% 21%	4580
	11th to 20th	Night	19% 15% 4 9% 5% 5% 5% 4 6% 5% 22%	5208
	21st to end	Night	26% 14% 4 10% 5% 5% 5% 4 5% 5% 17%	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	54% 12% 13% 13% 6% 1	2598
	11th to 20th	Day	68% 9% 12% 6% 5%	2671
	21st to end	Day	68% 7% 10% 11% 4 1	3133
February	1st to 10th	Day	60% 8% 12% 15% 3 1	1710
	11th to 20th	Day	57% 7% 10% 15% 10% 1	1672
	21st to end	Day	58% 6% 11% 16% 8% 1	1495
March	1st to 10th	Day	52% 6% 13% 19% 9% 1	3360
	11th to 20th	Day	49% 7% 17% 16% 10% 1	3480
	21st to end	Day	40% 5% 10% 26% 17% 2	3867
April	1st to 10th	Day	52% 5% 10% 20% 11% 2	2280
	11th to 20th	Day	48% 4 9% 22% 16% 1	2254
	21st to end	Day	50% 6% 9% 19% 13% 3	2456
May	1st to 10th	Day	47% 7% 11% 21% 12% 2	4266
	11th to 20th	Day	46% 11% 15% 18% 10% 1	4402
	21st to end	Day	34% 8% 14% 26% 15% 3	4837
June	1st to 10th	Day	44% 8% 13% 20% 14% 1	2660
	11th to 20th	Day	36% 7% 15% 27% 14% 1	2660
	21st to end	Day	39% 8% 15% 23% 15% 1	2660
July	1st to 10th	Day	34% 5% 14% 31% 15% 1	4370
	11th to 20th	Day	31% 7% 15% 37% 10%	4365
	21st to end	Day	33% 8% 15% 28% 14% 1	4553
August	1st to 10th	Day	42% 7% 14% 24% 13% 1	4091
	11th to 20th	Day	38% 7% 15% 30% 10%	4083
	21st to end	Day	34% 7% 13% 35% 10% 1	4252
September	1st to 10th	Day	36% 8% 14% 30% 12% 1	2280
	11th to 20th	Day	31% 6% 15% 32% 15% 1	2267
	21st to end	Day	41% 8% 14% 27% 10%	1990
October	1st to 10th	Day	38% 6% 12% 30% 13% 1	3160
	11th to 20th	Day	40% 5% 16% 24% 14% 1	3140
	21st to end	Day	48% 6% 16% 21% 9%	3407
November	1st to 10th	Day	46% 7% 15% 20% 11% 1	1691
	11th to 20th	Day	50% 7% 17% 16% 10% 1	1600
	21st to end	Day	58% 8% 13% 16% 6%	1600
December	1st to 10th	Day	63% 7% 16% 11% 3	2600
	11th to 20th	Day	60% 7% 10% 15% 8%	2986
	21st to end	Day	66% 7% 9% 12% 7%	3960
January	1st to 10th	Night	60% 10% 13% 12% 5%	4569
	11th to 20th	Night	63% 12% 13% 8% 4 1	4506
	21st to end	Night	64% 10% 10% 11% 5%	4546
February	1st to 10th	Night	60% 7% 13% 15% 4	2470
	11th to 20th	Night	55% 7% 11% 17% 9% 1	2405
	21st to end	Night	52% 6% 14% 21% 8% 1	1958
March	1st to 10th	Night	53% 6% 10% 21% 10% 1	3489
	11th to 20th	Night	49% 7% 14% 18% 11% 1	3480
	21st to end	Night	38% 4 11% 24% 21% 3	3764
April	1st to 10th	Night	49% 5% 10% 20% 14% 2	1900
	11th to 20th	Night	47% 6% 9% 18% 18% 2	1875
	21st to end	Night	46% 7% 12% 17% 16% 3	1739
May	1st to 10th	Night	49% 6% 12% 16% 15% 2	2694
	11th to 20th	Night	44% 8% 15% 20% 10% 2	2496
	21st to end	Night	34% 7% 12% 24% 20% 2	2739
June	1st to 10th	Night	42% 7% 12% 23% 14% 3	1520
	11th to 20th	Night	35% 6% 14% 21% 22% 3	1520
	21st to end	Night	41% 7% 12% 23% 16% 2	1520
July	1st to 10th	Night	34% 6% 13% 24% 22% 1	2480
	11th to 20th	Night	34% 5% 12% 33% 15%	2472
	21st to end	Night	31% 6% 16% 33% 13% 2	2755
August	1st to 10th	Night	35% 6% 13% 28% 16% 2	2814
	11th to 20th	Night	36% 5% 17% 29% 12% 2	2812
	21st to end	Night	30% 6% 13% 36% 14% 1	3371
September	1st to 10th	Night	35% 7% 15% 27% 14% 1	1900
	11th to 20th	Night	31% 6% 13% 32% 18% 1	1909
	21st to end	Night	43% 5% 11% 28% 14% 1	2190
October	1st to 10th	Night	36% 5% 9% 27% 22% 1	3800
	11th to 20th	Night	42% 5% 14% 30% 9%	3779
	21st to end	Night	42% 6% 15% 27% 10% 1	4141
November	1st to 10th	Night	45% 7% 14% 21% 13% 1	2604
	11th to 20th	Night	49% 7% 14% 19% 11% 1	2800
	21st to end	Night	54% 10% 16% 15% 4 1	2800
December	1st to 10th	Night	59% 7% 15% 16% 4	4580
	11th to 20th	Night	58% 8% 11% 12% 10% 1	5208
	21st to end	Night	67% 6% 9% 11% 6%	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% 20% 33% 17% 7% 2	200
	11th to 20th	Day	24% 24% 30% 16% 4 1	223
	21st to end	Day	22% 22% 30% 21% 4 1	300
February	1st to 10th	Day	23% 25% 35% 13% 4	270
	11th to 20th	Day	21% 22% 39% 17% 2	270
	21st to end	Day	29% 21% 33% 15% 2	307
March	1st to 10th	Day	20% 17% 37% 20% 6% 1	460
	11th to 20th	Day	23% 17% 31% 23% 4 1	460
	21st to end	Day	27% 11% 36% 18% 8%	506
April	1st to 10th	Day	22% 13% 34% 22% 7% 1	460
	11th to 20th	Day	24% 13% 30% 19% 9% 4	452
	21st to end	Day	23% 11% 35% 22% 8% 2	460
May	1st to 10th	Day	22% 8% 28% 28% 10% 5%	463
	11th to 20th	Day	16% 7% 25% 32% 15% 4	487
	21st to end	Day	13% 5% 26% 32% 20% 5%	529
June	1st to 10th	Day	18% 9% 25% 30% 17% 2	490
	11th to 20th	Day	11% 6% 28% 32% 19% 4	490
	21st to end	Day	19% 7% 25% 30% 16% 4	490
July	1st to 10th	Day	9% 3 20% 35% 27% 5%	485
	11th to 20th	Day	13% 4 23% 32% 20% 8%	484
	21st to end	Day	9% 4 24% 42% 15% 6%	532
August	1st to 10th	Day	12% 4 21% 32% 23% 8%	486
	11th to 20th	Day	11% 4 20% 32% 26% 8%	479
	21st to end	Day	10% 6% 23% 35% 18% 7%	539
September	1st to 10th	Day	12% 6% 25% 29% 20% 8%	490
	11th to 20th	Day	10% 4 18% 30% 23% 14%	490
	21st to end	Day	16% 7% 27% 28% 19% 5%	490
October	1st to 10th	Day	20% 12% 28% 18% 18% 5%	490
	11th to 20th	Day	20% 13% 31% 24% 9% 2	490
	21st to end	Day	26% 16% 28% 22% 7%	534
November	1st to 10th	Day	22% 18% 26% 23% 9% 1	325
	11th to 20th	Day	20% 21% 35% 20% 4 1	200
	21st to end	Day	26% 16% 33% 18% 7% 1	200
December	1st to 10th	Day	26% 19% 30% 19% 8%	200
	11th to 20th	Day	29% 26% 30% 13% 3 1	200
	21st to end	Day	32% 19% 33% 14% 1 1	220
January	1st to 10th	Night	25% 16% 35% 18% 6%	800
	11th to 20th	Night	26% 16% 30% 23% 5%	800
	21st to end	Night	20% 14% 32% 27% 6% 1	841
February	1st to 10th	Night	23% 15% 39% 21% 3	760
	11th to 20th	Night	21% 14% 38% 22% 4	743
	21st to end	Night	23% 16% 37% 21% 4	628
March	1st to 10th	Night	20% 14% 33% 28% 4 1	760
	11th to 20th	Night	22% 14% 34% 25% 5% 1	760
	21st to end	Night	20% 8% 34% 29% 9% 1	836
April	1st to 10th	Night	22% 9% 35% 28% 7%	760
	11th to 20th	Night	21% 10% 32% 25% 11% 1	751
	21st to end	Night	20% 9% 28% 30% 13% 1	599
May	1st to 10th	Night	21% 3 25% 32% 15% 3	489
	11th to 20th	Night	15% 3 26% 39% 17%	374
	21st to end	Night	11% 4 26% 34% 23% 2	411
June	1st to 10th	Night	12% 3 28% 39% 17% 1	380
	11th to 20th	Night	10% 4 23% 39% 23% 2	380
	21st to end	Night	13% 5% 20% 42% 19% 1	380
July	1st to 10th	Night	6% 2 22% 44% 24% 3	370
	11th to 20th	Night	11% 3 18% 39% 27% 2	368
	21st to end	Night	9% 2 20% 45% 22% 3	474
August	1st to 10th	Night	11% 2 14% 38% 31% 3	567
	11th to 20th	Night	9% 3 18% 35% 31% 4	564
	21st to end	Night	11% 3 19% 37% 28% 2	770
September	1st to 10th	Night	13% 3 21% 33% 25% 4	760
	11th to 20th	Night	13% 4 16% 31% 30% 7%	760
	21st to end	Night	16% 7% 22% 29% 22% 5%	760
October	1st to 10th	Night	19% 6% 28% 25% 20% 3	760
	11th to 20th	Night	23% 10% 27% 29% 9% 1	760
	21st to end	Night	24% 11% 32% 26% 6%	836
November	1st to 10th	Night	23% 11% 25% 29% 11% 1	780
	11th to 20th	Night	21% 15% 32% 24% 6% 1	800
	21st to end	Night	25% 13% 32% 23% 7%	800
December	1st to 10th	Night	27% 17% 33% 18% 5%	800
	11th to 20th	Night	25% 14% 38% 19% 5%	800
	21st to end	Night	31% 13% 35% 17% 4	880

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