Cabin-1800's Site
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Cabin-1800's Site clear sky chart from 2005-11-06 to 2024-11-25.

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	12% 7% 2 7% 5% 5% 5% 4 6% 7% 41%	100373
	Night	12% 7% 2 6% 4 4 4 4 5% 7% 45%	99078
Transparency	Day	39% 7% 11% 18% 18% 7%	104049
	Night	36% 7% 11% 19% 19% 9%	103542
Seeing	Day	15% 8% 21% 25% 21% 9%	15037
	Night	15% 8% 24% 30% 20% 4	24762

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% 13% 2 5% 4 3 3 3 5% 6% 41%	2847
	11th to 20th	Day	14% 6% 2 5% 4 3 3 3 4 6% 50%	2850
	21st to end	Day	22% 10% 2 5% 3 3 3 3 4 6% 39%	3197
February	1st to 10th	Day	17% 9% 3 6% 5% 4 3 3 4 6% 39%	1900
	11th to 20th	Day	19% 8% 3 6% 4 3 3 3 4 5% 43%	1848
	21st to end	Day	15% 10% 3 9% 5% 3 4 3 4 5% 39%	1570
March	1st to 10th	Day	21% 10% 4 8% 5% 4 4 4 5% 6% 28%	3327
	11th to 20th	Day	16% 8% 3 8% 5% 4 4 3 6% 7% 34%	3480
	21st to end	Day	12% 10% 3 9% 5% 5% 5% 3 4 4 40%	3966
April	1st to 10th	Day	11% 7% 3 6% 5% 4 5% 5% 6% 7% 43%	2280
	11th to 20th	Day	15% 7% 3 6% 4 4 4 4 5% 7% 40%	2254
	21st to end	Day	8% 5% 2 6% 5% 5% 5% 4 8% 7% 45%	2280
May	1st to 10th	Day	11% 6% 2 7% 5% 6% 5% 5% 7% 9% 37%	3765
	11th to 20th	Day	13% 7% 2 6% 5% 4 4 4 5% 7% 43%	3764
	21st to end	Day	7% 5% 2 6% 6% 5% 5% 5% 8% 11% 41%	4129
June	1st to 10th	Day	9% 5% 3 7% 7% 7% 7% 5% 7% 10% 34%	2554
	11th to 20th	Day	5% 5% 2 8% 7% 7% 7% 5% 8% 9% 39%	2616
	21st to end	Day	7% 4 2 8% 8% 7% 8% 5% 9% 9% 33%	2470
July	1st to 10th	Day	6% 6% 2 8% 8% 8% 8% 6% 9% 11% 28%	4120
	11th to 20th	Day	8% 6% 3 8% 7% 7% 9% 6% 10% 11% 26%	4120
	21st to end	Day	13% 9% 4 9% 7% 7% 7% 5% 6% 9% 25%	4316
August	1st to 10th	Day	3 3 2 4 4 4 4 3 5% 7% 61%	3785
	11th to 20th	Day	6% 4 2 6% 5% 5% 5% 5% 6% 10% 47%	3729
	21st to end	Day	10% 6% 3 7% 6% 5% 5% 5% 5% 7% 41%	4180
September	1st to 10th	Day	11% 6% 2 7% 6% 6% 5% 5% 7% 8% 37%	2280
	11th to 20th	Day	13% 6% 2 7% 5% 5% 5% 3 4 5% 42%	2251
	21st to end	Day	11% 4 2 5% 4 4 4 4 5% 7% 52%	2090
October	1st to 10th	Day	6% 4 2 5% 4 4 4 4 5% 6% 56%	1940
	11th to 20th	Day	9% 5% 2 5% 3 3 3 2 5% 6% 56%	1900
	21st to end	Day	12% 5% 1 5% 3 3 3 2 3 5% 57%	2090
November	1st to 10th	Day	10% 5% 2 7% 4 3 4 3 5% 6% 50%	1958
	11th to 20th	Day	11% 7% 2 4 3 4 4 2 4 6% 52%	1885
	21st to end	Day	14% 6% 2 6% 4 3 4 2 4 6% 49%	1746
December	1st to 10th	Day	15% 8% 3 6% 4 3 3 3 5% 6% 42%	1710
	11th to 20th	Day	15% 10% 3 7% 5% 4 4 3 4 5% 41%	2091
	21st to end	Day	14% 7% 2 6% 4 4 4 4 5% 6% 43%	3085
January	1st to 10th	Night	17% 10% 2 5% 3 3 3 3 4 5% 44%	4108
	11th to 20th	Night	15% 7% 3 6% 3 2 3 2 4 5% 50%	4120
	21st to end	Night	25% 11% 2 4 2 2 2 2 3 4 42%	4464
February	1st to 10th	Night	15% 10% 3 6% 4 4 3 3 5% 6% 41%	2280
	11th to 20th	Night	19% 9% 3 4 4 3 3 2 4 5% 44%	2218
	21st to end	Night	19% 9% 3 7% 4 4 3 3 5% 5% 38%	1884
March	1st to 10th	Night	20% 12% 3 7% 4 3 3 4 5% 6% 32%	3554
	11th to 20th	Night	12% 8% 2 7% 5% 5% 5% 4 6% 8% 38%	3480
	21st to end	Night	16% 7% 2 6% 5% 4 5% 4 5% 6% 39%	3663
April	1st to 10th	Night	11% 6% 2 5% 4 4 4 4 6% 7% 46%	1900
	11th to 20th	Night	12% 5% 2 6% 4 4 4 4 7% 7% 46%	1875
	21st to end	Night	7% 4 2 4 4 4 4 3 5% 8% 54%	1900
May	1st to 10th	Night	10% 7% 2 5% 4 4 4 4 6% 9% 46%	3130
	11th to 20th	Night	16% 5% 2 5% 4 4 4 3 4 7% 44%	3129
	21st to end	Night	6% 5% 2 6% 6% 5% 5% 4 7% 11% 42%	3426
June	1st to 10th	Night	8% 6% 2 6% 5% 4 6% 5% 8% 8% 42%	1606
	11th to 20th	Night	4 4 2 5% 4 4 5% 5% 7% 10% 50%	1546
	21st to end	Night	4 4 2 6% 5% 4 5% 6% 9% 12% 42%	1710
July	1st to 10th	Night	6% 6% 3 8% 5% 5% 7% 5% 8% 11% 37%	2850
	11th to 20th	Night	7% 5% 2 7% 6% 6% 8% 7% 10% 12% 31%	2850
	21st to end	Night	9% 9% 3 8% 6% 5% 5% 5% 7% 11% 31%	3241
August	1st to 10th	Night	2 2 1 3 2 3 4 4 6% 11% 62%	3144
	11th to 20th	Night	4 3 1 5% 4 5% 6% 5% 8% 13% 46%	3100
	21st to end	Night	7% 5% 3 8% 5% 5% 5% 5% 6% 7% 44%	3476
September	1st to 10th	Night	8% 5% 2 6% 5% 4 5% 5% 7% 9% 44%	1900
	11th to 20th	Night	9% 6% 3 6% 4 4 4 4 6% 9% 46%	1909
	21st to end	Night	9% 4 2 5% 3 3 4 3 5% 6% 55%	2090
October	1st to 10th	Night	6% 3 1 5% 3 3 3 4 5% 6% 61%	2235
	11th to 20th	Night	9% 5% 2 5% 3 3 3 3 4 5% 57%	2280
	21st to end	Night	13% 6% 2 4 2 3 3 3 3 4 57%	2508
November	1st to 10th	Night	12% 5% 2 4 3 3 3 3 4 5% 56%	2340
	11th to 20th	Night	12% 5% 2 5% 3 3 3 2 4 6% 55%	2511
	21st to end	Night	12% 7% 2 5% 3 4 3 3 5% 6% 49%	2529
December	1st to 10th	Night	16% 8% 2 5% 4 3 3 2 4 6% 46%	2470
	11th to 20th	Night	14% 9% 3 6% 4 4 3 3 5% 5% 43%	3088
	21st to end	Night	17% 12% 3 6% 3 3 3 2 3 5% 42%	4564

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	43% 5% 5% 16% 25% 6%	2848
	11th to 20th	Day	36% 5% 6% 20% 24% 9%	2850
	21st to end	Day	48% 3 6% 13% 22% 9%	3197
February	1st to 10th	Day	48% 4 7% 14% 17% 10%	1900
	11th to 20th	Day	45% 4 5% 16% 21% 9%	1858
	21st to end	Day	47% 5% 7% 14% 18% 9%	1570
March	1st to 10th	Day	54% 7% 8% 12% 14% 6%	3327
	11th to 20th	Day	47% 7% 8% 18% 16% 5%	3480
	21st to end	Day	46% 6% 8% 15% 20% 7%	3966
April	1st to 10th	Day	39% 7% 9% 18% 18% 9%	2280
	11th to 20th	Day	42% 6% 8% 17% 20% 7%	2254
	21st to end	Day	34% 8% 11% 21% 23% 4	2280
May	1st to 10th	Day	38% 11% 14% 21% 12% 3	3800
	11th to 20th	Day	39% 7% 11% 19% 22% 2	3768
	21st to end	Day	32% 11% 17% 28% 10% 2	4141
June	1st to 10th	Day	42% 11% 14% 21% 11% 1	2554
	11th to 20th	Day	38% 10% 14% 22% 14% 3	2616
	21st to end	Day	42% 12% 16% 18% 10% 2	2470
July	1st to 10th	Day	45% 13% 19% 17% 5%	4056
	11th to 20th	Day	45% 13% 24% 14% 3 1	4051
	21st to end	Day	54% 11% 12% 17% 6% 1	4179
August	1st to 10th	Day	23% 7% 12% 28% 28% 2	3773
	11th to 20th	Day	30% 10% 17% 29% 12% 1	3765
	21st to end	Day	41% 8% 14% 19% 16% 3	4179
September	1st to 10th	Day	41% 11% 14% 15% 15% 5%	2280
	11th to 20th	Day	42% 7% 10% 16% 18% 7%	2251
	21st to end	Day	31% 6% 12% 23% 18% 10%	2090
October	1st to 10th	Day	21% 4 7% 12% 23% 32%	3226
	11th to 20th	Day	18% 2 6% 13% 29% 31%	3140
	21st to end	Day	22% 3 4 15% 36% 20%	3428
November	1st to 10th	Day	34% 5% 8% 16% 26% 11%	1958
	11th to 20th	Day	34% 3 7% 15% 27% 14%	1885
	21st to end	Day	38% 4 8% 17% 23% 11%	1746
December	1st to 10th	Day	42% 6% 8% 18% 18% 8%	1710
	11th to 20th	Day	46% 5% 6% 13% 24% 7%	2088
	21st to end	Day	40% 6% 7% 19% 25% 3	3085
January	1st to 10th	Night	42% 4 5% 15% 25% 9%	4108
	11th to 20th	Night	37% 5% 7% 18% 22% 11%	4120
	21st to end	Night	47% 4 5% 16% 23% 6%	4464
February	1st to 10th	Night	44% 4 8% 15% 19% 9%	2280
	11th to 20th	Night	44% 4 5% 15% 22% 10%	2219
	21st to end	Night	48% 5% 8% 13% 15% 11%	1884
March	1st to 10th	Night	50% 9% 9% 13% 14% 6%	3554
	11th to 20th	Night	42% 8% 10% 17% 17% 6%	3480
	21st to end	Night	42% 8% 9% 16% 20% 4	3663
April	1st to 10th	Night	35% 8% 11% 18% 18% 10%	1900
	11th to 20th	Night	36% 7% 9% 19% 19% 10%	1875
	21st to end	Night	27% 6% 11% 23% 27% 6%	1900
May	1st to 10th	Night	33% 9% 14% 23% 17% 4	3160
	11th to 20th	Night	38% 7% 11% 25% 18% 1	3130
	21st to end	Night	32% 11% 16% 26% 13% 2	3435
June	1st to 10th	Night	36% 10% 17% 22% 14% 2	1606
	11th to 20th	Night	27% 8% 19% 28% 14% 3	1546
	21st to end	Night	29% 11% 23% 22% 13% 2	1710
July	1st to 10th	Night	39% 14% 22% 20% 4 1	2805
	11th to 20th	Night	39% 15% 25% 16% 4 1	2796
	21st to end	Night	46% 13% 16% 18% 6% 1	3138
August	1st to 10th	Night	16% 8% 19% 32% 24% 1	3132
	11th to 20th	Night	26% 11% 21% 30% 10% 1	3130
	21st to end	Night	37% 9% 13% 19% 18% 4	3476
September	1st to 10th	Night	33% 12% 16% 19% 15% 6%	1900
	11th to 20th	Night	34% 8% 15% 16% 19% 8%	1909
	21st to end	Night	29% 5% 10% 24% 22% 10%	2090
October	1st to 10th	Night	20% 4 9% 12% 24% 32%	3729
	11th to 20th	Night	19% 3 4 19% 22% 34%	3779
	21st to end	Night	20% 3 5% 12% 38% 21%	4120
November	1st to 10th	Night	31% 4 9% 18% 25% 14%	2340
	11th to 20th	Night	32% 3 6% 15% 28% 15%	2511
	21st to end	Night	36% 4 7% 18% 22% 13%	2529
December	1st to 10th	Night	39% 5% 10% 19% 20% 7%	2470
	11th to 20th	Night	43% 5% 6% 13% 24% 9%	3090
	21st to end	Night	45% 4 6% 18% 22% 6%	4564

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% 21% 35% 15% 3 1	270
	11th to 20th	Day	27% 16% 36% 16% 3 1	270
	21st to end	Day	29% 20% 37% 12% 2	297
February	1st to 10th	Day	29% 20% 33% 13% 5% 1	270
	11th to 20th	Day	36% 17% 33% 11% 3	270
	21st to end	Day	28% 25% 38% 8%	225
March	1st to 10th	Day	27% 14% 40% 16% 4	270
	11th to 20th	Day	27% 19% 37% 14% 3 1	270
	21st to end	Day	22% 17% 34% 23% 4	385
April	1st to 10th	Day	18% 16% 35% 23% 7% 1	460
	11th to 20th	Day	23% 13% 31% 21% 12% 1	452
	21st to end	Day	12% 10% 35% 29% 14% 1	460
May	1st to 10th	Day	15% 8% 31% 28% 14% 4	463
	11th to 20th	Day	13% 5% 25% 30% 20% 7%	487
	21st to end	Day	8% 5% 21% 34% 24% 10%	529
June	1st to 10th	Day	12% 3 14% 35% 26% 10%	645
	11th to 20th	Day	6% 3 15% 37% 31% 9%	680
	21st to end	Day	9% 2 13% 30% 30% 18%	680
July	1st to 10th	Day	6% 2 7% 28% 41% 16%	671
	11th to 20th	Day	7% 2 4 25% 42% 20%	668
	21st to end	Day	9% 3 5% 28% 37% 17%	652
August	1st to 10th	Day	5% 2 7% 24% 34% 28%	486
	11th to 20th	Day	9% 2 3 25% 40% 22%	479
	21st to end	Day	13% 3 6% 22% 33% 22%	539
September	1st to 10th	Day	12% 3 6% 27% 34% 18%	490
	11th to 20th	Day	13% 2 12% 24% 34% 14%	490
	21st to end	Day	12% 4 12% 30% 30% 12%	490
October	1st to 10th	Day	8% 4 22% 39% 19% 8%	340
	11th to 20th	Day	9% 6% 24% 36% 17% 7%	300
	21st to end	Day	20% 10% 22% 26% 17% 5%	330
November	1st to 10th	Day	14% 9% 30% 33% 11% 3	300
	11th to 20th	Day	18% 8% 38% 26% 8% 1	300
	21st to end	Day	16% 16% 35% 19% 11% 3	282
December	1st to 10th	Day	24% 12% 34% 23% 6% 1	270
	11th to 20th	Day	22% 16% 39% 17% 6% 1	270
	21st to end	Day	28% 15% 36% 13% 7%	297
January	1st to 10th	Night	24% 13% 35% 23% 5%	950
	11th to 20th	Night	22% 15% 30% 25% 7%	950
	21st to end	Night	25% 15% 33% 24% 3	1003
February	1st to 10th	Night	22% 15% 36% 23% 4	760
	11th to 20th	Night	27% 16% 30% 23% 5%	743
	21st to end	Night	26% 19% 35% 18% 2	628
March	1st to 10th	Night	19% 14% 36% 25% 6%	659
	11th to 20th	Night	19% 11% 36% 28% 5%	570
	21st to end	Night	19% 8% 38% 27% 8%	627
April	1st to 10th	Night	15% 12% 37% 26% 9% 1	570
	11th to 20th	Night	15% 11% 35% 29% 10% 1	563
	21st to end	Night	9% 9% 35% 33% 13% 1	570
May	1st to 10th	Night	10% 5% 31% 38% 14% 2	570
	11th to 20th	Night	12% 3 26% 37% 20% 3	561
	21st to end	Night	6% 2 22% 39% 27% 5%	617
June	1st to 10th	Night	10% 1 11% 41% 30% 6%	570
	11th to 20th	Night	5% 2 9% 38% 39% 7%	570
	21st to end	Night	6% 2 10% 31% 36% 15%	570
July	1st to 10th	Night	5% 2 6% 29% 46% 12%	555
	11th to 20th	Night	5% 2 5% 27% 44% 17%	552
	21st to end	Night	5% 2 5% 33% 43% 12%	624
August	1st to 10th	Night	4 1 3 25% 48% 19%	567
	11th to 20th	Night	7% 2 5% 25% 47% 15%	564
	21st to end	Night	12% 2 6% 24% 40% 17%	627
September	1st to 10th	Night	9% 3 6% 31% 42% 9%	570
	11th to 20th	Night	9% 3 9% 28% 41% 10%	579
	21st to end	Night	11% 2 11% 34% 36% 6%	760
October	1st to 10th	Night	7% 2 17% 37% 33% 5%	760
	11th to 20th	Night	12% 6% 23% 35% 22% 2	760
	21st to end	Night	18% 4 23% 32% 20% 3	836
November	1st to 10th	Night	14% 6% 22% 38% 17% 3	780
	11th to 20th	Night	15% 8% 28% 35% 13% 1	800
	21st to end	Night	16% 10% 30% 29% 14% 2	779
December	1st to 10th	Night	22% 10% 27% 31% 8%	760
	11th to 20th	Night	21% 9% 35% 28% 7%	793
	21st to end	Night	26% 11% 32% 25% 6%	1045

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