Cliff's Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Cliff's 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	16% 10% 4 8% 5% 5% 5% 4 5% 7% 31%	109682
	Night	16% 11% 4 8% 5% 4 4 3 5% 6% 33%	108603
Transparency	Day	50% 6% 11% 20% 11% 1	113356
	Night	50% 6% 11% 20% 12% 2	113056
Seeing	Day	19% 10% 26% 23% 15% 7%	15597
	Night	19% 8% 28% 30% 13% 1	25243

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% 19% 8% 20% 8% 5% 4 3 4 4 10%	4056
	11th to 20th	Day	15% 21% 6% 13% 7% 6% 7% 3 3 4 16%	4060
	21st to end	Day	22% 17% 9% 11% 7% 6% 5% 3 4 3 13%	4466
February	1st to 10th	Day	19% 15% 5% 10% 7% 5% 4 3 5% 4 22%	1755
	11th to 20th	Day	15% 13% 6% 13% 6% 5% 4 3 4 6% 23%	1817
	21st to end	Day	18% 14% 4 10% 5% 4 4 3 5% 5% 27%	1570
March	1st to 10th	Day	22% 15% 6% 10% 5% 4 4 3 4 5% 22%	3247
	11th to 20th	Day	13% 12% 4 9% 5% 5% 4 3 5% 6% 34%	3480
	21st to end	Day	22% 11% 5% 9% 5% 4 4 3 4 5% 28%	3815
April	1st to 10th	Day	15% 14% 4 7% 5% 5% 4 3 5% 8% 30%	2151
	11th to 20th	Day	19% 9% 3 7% 5% 4 4 4 6% 5% 35%	2400
	21st to end	Day	20% 9% 3 7% 5% 5% 4 4 5% 6% 33%	2470
May	1st to 10th	Day	22% 10% 3 7% 4 5% 5% 4 5% 6% 29%	4082
	11th to 20th	Day	21% 8% 3 7% 4 4 3 3 5% 7% 35%	4081
	21st to end	Day	12% 6% 3 6% 5% 5% 6% 5% 7% 10% 37%	4476
June	1st to 10th	Day	12% 6% 2 7% 6% 5% 5% 4 6% 9% 38%	2470
	11th to 20th	Day	12% 6% 2 6% 6% 6% 4 4 7% 10% 37%	2586
	21st to end	Day	12% 5% 2 5% 5% 5% 5% 5% 7% 10% 39%	2660
July	1st to 10th	Day	11% 7% 2 7% 5% 5% 5% 4 6% 9% 38%	4440
	11th to 20th	Day	10% 5% 1 5% 4 4 5% 4 8% 11% 44%	4152
	21st to end	Day	10% 6% 2 6% 5% 5% 5% 5% 7% 9% 40%	4466
August	1st to 10th	Day	10% 5% 2 7% 5% 5% 6% 5% 7% 9% 38%	4104
	11th to 20th	Day	9% 5% 2 7% 5% 5% 5% 5% 7% 10% 40%	4044
	21st to end	Day	7% 5% 2 7% 5% 5% 6% 4 7% 9% 44%	4470
September	1st to 10th	Day	12% 5% 2 6% 4 4 4 3 5% 7% 47%	2107
	11th to 20th	Day	10% 6% 2 5% 4 5% 4 4 5% 9% 46%	2090
	21st to end	Day	19% 7% 2 6% 4 4 4 3 4 6% 42%	2090
October	1st to 10th	Day	15% 6% 3 7% 4 4 4 3 6% 6% 43%	2035
	11th to 20th	Day	16% 8% 2 6% 4 3 4 4 6% 7% 39%	1900
	21st to end	Day	21% 10% 4 6% 4 5% 5% 3 5% 6% 31%	2011
November	1st to 10th	Day	12% 9% 2 5% 5% 4 5% 3 4 6% 45%	1762
	11th to 20th	Day	17% 13% 5% 7% 4 5% 4 3 5% 6% 32%	1800
	21st to end	Day	19% 16% 5% 9% 4 4 3 3 4 7% 26%	1800
December	1st to 10th	Day	19% 13% 6% 15% 6% 3 4 3 4 4 21%	2930
	11th to 20th	Day	22% 17% 7% 11% 4 4 4 3 4 5% 20%	3373
	21st to end	Day	26% 21% 7% 12% 6% 4 3 2 3 4 12%	4466
January	1st to 10th	Night	16% 19% 8% 18% 5% 4 4 3 4 4 15%	5889
	11th to 20th	Night	13% 19% 9% 15% 8% 7% 5% 3 3 4 13%	5900
	21st to end	Night	23% 16% 6% 10% 6% 3 3 3 3 4 21%	6490
February	1st to 10th	Night	21% 15% 4 10% 6% 5% 5% 3 5% 4 24%	2536
	11th to 20th	Night	17% 14% 5% 9% 6% 4 5% 4 5% 5% 26%	2247
	21st to end	Night	20% 15% 5% 10% 5% 4 3 3 3 4 26%	1884
March	1st to 10th	Night	20% 15% 5% 7% 5% 3 4 3 4 5% 31%	3605
	11th to 20th	Night	14% 9% 3 6% 4 4 3 3 5% 6% 42%	3480
	21st to end	Night	19% 9% 3 8% 5% 4 3 3 5% 5% 38%	3817
April	1st to 10th	Night	17% 11% 3 7% 4 5% 4 4 5% 7% 35%	2046
	11th to 20th	Night	17% 8% 3 7% 4 4 4 3 6% 7% 36%	1727
	21st to end	Night	17% 8% 2 7% 5% 5% 4 3 6% 7% 36%	1710
May	1st to 10th	Night	25% 10% 3 7% 5% 4 4 3 6% 6% 28%	2813
	11th to 20th	Night	19% 7% 3 6% 4 3 3 3 5% 7% 40%	2812
	21st to end	Night	10% 5% 2 6% 4 3 4 4 6% 9% 46%	3079
June	1st to 10th	Night	9% 8% 2 7% 4 4 5% 4 5% 7% 45%	1710
	11th to 20th	Night	9% 5% 1 5% 4 5% 5% 4 7% 10% 44%	1587
	21st to end	Night	9% 5% 2 5% 5% 4 5% 5% 7% 10% 44%	1520
July	1st to 10th	Night	12% 5% 2 5% 4 4 4 4 6% 8% 46%	2530
	11th to 20th	Night	9% 4 1 4 4 4 4 4 7% 8% 50%	2803
	21st to end	Night	9% 5% 2 7% 5% 4 5% 4 6% 9% 44%	3087
August	1st to 10th	Night	10% 6% 3 8% 5% 5% 5% 5% 7% 9% 38%	2825
	11th to 20th	Night	9% 8% 4 8% 5% 4 4 4 6% 7% 43%	2785
	21st to end	Night	5% 3 2 5% 4 5% 5% 5% 6% 8% 50%	3182
September	1st to 10th	Night	11% 6% 2 6% 3 3 4 3 5% 6% 51%	2053
	11th to 20th	Night	11% 4 2 5% 3 3 3 2 5% 7% 56%	2090
	21st to end	Night	16% 6% 2 6% 4 4 4 3 5% 6% 45%	2090
October	1st to 10th	Night	10% 5% 2 6% 4 4 4 4 5% 7% 50%	2144
	11th to 20th	Night	18% 8% 3 5% 5% 4 4 3 4 6% 41%	2280
	21st to end	Night	22% 8% 2 6% 5% 4 4 3 5% 6% 36%	2584
November	1st to 10th	Night	15% 7% 3 6% 4 4 4 3 5% 5% 45%	2536
	11th to 20th	Night	17% 13% 4 7% 4 4 3 2 5% 7% 34%	2600
	21st to end	Night	20% 14% 4 7% 5% 5% 4 3 5% 6% 27%	2600
December	1st to 10th	Night	17% 12% 6% 11% 6% 5% 5% 4 5% 6% 23%	4250
	11th to 20th	Night	20% 18% 8% 11% 4 3 3 3 4 4 21%	4822
	21st to end	Night	26% 21% 6% 10% 4 4 3 3 4 5% 14%	6490

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	76% 6% 6% 9% 3	4057
	11th to 20th	Day	74% 4 8% 6% 8%	4060
	21st to end	Day	77% 4 6% 8% 5%	4466
February	1st to 10th	Day	65% 5% 10% 12% 7%	1755
	11th to 20th	Day	64% 5% 7% 16% 8% 1	1827
	21st to end	Day	59% 5% 13% 15% 7% 2	1570
March	1st to 10th	Day	65% 5% 9% 17% 4	3247
	11th to 20th	Day	50% 6% 9% 26% 8% 1	3480
	21st to end	Day	57% 5% 11% 20% 6%	3815
April	1st to 10th	Day	53% 6% 12% 18% 11% 1	2151
	11th to 20th	Day	49% 6% 12% 18% 13% 2	2400
	21st to end	Day	51% 6% 10% 20% 11% 2	2470
May	1st to 10th	Day	53% 8% 11% 21% 7% 1	4120
	11th to 20th	Day	48% 7% 13% 23% 9% 1	4085
	21st to end	Day	39% 11% 13% 22% 13% 4	4489
June	1st to 10th	Day	42% 7% 11% 24% 15% 1	2470
	11th to 20th	Day	41% 8% 14% 22% 14% 1	2586
	21st to end	Day	38% 9% 15% 21% 15% 3	2660
July	1st to 10th	Day	42% 7% 11% 22% 16% 2	4371
	11th to 20th	Day	31% 9% 14% 25% 19% 2	4082
	21st to end	Day	37% 9% 14% 19% 18% 4	4324
August	1st to 10th	Day	37% 10% 15% 24% 13% 1	4091
	11th to 20th	Day	35% 9% 12% 28% 15% 1	4083
	21st to end	Day	33% 9% 13% 30% 12% 3	4469
September	1st to 10th	Day	37% 6% 12% 23% 19% 4	2107
	11th to 20th	Day	35% 7% 11% 25% 19% 3	2090
	21st to end	Day	44% 5% 10% 21% 19% 1	2090
October	1st to 10th	Day	28% 5% 12% 33% 22% 1	3386
	11th to 20th	Day	36% 6% 9% 26% 21% 2	3140
	21st to end	Day	42% 5% 11% 23% 18% 1	3287
November	1st to 10th	Day	41% 5% 11% 25% 16% 2	1762
	11th to 20th	Day	53% 4 10% 20% 11% 2	1800
	21st to end	Day	60% 4 10% 17% 8% 1	1800
December	1st to 10th	Day	66% 5% 14% 13% 3	2930
	11th to 20th	Day	67% 5% 10% 13% 5%	3370
	21st to end	Day	78% 4 6% 8% 4	4466
January	1st to 10th	Night	72% 5% 7% 12% 4	5889
	11th to 20th	Night	76% 4 7% 7% 5%	5900
	21st to end	Night	67% 3 11% 11% 6% 1	6490
February	1st to 10th	Night	63% 5% 10% 15% 7% 1	2536
	11th to 20th	Night	58% 5% 10% 16% 9% 1	2248
	21st to end	Night	62% 4 8% 14% 9% 1	1884
March	1st to 10th	Night	56% 5% 10% 19% 9% 1	3605
	11th to 20th	Night	40% 7% 10% 29% 11% 2	3480
	21st to end	Night	49% 4 9% 26% 11% 1	3817
April	1st to 10th	Night	48% 6% 12% 18% 14% 2	2046
	11th to 20th	Night	47% 4 11% 18% 17% 3	1727
	21st to end	Night	47% 5% 12% 19% 15% 2	1710
May	1st to 10th	Night	55% 7% 9% 19% 8% 2	2840
	11th to 20th	Night	43% 7% 12% 26% 11% 1	2813
	21st to end	Night	31% 10% 13% 27% 16% 2	3087
June	1st to 10th	Night	37% 7% 12% 23% 19% 2	1710
	11th to 20th	Night	33% 8% 14% 26% 16% 3	1587
	21st to end	Night	34% 9% 14% 24% 16% 4	1520
July	1st to 10th	Night	35% 7% 12% 24% 16% 6%	2490
	11th to 20th	Night	28% 8% 13% 31% 18% 1	2751
	21st to end	Night	35% 8% 14% 20% 18% 4	2988
August	1st to 10th	Night	39% 9% 14% 22% 14% 1	2814
	11th to 20th	Night	39% 8% 10% 25% 18% 1	2812
	21st to end	Night	29% 8% 15% 29% 17% 3	3182
September	1st to 10th	Night	33% 6% 12% 25% 18% 6%	2053
	11th to 20th	Night	29% 4 10% 27% 26% 4	2090
	21st to end	Night	40% 6% 9% 21% 22% 2	2090
October	1st to 10th	Night	24% 5% 8% 30% 32% 1	3573
	11th to 20th	Night	33% 5% 10% 27% 22% 3	3779
	21st to end	Night	37% 4 8% 28% 20% 4	4245
November	1st to 10th	Night	40% 4 12% 23% 17% 2	2536
	11th to 20th	Night	50% 4 9% 21% 12% 3	2600
	21st to end	Night	58% 4 12% 18% 7% 1	2600
December	1st to 10th	Night	61% 6% 11% 16% 6%	4250
	11th to 20th	Night	67% 5% 12% 12% 5%	4824
	21st to end	Night	73% 4 9% 10% 4	6490

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	27% 17% 33% 17% 5% 1	300
	11th to 20th	Day	24% 20% 42% 10% 4	300
	21st to end	Day	28% 17% 31% 18% 5%	330
February	1st to 10th	Day	28% 18% 35% 16% 2 1	285
	11th to 20th	Day	25% 21% 34% 17% 3	270
	21st to end	Day	24% 20% 41% 13% 1	225
March	1st to 10th	Day	23% 16% 35% 20% 5%	345
	11th to 20th	Day	24% 16% 36% 18% 5% 1	460
	21st to end	Day	30% 13% 30% 22% 5%	506
April	1st to 10th	Day	23% 13% 33% 21% 8% 2	460
	11th to 20th	Day	21% 17% 27% 21% 9% 5%	452
	21st to end	Day	23% 12% 34% 21% 8% 2	460
May	1st to 10th	Day	25% 10% 30% 23% 9% 3	463
	11th to 20th	Day	24% 7% 24% 25% 15% 6%	487
	21st to end	Day	12% 7% 24% 24% 22% 11%	529
June	1st to 10th	Day	12% 7% 22% 32% 21% 6%	490
	11th to 20th	Day	13% 6% 24% 29% 22% 6%	606
	21st to end	Day	13% 7% 28% 27% 19% 6%	680
July	1st to 10th	Day	12% 4 16% 28% 23% 17%	671
	11th to 20th	Day	11% 3 20% 27% 25% 15%	502
	21st to end	Day	12% 3 17% 24% 29% 15%	532
August	1st to 10th	Day	10% 3 15% 25% 31% 15%	486
	11th to 20th	Day	13% 3 17% 29% 27% 11%	479
	21st to end	Day	10% 3 16% 24% 31% 16%	539
September	1st to 10th	Day	13% 4 17% 27% 27% 13%	490
	11th to 20th	Day	11% 2 14% 24% 31% 18%	490
	21st to end	Day	20% 9% 18% 26% 16% 10%	490
October	1st to 10th	Day	12% 8% 22% 29% 19% 10%	490
	11th to 20th	Day	16% 12% 31% 24% 13% 3	490
	21st to end	Day	25% 14% 32% 22% 6% 1	460
November	1st to 10th	Day	13% 16% 33% 24% 10% 4	300
	11th to 20th	Day	24% 17% 30% 18% 11%	300
	21st to end	Day	28% 16% 37% 16% 3	300
December	1st to 10th	Day	24% 17% 37% 15% 5% 2	300
	11th to 20th	Day	25% 17% 38% 17% 3	300
	21st to end	Day	32% 16% 32% 17% 3	330
January	1st to 10th	Night	24% 14% 37% 21% 3	800
	11th to 20th	Night	28% 14% 37% 17% 4 1	800
	21st to end	Night	25% 15% 34% 24% 3	880
February	1st to 10th	Night	26% 11% 36% 24% 2	781
	11th to 20th	Night	25% 12% 36% 23% 4	743
	21st to end	Night	29% 11% 40% 18% 3	628
March	1st to 10th	Night	24% 12% 33% 25% 6%	760
	11th to 20th	Night	21% 12% 34% 26% 6% 1	760
	21st to end	Night	24% 10% 33% 27% 6%	836
April	1st to 10th	Night	22% 10% 37% 26% 5% 1	716
	11th to 20th	Night	20% 10% 36% 24% 9% 1	563
	21st to end	Night	20% 9% 36% 26% 8% 1	570
May	1st to 10th	Night	22% 8% 29% 31% 9% 1	570
	11th to 20th	Night	22% 5% 25% 33% 13% 1	561
	21st to end	Night	10% 3 23% 40% 23% 2	617
June	1st to 10th	Night	9% 5% 23% 43% 19% 1	570
	11th to 20th	Night	11% 4 18% 45% 21% 1	570
	21st to end	Night	10% 3 22% 41% 23% 2	570
July	1st to 10th	Night	11% 2 14% 39% 30% 4	555
	11th to 20th	Night	7% 4 14% 40% 30% 4	552
	21st to end	Night	8% 3 12% 43% 32% 2	624
August	1st to 10th	Night	8% 2 10% 45% 31% 4	567
	11th to 20th	Night	9% 2 12% 42% 30% 5%	564
	21st to end	Night	8% 1 13% 37% 36% 5%	627
September	1st to 10th	Night	12% 3 16% 36% 28% 5%	723
	11th to 20th	Night	12% 3 13% 36% 31% 5%	760
	21st to end	Night	17% 6% 20% 33% 21% 3	760
October	1st to 10th	Night	12% 5% 24% 35% 20% 4	760
	11th to 20th	Night	20% 9% 29% 29% 12% 1	760
	21st to end	Night	26% 11% 29% 26% 9%	836
November	1st to 10th	Night	18% 7% 31% 30% 13% 1	780
	11th to 20th	Night	22% 14% 31% 26% 7% 1	800
	21st to end	Night	26% 11% 31% 26% 6% 1	800
December	1st to 10th	Night	26% 12% 35% 22% 5%	800
	11th to 20th	Night	27% 10% 36% 23% 4	800
	21st to end	Night	31% 12% 31% 22% 3	880

Page generated at 2025-02-03 03:05:54. Version ac07

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