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 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	16% 10% 4 9% 5% 5% 5% 4 5% 7% 31%	133146
	Night	16% 11% 4 9% 5% 4 4 3 5% 6% 33%	130045
Transparency	Day	50% 6% 11% 20% 11% 1	136776
	Night	50% 6% 10% 20% 12% 2	134473
Seeing	Day	19% 11% 27% 23% 15% 6%	17385
	Night	19% 9% 28% 30% 13% 1	26961

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	18% 19% 7% 17% 8% 5% 4 3 4 4 11%	5258
	11th to 20th	Day	13% 19% 6% 14% 8% 7% 6% 3 3 4 17%	5280
	21st to end	Day	20% 16% 8% 12% 8% 6% 5% 4 4 4 13%	5780
February	1st to 10th	Day	18% 15% 6% 11% 7% 5% 4 3 5% 4 22%	1890
	11th to 20th	Day	16% 14% 7% 13% 6% 5% 4 3 5% 6% 22%	2015
	21st to end	Day	17% 13% 4 11% 5% 4 4 3 5% 6% 27%	1730
March	1st to 10th	Day	18% 12% 7% 13% 5% 4 4 3 4 4 25%	4767
	11th to 20th	Day	12% 10% 4 9% 5% 4 4 3 5% 6% 38%	5090
	21st to end	Day	21% 12% 5% 9% 5% 5% 5% 3 4 5% 27%	5586
April	1st to 10th	Day	16% 14% 4 7% 5% 5% 4 4 5% 7% 29%	2377
	11th to 20th	Day	17% 9% 3 8% 5% 4 4 4 5% 6% 35%	2656
	21st to end	Day	19% 8% 3 7% 5% 5% 4 4 5% 6% 34%	2730
May	1st to 10th	Day	23% 10% 3 8% 4 5% 5% 4 5% 6% 28%	5992
	11th to 20th	Day	19% 9% 3 7% 5% 5% 4 3 5% 7% 34%	5991
	21st to end	Day	17% 7% 3 6% 4 5% 5% 4 6% 8% 35%	6576
June	1st to 10th	Day	13% 6% 2 7% 6% 5% 5% 4 6% 9% 38%	2600
	11th to 20th	Day	13% 7% 2 6% 6% 6% 4 4 7% 10% 36%	2722
	21st to end	Day	11% 5% 2 5% 5% 5% 5% 5% 7% 10% 39%	2800
July	1st to 10th	Day	9% 6% 2 7% 5% 4 5% 4 6% 9% 42%	6360
	11th to 20th	Day	11% 5% 2 7% 5% 5% 6% 5% 8% 10% 37%	5946
	21st to end	Day	11% 6% 2 6% 5% 5% 5% 5% 7% 9% 39%	6424
August	1st to 10th	Day	10% 5% 2 7% 5% 5% 6% 5% 7% 9% 39%	4234
	11th to 20th	Day	9% 5% 2 7% 5% 5% 5% 5% 7% 10% 40%	4174
	21st to end	Day	7% 5% 2 7% 5% 5% 5% 4 7% 9% 44%	4611
September	1st to 10th	Day	12% 5% 2 6% 4 4 4 3 5% 7% 48%	2218
	11th to 20th	Day	10% 6% 2 5% 4 5% 4 4 5% 8% 47%	2200
	21st to end	Day	19% 7% 2 6% 4 4 4 3 4 5% 42%	2200
October	1st to 10th	Day	15% 6% 3 6% 4 4 4 3 5% 6% 44%	2142
	11th to 20th	Day	16% 8% 2 6% 4 3 4 4 6% 7% 40%	2000
	21st to end	Day	21% 10% 3 7% 4 5% 4 3 5% 7% 31%	2117
November	1st to 10th	Day	12% 9% 2 6% 5% 4 5% 3 4 6% 44%	1852
	11th to 20th	Day	17% 13% 5% 7% 4 5% 4 3 4 5% 33%	1890
	21st to end	Day	19% 17% 6% 9% 4 4 3 3 4 7% 26%	1890
December	1st to 10th	Day	19% 14% 6% 16% 6% 4 4 3 4 4 21%	3020
	11th to 20th	Day	21% 17% 7% 11% 4 4 4 3 4 5% 20%	3463
	21st to end	Day	26% 21% 7% 12% 5% 4 3 2 3 4 12%	4565
January	1st to 10th	Night	16% 18% 8% 17% 6% 5% 4 3 4 5% 14%	7635
	11th to 20th	Night	11% 18% 9% 16% 8% 7% 6% 3 3 4 15%	7680
	21st to end	Night	19% 15% 6% 10% 6% 5% 4 3 4 6% 23%	8401
February	1st to 10th	Night	19% 14% 4 11% 6% 4 4 3 4 5% 25%	2730
	11th to 20th	Night	17% 14% 5% 10% 6% 4 5% 4 5% 5% 25%	2489
	21st to end	Night	19% 15% 5% 10% 5% 4 3 3 3 4 27%	2076
March	1st to 10th	Night	20% 13% 4 8% 4 3 4 3 4 4 33%	5305
	11th to 20th	Night	13% 8% 3 6% 4 4 4 4 6% 8% 41%	5090
	21st to end	Night	19% 10% 3 8% 5% 4 4 3 4 5% 36%	5588
April	1st to 10th	Night	18% 11% 3 7% 4 5% 4 4 4 6% 34%	2261
	11th to 20th	Night	16% 8% 3 7% 4 4 4 4 6% 7% 37%	1911
	21st to end	Night	17% 9% 2 7% 4 5% 4 3 6% 7% 36%	1890
May	1st to 10th	Night	24% 10% 3 7% 5% 4 4 3 5% 6% 27%	4123
	11th to 20th	Night	17% 7% 3 7% 4 3 4 3 5% 8% 38%	4122
	21st to end	Night	14% 6% 2 7% 5% 4 5% 4 5% 8% 40%	4520
June	1st to 10th	Night	9% 8% 2 7% 4 4 5% 4 5% 7% 46%	1800
	11th to 20th	Night	10% 5% 1 6% 5% 5% 5% 4 7% 10% 44%	1670
	21st to end	Night	9% 4 2 5% 5% 4 5% 4 7% 10% 44%	1600
July	1st to 10th	Night	10% 5% 2 6% 4 4 4 4 6% 9% 45%	3618
	11th to 20th	Night	10% 5% 2 6% 5% 5% 5% 5% 6% 8% 43%	4007
	21st to end	Night	10% 6% 3 7% 5% 4 5% 4 6% 9% 44%	4427
August	1st to 10th	Night	10% 6% 3 7% 5% 5% 5% 5% 7% 9% 39%	2915
	11th to 20th	Night	9% 8% 4 8% 5% 4 4 4 6% 7% 44%	2875
	21st to end	Night	5% 3 2 5% 4 5% 5% 5% 7% 8% 50%	3283
September	1st to 10th	Night	11% 6% 2 6% 3 3 4 3 5% 6% 52%	2161
	11th to 20th	Night	10% 4 2 5% 3 3 3 2 5% 7% 56%	2200
	21st to end	Night	16% 6% 2 6% 4 4 4 3 5% 6% 45%	2200
October	1st to 10th	Night	10% 5% 2 6% 4 4 5% 4 5% 6% 51%	2257
	11th to 20th	Night	17% 7% 3 6% 5% 4 4 3 4 6% 42%	2400
	21st to end	Night	22% 8% 2 6% 4 4 4 3 5% 6% 37%	2720
November	1st to 10th	Night	14% 7% 3 6% 4 4 3 3 5% 6% 45%	2666
	11th to 20th	Night	17% 13% 4 7% 4 4 3 2 5% 7% 34%	2730
	21st to end	Night	21% 14% 5% 7% 5% 5% 4 3 5% 6% 27%	2730
December	1st to 10th	Night	17% 12% 6% 11% 6% 5% 5% 4 5% 6% 23%	4380
	11th to 20th	Night	20% 18% 8% 11% 5% 4 3 3 4 4 21%	4952
	21st to end	Night	26% 21% 6% 10% 4 4 3 3 4 5% 15%	6633

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	75% 6% 9% 8% 2	5259
	11th to 20th	Day	73% 4 7% 7% 9%	5280
	21st to end	Day	74% 4 6% 11% 4	5780
February	1st to 10th	Day	65% 5% 10% 14% 6%	1890
	11th to 20th	Day	64% 4 8% 15% 7% 1	2025
	21st to end	Day	58% 5% 13% 15% 7% 2	1730
March	1st to 10th	Day	62% 4 8% 19% 6%	4767
	11th to 20th	Day	46% 5% 10% 29% 9% 1	5090
	21st to end	Day	58% 5% 10% 19% 7% 1	5586
April	1st to 10th	Day	53% 6% 12% 17% 11% 1	2377
	11th to 20th	Day	49% 7% 12% 18% 13% 2	2656
	21st to end	Day	49% 6% 10% 21% 11% 2	2730
May	1st to 10th	Day	55% 8% 10% 16% 10% 1	6027
	11th to 20th	Day	49% 7% 13% 21% 10% 1	5995
	21st to end	Day	43% 9% 11% 23% 11% 3	6590
June	1st to 10th	Day	42% 7% 11% 24% 15% 1	2600
	11th to 20th	Day	42% 8% 14% 22% 13% 1	2722
	21st to end	Day	37% 9% 15% 21% 15% 3	2800
July	1st to 10th	Day	38% 7% 10% 28% 15% 2	6291
	11th to 20th	Day	37% 10% 15% 22% 15% 1	5876
	21st to end	Day	38% 8% 13% 20% 18% 3	6240
August	1st to 10th	Day	37% 10% 15% 24% 13% 1	4221
	11th to 20th	Day	35% 9% 12% 28% 15% 1	4213
	21st to end	Day	33% 9% 13% 30% 13% 3	4610
September	1st to 10th	Day	35% 6% 12% 24% 20% 4	2218
	11th to 20th	Day	34% 7% 11% 25% 20% 3	2200
	21st to end	Day	44% 5% 9% 21% 20% 1	2200
October	1st to 10th	Day	27% 5% 12% 33% 22% 1	3493
	11th to 20th	Day	36% 6% 10% 26% 21% 2	3240
	21st to end	Day	42% 5% 10% 23% 18% 1	3393
November	1st to 10th	Day	42% 5% 11% 25% 16% 2	1852
	11th to 20th	Day	53% 4 10% 20% 10% 2	1890
	21st to end	Day	61% 4 10% 17% 8% 1	1890
December	1st to 10th	Day	66% 5% 14% 12% 3	3020
	11th to 20th	Day	67% 5% 10% 14% 5%	3460
	21st to end	Day	77% 4 7% 8% 4	4565
January	1st to 10th	Night	72% 5% 7% 12% 3	7636
	11th to 20th	Night	73% 4 7% 9% 5% 1	7680
	21st to end	Night	63% 4 11% 14% 7% 1	8401
February	1st to 10th	Night	62% 5% 10% 16% 7% 1	2730
	11th to 20th	Night	59% 5% 10% 16% 8% 1	2490
	21st to end	Night	61% 5% 9% 15% 9% 1	2076
March	1st to 10th	Night	54% 5% 10% 21% 9% 1	5305
	11th to 20th	Night	38% 7% 11% 31% 11% 1	5090
	21st to end	Night	50% 4 9% 22% 14% 1	5588
April	1st to 10th	Night	49% 6% 11% 18% 13% 2	2261
	11th to 20th	Night	47% 5% 11% 18% 17% 3	1911
	21st to end	Night	47% 5% 11% 20% 15% 1	1890
May	1st to 10th	Night	56% 7% 10% 15% 10% 2	4150
	11th to 20th	Night	43% 7% 10% 24% 14% 1	4123
	21st to end	Night	39% 8% 13% 26% 12% 2	4528
June	1st to 10th	Night	37% 7% 12% 23% 19% 2	1800
	11th to 20th	Night	34% 8% 14% 25% 16% 3	1670
	21st to end	Night	33% 9% 14% 24% 16% 4	1600
July	1st to 10th	Night	34% 8% 12% 26% 17% 4	3580
	11th to 20th	Night	35% 10% 14% 27% 14% 1	3958
	21st to end	Night	35% 7% 14% 20% 21% 3	4297
August	1st to 10th	Night	38% 9% 14% 23% 14% 1	2904
	11th to 20th	Night	38% 8% 10% 26% 18% 1	2902
	21st to end	Night	28% 8% 15% 29% 17% 3	3283
September	1st to 10th	Night	32% 6% 12% 26% 19% 6%	2161
	11th to 20th	Night	28% 4 10% 28% 26% 3	2200
	21st to end	Night	40% 6% 9% 20% 22% 2	2200
October	1st to 10th	Night	24% 5% 8% 29% 32% 1	3686
	11th to 20th	Night	33% 5% 9% 27% 22% 3	3899
	21st to end	Night	37% 4 8% 28% 20% 4	4381
November	1st to 10th	Night	40% 5% 12% 24% 17% 2	2666
	11th to 20th	Night	50% 5% 9% 22% 12% 3	2730
	21st to end	Night	58% 4 12% 18% 7% 1	2730
December	1st to 10th	Night	61% 5% 11% 16% 6%	4380
	11th to 20th	Night	66% 5% 12% 12% 5%	4954
	21st to end	Night	73% 4 9% 11% 4	6633

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	28% 16% 34% 17% 5% 1	330
	11th to 20th	Day	25% 22% 40% 10% 4	330
	21st to end	Day	28% 18% 30% 19% 5%	363
February	1st to 10th	Day	25% 17% 36% 18% 2 2	330
	11th to 20th	Day	26% 21% 32% 18% 3	330
	21st to end	Day	21% 22% 42% 14% 1	273
March	1st to 10th	Day	24% 16% 37% 19% 4	413
	11th to 20th	Day	25% 16% 35% 17% 5% 1	540
	21st to end	Day	28% 16% 30% 20% 5%	594
April	1st to 10th	Day	24% 14% 34% 19% 7% 2	540
	11th to 20th	Day	21% 17% 29% 20% 9% 4	532
	21st to end	Day	21% 13% 32% 22% 9% 3	540
May	1st to 10th	Day	26% 11% 31% 21% 8% 3	543
	11th to 20th	Day	23% 7% 26% 25% 14% 5%	567
	21st to end	Day	14% 8% 25% 23% 20% 9%	617
June	1st to 10th	Day	12% 7% 22% 32% 21% 6%	530
	11th to 20th	Day	14% 6% 24% 28% 22% 6%	652
	21st to end	Day	12% 7% 27% 27% 20% 7%	730
July	1st to 10th	Day	11% 4 16% 29% 24% 17%	721
	11th to 20th	Day	11% 4 20% 26% 25% 14%	543
	21st to end	Day	13% 3 16% 24% 29% 15%	576
August	1st to 10th	Day	10% 2 15% 25% 32% 16%	526
	11th to 20th	Day	12% 3 16% 29% 27% 11%	519
	21st to end	Day	10% 4 16% 25% 31% 15%	583
September	1st to 10th	Day	12% 4 17% 25% 27% 14%	530
	11th to 20th	Day	10% 2 13% 23% 32% 20%	530
	21st to end	Day	19% 9% 18% 25% 18% 11%	530
October	1st to 10th	Day	12% 8% 21% 29% 19% 10%	530
	11th to 20th	Day	16% 12% 32% 26% 13% 3	530
	21st to end	Day	25% 14% 33% 22% 5% 1	500
November	1st to 10th	Day	14% 16% 34% 24% 9% 4	330
	11th to 20th	Day	24% 18% 31% 17% 10%	330
	21st to end	Day	28% 16% 37% 16% 3	330
December	1st to 10th	Day	23% 17% 38% 15% 5% 2	330
	11th to 20th	Day	25% 17% 38% 17% 3	330
	21st to end	Day	33% 16% 31% 17% 3	363
January	1st to 10th	Night	24% 14% 37% 22% 3	840
	11th to 20th	Night	27% 15% 37% 16% 3 1	840
	21st to end	Night	24% 15% 35% 23% 3	924
February	1st to 10th	Night	25% 11% 36% 25% 2	840
	11th to 20th	Night	25% 13% 35% 24% 3	823
	21st to end	Night	28% 11% 40% 18% 3	692
March	1st to 10th	Night	25% 12% 33% 24% 6%	840
	11th to 20th	Night	21% 13% 35% 25% 5% 1	840
	21st to end	Night	23% 11% 34% 26% 6%	924
April	1st to 10th	Night	23% 11% 36% 25% 4 1	791
	11th to 20th	Night	19% 11% 37% 23% 8% 1	623
	21st to end	Night	20% 10% 36% 26% 7% 1	630
May	1st to 10th	Night	22% 8% 29% 31% 8% 1	630
	11th to 20th	Night	21% 6% 27% 33% 12% 1	621
	21st to end	Night	11% 3 23% 39% 21% 2	683
June	1st to 10th	Night	10% 5% 23% 43% 19% 1	600
	11th to 20th	Night	11% 4 18% 46% 21% 1	600
	21st to end	Night	10% 3 21% 40% 24% 2	600
July	1st to 10th	Night	11% 2 14% 40% 31% 3	585
	11th to 20th	Night	8% 4 14% 40% 30% 4	582
	21st to end	Night	9% 3 12% 43% 32% 2	657
August	1st to 10th	Night	8% 2 10% 45% 31% 4	597
	11th to 20th	Night	9% 2 12% 41% 31% 4	594
	21st to end	Night	7% 1 13% 38% 35% 5%	660
September	1st to 10th	Night	12% 3 16% 36% 28% 5%	761
	11th to 20th	Night	11% 3 13% 36% 32% 5%	800
	21st to end	Night	17% 5% 20% 33% 22% 3	800
October	1st to 10th	Night	12% 5% 23% 35% 21% 4	800
	11th to 20th	Night	20% 9% 29% 30% 12% 1	800
	21st to end	Night	25% 11% 29% 26% 8%	880
November	1st to 10th	Night	18% 7% 32% 30% 12% 1	820
	11th to 20th	Night	22% 14% 31% 25% 7% 1	840
	21st to end	Night	27% 11% 31% 26% 6%	840
December	1st to 10th	Night	26% 12% 35% 22% 5%	840
	11th to 20th	Night	26% 10% 36% 23% 4	840
	21st to end	Night	31% 12% 32% 22% 3	924

Page generated at 2026-05-29 01:53:24. Version ac07

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