The Rezervatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the The Rezervatory clear sky chart from 2005-11-06 to 2025-01-07.

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	11% 7% 3 7% 4 4 4 3 5% 7% 44%	104790
	Night	11% 7% 2 6% 4 3 4 3 5% 7% 48%	104360
Transparency	Day	38% 5% 13% 25% 16% 2	108467
	Night	35% 5% 13% 27% 18% 3	108810
Seeing	Day	14% 9% 30% 30% 15% 3	15996
	Night	13% 8% 33% 35% 11% 1	25971

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	9% 13% 6% 14% 9% 5% 4 3 4 6% 26%	2901
	11th to 20th	Day	9% 11% 5% 11% 4 5% 7% 4 5% 7% 33%	2850
	21st to end	Day	8% 13% 6% 13% 6% 6% 4 3 4 5% 31%	3135
February	1st to 10th	Day	11% 11% 5% 10% 7% 5% 4 3 6% 7% 32%	1710
	11th to 20th	Day	9% 12% 3 8% 5% 4 4 4 6% 7% 39%	1703
	21st to end	Day	11% 8% 3 7% 5% 4 4 3 6% 6% 43%	1570
March	1st to 10th	Day	12% 9% 3 6% 4 3 3 3 3 5% 49%	3376
	11th to 20th	Day	11% 8% 2 6% 4 4 3 3 4 5% 50%	3480
	21st to end	Day	22% 11% 3 9% 4 4 4 4 6% 7% 25%	3814
April	1st to 10th	Day	15% 8% 3 6% 4 4 3 3 5% 6% 43%	2137
	11th to 20th	Day	16% 9% 2 7% 4 4 5% 3 6% 6% 39%	2375
	21st to end	Day	20% 7% 2 6% 5% 4 4 3 5% 7% 38%	2470
May	1st to 10th	Day	16% 9% 3 7% 5% 5% 5% 4 6% 7% 35%	4082
	11th to 20th	Day	13% 8% 3 7% 4 4 3 3 5% 6% 45%	4082
	21st to end	Day	14% 6% 2 6% 4 4 5% 4 7% 7% 39%	4587
June	1st to 10th	Day	7% 4 2 5% 3 4 5% 4 7% 9% 49%	2660
	11th to 20th	Day	10% 5% 2 5% 4 4 4 4 6% 9% 48%	2660
	21st to end	Day	7% 5% 2 6% 4 4 5% 3 6% 8% 51%	2660
July	1st to 10th	Day	12% 5% 2 6% 4 4 4 3 6% 8% 45%	4136
	11th to 20th	Day	7% 4 2 5% 4 4 4 4 6% 7% 54%	4120
	21st to end	Day	5% 3 1 5% 4 4 4 4 7% 10% 54%	4465
August	1st to 10th	Day	6% 4 2 5% 4 5% 5% 5% 7% 10% 47%	4104
	11th to 20th	Day	13% 6% 2 6% 4 4 4 4 6% 8% 42%	4041
	21st to end	Day	7% 4 2 6% 4 4 5% 4 5% 7% 52%	4470
September	1st to 10th	Day	12% 5% 2 5% 3 3 3 3 5% 5% 54%	2196
	11th to 20th	Day	10% 5% 2 5% 4 4 4 3 4 7% 53%	2090
	21st to end	Day	10% 5% 1 4 3 3 4 3 4 6% 57%	2090
October	1st to 10th	Day	14% 6% 2 4 3 3 3 3 4 5% 52%	1952
	11th to 20th	Day	11% 6% 2 5% 3 3 3 3 3 5% 57%	1900
	21st to end	Day	12% 8% 2 6% 4 4 3 3 4 6% 47%	2090
November	1st to 10th	Day	10% 6% 3 5% 2 3 4 3 5% 6% 53%	1773
	11th to 20th	Day	10% 7% 2 8% 5% 4 4 3 4 6% 48%	1750
	21st to end	Day	13% 9% 3 5% 4 3 4 3 5% 7% 44%	1600
December	1st to 10th	Day	5% 6% 3 6% 5% 4 5% 4 5% 6% 53%	2610
	11th to 20th	Day	9% 9% 4 10% 5% 4 4 4 5% 7% 41%	3008
	21st to end	Day	17% 9% 4 14% 4 4 3 3 4 5% 31%	4143
January	1st to 10th	Night	8% 12% 4 17% 8% 5% 5% 3 5% 6% 28%	4187
	11th to 20th	Night	9% 10% 4 8% 4 4 8% 4 8% 6% 36%	4120
	21st to end	Night	11% 12% 6% 11% 4 4 4 4 5% 6% 33%	4532
February	1st to 10th	Night	11% 11% 4 8% 6% 4 4 4 5% 6% 37%	2470
	11th to 20th	Night	9% 9% 3 7% 5% 4 4 4 5% 8% 43%	2380
	21st to end	Night	10% 8% 3 6% 4 3 4 3 4 5% 50%	1884
March	1st to 10th	Night	12% 6% 3 5% 4 3 3 3 3 5% 53%	3479
	11th to 20th	Night	11% 6% 2 5% 2 3 3 3 4 6% 55%	3480
	21st to end	Night	18% 8% 3 8% 5% 4 4 4 6% 7% 33%	3817
April	1st to 10th	Night	12% 8% 3 6% 4 3 4 3 5% 6% 47%	2041
	11th to 20th	Night	12% 7% 2 6% 4 3 4 3 4 6% 48%	1752
	21st to end	Night	16% 7% 2 5% 5% 4 3 3 4 6% 45%	1710
May	1st to 10th	Night	18% 6% 2 5% 3 4 4 3 4 7% 44%	2823
	11th to 20th	Night	11% 5% 2 4 4 3 4 3 5% 7% 51%	2823
	21st to end	Night	16% 7% 2 5% 4 4 4 3 4 5% 45%	2977
June	1st to 10th	Night	5% 4 1 5% 5% 3 5% 4 5% 9% 54%	1520
	11th to 20th	Night	8% 5% 1 4 3 4 4 4 6% 10% 50%	1520
	21st to end	Night	4 3 1 4 3 4 4 4 7% 7% 57%	1520
July	1st to 10th	Night	8% 5% 1 5% 3 3 4 4 5% 8% 54%	2814
	11th to 20th	Night	5% 3 1 3 2 2 3 3 5% 9% 63%	2850
	21st to end	Night	3 2 2 4 3 3 4 3 6% 9% 60%	3098
August	1st to 10th	Night	4 3 1 4 3 4 4 3 6% 11% 58%	2836
	11th to 20th	Night	11% 5% 1 5% 4 3 4 3 5% 9% 50%	2798
	21st to end	Night	7% 4 2 5% 3 3 4 3 6% 7% 55%	3193
September	1st to 10th	Night	9% 3 1 4 3 3 2 3 5% 6% 61%	1981
	11th to 20th	Night	9% 3 1 3 2 3 3 3 6% 8% 59%	2090
	21st to end	Night	10% 4 2 3 3 3 3 2 5% 6% 60%	2090
October	1st to 10th	Night	14% 5% 2 3 3 4 4 4 5% 5% 52%	2209
	11th to 20th	Night	13% 5% 2 4 3 3 3 2 4 6% 56%	2280
	21st to end	Night	14% 6% 2 5% 3 3 3 3 4 6% 52%	2508
November	1st to 10th	Night	9% 5% 2 4 3 3 4 3 5% 6% 57%	2521
	11th to 20th	Night	9% 8% 2 6% 3 3 3 2 5% 6% 53%	2648
	21st to end	Night	11% 8% 2 5% 3 3 3 3 5% 6% 51%	2800
December	1st to 10th	Night	5% 6% 2 5% 4 4 4 4 5% 7% 53%	4580
	11th to 20th	Night	10% 8% 3 8% 4 3 4 3 5% 8% 45%	5199
	21st to end	Night	17% 13% 5% 10% 4 3 3 2 4 5% 33%	6830

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	59% 3 13% 17% 7%	2902
	11th to 20th	Day	51% 3 17% 20% 9%	2850
	21st to end	Day	54% 4 15% 17% 9%	3135
February	1st to 10th	Day	50% 5% 15% 18% 11% 1	1710
	11th to 20th	Day	43% 6% 12% 22% 14% 3	1712
	21st to end	Day	41% 4 13% 24% 14% 3	1570
March	1st to 10th	Day	39% 3 10% 27% 18% 3	3376
	11th to 20th	Day	37% 4 11% 30% 17% 2	3480
	21st to end	Day	56% 6% 21% 13% 4	3814
April	1st to 10th	Day	42% 4 13% 26% 14% 2	2137
	11th to 20th	Day	44% 5% 13% 22% 13% 3	2375
	21st to end	Day	46% 5% 11% 24% 13% 1	2470
May	1st to 10th	Day	45% 6% 11% 22% 13% 2	4120
	11th to 20th	Day	40% 5% 11% 24% 18% 2	4086
	21st to end	Day	39% 8% 13% 24% 14% 1	4600
June	1st to 10th	Day	30% 7% 18% 30% 13% 2	2660
	11th to 20th	Day	33% 6% 13% 31% 15% 2	2660
	21st to end	Day	31% 6% 15% 29% 15% 4	2660
July	1st to 10th	Day	36% 5% 14% 30% 14% 1	4072
	11th to 20th	Day	28% 5% 13% 31% 19% 3	4051
	21st to end	Day	23% 9% 17% 28% 21% 2	4325
August	1st to 10th	Day	30% 7% 16% 28% 17% 3	4090
	11th to 20th	Day	39% 6% 13% 26% 15% 1	4080
	21st to end	Day	31% 7% 14% 26% 16% 6%	4469
September	1st to 10th	Day	32% 5% 11% 29% 18% 5%	2196
	11th to 20th	Day	31% 4 12% 33% 19% 1	2090
	21st to end	Day	29% 4 10% 32% 25% 1	2090
October	1st to 10th	Day	21% 3 7% 20% 46% 3	3238
	11th to 20th	Day	22% 3 12% 30% 26% 7%	3140
	21st to end	Day	35% 4 10% 26% 21% 3	3428
November	1st to 10th	Day	32% 4 15% 27% 20% 2	1773
	11th to 20th	Day	38% 4 13% 25% 19% 1	1750
	21st to end	Day	40% 4 12% 29% 14% 1	1600
December	1st to 10th	Day	31% 4 18% 29% 16% 1	2610
	11th to 20th	Day	44% 3 18% 25% 9%	3005
	21st to end	Day	55% 3 12% 22% 7% 1	4143
January	1st to 10th	Night	56% 5% 12% 17% 10% 1	4187
	11th to 20th	Night	45% 3 16% 24% 10% 1	4120
	21st to end	Night	50% 5% 15% 20% 9% 1	4532
February	1st to 10th	Night	47% 5% 13% 20% 12% 2	2470
	11th to 20th	Night	39% 5% 14% 23% 15% 3	2381
	21st to end	Night	35% 4 11% 25% 20% 4	1884
March	1st to 10th	Night	34% 4 9% 27% 23% 3	3479
	11th to 20th	Night	32% 4 9% 30% 21% 5%	3480
	21st to end	Night	48% 7% 22% 17% 5% 1	3817
April	1st to 10th	Night	37% 6% 10% 27% 18% 3	2041
	11th to 20th	Night	38% 5% 11% 26% 18% 3	1752
	21st to end	Night	40% 5% 12% 23% 18% 1	1710
May	1st to 10th	Night	40% 5% 11% 25% 16% 2	2850
	11th to 20th	Night	33% 6% 12% 25% 23% 1	2824
	21st to end	Night	40% 6% 12% 21% 18% 3	2985
June	1st to 10th	Night	27% 6% 15% 33% 16% 3	1520
	11th to 20th	Night	28% 7% 14% 27% 20% 5%	1520
	21st to end	Night	23% 6% 15% 33% 20% 3	1520
July	1st to 10th	Night	29% 5% 14% 33% 17% 3	2769
	11th to 20th	Night	20% 4 16% 36% 21% 3	2796
	21st to end	Night	19% 7% 16% 33% 23% 2	2999
August	1st to 10th	Night	21% 7% 16% 36% 19% 2	2826
	11th to 20th	Night	32% 6% 14% 27% 19% 2	2825
	21st to end	Night	28% 5% 15% 27% 17% 8%	3193
September	1st to 10th	Night	24% 4 12% 30% 24% 7%	1981
	11th to 20th	Night	23% 6% 12% 35% 20% 4	2090
	21st to end	Night	27% 4 8% 28% 28% 5%	2090
October	1st to 10th	Night	20% 3 7% 17% 48% 5%	3690
	11th to 20th	Night	22% 3 7% 30% 30% 8%	3779
	21st to end	Night	33% 3 10% 31% 22% 2	4120
November	1st to 10th	Night	27% 4 16% 25% 23% 4	2521
	11th to 20th	Night	32% 3 12% 30% 19% 3	2648
	21st to end	Night	35% 4 13% 29% 19% 2	2800
December	1st to 10th	Night	30% 4 14% 34% 16% 1	4580
	11th to 20th	Night	38% 5% 16% 30% 12%	5201
	21st to end	Night	54% 3 11% 23% 8% 1	6830

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	17% 12% 36% 24% 9% 2	288
	11th to 20th	Day	19% 14% 34% 26% 6% 1	270
	21st to end	Day	15% 9% 43% 23% 8% 2	297
February	1st to 10th	Day	14% 20% 42% 22% 2	270
	11th to 20th	Day	15% 13% 39% 24% 7% 2	270
	21st to end	Day	15% 19% 40% 20% 6%	225
March	1st to 10th	Day	16% 14% 41% 23% 5% 1	270
	11th to 20th	Day	26% 11% 34% 21% 7% 1	270
	21st to end	Day	30% 14% 35% 16% 4 1	297
April	1st to 10th	Day	19% 17% 41% 19% 3	317
	11th to 20th	Day	20% 14% 38% 20% 8%	573
	21st to end	Day	24% 15% 30% 22% 8% 2	650
May	1st to 10th	Day	19% 13% 33% 24% 9% 1	653
	11th to 20th	Day	17% 10% 30% 27% 14% 2	675
	21st to end	Day	18% 8% 27% 31% 15% 2	734
June	1st to 10th	Day	7% 5% 23% 35% 25% 5%	680
	11th to 20th	Day	8% 6% 30% 35% 17% 4	680
	21st to end	Day	7% 6% 31% 38% 15% 3	680
July	1st to 10th	Day	10% 3 15% 34% 29% 9%	671
	11th to 20th	Day	6% 4 20% 40% 25% 5%	668
	21st to end	Day	6% 3 18% 41% 28% 4	739
August	1st to 10th	Day	7% 2 20% 43% 24% 4	676
	11th to 20th	Day	8% 3 19% 41% 27% 3	665
	21st to end	Day	9% 3 21% 39% 24% 4	712
September	1st to 10th	Day	13% 6% 24% 35% 18% 4	406
	11th to 20th	Day	7% 3 30% 35% 20% 4	300
	21st to end	Day	12% 9% 31% 29% 15% 4	300
October	1st to 10th	Day	20% 6% 26% 28% 16% 5%	300
	11th to 20th	Day	11% 16% 40% 23% 10% 1	300
	21st to end	Day	17% 13% 37% 21% 10% 2	330
November	1st to 10th	Day	17% 13% 37% 22% 9% 3	300
	11th to 20th	Day	14% 15% 41% 23% 6% 1	300
	21st to end	Day	20% 14% 32% 24% 9% 1	300
December	1st to 10th	Day	9% 11% 40% 32% 6% 2	300
	11th to 20th	Day	16% 11% 38% 27% 7% 1	300
	21st to end	Day	21% 12% 42% 20% 5% 1	330
January	1st to 10th	Night	13% 11% 34% 34% 7%	981
	11th to 20th	Night	15% 12% 34% 31% 7%	950
	21st to end	Night	17% 8% 39% 31% 6%	1045
February	1st to 10th	Night	15% 11% 39% 31% 4	950
	11th to 20th	Night	12% 10% 41% 31% 5%	906
	21st to end	Night	15% 11% 41% 28% 5%	628
March	1st to 10th	Night	14% 13% 37% 27% 8%	608
	11th to 20th	Night	20% 7% 38% 29% 7%	570
	21st to end	Night	20% 12% 39% 24% 6%	627
April	1st to 10th	Night	16% 14% 40% 24% 5%	570
	11th to 20th	Night	16% 13% 39% 28% 4 1	563
	21st to end	Night	19% 11% 35% 27% 7%	570
May	1st to 10th	Night	19% 9% 32% 32% 8%	570
	11th to 20th	Night	14% 9% 31% 34% 11%	561
	21st to end	Night	17% 8% 29% 33% 12% 1	617
June	1st to 10th	Night	7% 5% 25% 45% 18%	570
	11th to 20th	Night	10% 4 31% 43% 13% 1	570
	21st to end	Night	4 4 32% 44% 15% 1	570
July	1st to 10th	Night	7% 3 17% 44% 28% 1	555
	11th to 20th	Night	3 2 20% 50% 24% 1	552
	21st to end	Night	3 1 15% 54% 25% 1	624
August	1st to 10th	Night	5% 2 19% 49% 23% 2	567
	11th to 20th	Night	8% 2 18% 49% 23% 1	564
	21st to end	Night	8% 2 23% 40% 23% 3	627
September	1st to 10th	Night	11% 3 24% 40% 22% 1	570
	11th to 20th	Night	11% 2 29% 41% 17% 1	570
	21st to end	Night	11% 6% 31% 34% 16% 1	570
October	1st to 10th	Night	16% 7% 30% 33% 12% 2	689
	11th to 20th	Night	15% 11% 36% 31% 7%	760
	21st to end	Night	18% 9% 29% 34% 9%	836
November	1st to 10th	Night	10% 10% 33% 37% 9% 1	961
	11th to 20th	Night	13% 11% 40% 31% 6% 1	1000
	21st to end	Night	15% 11% 34% 31% 9% 1	1000
December	1st to 10th	Night	13% 10% 41% 30% 6%	1000
	11th to 20th	Night	15% 10% 34% 34% 7%	1000
	21st to end	Night	19% 11% 33% 31% 5%	1100

Page generated at 2025-01-04 02:24:01. Version ac07

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