CHR
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the CHR clear sky chart from 2005-11-06 to 2024-11-04.

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% 6% 2 6% 4 4 5% 4 6% 8% 41%	101207
	Night	14% 6% 2 6% 4 4 4 4 5% 7% 43%	97762
Transparency	Day	38% 7% 12% 24% 16% 3	104848
	Night	38% 6% 12% 24% 17% 3	102260
Seeing	Day	20% 12% 27% 23% 13% 5%	14960
	Night	18% 11% 27% 29% 13% 1	24502

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	17% 4 2 5% 4 3 4 4 5% 6% 45%	2649
	11th to 20th	Day	15% 10% 3 8% 4 4 4 3 5% 5% 39%	2840
	21st to end	Day	16% 10% 3 7% 5% 4 4 3 5% 6% 37%	3124
February	1st to 10th	Day	13% 8% 2 6% 5% 5% 4 5% 6% 6% 39%	1710
	11th to 20th	Day	17% 7% 2 6% 5% 4 4 3 5% 6% 39%	1686
	21st to end	Day	16% 10% 2 5% 5% 4 5% 3 4 6% 39%	1570
March	1st to 10th	Day	21% 8% 2 6% 4 5% 5% 3 4 7% 35%	3430
	11th to 20th	Day	10% 5% 2 6% 4 5% 4 4 6% 7% 49%	3480
	21st to end	Day	19% 7% 2 6% 4 4 3 3 5% 7% 40%	3838
April	1st to 10th	Day	12% 7% 2 6% 6% 4 5% 4 6% 8% 41%	2280
	11th to 20th	Day	13% 7% 2 6% 4 4 4 3 4 6% 47%	2254
	21st to end	Day	18% 6% 2 7% 4 5% 4 3 5% 6% 41%	2280
May	1st to 10th	Day	22% 7% 3 6% 4 5% 5% 4 5% 8% 30%	4078
	11th to 20th	Day	23% 6% 3 6% 4 4 4 4 6% 9% 30%	4222
	21st to end	Day	11% 5% 2 6% 4 4 5% 4 7% 10% 44%	4823
June	1st to 10th	Day	10% 5% 2 5% 5% 5% 4 5% 6% 10% 43%	2660
	11th to 20th	Day	10% 5% 2 6% 5% 4 5% 5% 7% 10% 41%	2660
	21st to end	Day	8% 4 2 5% 5% 4 5% 4 8% 10% 45%	2660
July	1st to 10th	Day	5% 4 2 6% 5% 5% 6% 6% 8% 10% 42%	4440
	11th to 20th	Day	8% 6% 2 6% 5% 5% 5% 5% 8% 11% 38%	4222
	21st to end	Day	10% 7% 3 7% 6% 5% 6% 5% 7% 8% 37%	4466
August	1st to 10th	Day	15% 5% 2 7% 5% 6% 5% 5% 7% 10% 33%	4104
	11th to 20th	Day	8% 5% 2 7% 4 5% 6% 5% 8% 9% 41%	3921
	21st to end	Day	11% 5% 2 5% 4 4 5% 4 6% 8% 48%	4180
September	1st to 10th	Day	14% 6% 2 5% 5% 3 4 3 5% 7% 45%	2280
	11th to 20th	Day	13% 5% 2 5% 3 4 4 3 5% 7% 48%	2276
	21st to end	Day	21% 7% 2 6% 4 4 4 3 5% 7% 38%	1983
October	1st to 10th	Day	17% 5% 2 4 4 3 4 4 5% 7% 46%	1900
	11th to 20th	Day	16% 5% 1 5% 4 4 3 3 4 5% 49%	1900
	21st to end	Day	14% 5% 2 4 3 3 4 3 5% 6% 51%	2090
November	1st to 10th	Day	12% 6% 1 4 4 4 3 3 6% 7% 51%	1826
	11th to 20th	Day	14% 5% 2 4 3 4 4 3 5% 6% 50%	1653
	21st to end	Day	15% 5% 2 4 3 5% 4 4 5% 6% 47%	1520
December	1st to 10th	Day	15% 8% 2 6% 4 4 5% 4 5% 6% 39%	1520
	11th to 20th	Day	15% 6% 2 5% 4 4 4 3 4 8% 43%	1910
	21st to end	Day	18% 6% 2 6% 5% 4 5% 4 7% 7% 35%	2772
January	1st to 10th	Night	13% 8% 2 5% 4 4 3 3 5% 6% 46%	4292
	11th to 20th	Night	19% 7% 2 5% 3 3 3 2 4 6% 46%	4120
	21st to end	Night	19% 9% 3 6% 3 3 3 3 4 6% 40%	4532
February	1st to 10th	Night	14% 8% 3 5% 5% 5% 5% 4 5% 6% 41%	2470
	11th to 20th	Night	18% 7% 2 6% 4 3 4 4 4 5% 42%	2380
	21st to end	Night	17% 9% 3 5% 5% 4 4 3 5% 6% 41%	1884
March	1st to 10th	Night	20% 7% 2 7% 4 4 5% 3 5% 6% 38%	3418
	11th to 20th	Night	11% 5% 1 5% 4 4 4 3 5% 7% 50%	3480
	21st to end	Night	14% 9% 2 5% 4 4 3 2 4 5% 47%	3793
April	1st to 10th	Night	14% 8% 2 6% 4 4 5% 4 6% 7% 40%	1900
	11th to 20th	Night	12% 6% 2 5% 4 3 4 4 6% 8% 45%	1875
	21st to end	Night	16% 5% 2 5% 4 5% 4 5% 6% 7% 44%	1900
May	1st to 10th	Night	22% 9% 2 6% 5% 4 5% 3 6% 7% 30%	2845
	11th to 20th	Night	24% 8% 4 6% 5% 4 4 3 5% 6% 32%	2668
	21st to end	Night	10% 5% 2 5% 4 4 5% 4 7% 10% 43%	2732
June	1st to 10th	Night	10% 5% 2 6% 5% 4 4 4 7% 8% 45%	1520
	11th to 20th	Night	12% 5% 2 6% 4 4 5% 6% 6% 8% 43%	1520
	21st to end	Night	8% 5% 2 5% 5% 4 5% 5% 7% 10% 45%	1520
July	1st to 10th	Night	6% 5% 3 6% 6% 5% 6% 5% 8% 10% 40%	2520
	11th to 20th	Night	6% 5% 2 7% 6% 6% 6% 6% 7% 10% 39%	2733
	21st to end	Night	9% 7% 2 8% 4 6% 5% 5% 7% 9% 36%	3087
August	1st to 10th	Night	15% 5% 2 8% 6% 5% 5% 4 7% 10% 31%	2825
	11th to 20th	Night	9% 6% 3 8% 5% 5% 6% 4 7% 9% 38%	2874
	21st to end	Night	12% 5% 2 5% 3 4 4 4 5% 9% 48%	3476
September	1st to 10th	Night	14% 4 2 7% 4 4 5% 4 5% 6% 45%	1900
	11th to 20th	Night	10% 7% 2 6% 3 4 3 3 4 7% 50%	1900
	21st to end	Night	20% 7% 2 6% 4 4 3 3 4 5% 40%	2195
October	1st to 10th	Night	20% 6% 2 4 3 3 4 3 4 6% 46%	2280
	11th to 20th	Night	14% 5% 2 5% 3 3 3 3 5% 5% 53%	2280
	21st to end	Night	14% 6% 2 4 4 4 3 3 5% 7% 51%	2508
November	1st to 10th	Night	11% 5% 2 5% 4 3 3 3 4 6% 53%	2324
	11th to 20th	Night	14% 7% 2 4 3 3 3 4 5% 6% 50%	2522
	21st to end	Night	13% 4 2 5% 3 5% 4 4 5% 7% 47%	2660
December	1st to 10th	Night	16% 7% 2 7% 4 4 4 3 5% 6% 42%	2660
	11th to 20th	Night	16% 6% 2 6% 4 4 4 3 5% 7% 43%	3285
	21st to end	Night	16% 7% 3 6% 4 5% 5% 5% 6% 8% 36%	4884

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	38% 6% 8% 30% 14% 4	2650
	11th to 20th	Day	45% 5% 9% 20% 15% 5%	2840
	21st to end	Day	47% 6% 10% 23% 12% 2	3124
February	1st to 10th	Day	43% 6% 13% 23% 12% 3	1710
	11th to 20th	Day	44% 5% 10% 23% 16% 3	1695
	21st to end	Day	45% 6% 11% 19% 15% 4	1570
March	1st to 10th	Day	47% 6% 10% 20% 13% 3	3430
	11th to 20th	Day	32% 7% 9% 27% 21% 4	3480
	21st to end	Day	45% 6% 11% 20% 15% 3	3838
April	1st to 10th	Day	39% 7% 11% 19% 19% 5%	2280
	11th to 20th	Day	37% 5% 11% 21% 20% 6%	2254
	21st to end	Day	43% 6% 10% 22% 15% 4	2280
May	1st to 10th	Day	49% 9% 11% 20% 10% 1	4116
	11th to 20th	Day	48% 9% 13% 22% 8% 1	4226
	21st to end	Day	32% 9% 16% 24% 17% 2	4837
June	1st to 10th	Day	35% 8% 15% 25% 16% 1	2660
	11th to 20th	Day	35% 11% 14% 24% 14% 2	2660
	21st to end	Day	31% 9% 17% 25% 16% 2	2660
July	1st to 10th	Day	32% 11% 19% 23% 11% 5%	4370
	11th to 20th	Day	36% 9% 16% 28% 10% 1	4149
	21st to end	Day	42% 8% 10% 22% 17% 1	4324
August	1st to 10th	Day	43% 8% 17% 23% 8%	4091
	11th to 20th	Day	35% 9% 14% 27% 15% 1	3960
	21st to end	Day	33% 6% 15% 27% 17% 2	4180
September	1st to 10th	Day	38% 7% 12% 18% 20% 5%	2280
	11th to 20th	Day	34% 6% 9% 27% 21% 3	2276
	21st to end	Day	45% 7% 10% 19% 17% 2	1983
October	1st to 10th	Day	37% 4 7% 19% 28% 5%	3160
	11th to 20th	Day	26% 4 8% 28% 25% 10%	3140
	21st to end	Day	22% 3 10% 30% 28% 7%	3428
November	1st to 10th	Day	33% 5% 11% 26% 21% 5%	1826
	11th to 20th	Day	35% 4 11% 23% 21% 5%	1653
	21st to end	Day	38% 5% 10% 28% 17% 2	1520
December	1st to 10th	Day	43% 7% 12% 25% 11% 3	1520
	11th to 20th	Day	39% 5% 11% 31% 11% 4	1906
	21st to end	Day	43% 8% 15% 24% 10% 1	2772
January	1st to 10th	Night	36% 6% 9% 27% 18% 3	4292
	11th to 20th	Night	40% 4 10% 23% 19% 3	4120
	21st to end	Night	44% 5% 9% 26% 15% 1	4532
February	1st to 10th	Night	42% 6% 13% 24% 14% 2	2470
	11th to 20th	Night	43% 5% 9% 22% 16% 5%	2382
	21st to end	Night	43% 5% 10% 21% 15% 5%	1884
March	1st to 10th	Night	45% 7% 10% 25% 12% 2	3418
	11th to 20th	Night	31% 7% 10% 27% 22% 4	3480
	21st to end	Night	39% 5% 9% 23% 19% 5%	3793
April	1st to 10th	Night	41% 7% 9% 20% 19% 4	1900
	11th to 20th	Night	36% 7% 12% 20% 19% 6%	1875
	21st to end	Night	38% 7% 12% 25% 13% 5%	1900
May	1st to 10th	Night	51% 8% 11% 17% 10% 3	2872
	11th to 20th	Night	51% 8% 12% 19% 9% 1	2669
	21st to end	Night	32% 11% 17% 25% 12% 3	2739
June	1st to 10th	Night	35% 8% 12% 25% 17% 2	1520
	11th to 20th	Night	36% 8% 15% 24% 14% 2	1520
	21st to end	Night	33% 8% 16% 23% 17% 3	1520
July	1st to 10th	Night	37% 9% 19% 18% 13% 4	2480
	11th to 20th	Night	38% 9% 19% 22% 11% 1	2683
	21st to end	Night	41% 8% 15% 19% 16% 1	2988
August	1st to 10th	Night	45% 8% 16% 22% 8% 1	2814
	11th to 20th	Night	40% 7% 12% 26% 14% 1	2901
	21st to end	Night	32% 7% 13% 30% 16% 2	3476
September	1st to 10th	Night	39% 7% 10% 20% 19% 5%	1900
	11th to 20th	Night	34% 4 9% 27% 22% 3	1900
	21st to end	Night	45% 6% 10% 19% 17% 3	2195
October	1st to 10th	Night	41% 4 10% 21% 21% 4	3800
	11th to 20th	Night	24% 4 12% 22% 32% 8%	3779
	21st to end	Night	23% 3 9% 32% 26% 7%	4120
November	1st to 10th	Night	32% 5% 10% 25% 22% 6%	2324
	11th to 20th	Night	35% 5% 7% 24% 22% 6%	2522
	21st to end	Night	36% 5% 11% 25% 20% 4	2660
December	1st to 10th	Night	43% 5% 11% 24% 14% 2	2660
	11th to 20th	Night	41% 5% 10% 24% 17% 4	3288
	21st to end	Night	41% 9% 16% 20% 12% 1	4884

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	31% 23% 28% 14% 2	218
	11th to 20th	Day	36% 21% 28% 14% 1	270
	21st to end	Day	29% 29% 27% 11% 3 1	297
February	1st to 10th	Day	23% 26% 36% 11% 4	270
	11th to 20th	Day	36% 26% 31% 5% 2	293
	21st to end	Day	30% 19% 32% 16% 2	382
March	1st to 10th	Day	27% 21% 30% 18% 3	460
	11th to 20th	Day	26% 23% 30% 16% 4 2	460
	21st to end	Day	23% 20% 34% 16% 5% 1	506
April	1st to 10th	Day	23% 23% 30% 16% 8% 1	460
	11th to 20th	Day	19% 18% 29% 19% 12% 3	452
	21st to end	Day	25% 15% 31% 17% 9% 3	460
May	1st to 10th	Day	24% 16% 28% 19% 10% 4	463
	11th to 20th	Day	22% 10% 29% 21% 13% 4	487
	21st to end	Day	20% 6% 23% 24% 20% 6%	529
June	1st to 10th	Day	14% 8% 27% 26% 19% 6%	490
	11th to 20th	Day	16% 6% 27% 30% 16% 6%	490
	21st to end	Day	12% 5% 31% 30% 15% 7%	490
July	1st to 10th	Day	13% 3 16% 31% 27% 10%	485
	11th to 20th	Day	7% 2 22% 33% 28% 8%	484
	21st to end	Day	14% 2 22% 33% 19% 9%	532
August	1st to 10th	Day	14% 3 18% 29% 23% 13%	486
	11th to 20th	Day	13% 3 21% 30% 21% 11%	479
	21st to end	Day	10% 3 21% 32% 21% 13%	539
September	1st to 10th	Day	20% 6% 18% 25% 21% 10%	490
	11th to 20th	Day	16% 4 19% 32% 18% 12%	490
	21st to end	Day	28% 8% 22% 25% 13% 3	490
October	1st to 10th	Day	21% 6% 21% 26% 19% 7%	490
	11th to 20th	Day	18% 14% 24% 28% 13% 3	490
	21st to end	Day	18% 13% 32% 25% 11% 2	539
November	1st to 10th	Day	19% 11% 31% 25% 10% 4	438
	11th to 20th	Day	20% 14% 35% 19% 10% 1	313
	21st to end	Day	21% 19% 35% 16% 8% 1	180
December	1st to 10th	Day	20% 23% 37% 16% 4 1	180
	11th to 20th	Day	22% 28% 27% 18% 4	180
	21st to end	Day	23% 21% 32% 17% 7% 1	198
January	1st to 10th	Night	22% 21% 35% 19% 3	760
	11th to 20th	Night	25% 16% 30% 25% 5%	760
	21st to end	Night	20% 20% 37% 22% 2	836
February	1st to 10th	Night	19% 16% 34% 28% 3	760
	11th to 20th	Night	25% 24% 33% 16% 2	743
	21st to end	Night	26% 18% 35% 18% 3	628
March	1st to 10th	Night	22% 19% 35% 21% 3	760
	11th to 20th	Night	24% 19% 30% 21% 5% 1	760
	21st to end	Night	20% 18% 36% 20% 5%	836
April	1st to 10th	Night	19% 20% 34% 23% 4	760
	11th to 20th	Night	17% 12% 35% 27% 7% 1	751
	21st to end	Night	20% 12% 29% 29% 9%	760
May	1st to 10th	Night	23% 7% 25% 32% 13% 1	589
	11th to 20th	Night	22% 5% 23% 38% 12% 1	482
	21st to end	Night	16% 4 21% 39% 19% 1	411
June	1st to 10th	Night	12% 4 24% 39% 21% 1	380
	11th to 20th	Night	15% 3 19% 40% 22% 2	380
	21st to end	Night	9% 3 21% 48% 18% 1	380
July	1st to 10th	Night	12% 2 14% 39% 28% 5%	370
	11th to 20th	Night	7% 1 12% 42% 34% 4	488
	21st to end	Night	10% 2 15% 42% 28% 4	624
August	1st to 10th	Night	12% 1 11% 41% 30% 4	567
	11th to 20th	Night	11% 3 11% 41% 31% 3	614
	21st to end	Night	13% 2 15% 39% 27% 3	836
September	1st to 10th	Night	17% 2 17% 33% 26% 4	760
	11th to 20th	Night	13% 4 17% 33% 29% 4	760
	21st to end	Night	24% 5% 20% 29% 21% 1	760
October	1st to 10th	Night	21% 5% 21% 29% 20% 4	760
	11th to 20th	Night	17% 13% 23% 32% 15% 1	760
	21st to end	Night	19% 11% 29% 28% 12% 1	836
November	1st to 10th	Night	15% 11% 31% 31% 11% 1	755
	11th to 20th	Night	18% 15% 30% 28% 8% 1	760
	21st to end	Night	17% 15% 32% 28% 8% 1	760
December	1st to 10th	Night	22% 18% 33% 21% 7% 1	760
	11th to 20th	Night	19% 14% 38% 25% 4	760
	21st to end	Night	21% 14% 33% 29% 3	836

Page generated at 2024-11-01 22:47:32. Version ac07

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