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 2025-07-21.

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%	120206
	Night	14% 6% 2 6% 4 4 4 4 5% 7% 43%	118939
Transparency	Day	38% 7% 12% 23% 16% 4	123845
	Night	38% 6% 11% 23% 18% 4	123437
Seeing	Day	20% 12% 27% 23% 13% 5%	15843
	Night	19% 12% 27% 29% 12% 1	25397

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% 4 2 5% 4 3 4 3 5% 6% 49%	3799
	11th to 20th	Day	14% 8% 3 6% 4 4 4 3 4 4 47%	4060
	21st to end	Day	14% 9% 3 6% 4 4 4 3 5% 6% 43%	4466
February	1st to 10th	Day	13% 8% 3 6% 5% 5% 4 5% 6% 6% 39%	1800
	11th to 20th	Day	18% 7% 2 6% 5% 4 4 3 5% 6% 39%	1778
	21st to end	Day	16% 10% 2 5% 5% 4 5% 3 4 6% 41%	1650
March	1st to 10th	Day	18% 6% 2 5% 4 5% 5% 3 4 6% 41%	4930
	11th to 20th	Day	9% 6% 2 6% 4 4 4 3 5% 7% 49%	4980
	21st to end	Day	15% 6% 2 6% 5% 5% 5% 4 6% 7% 40%	5502
April	1st to 10th	Day	13% 7% 2 6% 6% 4 5% 4 6% 7% 40%	2400
	11th to 20th	Day	13% 7% 2 6% 4 4 4 3 5% 6% 46%	2374
	21st to end	Day	17% 6% 2 7% 4 5% 4 3 5% 7% 41%	2400
May	1st to 10th	Day	18% 7% 3 6% 4 5% 6% 5% 6% 10% 31%	5858
	11th to 20th	Day	22% 6% 2 6% 4 4 4 4 7% 10% 30%	6058
	21st to end	Day	15% 5% 2 6% 4 4 5% 4 7% 9% 38%	6934
June	1st to 10th	Day	10% 5% 2 5% 5% 5% 4 5% 6% 9% 43%	2800
	11th to 20th	Day	11% 5% 2 6% 5% 4 5% 5% 7% 10% 41%	2800
	21st to end	Day	8% 4 2 5% 5% 4 5% 4 7% 10% 46%	2800
July	1st to 10th	Day	5% 4 2 6% 5% 5% 6% 6% 8% 10% 42%	4580
	11th to 20th	Day	8% 6% 2 6% 6% 5% 6% 5% 8% 12% 38%	4355
	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%	1892
	11th to 20th	Day	14% 5% 2 4 3 4 4 3 5% 6% 50%	1740
	21st to end	Day	15% 5% 2 4 4 5% 4 4 5% 7% 46%	1600
December	1st to 10th	Day	11% 5% 2 5% 4 4 5% 4 5% 6% 49%	2600
	11th to 20th	Day	18% 8% 2 5% 4 4 4 4 4 7% 41%	2990
	21st to end	Day	17% 7% 3 7% 5% 4 5% 4 6% 8% 36%	3960
January	1st to 10th	Night	11% 7% 2 5% 4 4 4 3 5% 6% 50%	6142
	11th to 20th	Night	14% 6% 2 5% 3 3 3 3 4 6% 51%	5900
	21st to end	Night	14% 8% 3 6% 3 3 3 3 4 6% 48%	6490
February	1st to 10th	Night	14% 8% 3 5% 5% 4 5% 4 5% 7% 41%	2600
	11th to 20th	Night	18% 7% 2 6% 4 3 4 4 5% 5% 42%	2508
	21st to end	Night	16% 8% 3 5% 5% 4 4 3 5% 6% 41%	1980
March	1st to 10th	Night	15% 6% 2 6% 4 3 4 3 4 6% 47%	4918
	11th to 20th	Night	10% 6% 3 5% 4 4 4 3 5% 7% 50%	4980
	21st to end	Night	11% 7% 2 6% 5% 4 5% 4 5% 7% 44%	5429
April	1st to 10th	Night	15% 8% 2 6% 4 4 5% 4 6% 6% 40%	2000
	11th to 20th	Night	13% 6% 2 6% 4 3 4 4 6% 8% 43%	1975
	21st to end	Night	15% 5% 2 5% 4 4 4 5% 6% 7% 44%	2000
May	1st to 10th	Night	21% 9% 3 7% 5% 4 5% 4 6% 7% 29%	4079
	11th to 20th	Night	24% 8% 3 5% 4 4 5% 4 6% 8% 30%	3832
	21st to end	Night	17% 7% 3 6% 4 4 5% 4 7% 8% 35%	3920
June	1st to 10th	Night	10% 5% 2 6% 5% 4 4 4 7% 8% 45%	1600
	11th to 20th	Night	12% 5% 2 6% 4 5% 5% 6% 6% 8% 42%	1600
	21st to end	Night	8% 5% 2 5% 5% 4 5% 5% 7% 10% 45%	1600
July	1st to 10th	Night	6% 5% 3 7% 6% 5% 6% 5% 7% 10% 40%	2600
	11th to 20th	Night	6% 5% 2 7% 6% 6% 6% 6% 7% 10% 38%	2820
	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%	2405
	11th to 20th	Night	14% 7% 2 4 3 3 3 4 5% 6% 49%	2655
	21st to end	Night	13% 4 2 5% 4 5% 4 4 5% 7% 47%	2800
December	1st to 10th	Night	10% 5% 2 5% 4 3 4 3 5% 7% 52%	4580
	11th to 20th	Night	19% 6% 2 5% 4 4 4 3 5% 7% 40%	5205
	21st to end	Night	16% 7% 3 6% 5% 5% 5% 5% 6% 8% 35%	6996

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	35% 5% 6% 27% 20% 7%	3800
	11th to 20th	Day	40% 4 7% 22% 18% 10%	4060
	21st to end	Day	41% 5% 10% 27% 14% 4	4466
February	1st to 10th	Day	43% 6% 13% 23% 12% 2	1800
	11th to 20th	Day	45% 5% 10% 22% 16% 3	1787
	21st to end	Day	44% 6% 11% 20% 15% 4	1650
March	1st to 10th	Day	42% 5% 9% 21% 15% 8%	4930
	11th to 20th	Day	33% 6% 10% 26% 20% 6%	4980
	21st to end	Day	43% 5% 11% 18% 17% 6%	5502
April	1st to 10th	Day	40% 7% 11% 19% 18% 5%	2400
	11th to 20th	Day	38% 5% 11% 21% 20% 5%	2374
	21st to end	Day	43% 6% 10% 22% 16% 4	2400
May	1st to 10th	Day	46% 9% 15% 18% 10% 2	5893
	11th to 20th	Day	47% 8% 12% 24% 9% 1	6062
	21st to end	Day	39% 8% 15% 21% 16% 1	6949
June	1st to 10th	Day	35% 8% 15% 24% 17% 1	2800
	11th to 20th	Day	36% 11% 14% 23% 14% 2	2800
	21st to end	Day	31% 9% 16% 25% 17% 2	2800
July	1st to 10th	Day	33% 11% 18% 22% 12% 5%	4510
	11th to 20th	Day	36% 9% 16% 28% 10% 1	4282
	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%	1892
	11th to 20th	Day	35% 4 12% 23% 21% 5%	1740
	21st to end	Day	38% 6% 10% 27% 17% 2	1600
December	1st to 10th	Day	35% 5% 12% 29% 15% 4	2600
	11th to 20th	Day	43% 5% 14% 26% 10% 3	2986
	21st to end	Day	43% 7% 15% 20% 14% 1	3960
January	1st to 10th	Night	34% 5% 7% 25% 24% 4	6142
	11th to 20th	Night	35% 4 8% 24% 23% 7%	5900
	21st to end	Night	38% 4 8% 29% 18% 3	6490
February	1st to 10th	Night	42% 6% 13% 24% 14% 2	2600
	11th to 20th	Night	43% 5% 9% 22% 16% 5%	2510
	21st to end	Night	43% 5% 10% 22% 16% 5%	1980
March	1st to 10th	Night	38% 5% 8% 25% 15% 8%	4918
	11th to 20th	Night	33% 6% 11% 25% 20% 6%	4980
	21st to end	Night	39% 5% 10% 22% 20% 5%	5429
April	1st to 10th	Night	42% 6% 9% 20% 19% 4	2000
	11th to 20th	Night	37% 7% 12% 20% 19% 6%	1975
	21st to end	Night	37% 7% 12% 25% 14% 4	2000
May	1st to 10th	Night	52% 7% 13% 16% 8% 4	4106
	11th to 20th	Night	51% 7% 11% 20% 10% 1	3833
	21st to end	Night	43% 9% 14% 23% 9% 2	3927
June	1st to 10th	Night	35% 8% 12% 25% 17% 2	1600
	11th to 20th	Night	38% 8% 15% 23% 14% 2	1600
	21st to end	Night	33% 8% 16% 23% 18% 3	1600
July	1st to 10th	Night	38% 9% 19% 18% 13% 4	2560
	11th to 20th	Night	38% 9% 19% 22% 11% 1	2770
	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%	2405
	11th to 20th	Night	35% 5% 7% 25% 22% 6%	2655
	21st to end	Night	36% 5% 11% 24% 20% 4	2800
December	1st to 10th	Night	32% 5% 13% 28% 20% 2	4580
	11th to 20th	Night	44% 4 12% 20% 16% 4	5208
	21st to end	Night	43% 8% 16% 19% 14% 1	6996

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	35% 22% 27% 13% 2	243
	11th to 20th	Day	37% 21% 28% 13% 1	300
	21st to end	Day	28% 29% 28% 12% 3 1	330
February	1st to 10th	Day	25% 26% 35% 10% 3	300
	11th to 20th	Day	40% 24% 29% 5% 2	325
	21st to end	Day	29% 19% 32% 17% 2	414
March	1st to 10th	Day	27% 21% 31% 18% 3	500
	11th to 20th	Day	26% 22% 29% 16% 5% 2	500
	21st to end	Day	23% 21% 34% 16% 5% 1	550
April	1st to 10th	Day	25% 22% 29% 15% 7% 1	500
	11th to 20th	Day	21% 18% 29% 18% 12% 3	492
	21st to end	Day	25% 15% 31% 17% 9% 3	500
May	1st to 10th	Day	23% 17% 28% 19% 9% 4	503
	11th to 20th	Day	24% 10% 29% 21% 13% 4	527
	21st to end	Day	21% 7% 24% 23% 20% 6%	573
June	1st to 10th	Day	13% 8% 28% 26% 19% 6%	530
	11th to 20th	Day	17% 6% 26% 30% 16% 6%	530
	21st to end	Day	11% 5% 29% 31% 17% 7%	530
July	1st to 10th	Day	13% 3 16% 31% 26% 10%	525
	11th to 20th	Day	7% 2 22% 33% 28% 9%	524
	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	18% 12% 31% 25% 10% 4	462
	11th to 20th	Day	20% 14% 37% 19% 9% 1	340
	21st to end	Day	24% 21% 33% 15% 7% 1	200
December	1st to 10th	Day	20% 24% 38% 15% 4 1	200
	11th to 20th	Day	23% 26% 29% 18% 5%	200
	21st to end	Day	25% 20% 31% 18% 6%	220
January	1st to 10th	Night	23% 21% 35% 18% 3	800
	11th to 20th	Night	25% 16% 30% 24% 5%	800
	21st to end	Night	19% 19% 38% 21% 2	880
February	1st to 10th	Night	19% 17% 35% 27% 2	800
	11th to 20th	Night	26% 24% 32% 16% 2	783
	21st to end	Night	26% 17% 35% 18% 3	660
March	1st to 10th	Night	22% 19% 35% 21% 3	800
	11th to 20th	Night	24% 20% 30% 21% 6% 1	800
	21st to end	Night	20% 20% 36% 20% 5%	880
April	1st to 10th	Night	20% 20% 34% 22% 4	800
	11th to 20th	Night	18% 13% 34% 27% 7% 1	791
	21st to end	Night	20% 12% 29% 30% 9%	800
May	1st to 10th	Night	23% 7% 25% 32% 12% 1	620
	11th to 20th	Night	22% 5% 23% 38% 12% 1	508
	21st to end	Night	16% 4 23% 38% 18% 1	433
June	1st to 10th	Night	12% 4 24% 39% 21% 1	400
	11th to 20th	Night	16% 3 19% 40% 21% 2	400
	21st to end	Night	9% 3 20% 47% 19% 2	400
July	1st to 10th	Night	12% 2 14% 39% 28% 5%	390
	11th to 20th	Night	8% 1 11% 41% 34% 4	515
	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% 12% 31% 31% 11% 1	780
	11th to 20th	Night	18% 15% 31% 28% 8% 1	800
	21st to end	Night	17% 16% 32% 27% 8% 1	800
December	1st to 10th	Night	22% 17% 33% 21% 6% 1	800
	11th to 20th	Night	20% 14% 36% 26% 4	800
	21st to end	Night	21% 14% 33% 30% 3	880

Page generated at 2025-07-18 01:10:01. Version ac07

Copyright © 2025 Attilla Danko.