The Icebox
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the The Icebox clear sky chart from 2005-11-06 to 2025-10-04.

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	3 7% 4 10% 6% 6% 5% 4 6% 7% 42%	135302
	Night	2 6% 4 10% 6% 5% 5% 4 6% 7% 46%	131533
Transparency	Day	34% 8% 12% 21% 18% 6%	130232
	Night	30% 7% 14% 24% 19% 6%	127620
Seeing	Day	17% 28% 30% 17% 7% 1	16861
	Night	10% 26% 30% 23% 9% 1	26822

By Month

Percentage Cloud Cover SW (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	4 4 23% 14% 6% 4 3 4 5% 33%	3638
	11th to 20th	Day	5% 6% 18% 10% 13% 7% 4 4 4 28%	3847
	21st to end	Day	6% 8% 15% 5% 9% 5% 2 4 5% 41%	4510
February	1st to 10th	Day	10% 8% 17% 8% 7% 5% 5% 6% 5% 29%	1800
	11th to 20th	Day	8% 6% 17% 9% 6% 5% 3 5% 6% 35%	1876
	21st to end	Day	6% 6% 18% 12% 6% 5% 4 4 5% 34%	1650
March	1st to 10th	Day	6% 8% 15% 6% 5% 5% 3 4 4 42%	4561
	11th to 20th	Day	1 7% 6% 16% 5% 4 4 3 6% 9% 38%	5000
	21st to end	Day	1 12% 7% 16% 8% 6% 5% 3 5% 6% 30%	5744
April	1st to 10th	Day	2 11% 5% 10% 7% 6% 5% 4 6% 7% 38%	2400
	11th to 20th	Day	3 17% 5% 11% 8% 4 4 3 5% 7% 32%	2371
	21st to end	Day	4 15% 5% 10% 6% 5% 5% 5% 6% 7% 31%	2400
May	1st to 10th	Day	3 12% 7% 13% 8% 7% 6% 4 6% 6% 28%	5773
	11th to 20th	Day	4 11% 4 9% 6% 6% 6% 5% 7% 8% 33%	6298
	21st to end	Day	3 7% 3 7% 5% 5% 7% 5% 8% 8% 41%	6925
June	1st to 10th	Day	5% 6% 2 6% 6% 5% 5% 5% 7% 8% 45%	2800
	11th to 20th	Day	6% 5% 2 6% 4 5% 5% 4 7% 8% 49%	2800
	21st to end	Day	4 5% 2 5% 5% 5% 5% 6% 6% 8% 49%	2800
July	1st to 10th	Day	4 4 1 5% 4 4 6% 5% 7% 9% 52%	6339
	11th to 20th	Day	2 3 2 5% 5% 6% 8% 7% 10% 9% 41%	6340
	21st to end	Day	5% 5% 2 7% 6% 6% 7% 5% 7% 8% 41%	6660
August	1st to 10th	Day	4 4 2 6% 5% 6% 6% 5% 8% 11% 43%	5779
	11th to 20th	Day	5% 4 2 6% 5% 5% 6% 5% 6% 8% 47%	5461
	21st to end	Day	6% 6% 3 8% 7% 6% 6% 5% 7% 7% 38%	5984
September	1st to 10th	Day	7% 5% 1 5% 4 4 5% 3 6% 7% 53%	2400
	11th to 20th	Day	6% 6% 2 6% 5% 4 5% 4 5% 5% 51%	2400
	21st to end	Day	7% 7% 2 5% 3 3 3 3 4 7% 58%	2167
October	1st to 10th	Day	3 5% 1 5% 4 2 2 2 3 4 69%	3200
	11th to 20th	Day	2 5% 2 7% 6% 5% 5% 5% 7% 7% 49%	3150
	21st to end	Day	3 10% 4 7% 4 3 3 3 4 6% 53%	3381
November	1st to 10th	Day	1 7% 4 8% 5% 5% 4 3 5% 6% 52%	1763
	11th to 20th	Day	1 10% 4 8% 6% 3 3 4 5% 7% 49%	1800
	21st to end	Day	1 8% 4 10% 5% 3 3 4 5% 7% 49%	1635
December	1st to 10th	Day	5% 3 10% 11% 6% 3 2 4 4 52%	2620
	11th to 20th	Day	6% 6% 11% 6% 3 3 3 6% 9% 47%	3026
	21st to end	Day	7% 6% 16% 8% 7% 5% 5% 5% 6% 33%	4004
January	1st to 10th	Night	4 5% 21% 11% 7% 5% 4 5% 5% 34%	6327
	11th to 20th	Night	3 5% 18% 13% 13% 6% 3 3 4 31%	6177
	21st to end	Night	6% 6% 16% 8% 8% 4 4 4 4 40%	6468
February	1st to 10th	Night	8% 5% 17% 9% 8% 5% 4 5% 6% 32%	2600
	11th to 20th	Night	7% 5% 16% 9% 7% 4 4 6% 6% 36%	2410
	21st to end	Night	7% 6% 16% 10% 7% 5% 4 6% 5% 35%	1980
March	1st to 10th	Night	8% 7% 15% 6% 5% 4 3 4 5% 42%	5313
	11th to 20th	Night	1 7% 6% 16% 4 3 3 3 5% 6% 46%	5000
	21st to end	Night	1 13% 7% 15% 6% 6% 5% 4 6% 6% 32%	5219
April	1st to 10th	Night	1 10% 4 9% 6% 4 4 4 5% 7% 46%	2000
	11th to 20th	Night	2 11% 4 11% 6% 5% 5% 3 6% 6% 41%	1977
	21st to end	Night	4 13% 5% 7% 5% 4 3 3 6% 7% 43%	2000
May	1st to 10th	Night	2 10% 5% 10% 5% 6% 5% 4 6% 9% 38%	4190
	11th to 20th	Night	4 12% 3 7% 4 4 5% 4 6% 8% 42%	3617
	21st to end	Night	3 8% 2 6% 4 4 4 4 5% 8% 52%	3972
June	1st to 10th	Night	4 5% 1 4 3 3 4 4 7% 8% 58%	1600
	11th to 20th	Night	4 3 1 3 3 4 3 4 6% 7% 62%	1600
	21st to end	Night	2 3 1 4 2 4 3 4 8% 7% 61%	1600
July	1st to 10th	Night	3 4 2 4 3 4 4 4 7% 9% 56%	3640
	11th to 20th	Night	1 2 1 4 4 5% 7% 6% 10% 11% 49%	3640
	21st to end	Night	2 3 1 4 4 5% 6% 6% 7% 10% 52%	4196
August	1st to 10th	Night	3 3 2 5% 3 5% 6% 5% 8% 10% 49%	4033
	11th to 20th	Night	2 3 1 4 3 4 5% 5% 7% 9% 56%	4373
	21st to end	Night	4 6% 3 8% 6% 5% 6% 4 7% 8% 43%	4993
September	1st to 10th	Night	4 3 1 5% 5% 3 4 4 7% 7% 57%	2000
	11th to 20th	Night	3 5% 1 5% 3 3 4 3 5% 8% 60%	2000
	21st to end	Night	5% 5% 2 4 3 4 4 3 4 6% 61%	2228
October	1st to 10th	Night	2 4 1 3 3 2 2 2 3 5% 71%	3836
	11th to 20th	Night	2 4 2 5% 4 4 4 3 6% 8% 59%	3778
	21st to end	Night	2 7% 3 7% 4 3 3 2 5% 6% 59%	4145
November	1st to 10th	Night	1 7% 3 7% 4 4 3 4 5% 8% 55%	2535
	11th to 20th	Night	1 9% 5% 8% 6% 5% 4 3 5% 6% 49%	2600
	21st to end	Night	1 7% 4 11% 6% 3 4 4 6% 7% 48%	2761
December	1st to 10th	Night	4 4 9% 9% 5% 4 3 4 5% 55%	4570
	11th to 20th	Night	4 3 14% 9% 6% 4 4 6% 7% 42%	5181
	21st to end	Night	1 11% 8% 15% 8% 7% 5% 4 6% 7% 29%	6974

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	40% 8% 14% 19% 14% 4	3639
	11th to 20th	Day	45% 9% 16% 15% 13% 2	3847
	21st to end	Day	36% 6% 13% 18% 19% 8%	4510
February	1st to 10th	Day	46% 7% 15% 14% 13% 4	1800
	11th to 20th	Day	44% 5% 13% 19% 15% 5%	1885
	21st to end	Day	44% 6% 12% 16% 18% 5%	1650
March	1st to 10th	Day	37% 5% 9% 20% 18% 11%	4561
	11th to 20th	Day	39% 10% 9% 26% 15% 2	5000
	21st to end	Day	44% 7% 17% 22% 9% 1	5744
April	1st to 10th	Day	39% 6% 14% 21% 15% 5%	2400
	11th to 20th	Day	49% 6% 12% 16% 15% 3	2374
	21st to end	Day	48% 7% 12% 23% 10% 1	2400
May	1st to 10th	Day	40% 12% 12% 19% 16% 1	5773
	11th to 20th	Day	39% 9% 13% 21% 16% 1	6302
	21st to end	Day	28% 9% 14% 25% 21% 3	6926
June	1st to 10th	Day	33% 9% 14% 22% 17% 5%	2800
	11th to 20th	Day	30% 8% 10% 16% 25% 11%	2800
	21st to end	Day	30% 8% 8% 20% 23% 11%	2800
July	1st to 10th	Day	23% 7% 9% 14% 23% 23%	6271
	11th to 20th	Day	28% 11% 14% 23% 16% 7%	6265
	21st to end	Day	34% 11% 12% 16% 16% 11%	6468
August	1st to 10th	Day	34% 10% 18% 17% 14% 7%	4208
	11th to 20th	Day	34% 9% 12% 16% 21% 8%	3954
	21st to end	Day	36% 9% 9% 14% 24% 8%	4311
September	1st to 10th	Day	29% 7% 9% 18% 25% 12%	2400
	11th to 20th	Day	31% 7% 11% 20% 24% 8%	2400
	21st to end	Day	27% 6% 8% 24% 29% 6%	2167
October	1st to 10th	Day	20% 5% 6% 25% 36% 9%	3200
	11th to 20th	Day	27% 8% 11% 30% 21% 4	3150
	21st to end	Day	29% 4 8% 34% 21% 4	3381
November	1st to 10th	Day	28% 7% 13% 23% 27% 4	1763
	11th to 20th	Day	27% 6% 15% 26% 19% 6%	1800
	21st to end	Day	31% 6% 11% 27% 17% 7%	1635
December	1st to 10th	Day	32% 5% 11% 33% 17% 3	2620
	11th to 20th	Day	28% 7% 17% 31% 16% 1	3024
	21st to end	Day	38% 8% 15% 18% 18% 3	4004
January	1st to 10th	Night	42% 6% 15% 19% 14% 5%	6327
	11th to 20th	Night	41% 6% 16% 21% 11% 4	6177
	21st to end	Night	34% 7% 13% 18% 19% 8%	6468
February	1st to 10th	Night	43% 7% 13% 19% 14% 3	2600
	11th to 20th	Night	40% 5% 14% 23% 15% 3	2411
	21st to end	Night	40% 6% 14% 18% 17% 6%	1980
March	1st to 10th	Night	37% 4 10% 17% 22% 10%	5313
	11th to 20th	Night	33% 8% 11% 22% 24% 3	5000
	21st to end	Night	38% 7% 21% 25% 9% 1	5219
April	1st to 10th	Night	32% 8% 15% 24% 17% 5%	2000
	11th to 20th	Night	37% 8% 14% 23% 16% 3	1977
	21st to end	Night	36% 7% 14% 28% 15% 1	2000
May	1st to 10th	Night	29% 10% 16% 29% 15% 1	4187
	11th to 20th	Night	32% 10% 12% 30% 16%	3618
	21st to end	Night	21% 6% 18% 31% 22% 2	3972
June	1st to 10th	Night	20% 8% 19% 31% 17% 5%	1600
	11th to 20th	Night	18% 8% 11% 22% 26% 14%	1600
	21st to end	Night	18% 7% 13% 24% 25% 14%	1600
July	1st to 10th	Night	21% 5% 10% 18% 25% 21%	3600
	11th to 20th	Night	18% 10% 21% 27% 18% 5%	3592
	21st to end	Night	20% 9% 21% 25% 13% 12%	4077
August	1st to 10th	Night	27% 9% 22% 18% 19% 4	2928
	11th to 20th	Night	24% 10% 22% 16% 19% 9%	3168
	21st to end	Night	28% 8% 13% 21% 25% 4	3597
September	1st to 10th	Night	23% 7% 13% 21% 27% 9%	2000
	11th to 20th	Night	21% 7% 12% 26% 24% 11%	2000
	21st to end	Night	22% 6% 10% 30% 25% 7%	2228
October	1st to 10th	Night	15% 5% 7% 29% 35% 10%	3836
	11th to 20th	Night	18% 6% 12% 41% 18% 4	3778
	21st to end	Night	23% 4 9% 32% 26% 4	4145
November	1st to 10th	Night	24% 6% 12% 27% 25% 6%	2535
	11th to 20th	Night	28% 7% 13% 26% 20% 5%	2600
	21st to end	Night	30% 6% 11% 27% 20% 5%	2761
December	1st to 10th	Night	25% 7% 13% 34% 18% 3	4570
	11th to 20th	Night	29% 9% 16% 26% 19% 2	5182
	21st to end	Night	43% 8% 12% 18% 16% 3	6974

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	23% 33% 26% 12% 5% 1	300
	11th to 20th	Day	20% 47% 21% 10% 1 1	300
	21st to end	Day	15% 46% 27% 9% 3	330
February	1st to 10th	Day	33% 44% 18% 4 1 1	300
	11th to 20th	Day	33% 41% 19% 5% 2	300
	21st to end	Day	34% 41% 18% 7% 1	249
March	1st to 10th	Day	24% 41% 23% 10% 2	322
	11th to 20th	Day	22% 35% 30% 11% 2	500
	21st to end	Day	27% 40% 25% 6% 2	550
April	1st to 10th	Day	26% 40% 26% 7% 1	500
	11th to 20th	Day	28% 42% 22% 6% 2	492
	21st to end	Day	21% 39% 29% 9% 2	500
May	1st to 10th	Day	25% 32% 25% 14% 4	587
	11th to 20th	Day	18% 23% 35% 17% 6% 1	725
	21st to end	Day	15% 23% 33% 20% 8% 1	789
June	1st to 10th	Day	10% 20% 32% 23% 12% 3	730
	11th to 20th	Day	11% 25% 34% 21% 7% 2	730
	21st to end	Day	8% 21% 35% 21% 13% 2	730
July	1st to 10th	Day	8% 11% 32% 30% 16% 3	721
	11th to 20th	Day	5% 11% 33% 30% 16% 4	718
	21st to end	Day	8% 9% 32% 31% 16% 4	691
August	1st to 10th	Day	12% 17% 34% 28% 8% 2	527
	11th to 20th	Day	8% 15% 38% 25% 11% 3	517
	21st to end	Day	9% 17% 34% 25% 11% 3	583
September	1st to 10th	Day	9% 18% 34% 24% 12% 4	530
	11th to 20th	Day	10% 21% 40% 21% 8% 1	530
	21st to end	Day	14% 21% 32% 21% 10% 3	341
October	1st to 10th	Day	13% 28% 24% 22% 10% 3	309
	11th to 20th	Day	18% 32% 30% 12% 6% 1	300
	21st to end	Day	26% 33% 26% 11% 3	330
November	1st to 10th	Day	22% 42% 19% 11% 5%	300
	11th to 20th	Day	25% 38% 27% 7% 3 1	300
	21st to end	Day	15% 42% 33% 10% 1	300
December	1st to 10th	Day	25% 35% 26% 10% 3	300
	11th to 20th	Day	20% 50% 23% 6% 2	300
	21st to end	Day	25% 38% 25% 9% 2	330
January	1st to 10th	Night	16% 34% 28% 16% 6% 1	1000
	11th to 20th	Night	16% 40% 26% 13% 4 1	928
	21st to end	Night	9% 42% 32% 13% 3	880
February	1st to 10th	Night	15% 44% 29% 9% 3	800
	11th to 20th	Night	13% 43% 29% 12% 2	784
	21st to end	Night	13% 43% 31% 11% 2	660
March	1st to 10th	Night	12% 37% 31% 15% 5%	800
	11th to 20th	Night	8% 31% 36% 21% 4	800
	21st to end	Night	10% 36% 34% 16% 3 1	765
April	1st to 10th	Night	10% 35% 38% 16% 2	600
	11th to 20th	Night	11% 32% 34% 17% 5%	593
	21st to end	Night	14% 24% 36% 18% 7% 1	600
May	1st to 10th	Night	10% 21% 41% 21% 8% 1	600
	11th to 20th	Night	11% 13% 33% 33% 9% 1	591
	21st to end	Night	8% 17% 30% 33% 10% 2	650
June	1st to 10th	Night	6% 9% 32% 33% 18% 2	600
	11th to 20th	Night	6% 14% 36% 33% 11% 1	600
	21st to end	Night	3 8% 28% 39% 22% 2	600
July	1st to 10th	Night	5% 3 21% 39% 30% 2	585
	11th to 20th	Night	1 3 21% 42% 29% 3	582
	21st to end	Night	4 3 13% 43% 33% 3	657
August	1st to 10th	Night	7% 3 20% 44% 24% 2	597
	11th to 20th	Night	4 4 21% 45% 22% 3	594
	21st to end	Night	4 4 21% 43% 25% 4	660
September	1st to 10th	Night	5% 5% 27% 41% 20% 4	600
	11th to 20th	Night	6% 7% 32% 35% 17% 3	600
	21st to end	Night	8% 10% 34% 30% 15% 3	640
October	1st to 10th	Night	7% 20% 31% 28% 11% 2	772
	11th to 20th	Night	10% 29% 30% 22% 7% 2	760
	21st to end	Night	12% 28% 36% 17% 6% 1	849
November	1st to 10th	Night	10% 36% 33% 15% 5% 1	975
	11th to 20th	Night	16% 37% 32% 11% 4 1	1000
	21st to end	Night	10% 33% 37% 17% 3	1000
December	1st to 10th	Night	14% 39% 32% 12% 3	1000
	11th to 20th	Night	14% 40% 31% 12% 3	1000
	21st to end	Night	16% 37% 31% 12% 4	1100

Page generated at 2025-09-30 23:39:01. Version ac07

Copyright © 2025 Attilla Danko.