GCC Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the GCC Observatory clear sky chart from 2005-11-06 to 2025-01-22.

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	17% 13% 5% 9% 5% 4 4 3 5% 6% 28%	104990
	Night	17% 14% 5% 10% 5% 4 4 3 5% 6% 27%	104323
Transparency	Day	55% 6% 11% 18% 9% 1	108633
	Night	57% 5% 10% 17% 9% 1	108819
Seeing	Day	25% 17% 29% 18% 9% 3	14766
	Night	25% 15% 27% 22% 9% 1	24465

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% 25% 12% 16% 5% 4 3 2 3 4 10%	2772
	11th to 20th	Day	12% 25% 9% 18% 8% 5% 3 2 3 4 12%	2930
	21st to end	Day	25% 19% 11% 18% 6% 3 3 2 2 2 7%	3133
February	1st to 10th	Day	16% 21% 7% 15% 9% 4 3 3 3 3 16%	1710
	11th to 20th	Day	15% 17% 8% 16% 9% 5% 4 3 3 4 16%	1663
	21st to end	Day	18% 19% 4 13% 6% 3 4 3 5% 4 21%	1533
March	1st to 10th	Day	21% 12% 7% 10% 4 4 3 3 4 6% 25%	3435
	11th to 20th	Day	11% 21% 7% 10% 4 3 3 3 4 5% 28%	3480
	21st to end	Day	22% 14% 5% 11% 4 3 3 2 4 5% 28%	3815
April	1st to 10th	Day	18% 15% 3 8% 5% 4 4 3 4 7% 29%	2199
	11th to 20th	Day	21% 13% 3 6% 5% 4 3 4 4 6% 32%	2321
	21st to end	Day	21% 13% 3 8% 4 4 3 3 4 5% 33%	2470
May	1st to 10th	Day	21% 10% 3 8% 6% 5% 4 3 5% 6% 30%	4082
	11th to 20th	Day	19% 11% 4 8% 4 4 4 4 6% 7% 30%	4130
	21st to end	Day	13% 6% 2 6% 5% 5% 4 4 6% 7% 41%	4823
June	1st to 10th	Day	16% 7% 2 7% 5% 5% 5% 4 6% 8% 35%	2660
	11th to 20th	Day	14% 7% 3 7% 5% 5% 4 5% 5% 8% 36%	2660
	21st to end	Day	16% 6% 2 6% 4 4 5% 5% 6% 9% 37%	2660
July	1st to 10th	Day	10% 6% 2 7% 5% 5% 6% 4 6% 10% 39%	4440
	11th to 20th	Day	13% 6% 2 6% 4 4 4 4 6% 10% 41%	4162
	21st to end	Day	11% 6% 2 6% 6% 5% 6% 5% 6% 8% 38%	4466
August	1st to 10th	Day	15% 8% 2 8% 6% 6% 6% 5% 7% 9% 31%	4104
	11th to 20th	Day	16% 7% 3 8% 5% 5% 6% 5% 8% 8% 29%	4044
	21st to end	Day	12% 6% 3 7% 5% 5% 5% 5% 6% 9% 38%	4350
September	1st to 10th	Day	18% 7% 3 6% 5% 4 4 3 5% 8% 37%	2280
	11th to 20th	Day	12% 6% 2 7% 4 4 5% 4 7% 7% 41%	2105
	21st to end	Day	20% 10% 3 8% 4 5% 5% 3 5% 6% 32%	2085
October	1st to 10th	Day	18% 7% 2 7% 5% 4 5% 4 6% 7% 34%	1900
	11th to 20th	Day	22% 11% 3 9% 4 5% 5% 3 4 5% 29%	1900
	21st to end	Day	25% 14% 3 8% 5% 5% 4 3 4 6% 23%	2090
November	1st to 10th	Day	14% 14% 4 8% 5% 4 5% 3 5% 6% 32%	1833
	11th to 20th	Day	18% 22% 4 9% 4 4 4 3 5% 6% 23%	1605
	21st to end	Day	19% 27% 6% 9% 4 4 4 3 4 6% 14%	1600
December	1st to 10th	Day	18% 29% 15% 14% 4 3 3 2 2 2 8%	2600
	11th to 20th	Day	22% 25% 9% 12% 6% 3 3 2 4 4 11%	2990
	21st to end	Day	25% 25% 8% 14% 5% 4 3 2 2 3 9%	3960
January	1st to 10th	Night	16% 23% 11% 20% 5% 3 2 2 3 3 10%	4389
	11th to 20th	Night	14% 21% 11% 20% 6% 5% 4 3 3 3 10%	4250
	21st to end	Night	25% 21% 9% 15% 6% 3 3 2 2 2 10%	4546
February	1st to 10th	Night	18% 20% 6% 13% 8% 5% 3 3 4 3 17%	2470
	11th to 20th	Night	15% 18% 7% 15% 7% 6% 5% 3 4 4 16%	2403
	21st to end	Night	19% 15% 5% 17% 5% 4 3 2 3 4 21%	1917
March	1st to 10th	Night	21% 16% 4 9% 5% 3 3 2 3 6% 27%	3418
	11th to 20th	Night	17% 17% 8% 8% 4 3 3 3 4 5% 27%	3480
	21st to end	Night	18% 13% 5% 9% 4 3 3 3 3 4 34%	3817
April	1st to 10th	Night	18% 17% 4 7% 4 4 4 4 4 5% 29%	1980
	11th to 20th	Night	19% 10% 3 7% 5% 5% 4 4 5% 6% 32%	1824
	21st to end	Night	15% 12% 4 7% 5% 5% 5% 3 5% 8% 33%	1710
May	1st to 10th	Night	26% 9% 2 5% 4 4 4 3 4 6% 31%	2813
	11th to 20th	Night	16% 10% 3 7% 4 4 4 3 5% 8% 35%	2759
	21st to end	Night	14% 5% 2 4 4 4 4 3 6% 11% 44%	2732
June	1st to 10th	Night	11% 6% 2 8% 4 5% 4 5% 7% 9% 39%	1520
	11th to 20th	Night	11% 6% 3 7% 5% 4 5% 4 5% 8% 42%	1520
	21st to end	Night	13% 6% 3 6% 4 5% 5% 4 6% 8% 40%	1520
July	1st to 10th	Night	13% 8% 3 7% 5% 4 3 3 5% 8% 41%	2520
	11th to 20th	Night	12% 5% 2 5% 4 5% 4 3 6% 8% 45%	2794
	21st to end	Night	9% 6% 2 6% 4 5% 5% 4 7% 8% 44%	3087
August	1st to 10th	Night	12% 8% 3 8% 5% 4 5% 4 6% 7% 39%	2825
	11th to 20th	Night	16% 8% 2 6% 4 4 5% 4 5% 7% 38%	2785
	21st to end	Night	9% 5% 2 7% 5% 5% 5% 5% 7% 8% 43%	3272
September	1st to 10th	Night	14% 6% 2 5% 4 4 4 3 6% 8% 43%	1900
	11th to 20th	Night	13% 5% 2 5% 4 4 4 3 4 7% 49%	2071
	21st to end	Night	18% 9% 3 7% 5% 4 4 3 5% 7% 35%	2095
October	1st to 10th	Night	17% 7% 2 6% 4 3 4 3 5% 6% 41%	2280
	11th to 20th	Night	21% 10% 3 7% 5% 5% 4 4 6% 6% 30%	2280
	21st to end	Night	25% 13% 3 6% 4 4 4 3 4 6% 28%	2508
November	1st to 10th	Night	15% 12% 3 8% 4 4 4 3 6% 7% 33%	2465
	11th to 20th	Night	19% 19% 4 8% 5% 5% 5% 3 5% 5% 22%	2792
	21st to end	Night	19% 22% 6% 8% 5% 4 4 3 4 5% 19%	2800
December	1st to 10th	Night	16% 23% 12% 13% 5% 4 4 3 3 4 12%	4580
	11th to 20th	Night	20% 25% 10% 12% 5% 4 3 2 3 4 14%	5205
	21st to end	Night	22% 23% 8% 12% 6% 4 4 3 4 4 12%	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	80% 4 6% 7% 3	2773
	11th to 20th	Day	79% 3 7% 9% 2	2930
	21st to end	Day	86% 3 5% 5% 2	3133
February	1st to 10th	Day	76% 4 7% 9% 4	1710
	11th to 20th	Day	74% 4 5% 11% 5%	1672
	21st to end	Day	66% 4 9% 14% 5% 2	1533
March	1st to 10th	Day	60% 5% 9% 22% 4	3435
	11th to 20th	Day	59% 5% 15% 15% 5% 1	3480
	21st to end	Day	60% 5% 6% 19% 10% 1	3815
April	1st to 10th	Day	55% 5% 12% 16% 12% 1	2199
	11th to 20th	Day	53% 5% 9% 20% 11% 2	2321
	21st to end	Day	54% 4 10% 19% 10% 3	2470
May	1st to 10th	Day	54% 5% 8% 23% 9% 1	4120
	11th to 20th	Day	51% 8% 12% 20% 8% 1	4134
	21st to end	Day	40% 8% 13% 21% 15% 4	4837
June	1st to 10th	Day	45% 6% 12% 23% 12% 1	2660
	11th to 20th	Day	44% 7% 13% 22% 13% 1	2660
	21st to end	Day	42% 7% 12% 24% 13% 1	2660
July	1st to 10th	Day	39% 7% 13% 24% 16% 2	4370
	11th to 20th	Day	38% 6% 13% 25% 17% 1	4091
	21st to end	Day	41% 8% 13% 21% 16% 1	4324
August	1st to 10th	Day	47% 7% 14% 23% 9%	4091
	11th to 20th	Day	47% 9% 14% 24% 6% 1	4083
	21st to end	Day	40% 8% 12% 27% 12% 1	4350
September	1st to 10th	Day	44% 7% 12% 21% 14% 2	2280
	11th to 20th	Day	41% 7% 11% 24% 16% 1	2105
	21st to end	Day	51% 6% 11% 21% 10%	2085
October	1st to 10th	Day	39% 6% 14% 23% 16% 1	3160
	11th to 20th	Day	47% 5% 13% 18% 12% 4	3140
	21st to end	Day	48% 5% 14% 23% 9% 1	3428
November	1st to 10th	Day	52% 7% 10% 18% 11% 2	1833
	11th to 20th	Day	63% 5% 9% 14% 7% 2	1605
	21st to end	Day	73% 5% 9% 9% 4 1	1600
December	1st to 10th	Day	85% 4 5% 5% 1	2600
	11th to 20th	Day	77% 4 6% 11% 1	2986
	21st to end	Day	83% 3 7% 6% 1	3960
January	1st to 10th	Night	81% 3 6% 7% 2	4389
	11th to 20th	Night	80% 4 7% 7% 1	4250
	21st to end	Night	82% 3 6% 7% 1	4546
February	1st to 10th	Night	73% 5% 8% 11% 4	2470
	11th to 20th	Night	73% 3 8% 12% 4	2405
	21st to end	Night	68% 3 9% 12% 7% 1	1917
March	1st to 10th	Night	60% 4 10% 20% 6%	3418
	11th to 20th	Night	60% 4 12% 16% 7% 1	3480
	21st to end	Night	55% 5% 8% 18% 13% 2	3817
April	1st to 10th	Night	57% 5% 10% 16% 11% 1	1980
	11th to 20th	Night	51% 6% 10% 18% 12% 3	1824
	21st to end	Night	49% 6% 12% 19% 10% 3	1710
May	1st to 10th	Night	53% 5% 9% 19% 11% 3	2840
	11th to 20th	Night	47% 7% 13% 21% 11% 2	2760
	21st to end	Night	34% 7% 13% 27% 15% 3	2739
June	1st to 10th	Night	40% 8% 12% 24% 15% 2	1520
	11th to 20th	Night	39% 7% 13% 23% 16% 2	1520
	21st to end	Night	40% 8% 13% 23% 16% 1	1520
July	1st to 10th	Night	42% 5% 11% 21% 18% 2	2480
	11th to 20th	Night	36% 7% 12% 27% 16% 2	2742
	21st to end	Night	36% 9% 13% 20% 21% 1	2988
August	1st to 10th	Night	43% 7% 11% 26% 11% 1	2814
	11th to 20th	Night	43% 8% 13% 28% 8% 1	2812
	21st to end	Night	35% 9% 15% 28% 12% 1	3272
September	1st to 10th	Night	38% 7% 13% 25% 14% 3	1900
	11th to 20th	Night	35% 6% 12% 26% 21% 1	2071
	21st to end	Night	48% 6% 12% 20% 13% 1	2095
October	1st to 10th	Night	39% 7% 10% 22% 22% 1	3800
	11th to 20th	Night	47% 6% 12% 15% 15% 4	3779
	21st to end	Night	43% 4 15% 23% 14% 1	4120
November	1st to 10th	Night	50% 6% 11% 21% 10% 2	2465
	11th to 20th	Night	64% 5% 8% 13% 6% 3	2792
	21st to end	Night	68% 5% 9% 13% 3 1	2800
December	1st to 10th	Night	77% 4 9% 8% 2	4580
	11th to 20th	Night	77% 3 8% 10% 2	5208
	21st to end	Night	77% 5% 8% 10% 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	32% 30% 27% 8% 3	255
	11th to 20th	Day	34% 32% 25% 5% 2 1	300
	21st to end	Day	33% 30% 28% 7% 2	300
February	1st to 10th	Day	38% 23% 33% 6%	270
	11th to 20th	Day	39% 26% 29% 6% 1	270
	21st to end	Day	44% 25% 26% 5%	236
March	1st to 10th	Day	32% 24% 31% 10% 3	460
	11th to 20th	Day	33% 21% 33% 10% 2 1	460
	21st to end	Day	37% 21% 25% 14% 2	506
April	1st to 10th	Day	28% 26% 28% 12% 5% 1	460
	11th to 20th	Day	33% 21% 25% 13% 7% 2	452
	21st to end	Day	30% 20% 30% 13% 5% 1	460
May	1st to 10th	Day	29% 18% 29% 16% 5% 2	463
	11th to 20th	Day	24% 18% 26% 21% 10% 1	487
	21st to end	Day	16% 11% 32% 24% 13% 4	529
June	1st to 10th	Day	19% 15% 30% 25% 8% 3	490
	11th to 20th	Day	21% 11% 27% 26% 12% 2	490
	21st to end	Day	18% 13% 33% 24% 10% 2	490
July	1st to 10th	Day	10% 10% 26% 29% 17% 7%	485
	11th to 20th	Day	15% 10% 29% 27% 14% 5%	484
	21st to end	Day	11% 9% 35% 26% 15% 4	532
August	1st to 10th	Day	14% 8% 27% 27% 17% 6%	486
	11th to 20th	Day	17% 6% 27% 28% 13% 8%	479
	21st to end	Day	14% 8% 28% 29% 16% 5%	539
September	1st to 10th	Day	20% 9% 26% 25% 17% 4	490
	11th to 20th	Day	11% 6% 26% 30% 20% 7%	490
	21st to end	Day	24% 14% 28% 19% 11% 4	490
October	1st to 10th	Day	21% 16% 27% 20% 13% 3	490
	11th to 20th	Day	27% 17% 33% 14% 6% 2	490
	21st to end	Day	35% 18% 30% 14% 2	539
November	1st to 10th	Day	29% 20% 29% 15% 5% 1	371
	11th to 20th	Day	36% 26% 26% 9% 3	203
	21st to end	Day	35% 24% 35% 8%	200
December	1st to 10th	Day	39% 29% 25% 4 4 1	200
	11th to 20th	Day	42% 23% 28% 7% 2	200
	21st to end	Day	36% 24% 33% 6% 1	220
January	1st to 10th	Night	33% 26% 29% 9% 3	800
	11th to 20th	Night	36% 22% 28% 13% 1	800
	21st to end	Night	26% 27% 31% 14% 2	841
February	1st to 10th	Night	34% 21% 29% 14% 1	760
	11th to 20th	Night	29% 25% 31% 12% 1	743
	21st to end	Night	32% 23% 32% 12% 2	628
March	1st to 10th	Night	27% 21% 32% 15% 4	760
	11th to 20th	Night	29% 19% 30% 17% 4	760
	21st to end	Night	28% 18% 32% 19% 4 1	836
April	1st to 10th	Night	28% 18% 31% 19% 5%	760
	11th to 20th	Night	28% 13% 31% 20% 7% 1	700
	21st to end	Night	22% 14% 33% 26% 5% 1	570
May	1st to 10th	Night	25% 10% 28% 27% 9% 1	570
	11th to 20th	Night	19% 9% 31% 31% 8% 1	534
	21st to end	Night	13% 8% 28% 31% 18% 2	411
June	1st to 10th	Night	14% 7% 33% 33% 14%	380
	11th to 20th	Night	12% 9% 26% 40% 12%	380
	21st to end	Night	16% 6% 29% 34% 15% 1	380
July	1st to 10th	Night	11% 4 20% 41% 19% 5%	370
	11th to 20th	Night	12% 4 21% 34% 25% 3	521
	21st to end	Night	12% 3 20% 39% 25% 1	624
August	1st to 10th	Night	12% 5% 18% 38% 26% 2	567
	11th to 20th	Night	13% 4 18% 33% 28% 4	564
	21st to end	Night	14% 5% 22% 31% 25% 3	710
September	1st to 10th	Night	17% 8% 19% 31% 22% 3	760
	11th to 20th	Night	14% 5% 19% 33% 24% 4	760
	21st to end	Night	23% 11% 21% 27% 14% 3	760
October	1st to 10th	Night	20% 8% 28% 25% 16% 2	760
	11th to 20th	Night	28% 14% 28% 21% 8% 1	760
	21st to end	Night	35% 14% 30% 18% 3	836
November	1st to 10th	Night	25% 16% 28% 24% 7%	780
	11th to 20th	Night	33% 21% 28% 13% 5% 1	800
	21st to end	Night	34% 21% 29% 12% 4	800
December	1st to 10th	Night	37% 23% 27% 10% 3	800
	11th to 20th	Night	32% 28% 25% 12% 2	800
	21st to end	Night	35% 21% 30% 13% 1	880

Page generated at 2025-01-19 03:06:48. Version ac07

Copyright © 2025 Attilla Danko.