Gateway
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Gateway clear sky chart from 2005-11-06 to 2024-11-25.

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 5% 2 5% 4 4 5% 4 6% 8% 56%	105156
	Night	3 5% 2 5% 4 4 4 4 6% 7% 57%	103055
Transparency	Day	24% 3 8% 24% 25% 15%	104921
	Night	25% 3 10% 26% 24% 13%	102879
Seeing	Day	8% 12% 28% 30% 18% 4	15284
	Night	6% 4 18% 41% 26% 5%	25508

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	2 4 3 6% 4 5% 5% 5% 9% 8% 48%	2858
	11th to 20th	Day	2 5% 2 7% 5% 5% 6% 5% 8% 9% 46%	2860
	21st to end	Day	4 11% 3 5% 3 3 3 3 5% 6% 54%	3146
February	1st to 10th	Day	2 8% 3 6% 5% 3 4 4 6% 9% 49%	1710
	11th to 20th	Day	4 10% 3 6% 4 4 4 5% 5% 7% 49%	1665
	21st to end	Day	3 9% 3 6% 5% 4 4 3 6% 8% 49%	1603
March	1st to 10th	Day	2 8% 2 7% 5% 6% 5% 4 6% 7% 47%	3444
	11th to 20th	Day	3 11% 3 7% 4 5% 5% 5% 7% 8% 42%	3490
	21st to end	Day	3 11% 4 9% 6% 6% 5% 5% 5% 7% 39%	3825
April	1st to 10th	Day	3 5% 2 5% 5% 4 4 3 6% 7% 55%	2228
	11th to 20th	Day	3 4 2 6% 4 4 6% 4 6% 8% 52%	2251
	21st to end	Day	3 5% 2 5% 4 5% 6% 4 6% 8% 53%	2348
May	1st to 10th	Day	3 4 2 6% 5% 5% 6% 5% 7% 8% 49%	4110
	11th to 20th	Day	2 4 1 4 4 4 5% 4 6% 9% 56%	4071
	21st to end	Day	3 3 2 5% 3 4 5% 4 6% 9% 57%	4694
June	1st to 10th	Day	3 3 1 3 3 3 4 3 6% 8% 65%	2660
	11th to 20th	Day	3 3 1 3 3 3 4 3 4 5% 70%	2660
	21st to end	Day	1 2 1 3 3 3 4 3 5% 7% 68%	2660
July	1st to 10th	Day	2 2 1 2 3 3 3 3 5% 6% 71%	4216
	11th to 20th	Day	2 2 1 3 3 4 5% 5% 9% 12% 54%	4110
	21st to end	Day	2 2 1 4 3 4 5% 4 6% 9% 59%	4454
August	1st to 10th	Day	3 3 1 4 4 5% 7% 6% 10% 12% 45%	4092
	11th to 20th	Day	3 2 1 4 4 5% 6% 5% 8% 8% 54%	4028
	21st to end	Day	3 3 1 4 4 4 5% 4 6% 8% 56%	4190
September	1st to 10th	Day	5% 3 1 4 3 3 5% 4 5% 6% 62%	2280
	11th to 20th	Day	3 3 1 4 4 4 5% 4 6% 7% 59%	2164
	21st to end	Day	5% 3 1 4 2 3 2 3 4 7% 67%	2062
October	1st to 10th	Day	4 2 1 3 2 2 3 2 3 4 74%	3170
	11th to 20th	Day	5% 4 2 4 3 4 4 4 6% 9% 56%	3150
	21st to end	Day	3 3 1 3 2 3 3 2 4 7% 68%	3438
November	1st to 10th	Day	3 3 2 3 3 3 4 3 4 6% 66%	1959
	11th to 20th	Day	2 4 1 4 3 3 3 2 4 6% 68%	1725
	21st to end	Day	3 6% 1 4 3 3 3 3 6% 6% 62%	1552
December	1st to 10th	Day	3 5% 2 7% 4 5% 3 3 5% 6% 56%	1520
	11th to 20th	Day	4 8% 3 6% 5% 4 5% 5% 6% 8% 47%	1926
	21st to end	Day	4 8% 2 5% 3 3 5% 4 6% 8% 51%	2837
January	1st to 10th	Night	2 5% 3 8% 4 4 5% 4 5% 6% 53%	4097
	11th to 20th	Night	2 9% 4 7% 4 4 5% 5% 7% 8% 45%	4110
	21st to end	Night	4 10% 3 7% 4 4 4 3 5% 7% 49%	4521
February	1st to 10th	Night	2 5% 2 5% 4 4 4 4 6% 8% 56%	2470
	11th to 20th	Night	3 8% 2 6% 4 3 5% 4 6% 8% 53%	2402
	21st to end	Night	2 8% 3 7% 4 4 4 3 6% 7% 52%	1844
March	1st to 10th	Night	2 7% 2 6% 5% 5% 5% 3 5% 8% 52%	3430
	11th to 20th	Night	3 8% 2 8% 4 4 5% 4 6% 7% 47%	3490
	21st to end	Night	2 7% 3 9% 6% 6% 6% 5% 7% 7% 41%	3828
April	1st to 10th	Night	4 7% 2 4 5% 4 5% 4 6% 7% 54%	1952
	11th to 20th	Night	4 7% 2 4 4 4 5% 4 5% 7% 55%	1877
	21st to end	Night	4 5% 2 4 4 4 5% 4 7% 9% 53%	1828
May	1st to 10th	Night	5% 4 2 6% 4 5% 6% 4 5% 7% 52%	2860
	11th to 20th	Night	3 3 1 4 3 4 5% 5% 7% 9% 56%	2834
	21st to end	Night	5% 3 2 4 4 3 3 3 4 7% 62%	2888
June	1st to 10th	Night	2 2 1 3 3 4 4 4 5% 8% 65%	1520
	11th to 20th	Night	4 3 2 4 3 2 4 3 5% 7% 65%	1520
	21st to end	Night	3 2 1 3 2 2 3 3 4 8% 69%	1520
July	1st to 10th	Night	2 2 1 2 2 2 3 3 4 7% 73%	2749
	11th to 20th	Night	1 2 1 3 3 4 5% 5% 9% 12% 55%	2860
	21st to end	Night	2 3 1 4 4 4 5% 4 6% 9% 58%	3109
August	1st to 10th	Night	2 3 2 5% 5% 6% 7% 6% 9% 11% 45%	2847
	11th to 20th	Night	3 2 1 4 4 4 4 5% 7% 9% 56%	2811
	21st to end	Night	3 3 1 4 3 4 4 5% 7% 7% 58%	3475
September	1st to 10th	Night	4 3 1 3 3 3 4 3 6% 7% 62%	1900
	11th to 20th	Night	3 2 1 3 3 3 3 3 5% 8% 66%	2012
	21st to end	Night	5% 3 1 3 2 2 3 2 3 5% 71%	2118
October	1st to 10th	Night	3 2 1 3 2 2 2 2 3 5% 75%	3800
	11th to 20th	Night	4 4 1 3 3 4 4 3 5% 8% 62%	3778
	21st to end	Night	3 4 2 3 2 3 3 2 4 6% 68%	4120
November	1st to 10th	Night	2 4 1 4 3 3 3 3 5% 6% 65%	2340
	11th to 20th	Night	2 3 1 3 3 3 3 3 4 6% 70%	2647
	21st to end	Night	3 6% 2 5% 3 3 3 4 5% 6% 60%	2723
December	1st to 10th	Night	4 7% 2 4 4 3 4 3 5% 7% 58%	2660
	11th to 20th	Night	4 8% 2 5% 4 4 4 5% 7% 10% 47%	3271
	21st to end	Night	4 8% 3 6% 4 4 4 4 5% 7% 52%	4844

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	29% 3 18% 37% 12% 2	2859
	11th to 20th	Day	29% 3 23% 31% 13% 1	2860
	21st to end	Day	32% 2 12% 37% 14% 3	3146
February	1st to 10th	Day	29% 3 20% 25% 20% 3	1710
	11th to 20th	Day	32% 3 11% 29% 21% 3	1674
	21st to end	Day	31% 3 9% 27% 24% 6%	1603
March	1st to 10th	Day	32% 2 6% 33% 24% 3	3444
	11th to 20th	Day	38% 1 10% 34% 13% 4	3490
	21st to end	Day	44% 2 7% 26% 18% 3	3825
April	1st to 10th	Day	28% 3 7% 25% 25% 12%	2228
	11th to 20th	Day	27% 4 6% 26% 26% 10%	2254
	21st to end	Day	26% 3 6% 26% 31% 8%	2348
May	1st to 10th	Day	26% 4 4 19% 37% 11%	4110
	11th to 20th	Day	23% 3 4 23% 35% 12%	4074
	21st to end	Day	21% 3 3 17% 43% 12%	4694
June	1st to 10th	Day	17% 2 4 20% 34% 23%	2660
	11th to 20th	Day	17% 3 3 11% 27% 40%	2660
	21st to end	Day	15% 3 2 14% 24% 41%	2660
July	1st to 10th	Day	14% 4 4 10% 14% 54%	4152
	11th to 20th	Day	18% 5% 8% 23% 26% 20%	4041
	21st to end	Day	19% 4 10% 21% 19% 28%	4314
August	1st to 10th	Day	26% 6% 16% 22% 14% 15%	4078
	11th to 20th	Day	25% 6% 10% 19% 22% 18%	4067
	21st to end	Day	22% 7% 10% 20% 23% 18%	4189
September	1st to 10th	Day	22% 3 6% 14% 26% 28%	2280
	11th to 20th	Day	23% 4 7% 18% 25% 24%	2164
	21st to end	Day	19% 2 5% 15% 40% 19%	2062
October	1st to 10th	Day	16% 1 4 14% 33% 32%	3170
	11th to 20th	Day	23% 4 3 28% 34% 8%	3150
	21st to end	Day	16% 4 7% 34% 34% 5%	3438
November	1st to 10th	Day	19% 3 7% 32% 31% 8%	1959
	11th to 20th	Day	18% 2 8% 36% 31% 4	1725
	21st to end	Day	22% 1 11% 32% 27% 5%	1552
December	1st to 10th	Day	28% 3 19% 28% 17% 4	1520
	11th to 20th	Day	32% 4 14% 37% 12% 1	1924
	21st to end	Day	31% 2 16% 19% 26% 4	2837
January	1st to 10th	Night	30% 1 15% 39% 14% 1	4097
	11th to 20th	Night	33% 2 21% 30% 11% 2	4110
	21st to end	Night	37% 2 16% 32% 12% 2	4521
February	1st to 10th	Night	24% 3 17% 35% 17% 3	2470
	11th to 20th	Night	29% 3 12% 31% 23% 3	2403
	21st to end	Night	31% 2 8% 30% 26% 4	1844
March	1st to 10th	Night	29% 1 6% 34% 27% 3	3430
	11th to 20th	Night	35% 1 8% 32% 21% 3	3490
	21st to end	Night	37% 3 9% 28% 21% 2	3828
April	1st to 10th	Night	28% 2 6% 25% 28% 11%	1952
	11th to 20th	Night	27% 3 6% 23% 28% 12%	1877
	21st to end	Night	26% 3 6% 26% 31% 8%	1828
May	1st to 10th	Night	28% 3 5% 17% 36% 11%	2860
	11th to 20th	Night	22% 3 6% 26% 35% 9%	2835
	21st to end	Night	23% 3 3 16% 39% 16%	2888
June	1st to 10th	Night	16% 3 3 18% 35% 26%	1520
	11th to 20th	Night	18% 3 5% 13% 21% 40%	1520
	21st to end	Night	15% 2 3 13% 25% 42%	1520
July	1st to 10th	Night	12% 3 6% 9% 13% 56%	2704
	11th to 20th	Night	18% 4 6% 27% 25% 20%	2806
	21st to end	Night	21% 2 11% 18% 20% 26%	3011
August	1st to 10th	Night	28% 6% 17% 18% 15% 16%	2838
	11th to 20th	Night	23% 7% 14% 16% 20% 21%	2838
	21st to end	Night	21% 6% 10% 18% 24% 21%	3475
September	1st to 10th	Night	20% 3 6% 15% 23% 33%	1900
	11th to 20th	Night	17% 3 10% 19% 24% 27%	2012
	21st to end	Night	17% 2 5% 14% 39% 23%	2118
October	1st to 10th	Night	14% 1 3 14% 31% 36%	3800
	11th to 20th	Night	19% 4 6% 27% 32% 10%	3778
	21st to end	Night	17% 4 5% 38% 31% 5%	4120
November	1st to 10th	Night	19% 2 8% 29% 31% 11%	2340
	11th to 20th	Night	16% 3 7% 35% 34% 5%	2647
	21st to end	Night	24% 2 8% 29% 29% 7%	2723
December	1st to 10th	Night	26% 3 17% 29% 21% 5%	2660
	11th to 20th	Night	30% 4 14% 35% 16% 1	3272
	21st to end	Night	31% 3 15% 23% 24% 5%	4844

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	9% 9% 34% 29% 16% 4	270
	11th to 20th	Day	9% 7% 39% 29% 14% 1	270
	21st to end	Day	6% 9% 29% 38% 17% 1	297
February	1st to 10th	Day	7% 16% 37% 29% 10% 1	270
	11th to 20th	Day	10% 14% 41% 25% 10% 1	270
	21st to end	Day	12% 19% 34% 25% 9% 1	301
March	1st to 10th	Day	14% 22% 32% 23% 8% 2	460
	11th to 20th	Day	12% 17% 34% 25% 10% 2	460
	21st to end	Day	12% 33% 30% 19% 5% 1	506
April	1st to 10th	Day	12% 21% 34% 21% 10% 2	460
	11th to 20th	Day	14% 25% 36% 18% 6% 1	452
	21st to end	Day	12% 22% 32% 25% 7% 2	460
May	1st to 10th	Day	12% 24% 35% 21% 7% 1	463
	11th to 20th	Day	7% 15% 34% 28% 14% 2	488
	21st to end	Day	8% 14% 28% 27% 18% 4	655
June	1st to 10th	Day	4 9% 24% 36% 23% 4	680
	11th to 20th	Day	7% 10% 29% 30% 20% 4	680
	21st to end	Day	3 9% 27% 36% 22% 4	680
July	1st to 10th	Day	2 5% 22% 37% 27% 6%	553
	11th to 20th	Day	2 2 16% 42% 32% 6%	484
	21st to end	Day	4 1 15% 39% 35% 6%	531
August	1st to 10th	Day	5% 2 21% 37% 29% 6%	487
	11th to 20th	Day	4 3 21% 39% 27% 5%	477
	21st to end	Day	5% 2 22% 38% 29% 4	539
September	1st to 10th	Day	4 4 24% 35% 24% 8%	490
	11th to 20th	Day	7% 11% 27% 35% 16% 4	490
	21st to end	Day	8% 11% 21% 33% 21% 6%	462
October	1st to 10th	Day	12% 11% 23% 21% 22% 12%	300
	11th to 20th	Day	8% 10% 27% 29% 21% 5%	300
	21st to end	Day	6% 15% 27% 32% 18% 2	330
November	1st to 10th	Day	9% 17% 26% 28% 16% 4	300
	11th to 20th	Day	5% 12% 32% 34% 12% 6%	300
	21st to end	Day	8% 12% 30% 29% 17% 5%	282
December	1st to 10th	Day	8% 9% 33% 29% 16% 6%	270
	11th to 20th	Day	6% 12% 31% 31% 17% 3	270
	21st to end	Day	12% 12% 35% 29% 8% 3	297
January	1st to 10th	Night	5% 3 18% 46% 24% 4	760
	11th to 20th	Night	6% 4 20% 43% 24% 3	760
	21st to end	Night	7% 4 20% 44% 21% 3	836
February	1st to 10th	Night	4 3 26% 48% 17% 3	760
	11th to 20th	Night	7% 5% 26% 45% 15% 3	744
	21st to end	Night	7% 5% 27% 49% 11% 2	628
March	1st to 10th	Night	7% 5% 26% 44% 16% 2	760
	11th to 20th	Night	6% 5% 18% 45% 24% 3	760
	21st to end	Night	7% 7% 25% 43% 16% 1	836
April	1st to 10th	Night	6% 9% 27% 42% 14% 1	622
	11th to 20th	Night	10% 8% 25% 40% 15% 2	563
	21st to end	Night	7% 8% 26% 37% 17% 5%	570
May	1st to 10th	Night	8% 8% 19% 44% 20% 2	570
	11th to 20th	Night	5% 5% 14% 42% 28% 5%	561
	21st to end	Night	7% 6% 21% 35% 27% 4	617
June	1st to 10th	Night	3 3 15% 41% 34% 5%	570
	11th to 20th	Night	6% 4 18% 44% 25% 4	570
	21st to end	Night	4 2 10% 41% 38% 5%	570
July	1st to 10th	Night	3 2 8% 33% 46% 8%	555
	11th to 20th	Night	2 1 3 34% 49% 10%	552
	21st to end	Night	4 2 4 27% 52% 12%	624
August	1st to 10th	Night	3 2 5% 32% 50% 8%	567
	11th to 20th	Night	5% 6% 41% 40% 7%	564
	21st to end	Night	4 1 7% 36% 42% 10%	627
September	1st to 10th	Night	5% 2 7% 38% 41% 8%	570
	11th to 20th	Night	4 2 12% 39% 35% 8%	682
	21st to end	Night	7% 2 14% 33% 34% 10%	760
October	1st to 10th	Night	8% 3 15% 39% 28% 6%	760
	11th to 20th	Night	4 3 15% 43% 28% 7%	760
	21st to end	Night	7% 5% 17% 40% 26% 5%	836
November	1st to 10th	Night	5% 4 18% 42% 27% 5%	780
	11th to 20th	Night	4 3 21% 47% 21% 5%	912
	21st to end	Night	7% 6% 21% 39% 23% 4	973
December	1st to 10th	Night	7% 3 20% 48% 19% 3	950
	11th to 20th	Night	10% 4 22% 41% 21% 2	950
	21st to end	Night	10% 6% 22% 45% 14% 2	1029

Page generated at 2024-11-22 06:02:24. Version ac07

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