Frog Island Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Frog Island Observatory clear sky chart from 2005-11-06 to 2024-11-28.

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	12% 8% 3 8% 5% 5% 5% 4 6% 8% 36%	101654
	Night	14% 10% 3 8% 5% 4 4 4 5% 7% 35%	97931
Transparency	Day	23% 48% 8% 13% 7%	105126
	Night	26% 48% 7% 12% 6% 1	102547
Seeing	Day	15% 11% 32% 28% 12% 3	16057
	Night	18% 9% 31% 30% 12% 1	25998

By Month

Percentage Cloud Cover NE (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	10% 13% 6% 11% 6% 5% 6% 4 7% 7% 24%	2527
	11th to 20th	Day	14% 12% 4 10% 8% 7% 6% 4 6% 8% 22%	2530
	21st to end	Day	13% 16% 6% 11% 7% 6% 6% 4 6% 5% 21%	2783
February	1st to 10th	Day	9% 10% 4 10% 8% 6% 6% 4 7% 7% 31%	1577
	11th to 20th	Day	10% 12% 4 7% 6% 7% 6% 5% 6% 6% 32%	1837
	21st to end	Day	12% 9% 4 12% 6% 5% 6% 5% 7% 7% 27%	1570
March	1st to 10th	Day	11% 10% 3 7% 4 4 4 4 6% 8% 39%	3118
	11th to 20th	Day	7% 9% 4 12% 7% 5% 6% 4 6% 7% 33%	3276
	21st to end	Day	16% 11% 3 9% 7% 6% 6% 4 6% 6% 25%	4124
April	1st to 10th	Day	15% 12% 3 9% 7% 5% 5% 4 5% 7% 28%	2280
	11th to 20th	Day	18% 9% 4 7% 5% 5% 5% 4 5% 6% 33%	2296
	21st to end	Day	17% 8% 2 7% 4 4 6% 4 5% 8% 35%	2470
May	1st to 10th	Day	13% 7% 3 7% 5% 5% 5% 4 5% 7% 41%	4186
	11th to 20th	Day	11% 7% 2 7% 5% 5% 5% 4 5% 8% 41%	4399
	21st to end	Day	11% 6% 3 7% 5% 5% 5% 4 6% 8% 40%	4837
June	1st to 10th	Day	11% 5% 2 6% 5% 4 5% 4 7% 8% 42%	2660
	11th to 20th	Day	12% 6% 2 7% 5% 4 4 5% 7% 7% 41%	2660
	21st to end	Day	12% 5% 2 7% 4 4 5% 5% 8% 9% 40%	2660
July	1st to 10th	Day	14% 5% 2 6% 4 4 4 4 6% 9% 41%	4440
	11th to 20th	Day	4 4 1 5% 4 3 4 4 7% 12% 52%	4440
	21st to end	Day	7% 5% 2 5% 4 4 4 4 7% 8% 50%	4814
August	1st to 10th	Day	7% 5% 2 7% 5% 5% 6% 5% 8% 10% 42%	4423
	11th to 20th	Day	11% 4 2 5% 3 3 4 4 6% 7% 50%	4056
	21st to end	Day	11% 6% 3 7% 4 4 4 3 5% 6% 46%	4198
September	1st to 10th	Day	11% 5% 2 6% 5% 5% 5% 4 6% 9% 44%	2280
	11th to 20th	Day	11% 5% 2 5% 4 4 5% 4 7% 9% 44%	2135
	21st to end	Day	12% 7% 2 6% 6% 5% 4 5% 6% 8% 38%	2090
October	1st to 10th	Day	15% 6% 2 6% 5% 4 5% 4 6% 8% 38%	2057
	11th to 20th	Day	20% 10% 4 8% 5% 5% 5% 4 6% 7% 26%	1783
	21st to end	Day	19% 11% 4 9% 5% 5% 5% 4 6% 7% 25%	1881
November	1st to 10th	Day	14% 10% 3 7% 5% 4 6% 4 5% 7% 36%	1762
	11th to 20th	Day	15% 12% 4 11% 6% 5% 5% 4 5% 6% 26%	1800
	21st to end	Day	17% 13% 4 8% 5% 5% 6% 5% 7% 7% 24%	1585
December	1st to 10th	Day	14% 12% 3 9% 7% 6% 6% 4 6% 7% 26%	1461
	11th to 20th	Day	16% 15% 4 11% 6% 6% 5% 4 5% 6% 22%	1876
	21st to end	Day	23% 21% 4 10% 6% 4 4 4 4 4 16%	2783
January	1st to 10th	Night	13% 15% 5% 11% 6% 4 4 3 4 6% 29%	4428
	11th to 20th	Night	11% 15% 6% 13% 7% 5% 6% 4 5% 8% 21%	4440
	21st to end	Night	13% 16% 6% 12% 6% 5% 5% 4 5% 6% 23%	4884
February	1st to 10th	Night	10% 13% 5% 9% 6% 4 5% 5% 5% 7% 31%	2599
	11th to 20th	Night	10% 12% 3 9% 6% 6% 5% 4 7% 8% 32%	2228
	21st to end	Night	13% 13% 4 10% 5% 6% 5% 4 6% 6% 28%	1884
March	1st to 10th	Night	13% 10% 3 7% 4 3 3 3 5% 8% 42%	3722
	11th to 20th	Night	9% 10% 4 9% 5% 5% 5% 4 5% 7% 36%	3682
	21st to end	Night	16% 10% 3 8% 6% 6% 5% 5% 5% 7% 30%	3535
April	1st to 10th	Night	18% 12% 4 8% 5% 4 5% 3 5% 7% 30%	1900
	11th to 20th	Night	19% 9% 3 7% 4 4 4 4 5% 6% 36%	1833
	21st to end	Night	16% 9% 3 7% 5% 5% 5% 3 5% 8% 35%	1710
May	1st to 10th	Night	17% 8% 2 6% 4 4 4 3 5% 7% 40%	2754
	11th to 20th	Night	14% 8% 2 9% 5% 5% 4 4 5% 7% 37%	2506
	21st to end	Night	17% 6% 2 6% 4 5% 6% 4 5% 7% 39%	2750
June	1st to 10th	Night	12% 6% 2 6% 4 5% 4 4 5% 7% 45%	1520
	11th to 20th	Night	14% 6% 3 5% 5% 6% 4 3 6% 8% 39%	1520
	21st to end	Night	14% 7% 2 5% 4 3 4 4 5% 8% 45%	1520
July	1st to 10th	Night	12% 5% 2 6% 5% 5% 5% 4 6% 7% 42%	2530
	11th to 20th	Night	5% 3 2 4 4 3 4 4 5% 8% 57%	2530
	21st to end	Night	8% 5% 2 6% 3 4 4 2 6% 7% 52%	2750
August	1st to 10th	Night	11% 5% 2 7% 4 5% 5% 4 5% 8% 45%	2517
	11th to 20th	Night	12% 4 2 5% 3 4 5% 3 5% 7% 51%	2772
	21st to end	Night	11% 5% 2 7% 4 4 4 3 5% 6% 48%	3442
September	1st to 10th	Night	9% 6% 2 6% 3 4 3 3 5% 7% 52%	1900
	11th to 20th	Night	12% 6% 2 5% 4 3 5% 3 6% 5% 49%	2025
	21st to end	Night	14% 7% 3 7% 5% 4 4 3 5% 7% 40%	2090
October	1st to 10th	Night	16% 8% 2 7% 3 4 4 4 5% 7% 40%	2121
	11th to 20th	Night	15% 9% 3 7% 5% 5% 5% 5% 6% 6% 34%	2396
	21st to end	Night	20% 11% 3 7% 4 4 4 4 6% 7% 30%	2717
November	1st to 10th	Night	14% 9% 3 7% 5% 4 5% 4 5% 7% 36%	2536
	11th to 20th	Night	16% 14% 3 8% 6% 5% 5% 4 5% 6% 29%	2600
	21st to end	Night	15% 13% 4 8% 5% 5% 5% 4 5% 7% 28%	2749
December	1st to 10th	Night	13% 13% 5% 9% 6% 5% 5% 4 5% 8% 27%	2674
	11th to 20th	Night	13% 15% 5% 12% 5% 5% 4 3 5% 6% 26%	3283
	21st to end	Night	25% 19% 6% 11% 4 4 3 3 3 4 18%	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	34% 41% 10% 11% 3	2528
	11th to 20th	Day	35% 43% 13% 7% 2	2530
	21st to end	Day	41% 44% 8% 5% 2	2783
February	1st to 10th	Day	17% 66% 6% 7% 4	1577
	11th to 20th	Day	19% 67% 5% 5% 4	1847
	21st to end	Day	16% 66% 4 8% 5% 1	1570
March	1st to 10th	Day	21% 43% 11% 14% 11%	3120
	11th to 20th	Day	28% 44% 10% 12% 5% 1	3276
	21st to end	Day	33% 46% 6% 10% 5%	4124
April	1st to 10th	Day	20% 66% 3 7% 3	2280
	11th to 20th	Day	18% 66% 2 7% 5% 1	2296
	21st to end	Day	15% 66% 3 8% 6% 1	2470
May	1st to 10th	Day	21% 43% 6% 18% 11% 1	4198
	11th to 20th	Day	22% 43% 9% 18% 7%	4403
	21st to end	Day	24% 45% 7% 11% 11% 2	4837
June	1st to 10th	Day	12% 66% 5% 10% 5% 1	2660
	11th to 20th	Day	16% 66% 5% 6% 6% 1	2660
	21st to end	Day	13% 65% 7% 11% 4	2660
July	1st to 10th	Day	28% 38% 11% 15% 7% 1	4371
	11th to 20th	Day	15% 39% 9% 28% 9% 1	4365
	21st to end	Day	18% 38% 8% 24% 13%	4662
August	1st to 10th	Day	23% 40% 11% 19% 7%	4408
	11th to 20th	Day	23% 39% 11% 20% 7%	4095
	21st to end	Day	26% 39% 10% 17% 7%	4197
September	1st to 10th	Day	17% 62% 6% 10% 4	2280
	11th to 20th	Day	13% 62% 8% 12% 5%	2135
	21st to end	Day	16% 62% 7% 12% 3	2090
October	1st to 10th	Day	15% 38% 8% 21% 18%	3421
	11th to 20th	Day	29% 39% 6% 18% 8%	2934
	21st to end	Day	33% 39% 10% 13% 4	3081
November	1st to 10th	Day	22% 58% 6% 10% 4	1762
	11th to 20th	Day	24% 57% 7% 8% 4 1	1800
	21st to end	Day	26% 59% 6% 6% 2 1	1585
December	1st to 10th	Day	23% 61% 5% 7% 3	1461
	11th to 20th	Day	32% 49% 7% 7% 4	1877
	21st to end	Day	46% 40% 4 6% 4	2783
January	1st to 10th	Night	34% 40% 8% 14% 4	4428
	11th to 20th	Night	38% 41% 12% 7% 2	4440
	21st to end	Night	40% 43% 10% 5% 2	4884
February	1st to 10th	Night	19% 65% 4 8% 5%	2599
	11th to 20th	Night	17% 65% 4 11% 4 1	2229
	21st to end	Night	17% 65% 4 8% 5% 1	1884
March	1st to 10th	Night	23% 41% 14% 15% 6% 1	3733
	11th to 20th	Night	27% 44% 7% 17% 5%	3683
	21st to end	Night	30% 45% 8% 10% 5% 1	3535
April	1st to 10th	Night	20% 65% 2 6% 5% 1	1900
	11th to 20th	Night	19% 66% 3 7% 4 2	1833
	21st to end	Night	17% 65% 4 7% 6% 1	1710
May	1st to 10th	Night	24% 42% 7% 17% 9% 1	2761
	11th to 20th	Night	23% 44% 9% 16% 7%	2507
	21st to end	Night	29% 43% 7% 10% 4 6%	2751
June	1st to 10th	Night	13% 66% 4 11% 7%	1520
	11th to 20th	Night	17% 66% 4 7% 6% 1	1520
	21st to end	Night	14% 63% 3 12% 7% 1	1520
July	1st to 10th	Night	31% 39% 10% 11% 7% 3	2490
	11th to 20th	Night	14% 37% 11% 27% 9% 1	2482
	21st to end	Night	21% 37% 7% 15% 19% 1	2662
August	1st to 10th	Night	26% 38% 9% 15% 12% 1	2508
	11th to 20th	Night	23% 39% 10% 21% 7%	2799
	21st to end	Night	25% 39% 9% 16% 11%	3442
September	1st to 10th	Night	15% 61% 7% 12% 5%	1900
	11th to 20th	Night	15% 61% 6% 12% 6%	2025
	21st to end	Night	17% 61% 7% 10% 5%	2090
October	1st to 10th	Night	22% 37% 5% 25% 10%	3537
	11th to 20th	Night	24% 38% 9% 17% 12%	3984
	21st to end	Night	32% 38% 9% 15% 7%	4467
November	1st to 10th	Night	23% 56% 6% 11% 4	2536
	11th to 20th	Night	22% 58% 7% 9% 4	2600
	21st to end	Night	25% 58% 6% 7% 2 1	2749
December	1st to 10th	Night	22% 61% 5% 9% 2	2674
	11th to 20th	Night	28% 51% 7% 10% 4	3281
	21st to end	Night	45% 41% 5% 6% 3	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	15% 14% 39% 23% 10%	270
	11th to 20th	Day	21% 15% 35% 21% 7% 1	270
	21st to end	Day	19% 10% 37% 28% 6%	297
February	1st to 10th	Day	14% 14% 45% 20% 5% 1	270
	11th to 20th	Day	11% 20% 43% 21% 4 1	270
	21st to end	Day	18% 14% 50% 16% 2	225
March	1st to 10th	Day	13% 16% 46% 20% 6%	270
	11th to 20th	Day	13% 16% 37% 24% 7% 2	334
	21st to end	Day	20% 14% 31% 25% 8% 1	506
April	1st to 10th	Day	22% 12% 36% 24% 6% 1	460
	11th to 20th	Day	20% 16% 30% 22% 10% 1	452
	21st to end	Day	18% 14% 33% 24% 10% 2	460
May	1st to 10th	Day	18% 14% 36% 21% 10% 2	502
	11th to 20th	Day	15% 10% 36% 27% 10% 2	675
	21st to end	Day	13% 9% 31% 30% 13% 4	734
June	1st to 10th	Day	11% 9% 36% 31% 11% 1	680
	11th to 20th	Day	16% 8% 28% 34% 13% 2	680
	21st to end	Day	14% 6% 28% 36% 15% 2	680
July	1st to 10th	Day	8% 4 25% 39% 20% 4	671
	11th to 20th	Day	7% 6% 26% 38% 17% 6%	668
	21st to end	Day	7% 5% 26% 34% 22% 7%	739
August	1st to 10th	Day	7% 3 23% 40% 21% 5%	676
	11th to 20th	Day	11% 4 19% 33% 26% 8%	491
	21st to end	Day	12% 9% 24% 26% 24% 6%	539
September	1st to 10th	Day	12% 7% 28% 28% 18% 6%	490
	11th to 20th	Day	11% 4 28% 34% 17% 7%	490
	21st to end	Day	14% 13% 25% 27% 17% 6%	490
October	1st to 10th	Day	18% 10% 33% 25% 12% 3	457
	11th to 20th	Day	18% 17% 38% 21% 6%	300
	21st to end	Day	26% 18% 31% 21% 4	330
November	1st to 10th	Day	15% 15% 38% 25% 7% 1	300
	11th to 20th	Day	21% 20% 38% 16% 5% 1	300
	21st to end	Day	22% 17% 38% 18% 5%	291
December	1st to 10th	Day	20% 19% 36% 17% 9%	241
	11th to 20th	Day	25% 17% 38% 15% 4 1	252
	21st to end	Day	28% 18% 33% 17% 4	297
January	1st to 10th	Night	17% 11% 40% 27% 6%	950
	11th to 20th	Night	20% 11% 38% 23% 7%	950
	21st to end	Night	19% 7% 36% 29% 9% 1	1045
February	1st to 10th	Night	15% 7% 40% 29% 7% 1	950
	11th to 20th	Night	16% 12% 38% 27% 6% 1	753
	21st to end	Night	19% 11% 37% 29% 4	628
March	1st to 10th	Night	17% 8% 37% 29% 9% 1	760
	11th to 20th	Night	15% 10% 41% 27% 7%	760
	21st to end	Night	18% 8% 31% 33% 9% 1	670
April	1st to 10th	Night	21% 8% 34% 29% 6% 1	570
	11th to 20th	Night	23% 8% 29% 28% 11% 1	563
	21st to end	Night	18% 9% 29% 35% 9% 1	570
May	1st to 10th	Night	19% 7% 30% 32% 10% 1	570
	11th to 20th	Night	19% 5% 29% 35% 11% 1	561
	21st to end	Night	15% 4 21% 40% 19% 2	617
June	1st to 10th	Night	12% 5% 25% 42% 15% 1	570
	11th to 20th	Night	17% 4 21% 39% 16% 2	570
	21st to end	Night	16% 3 21% 39% 19% 1	570
July	1st to 10th	Night	8% 3 17% 44% 26% 2	555
	11th to 20th	Night	9% 3 20% 38% 27% 2	552
	21st to end	Night	9% 3 21% 39% 24% 4	624
August	1st to 10th	Night	8% 2 20% 39% 27% 3	567
	11th to 20th	Night	11% 1 19% 36% 29% 4	564
	21st to end	Night	12% 4 24% 37% 23% 1	627
September	1st to 10th	Night	11% 5% 24% 32% 23% 5%	570
	11th to 20th	Night	13% 5% 25% 34% 19% 4	695
	21st to end	Night	16% 9% 25% 27% 20% 3	760
October	1st to 10th	Night	19% 8% 31% 26% 14% 2	760
	11th to 20th	Night	21% 12% 31% 26% 9%	760
	21st to end	Night	25% 12% 33% 23% 7%	836
November	1st to 10th	Night	18% 13% 36% 26% 7%	780
	11th to 20th	Night	22% 17% 34% 22% 6% 1	800
	21st to end	Night	22% 17% 33% 22% 6%	976
December	1st to 10th	Night	21% 16% 37% 19% 6%	950
	11th to 20th	Night	21% 14% 38% 20% 7% 1	950
	21st to end	Night	27% 11% 36% 22% 4	1045

Page generated at 2024-11-25 00:22:15. Version ac07

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