Frosty Drew Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Frosty Drew Observatory clear sky chart from 2005-11-06 to 2024-12-02.

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	18% 7% 2 6% 4 4 4 3 5% 7% 39%	101398
	Night	19% 8% 3 6% 4 4 4 3 5% 6% 39%	98255
Transparency	Day	25% 46% 6% 12% 9% 1	104975
	Night	25% 46% 6% 12% 9% 1	102767
Seeing	Day	23% 16% 33% 19% 7% 2	15225
	Night	22% 13% 31% 25% 8% 1	24191

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	16% 8% 2 6% 4 3 4 3 4 5% 45%	2517
	11th to 20th	Day	19% 8% 2 6% 4 4 4 3 5% 6% 39%	2520
	21st to end	Day	23% 11% 3 7% 3 3 3 3 3 5% 35%	2772
February	1st to 10th	Day	19% 7% 2 6% 4 5% 4 3 6% 6% 39%	1841
	11th to 20th	Day	14% 7% 3 6% 5% 4 5% 4 6% 6% 42%	1848
	21st to end	Day	21% 10% 3 5% 4 3 3 3 5% 6% 37%	1570
March	1st to 10th	Day	27% 11% 3 6% 4 3 2 2 4 5% 34%	3116
	11th to 20th	Day	11% 5% 2 4 4 4 4 4 6% 9% 48%	3313
	21st to end	Day	22% 9% 2 6% 4 4 4 3 5% 5% 35%	4060
April	1st to 10th	Day	19% 7% 3 5% 4 4 4 3 5% 6% 39%	2280
	11th to 20th	Day	19% 7% 2 6% 4 4 4 3 4 7% 41%	2254
	21st to end	Day	21% 6% 2 6% 4 4 4 4 5% 6% 37%	2446
May	1st to 10th	Day	27% 11% 3 6% 4 3 4 3 5% 6% 28%	4106
	11th to 20th	Day	22% 8% 3 8% 5% 4 4 3 4 6% 32%	4102
	21st to end	Day	16% 6% 2 5% 3 3 4 3 5% 8% 43%	4837
June	1st to 10th	Day	19% 6% 2 5% 4 5% 4 4 6% 9% 35%	2660
	11th to 20th	Day	16% 6% 2 5% 5% 4 5% 5% 6% 8% 38%	2660
	21st to end	Day	15% 6% 2 6% 4 4 4 4 6% 10% 39%	2660
July	1st to 10th	Day	18% 7% 2 7% 4 4 5% 3 5% 9% 34%	4440
	11th to 20th	Day	17% 6% 2 6% 4 4 4 3 6% 10% 38%	4440
	21st to end	Day	22% 7% 2 6% 4 5% 5% 4 6% 7% 30%	4814
August	1st to 10th	Day	18% 8% 3 8% 5% 4 5% 4 5% 8% 32%	4115
	11th to 20th	Day	13% 6% 2 6% 4 4 5% 4 6% 8% 40%	3894
	21st to end	Day	7% 3 2 5% 4 5% 6% 5% 8% 10% 44%	4180
September	1st to 10th	Day	16% 6% 2 6% 3 4 5% 3 4 6% 45%	2280
	11th to 20th	Day	13% 6% 2 4 3 3 3 3 4 7% 51%	2094
	21st to end	Day	19% 8% 2 5% 4 4 4 3 5% 5% 40%	2090
October	1st to 10th	Day	20% 7% 2 5% 4 3 3 3 5% 5% 43%	2090
	11th to 20th	Day	16% 5% 2 5% 3 3 4 3 5% 7% 47%	1881
	21st to end	Day	20% 5% 2 6% 4 4 4 2 5% 5% 44%	1881
November	1st to 10th	Day	15% 6% 2 6% 4 4 3 2 5% 4 49%	1761
	11th to 20th	Day	16% 5% 2 4 4 3 3 4 5% 5% 49%	1800
	21st to end	Day	18% 6% 3 5% 4 3 4 3 5% 6% 45%	1800
December	1st to 10th	Day	21% 7% 2 6% 3 3 4 4 5% 6% 39%	1598
	11th to 20th	Day	23% 8% 2 3 3 2 2 2 4 5% 45%	1906
	21st to end	Day	21% 10% 3 7% 5% 3 4 3 4 5% 36%	2772
January	1st to 10th	Night	17% 7% 2 4 3 3 3 3 3 6% 49%	4429
	11th to 20th	Night	17% 9% 3 7% 4 4 3 2 4 4 43%	4440
	21st to end	Night	23% 12% 4 7% 4 4 4 3 4 5% 31%	4884
February	1st to 10th	Night	15% 10% 3 7% 5% 3 3 2 4 6% 41%	2335
	11th to 20th	Night	18% 7% 2 5% 5% 3 3 3 4 6% 42%	2218
	21st to end	Night	19% 9% 2 5% 3 4 4 4 4 5% 40%	1884
March	1st to 10th	Night	28% 11% 2 5% 3 3 3 3 4 5% 34%	3724
	11th to 20th	Night	11% 7% 3 7% 4 4 5% 3 5% 6% 43%	3645
	21st to end	Night	24% 8% 2 5% 4 4 5% 4 5% 4 35%	3569
April	1st to 10th	Night	20% 9% 3 5% 4 4 4 3 5% 6% 39%	1900
	11th to 20th	Night	19% 8% 2 6% 4 4 4 3 5% 5% 40%	1875
	21st to end	Night	22% 7% 2 6% 5% 3 3 3 4 6% 39%	1734
May	1st to 10th	Night	25% 12% 4 8% 4 4 3 3 4 6% 27%	2834
	11th to 20th	Night	23% 11% 3 9% 5% 4 4 4 6% 6% 25%	2813
	21st to end	Night	18% 8% 3 6% 5% 4 4 4 6% 7% 35%	2745
June	1st to 10th	Night	19% 8% 3 6% 4 3 5% 4 5% 7% 37%	1520
	11th to 20th	Night	20% 7% 2 4 4 4 3 3 6% 9% 38%	1520
	21st to end	Night	17% 8% 2 6% 4 4 3 4 6% 10% 37%	1520
July	1st to 10th	Night	21% 9% 3 8% 5% 4 5% 4 5% 7% 29%	2520
	11th to 20th	Night	18% 7% 2 6% 5% 4 3 4 7% 9% 34%	2520
	21st to end	Night	21% 10% 2 7% 4 5% 4 4 5% 7% 30%	2739
August	1st to 10th	Night	24% 8% 2 7% 5% 5% 5% 4 7% 6% 27%	2782
	11th to 20th	Night	9% 7% 3 7% 5% 5% 6% 5% 8% 9% 36%	2931
	21st to end	Night	9% 4 1 5% 3 3 3 3 6% 9% 53%	3476
September	1st to 10th	Night	15% 7% 2 7% 4 5% 4 4 5% 6% 41%	1900
	11th to 20th	Night	14% 7% 2 5% 3 2 3 2 4 6% 51%	2085
	21st to end	Night	21% 7% 3 6% 5% 5% 4 3 5% 4 38%	2090
October	1st to 10th	Night	19% 9% 2 6% 4 4 3 3 4 6% 40%	2090
	11th to 20th	Night	15% 5% 3 5% 3 3 3 4 5% 6% 49%	2295
	21st to end	Night	20% 7% 2 5% 3 3 3 3 5% 7% 41%	2717
November	1st to 10th	Night	13% 6% 2 6% 4 4 3 3 5% 6% 48%	2537
	11th to 20th	Night	18% 7% 2 5% 3 3 3 3 4 5% 47%	2600
	21st to end	Night	15% 6% 2 6% 4 3 4 4 4 7% 46%	2600
December	1st to 10th	Night	21% 7% 3 5% 4 4 4 3 4 6% 41%	2612
	11th to 20th	Night	20% 7% 2 4 4 4 4 3 5% 5% 43%	3288
	21st to end	Night	21% 11% 3 7% 3 3 3 3 3 4 39%	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	27% 37% 4 19% 10% 2	2518
	11th to 20th	Day	29% 38% 5% 19% 8% 1	2520
	21st to end	Day	40% 39% 4 11% 6% 1	2772
February	1st to 10th	Day	17% 65% 6% 8% 3 1	1841
	11th to 20th	Day	17% 63% 2 8% 8% 2	1857
	21st to end	Day	16% 65% 5% 8% 4 1	1570
March	1st to 10th	Day	40% 41% 5% 7% 5% 2	3119
	11th to 20th	Day	17% 40% 7% 24% 10% 1	3313
	21st to end	Day	35% 40% 2 11% 9% 4	4060
April	1st to 10th	Day	19% 64% 4 6% 5% 1	2280
	11th to 20th	Day	15% 65% 3 8% 8%	2254
	21st to end	Day	19% 65% 3 7% 5% 1	2446
May	1st to 10th	Day	38% 40% 10% 7% 4	4120
	11th to 20th	Day	38% 40% 10% 9% 3	4106
	21st to end	Day	25% 40% 5% 17% 13%	4837
June	1st to 10th	Day	16% 66% 3 8% 7% 1	2660
	11th to 20th	Day	15% 65% 4 10% 6% 1	2660
	21st to end	Day	14% 64% 4 10% 8%	2660
July	1st to 10th	Day	32% 38% 8% 12% 10% 1	4370
	11th to 20th	Day	29% 36% 11% 15% 8% 1	4365
	21st to end	Day	37% 36% 8% 10% 8% 1	4661
August	1st to 10th	Day	36% 37% 10% 12% 4	4101
	11th to 20th	Day	27% 38% 10% 21% 4	3933
	21st to end	Day	19% 38% 10% 19% 13% 1	4180
September	1st to 10th	Day	16% 60% 4 11% 8% 1	2280
	11th to 20th	Day	15% 61% 4 9% 10% 2	2094
	21st to end	Day	21% 60% 4 8% 8%	2090
October	1st to 10th	Day	24% 37% 5% 15% 14% 5%	3480
	11th to 20th	Day	16% 37% 4 15% 25% 4	3110
	21st to end	Day	17% 36% 9% 20% 15% 2	3081
November	1st to 10th	Day	19% 56% 6% 8% 9% 1	1761
	11th to 20th	Day	15% 56% 4 9% 13% 3	1800
	21st to end	Day	17% 57% 4 14% 7% 2	1800
December	1st to 10th	Day	21% 59% 5% 6% 9%	1598
	11th to 20th	Day	23% 48% 5% 9% 11% 5%	1906
	21st to end	Day	34% 37% 5% 16% 7%	2772
January	1st to 10th	Night	24% 36% 6% 24% 7% 4	4429
	11th to 20th	Night	29% 37% 7% 16% 9% 2	4440
	21st to end	Night	43% 39% 5% 8% 5% 1	4884
February	1st to 10th	Night	17% 64% 5% 8% 4 1	2335
	11th to 20th	Night	16% 63% 3 6% 10% 2	2220
	21st to end	Night	16% 65% 3 7% 6% 2	1884
March	1st to 10th	Night	39% 40% 4 10% 5% 1	3734
	11th to 20th	Night	21% 40% 10% 15% 11% 1	3646
	21st to end	Night	35% 40% 3 6% 13% 3	3569
April	1st to 10th	Night	19% 65% 3 7% 5% 1	1900
	11th to 20th	Night	16% 66% 5% 6% 7% 1	1875
	21st to end	Night	18% 64% 3 8% 5% 1	1734
May	1st to 10th	Night	39% 40% 9% 9% 3	2840
	11th to 20th	Night	41% 42% 9% 6% 2	2814
	21st to end	Night	28% 40% 8% 16% 7% 1	2746
June	1st to 10th	Night	15% 65% 2 9% 8% 1	1520
	11th to 20th	Night	14% 66% 3 11% 6% 1	1520
	21st to end	Night	14% 63% 5% 11% 6% 1	1520
July	1st to 10th	Night	36% 37% 6% 11% 9%	2480
	11th to 20th	Night	30% 37% 11% 15% 7%	2472
	21st to end	Night	38% 36% 8% 9% 9% 1	2651
August	1st to 10th	Night	39% 38% 7% 10% 5% 1	2772
	11th to 20th	Night	30% 38% 9% 19% 4	2958
	21st to end	Night	13% 37% 11% 22% 16%	3476
September	1st to 10th	Night	17% 60% 4 11% 6% 1	1900
	11th to 20th	Night	15% 60% 5% 8% 11% 1	2085
	21st to end	Night	23% 60% 2 7% 8%	2090
October	1st to 10th	Night	25% 37% 7% 18% 12% 2	3480
	11th to 20th	Night	11% 37% 4 16% 29% 4	3805
	21st to end	Night	17% 36% 8% 20% 16% 2	4467
November	1st to 10th	Night	18% 56% 6% 9% 9% 1	2537
	11th to 20th	Night	17% 56% 3 8% 11% 4	2600
	21st to end	Night	16% 57% 6% 14% 6% 2	2600
December	1st to 10th	Night	21% 59% 4 8% 7%	2612
	11th to 20th	Night	22% 49% 6% 10% 12% 2	3288
	21st to end	Night	35% 37% 5% 15% 9%	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	24% 27% 31% 17% 2	180
	11th to 20th	Day	25% 33% 30% 8% 4	180
	21st to end	Day	24% 27% 37% 8% 4 1	198
February	1st to 10th	Day	28% 24% 34% 13% 1	413
	11th to 20th	Day	29% 27% 34% 9% 2	456
	21st to end	Day	29% 21% 35% 12% 3 1	382
March	1st to 10th	Day	27% 23% 31% 17% 2	460
	11th to 20th	Day	26% 25% 33% 12% 3 1	460
	21st to end	Day	33% 19% 31% 12% 3 1	506
April	1st to 10th	Day	31% 24% 27% 12% 5%	460
	11th to 20th	Day	28% 24% 31% 13% 3	452
	21st to end	Day	31% 18% 33% 12% 4 2	460
May	1st to 10th	Day	33% 20% 30% 12% 3	463
	11th to 20th	Day	24% 13% 45% 14% 3 1	487
	21st to end	Day	21% 10% 34% 28% 6% 2	529
June	1st to 10th	Day	20% 15% 35% 22% 7% 1	490
	11th to 20th	Day	18% 14% 37% 22% 7% 1	490
	21st to end	Day	19% 11% 37% 24% 6% 2	490
July	1st to 10th	Day	20% 6% 33% 28% 12% 1	485
	11th to 20th	Day	17% 7% 33% 29% 12% 2	484
	21st to end	Day	16% 11% 37% 26% 9% 1	532
August	1st to 10th	Day	16% 9% 33% 28% 10% 3	486
	11th to 20th	Day	18% 8% 32% 26% 12% 4	479
	21st to end	Day	12% 8% 34% 27% 14% 5%	539
September	1st to 10th	Day	20% 10% 27% 25% 14% 3	490
	11th to 20th	Day	16% 10% 30% 25% 12% 7%	490
	21st to end	Day	23% 9% 36% 20% 8% 3	490
October	1st to 10th	Day	25% 12% 28% 21% 10% 4	490
	11th to 20th	Day	21% 16% 31% 20% 11% 2	450
	21st to end	Day	25% 14% 31% 20% 7% 2	429
November	1st to 10th	Day	18% 18% 33% 21% 8% 2	395
	11th to 20th	Day	24% 25% 29% 17% 6%	400
	21st to end	Day	25% 19% 30% 18% 7% 1	400
December	1st to 10th	Day	25% 25% 31% 16% 4	252
	11th to 20th	Day	32% 26% 32% 8% 3	180
	21st to end	Day	25% 23% 34% 12% 5% 2	198
January	1st to 10th	Night	22% 21% 34% 20% 3	760
	11th to 20th	Night	29% 18% 33% 17% 3	760
	21st to end	Night	22% 22% 32% 21% 3	836
February	1st to 10th	Night	22% 20% 34% 22% 2	760
	11th to 20th	Night	25% 21% 38% 13% 2	743
	21st to end	Night	26% 21% 33% 18% 2	628
March	1st to 10th	Night	25% 19% 35% 18% 3	760
	11th to 20th	Night	23% 17% 37% 20% 3	760
	21st to end	Night	26% 17% 34% 18% 3 1	836
April	1st to 10th	Night	27% 15% 33% 22% 3	760
	11th to 20th	Night	23% 14% 37% 21% 5%	751
	21st to end	Night	25% 13% 35% 22% 5% 1	594
May	1st to 10th	Night	28% 11% 31% 23% 7% 1	570
	11th to 20th	Night	22% 14% 38% 22% 4	562
	21st to end	Night	22% 6% 28% 35% 8% 1	418
June	1st to 10th	Night	20% 5% 27% 36% 11%	380
	11th to 20th	Night	23% 6% 27% 34% 10%	380
	21st to end	Night	22% 3 26% 39% 10%	380
July	1st to 10th	Night	20% 4 22% 37% 16% 1	370
	11th to 20th	Night	18% 3 20% 43% 14% 2	368
	21st to end	Night	17% 5% 22% 43% 13% 1	416
August	1st to 10th	Night	15% 4 29% 39% 12% 1	537
	11th to 20th	Night	15% 4 26% 35% 18% 2	646
	21st to end	Night	14% 4 21% 38% 19% 3	836
September	1st to 10th	Night	18% 4 29% 31% 16% 2	760
	11th to 20th	Night	18% 5% 25% 31% 18% 4	760
	21st to end	Night	23% 7% 28% 27% 14% 2	760
October	1st to 10th	Night	23% 9% 28% 25% 13% 2	760
	11th to 20th	Night	19% 12% 33% 25% 10% 2	760
	21st to end	Night	24% 12% 34% 22% 8%	836
November	1st to 10th	Night	16% 14% 33% 24% 12% 1	780
	11th to 20th	Night	22% 18% 33% 19% 7% 1	800
	21st to end	Night	22% 19% 31% 21% 6% 1	800
December	1st to 10th	Night	27% 17% 31% 21% 4 1	768
	11th to 20th	Night	24% 20% 32% 19% 4	760
	21st to end	Night	25% 18% 36% 18% 3	836

Page generated at 2024-11-29 10:43:09. Version ac07

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