James Webb Space Telescope

It is said that the Hubble Space Telescope is the most important advance in astronomy and astrophysics since Galileo put lenses at the ends of a tube, and looked skyward.

Later this month, an Ariane 5 rocket will take off from the Spaceport in French Guiana, carrying what is likely to be the most important advance since Hubble – the James Webb Space Telescope (JWST).

JWST is not intended to replace Hubble, but to work in concert with it. Hubble works mostly in the visible light range, which collects amazing images, but is limited in the distance it can see. The JWST is designed to work with infrared light, extending the range of detection.

In our rapidly expanding universe, distant objects are retreating from us so rapidly that the light waves they emit are stretched (so-called 'red-shifted') beyond the wavelength of visible light. This is akin to the way the sound of a train is high pitched as it approaches, then drops to a lower pitch as it moves away.

Because JWST works primarily in the longer wavelength infrared range, it will allow us to see back to very near the beginning of our universe (within a hundred million years or so). JWST also has some benefit for viewing nearer objects, since infrared light is not blocked by the massive interstellar dust clouds that obscure many of the formations viewed, but obscured by Hubble.

The next few months will be very stressful for those involved in, or who care about, this project. There are many things that can go wrong, even after launch. JWST will have to reach, and settle at a stable 'Lagrange point' over a million miles from Earth. If a problem is found—as happened with Hubble—we will not have the option of sending a team to repair it. This link has some details about it's planned position, as well as some other trivia.

https://webb.nasa.gov/content/about/orbit.html

Kinetic Theory of Sweat - Part Deux

It is supposed to top 100 degrees today, which has me thinking about sweat.  And I’m sure it makes people curious about how it is that sweating cools us off.  You ARE super curious about that, right?   

As a recovering engineer (and a major Cliff Claven fan), I know you are curious, and I am here to satisfy your curiosity.  Rather than delve deeply into textbook explanations, let’s try a familiar sports metaphor.  

First of all, it isn’t the process of sweating that cools us off; we cool off when that sweat evaporates, and leaves our bodies, taking heat with it.  If it can’t evaporate, we don’t cool off.

(Brief technical bit - I promise it will be brief) Heat is a measure of the average kinetic energy of all the molecules in whatever is being measured.  The hotter something is, the faster the average motion of its molecules (and thus, kinetic energy) .  The key word here is AVERAGE.  There are billions of molecules of water in a bead of sweat, some moving fast, and some less so.  The temperature we sense is the average.  

Evaporation is when some of the fastest molecules actually move fast enough, they leave the puddle of sweat.  Because they are the fastest, when they leave, the AVERAGE speed of the ones that stay behind is lower, which, by definition means it is cooler.  This is exactly how those evaporative swamp coolers work … the ones that use a fan to blow hot air over a water.  

For those who have been missing professional baseball this summer, here’s a way of looking at it.  Suppose your home town has a minor league baseball team - a farm club connected with a major league team.  Think of kinetic energy as the skills of each player on the team.  As they train and develop, their skills improve … they get hot, and start winning.  The major league team who owns the club starts to pay attention - especially to the hottest players on the team - and one day, that player gets called up to ‘the show’.  Without this player, the AVERAGE talent of the team decreases … they aren’t quite as hot.  That player’s phase change from AAA to the majors sucks up a lot of heat from the team.  Maybe the coaching staff focuses more on the remaining players, and they get hot again … and the big leagues come and snag the next hottest player, and the team cools off again.  

This metaphor works as long as there is somewhere for the best players to go … as long as the majors have room to take them.  If the team is insulated, it remains hot.  

I’ll stretch the metaphor just a bit (and hope it still works), to see if we can address why sweating doesn’t help when it’s humid.  Let's say that the major league team calls up your best player, then sends down one, whose skills have cooled off a bit (by big-league standards).  The cooling of minor league team is now offset by the addition of this new player.  A minor league team operating in an environment where there is a surplus of highly-talented players in the majors can’t really cool off by sending up their hottest players, if they are just going to be replaced immediately with one who is just as hot.  

That’s all I’ve got for now.  Time to go out to the kiddie pool (would that be like Little League?)!  



I gave this whole thing a shot a few years ago.  Not sure if my current explanation is any better than my attempt was then:
https://askdoctorwizard.blogspot.com/2013/09/the-kinetic-theory-of-sweat.html

The Sinusoids of Fall

This time of year, it's not unusual to get the sense that the days are becoming shorter very quickly.  

This is not just an illusion, it is actually happening.  As we make our way from long summer days to the long nights of winter, the day-to-day reduction in day length is never greater than it is near the autumnal equinox - and conversely, the day-to-day increase is never greater than the first day of spring - the vernal equinox.  

This is an excellent illustration of what scientists call a sinusoidal function; a function whose form looks like an unending series of waves (technically, 'sine waves').  

Sinusoidal functions are are all over the damn place in nature and science - from the pendulum on a grandfather clock, the vertical motion of an engine's piston as the crankshaft turns, or the up-and-down motion of a weight hanging at the end of a spring.  

This is a graph of a simple sine wave.  Notice that when the wave is at the top and bottom of its path, it is just about level.  This could represent the summer and winter solstices.  The days are at their longest, and shortest, respectively, and hardly change at all from day to day.  Then the slope gradually increases, until it is steepest when the curve is exactly at zero (equal lengths day and night).  From there, the slope gradually becomes less steep, until it reaches the other extreme, at the top or bottom, at which time, the process begins in reverse.  

We may sense the stability of the solstices; as the enjoying the long days of early summer, or the interminable wait in December and January, waiting for the longer days of spring.  In both cases, things are not changing very rapidly.  

If this hasn't bored you to tears yet, consider this further illustration.  

The animation below dynamically illustrates the relationship between day length and the rate of change in day length, using two related sine waves.  Think of the sine wave along the bottom of the graphic as day length, and the vertical wave on the left as the rate of change in day length (what calculus nerds call the first derivative).  It is when the day length is at the middle that the change is at its maximum.  

I could go on, but that would surely induce a nap, if I haven't already. 

Fridge Thermodynamics

Alright, engineer cadets, and appliance-repair apprentice wannabees, I have a story problem for you.  

Imagine a hypothetical beer fridge.  Like many beer fridges, it is kept in the garage – though in the yard, at the end of an extension cord, next to the car up on blocks would work as well for this example … but I digress.  

So, in addition to beer, there is ice cream in the freezer.  Everything works fine, even in the heat of summer (the one in the yard is under s shade tree).  

But a strange thing happens when it gets chilly outside.  It’s all good until it gets down to about 40°F.  At that point, the refrigerator continues to work just fine, but the ice cream in the freezer softens up a little.  Then, when it gets to the mid-30s, everything in the freezer slowly melts.  When it gets below freezing, it hardens back up, but then softens up again between 33°F and a bit over 40°F.  When the weather gets warm again, it freezes up again just fine (though thawed and refrozen ice cream is never really the same again.  

Gravity Critique

Lost of people loved the movie Gravity.  I am not one of them. I get it ... big-name stars, and pretty special effects.  But if you're going to name your movie after a physical phenomenon, it somewhat behooves you to get that phenomenon right.

And they don't.  'Gravity' totally distorts gravity.  One of the key tension element in the movie is a marauding pile of space junk, left over from an explosion, that whips by every ninety minutes, threatening to kill the stranded astronauts.  From a cinematic perspective, I get it that you need tension - but this would not happen.  Here's why:

The orbit (direction, and altitude) of a spaceship , or anything else in orbit is the geometric sum of two components (vectors);

• Gravity, is causing it fall toward the Earth
• Inertia, that drives it forward in it’s current direction

In a stable orbit, the vectors sum to the shape of the orbit.  This can be visualized as a rectangle, with one side as the thrust vector, the other side as the gravity vector, and the diagonal as the actual direction (vector sum). 

The spaceship cannot control the magnitude of  the gravity vector, but blasting the engines increases the inertia vector. 
Accelerating, changes the relative effects of inertia and gravity, increasing the length of the rectangle, relative to the width … in effect raising the craft to a higher orbit (bigger circle, less curvature)

If two spacecraft had been orbiting side by side, then one accelerates, and moves to a higher orbit. 
Because the rate of falling (gravity) is less, relative to the inertia,the time required for each orbit is longer, and (counterintuitively),from the perspective of the other craft, the faster craft will appear to fall behind.  Taken to the extreme, if one of the craft accelerated enough, it would rise to the same orbit as the moon, and only circle the Earth once every 28 days. 

Unless something happens to slow the green spacecraft, it will remain at this higher orbit, well out of the way of the slower craft.  

The movie ‘Gravity’ gets this all wrong. 

In order to add tension, the story has a mass of space junk
whipping around every ninety minutes, and slamming into
the stranded astronauts (Presumably, the director chose a
ninety-minute cycle time because we baby boomers vaguely
remember that each of John Glenn’s orbits took about
ninety minutes). 

This would not happen.  If a nearby spaceship explodes,
and one is fortunate to avoid the initial blast, the danger is past. 
Whatever detritus blows off in the same direction as the orbit
will accelerate, and move to a higher orbit, where it will not
be a problem.  Anything that blows off in the opposite direction
from its orbit will descend to a lower orbit, consistent with its
net post-explosion velocity. And, anything that blows out sideways,
just goes off its own way.  

The only way these every-ninety-minute collisions could occur
would be if a giant scaffolding were erected to hold the stranded
astronauts in place, in the path of the space junk. 
And this scaffolding would have to be moved about 1,000 miles
east every hour to compensate for the rotation of the Earth
around its axis.  

Pluto

Recognizing with a bit of sadness the tenth anniversary of the demotion of Pluto from planet to ‘dwarf’ planet. 

Thanks, Neil deGrasse Tyson! 

Hardly seems fair; Pluto was just discovered in 1930, and the little fella didn’t even get one full lap as a planet before the naysayers got him. Of course, that would have taken until 2178, but what's the damn hurry?

Spring has Sprung

This March, Americans are of two minds regarding the change of seasons. There are those on the east coast and Midwest for whom this offers hope of the end of one of the most severe winters in history.  Here in the Pacific Northwest, the longer days and higher temperatures confirm what we have long feared – that this will be a year with no winter at all.  

But what exactly is it that’s causing these competing feelings of disappointment and optimism?  What does science have to say on the subject?  

Thanks for asking!

The answer lies in the intersection of biology and physics.  Humans are generally diurnal; we tend to be active during daylight hours and asleep at night.  Though we don’t hibernate (at least most of us don’t), our processes tend to slow down when the days become shorter, then pick up as day length increases.  As the days (okay … the daylight portion of the 24-hour day) lengthen, we begin the transition from our wintertime to summertime selves.  

It’s not so much the length of the days that causes this, but the day-over-day INCREASE in daylight hours that affects our moods.  And there is not time when this is more pronounced than the first day of spring – the vernal equinox.  This is where physics comes in.  

You probably recall from high school science that the seasons are caused by the tilt of the Earth’s axis, relative to our orbit around the Sun.  If you look at a chart of daytime versus nighttime throughout the year, the result becomes clear.  The graph is a classic sinusoidal function – like a radio or sound wave.  And one thing that’s fundamental to these functions is that, as the function reaches its neutral position (the spring and autumn equinoxes), the rate of change (the ‘first derivative’ in Calculus terms) hits is greatest.  

This effect applies everywhere except at the equator, where day time and nighttime hardly vary throughout the year.  At the other end, the Polar Regions go from nearly constant darkness in mid-winter to round-the-clock daylight in late June.   Scientifically speaking, the further from the equator one is, the greater the amplitude of the sinusoidal function.  
If that hasn’t adequately stolen the magic of the season from you, stay tuned. 

Next time we’ll cover how it’s the wind ABOVE one’s wings that provides the lift; so if a friend says you’re ‘the wind beneath my wings’, you’re actually being told you’re a drag.  Future lessons will cover how it’s generally darkest a really really long time before the dawn (around midnight).  

If this all seems a little much, there is an alternate explanation.  The Earth is coming to life in joyous celebration of Persephone’s annual release from captivity in the underworld.  

You’re welcome!  

Chain Reactions and Immunization

I remember learning about nuclear chain reactions when I was a kid. 

Basically, certain isotopes of Uranium (U-235, for example) are unstable, and if they’re hit by a neutron from another unstable U235 nucleus, it very rapidly decays into two smaller atoms, emitting more neutrons into its environment, and generating heat in the process. 

If there are no other vulnerable atoms in the vicinity, the neutrons just fly harmlessly off into space.  But if one of these neutrons impacts another U235 nucleus, it will also split, emitting even more reactive neutrons, and more energy.  If the concentration of U235 atoms is high enough, this reaction will sustain itself, generating heat to boil water, run a turbine and produce electrical power.  If it’s even higher … you get a nuclear bomb. 

To illustrate the concept, think of a place where people gather – Disneyland for example.  Say one kid with the measles shows up and sneezes, or wipes his nose, then touches a handrail.  If all the people around him are vaccinated, this is a non-event – except for that one kid.  But if the virus ejected in his sneeze comes into contact with an unvaccinated kid, he gets sick, too.  Then when he sneezes, the pattern is repeated.  If enough kids are not vaccinated, the result is an epidemic. 

Since the late 1950s, a range of vaccinations has been the norm for most people.  As a result, few people have been exposed to measles, or a number of other contagious, preventable diseases.  This lack of exposure has led to complacency, and a sense that vaccinations are really optional.  There is even a mythology—unfounded by empirical science—that vaccines are dangerous.   Vaccination for any individual is optional, and safe – as long as everybody else is vaccinated; because the individual is never exposed.  But there is now a concentration of non-vaccinated people within our population to create and sustain epidemics. 

At the concentration we’ve reached, widespread complacency puts us all at risk of a chain reaction of infection.  Many of these diseases are highly contagious, and we are all vulnerable – even if we exercise regularly, eat a diet high in omega-3 fatty acids, and—sad to say—even if we vote Democratic. 

Seriously … get vaccinated, and get your kids vaccinated. 

THE KINETIC THEORY OF SWEAT

As we enjoy this bit of toasty weather, do you ever stop to ask yourself why we sweat, and how it is that sweating cools us off? 

The reality is, sweating actually doesn’t cool us off – at least not directly. If the concept isn’t clear, spend a few days during a summer in Louisiana.  You’ll sweat like a pig and not cool off at all. You see, sweating itself doesn’t cool you off; we cool off only when sweat evaporates from our skin.   

To get this, there’s just a bit of basic physics involved.  I’ll try to keep it to a minimum, so please bear with me.

What we sense as temperature is actually the average kinetic (motion) energy of the molecules in our environment. As things heat up, the molecules speed up – and their average kinetic energy increases.  The key here is ‘average’.  There are billions of molecules in a single droplet of sweat, and their velocity (and thus, kinetic energy) ranges widely. 

One way to visualize this is to imagine a pool table with bazillions of teeny balls bouncing around. Whenever one of these balls hits a cushion or another ball, it transfers a bit of its energy to it. The larger the balls, or the faster they’re going, the more energy it transfers. That’s not a perfect analogy for kinetic energy, but it’s close enough for our purposes. On our pool table, some of the balls are moving slowly, some at a medium rate, and some really fast. Remember, that temperature is the AVERAGE of these. Every so often, one of the faster pool balls flies off the table (evaporates). Because it’s the fastest balls that are most likely to fly off the table, the AVERAGE speed of the balls that remain on the table is slower – that is to say that the average kinetic energy is lower, and the temperature decreases. 

Essentially, this is what happens when your sweat evaporates.  The fastest, hottest molecules of sweat evaporate, leaving the slower cooler ones next to your skin ... 'Ahhh!' 

So why doesn’t sweat work as well in high humidity? Well, as air picks up more and more water vapor, the process begins to work in reverse, with water vapor condensing onto our skin, heating it up.  As the air finally becomes saturated and can hold no more; what we call 100% humidity. At that point, a water molecule is as precisely as likely to condense into the droplet of sweat as one is to evaporate from it.  Since condensation has the opposite effect as evaporation, there is a balance between the cooling effect of evaporation, and the heating effect of condensation.  Sweat all you want – it’s going to just sit there on your body.  

As a bonus, the body reacts to the failure of this cooling mechanism by ... you guessed it, sweating even more.  You end up drenched in sweat that isn't doing you a bit of good.  

The technical term for this balance of heating and cooling is called equilibrium, but on a hot, humid day, you might know it better as Hell.  

Summer Sunshine and Heat

Why isn’t June 21 the hottest day of the year?

Sometimes it is—weather being a classic example of chaos theory, where anything can happen on any given day, based on the laws of probability—but generally the hottest time of year comes much later.

To understand why it generally is not, it’s important to first understand why you might think the Summer Solstice SHOULD be the hottest day.

The Earth is heated by the Sun – trivial, but critical.

The Sun imparts the most heat on that part of the Earth which is closest to it—not because it is literally closest to the Sun, but because the Earth is round, and the part which is closest to the Sun also happens to be oriented such that the sun is directly overheat (orthogonal to the surface, in math-speak).  In this orientation, the Sun’s rays are most concentrated there.

As the angle of the Earth’s surface in relation to the sun deflects away—which occurs during winter, or as the day moves from Noon toward evening,—the intensity of the solar radiation decreases, and there is less warming. 

Because the Earth is tilted, with respect to its orbit of the Sun, the Sun appears to be directly over different parts of the Earth at different times of the year. On the Summer Solstice (on or around June 21) the Sun appears directly over the Tropic of Cancer, at ~23 degrees North Latitude - around the middle of Mexico. On that day, the Northern Hemisphere experiences more direct exposure to solar radiation than it does on any other day of the year—and for more hours, since day length is also longest on the solstice. 

On the face of it, this suggests that this should be the hottest day of the year. So why isn’t that the case? For the same reason that an oven doesn’t reach its highest temperature as soon as you turn the flame up to the highest setting. At any given moment, the temperature is the result of not just the heat being imparted at that instant, but also any stored heat from before. 

Each day, as the Earth rotates, it goes through a heating and cooling cycle. During the day, as the Earth basks in the Sun’s radiation, it is warmed. Then, as night falls, that part of the Earth that moves to darkness radiates that heat back into space – causing the surface of the Earth to cool. 

On the first day of spring (the Vernal Equinox), the entire planet experiences approximately equal periods of night and day – so the heating and cooling cycles are more or less the same. After that, as the Earth’s orbit around the Sun orients the Northern Hemisphere more directly toward the Sun, the days (warming periods) get longer, and more intense, while the nighttime cooling periods become shorter. 

The effect of this heating and cooling is both immediate and cumulative. Directly beneath the Sun’s rays, the immediate effect of the radiation is more intense. But the cumulative effect is the net result of the heat that is picked up during the daytime, minus that which is radiated out to space at night. Though the Northern Hemisphere actually nets the most heat gain on June 21, the warmth of the surface of the planet is the net result of the days and weeks leading up to it. The decrease in day length after the solstice is gradual, and the heat which is added through the end of June, through July and August is also substantial, and the cooling periods of nighttime remain relatively brief – adding to the heat accumulation. 

Depending upon where one lives, it may be early to mid August before the net effect of these cycles reaches its peak – and temperatures reach their highest. Somewhere between late August and mid-September, the shift in heating/cooling cycles begins to cause temperatures to noticeably decrease. At the Autumnal Equinox—around September 21—the periods of day and night are again approximately equal. After that point, the periods of warming (daylight hours) diminish further, and the periods of cooling (nighttime) increase. 

This effect is not uniform everywhere, and many other factors (the Jet Stream, ocean currents, local geography, etc.) influence the climate of any given location.  This is really just a 'broad-brush' explanation. 

For example, this lag between the time of the greatest solar radiation (June 21), and the warmest time of the year is most pronounced near large bodies of water, such as oceans. Water has a tremendous ability to store heat, and oceans act as ‘thermal banks’, accumulating and storing solar energy. It takes longer for the oceans to warm than it does for land surfaces, but once heated, the oceans—and those areas whose climates are strongly influenced by the sea—remain warm longer than areas isolated from their influence. 

This is why coastal areas are subject to significantly smaller temperature swings than inland areas.  It is also why the coast often experiences the warmest weather in the late summer to early autumn – a time when inland areas have already begun to cool. 

The hurricanes that plague the US Atlantic and Gulf coasts illustrate this point. These storms are caused in part by the evaporation of warm tropical waters in the Atlantic, and are sustained and intensified as the storm’s wet air mass passes over the warm waters further north in the Atlantic, the Caribbean, and the Gulf of Mexico. Though the Atlantic hurricane officially begins June 1, the really powerful hurricanes rarely occur prior to mid or even late summer - when these waters have been heated enough to contribute the energy necessary to sustain and intensify these storms; and it doesn’t officially end until November 30, when the heat in these waters has dissipated to the point where they can no longer do so.

The Four-eyed Dilemma

Dear Doctor Wizard,
I’m confused. I love to play outside in the snow with my friends, but when I come back inside, my glasses fog up. My friends who don’t wear glasses always pick on me when this happens until I can see again. My teacher says that fog is just water, so what gives? It’s cold and wet outside, but warm and dry inside – so why do my glasses that were perfectly clear outside where it’s wet fog up when I come in where it’s dry?
Signed,
Bespectacled and Bruised

Dear Four-Eyes,
First of all … those aren’t your friends. A friend would help you remove your glasses, then start thumping you.
Now that we’ve got that cleared up, let’s look at the science question.
It’s actually wetter inside than outside – at least there’s more water in the air inside than out.
Now before you go all ‘liar liar, pants on fire’ on me, read on. Even though it feels drier, there’s more water in the air in the nice warm cabin than there is outside in the cold. If there weren’t, your glasses would not fog up. 
Why is that? It’s simple really (at least if you're Doctor Wizard). For the same reason you can dissolve more sugar in warm water than you can in cold water (you knew you could do that, right?), you can ‘dissolve’—or evaporate, to use the correct term—more water in warm air than you can in cold air (in both cases, this is related to the kinetic theory of matter, but if you share that little tidbit with your friends, they’ll beat you up even more. Trust me – I know.  Dr. Wizard was young once too).  So the dry air inside your house holds more water than the wet air outside. You just don't notice it, because water dissolved in air doesn’t feel ‘wet’.
The reason that water forms on your glasses when you come inside is because your glasses are still cold – nearly as cold as they were outside. And because your glasses are still cold, the air that touches the surface of them gets just about as cold. In physics, this little microscopic layer of air on your glasses is called the ‘boundary layer’, and if the glasses are colder than the 'dew point' for the inside air, that means there’s more water in the air than it can hold, and it condenses into dew (which is why they call it the dew point) on your glasses. But as soon as your glasses warm up to room temperature (or at least above the dew point of the inside air), the dew evaporates back into the room air – and your friends better watch out. 
Extra Credit: Read this to your dad – then ask him if he can tell you why frost forms on that mug they pull out of the freezer for him down at the tavern – even though there’s no frost on it when it’s still in the freezer. If he hasn’t had more than a couple, he should be able to figure it out. 

Poindexter Points:  The defrosters in most new cars pull in outside air, and pass it through the air conditioner first, before running it through the heater, and finally over the windshield.  This makes the defroster much quicker and more efficient.  Ten PP's if you can explain why.

Beneath the Wings

Hey Doc,

I love that Bette Midler song, ‘The Wind Beneath my Wings’, but I really don’t get it. What does she mean when she uses that expression?
Signed,
Puzzled Pilot

Hey Puz,
Good question. As best the Wiz can tell, The Divine Miss M. is making two statements:
1. ‘You are a good friend, and I can always count on you.’
2. ‘I don’t know squat about aerodynamics.’

While we appreciate what she is trying to say—and love that gal to pieces—we feel compelled to help out with the science. As Bernoulli’s principle clearly states, it is the wind above the wings that provides the lift. To the extent that wind beneath the wings has any effect at all, it is to bring down the flying object (bird, plane or Superman) and—aerodynamically at least—to be a drag.  We feel certain that this was not the message she meant to convey. 
We encourage La Midler to continue her exemplary work entertaining us – occasionally moving us to tears of joy or laughter, but to leave science to the professionals… like Doctor Wizard.

Signed,
Your Well-grounded Doctor

Darkest Before Dawn?

Dear Doctor Wizard,
Hi!  Thanks for taking my question.  I think you're really cool, and read your blog all the time.
Okay, here goes:
Is it really always darkest before the dawn? 
Signed, Nightowl 

Dear Nighty,
First of all, don't be so damned obsequious.  Nobody likes a suck-up, and readers will assume you're a plant, tossing up slow pitches for me to hit out of the park.  Besides, you couldn't have read my blog before, since yours is the first question.
That being said ... good question!  My advice, never say 'never', and rarely say 'always'.  As comforting as the thought may be that it's always darkest before the dawn, reality doesn't need to conform to our comfort.  In the most literal sense, it is darkest before the dawn.  But one should not infer from this that it's darkest immediately before the dawn.  In the absence of artificial light (nearby cities, campfires, etc), and normalizing for the stage of the moon, it's actually darkest about midway from dusk and dawn, when the sun is directly opposite one's position on the surface of the Earth - midnight ... way before dawn.  
If you still want to find words of assurance for a friend in need, it is (normalizing once again for transient phenomena such as weather fronts, etc.) generally coldest before the dawn ... or soon thereafter.
I hope that helps.
Cheerfully,
Dr. Wizard