Temperature Conversion Rule of Thumb

This is not the best way to convert between Fahrenheit and Celsius! But it is the quickest, especially if you are doing the math in your head.

F = 2C + 30

This means that the temperature in Fahrenheit is equal to 2 times the degree in Celsius + 30. So if it is 10 degrees Celsius then 10 doubled is 20 and then 20 plus 30 is 50 Fahrenheit.

You can also convert the other way with the equation below.

C = (F – 30)/2

Warning! This is not exact at all.

The real equations are F = (9/5)C + 32 and C = (5/9)(F-32)

So if the AWOS gives you the temperature in Celsius and you just want to decide if you will need a sweater, then go ahead and use this rule of thumb.

If you need the information for calculating performance, then this rule of thumb is inappropriate to use and there is a much better way.


Instrument Flight Rules

Instrument Flight Rules, commonly called IFR, are a set of rules that apply to planes flying by instrument reference. This is in opposition to VFR (Visual Flight Rules), flown by visual references. Basically, if you can see where you are going, then VFR is an option, but IFR is always an option.

This set of rules requires a pilot to have an instrument flight plan and follow a set of procedures that govern communication and navigation.

IFR flying requires constant communication with ATC and a mixture of visual traffic separation when you are in VMC and reliance on ATC for separation when you are in IMC.

How do I know if I’m flying IFR?

If you have to ask, you’re not flying IFR!

But seriously, to fly IFR you will need to file an IFR flight plan, get a clearance from air traffic control, and comply with that clearance. You can fly IFR in VMC, but you can only log IFR, or “flight by reference to instruments” time when you are flying in actual IMC.

Instrument Meteorological Conditions

Instrument meteorological conditions (IMC) exist when an aircraft is in weather conditions that are not within the VFR visibility and cloud clearance requirements.

This means that you are in IMC if:
• You are inside of a cloud
• The visibility is too low
• You are too close to a cloud

When you are in IMC you need to be flying an instrument flight plan. Flying VFR into IMC is very dangerous and there are a multitude of accidents that occur this way. Don’t be a statistic! Stay away from the clouds.

In fact, I recommend having personal minimums for visibility and cloud clearance that are even more restrictive than the rules.

Visual Flight Rules

Visual Flight Rules, commonly called VFR, are a set of rules that apply to planes flying by visual reference. This is in opposition to IFR (Instrument Flight Rules), flown without visual references.

This set of rules allows a pilot to fly in most airspace without a flight plan, clearance, or ATC communication.

VFR flying requires constant vigilance for other traffic and reliance on oneself for navigation.

When you are flying VFR you must be in VMC.

Of course, while flying VFR you may choose to use air traffic control services to:

  • Get help in an emergency
  • Get directions
  • Get traffic advisories
  • Get clearance through airspace or to land

How do I know if I’m flying VFR?

Did you file and open an instrument flight plan? If not, then you are flying VFR. You must comply with all VFR regulations including visibility and cloud clearance minimums.

Visual Meteorological Conditions

Visual Meteorological Conditions, or VMC, are a set of weather conditions that allow for flying VFR.

Generally, VMC means good weather, but it also technically means that you are far enough between the clouds to allow for safe VFR flying.

For example, when flying IFR you might be popping in and out of clouds every few minutes.

If ATC notifies you about traffic then you must try to find the other plane. However, this only applies if you are in visual conditions at that moment. If you happen to be inside of a cloud you just respond with “IMC”.

In addition, it is also possible that you are near clouds and you can’t see the other plane because he is in a cloud. I will always take a quick look but if there is a chance that I can’t see the other plane because there is a large cloud right where he should be, I will still say “IMC”.

I am letting the controller know that the conditions where I am flying are not sufficient for VFR, even though I may be outside of a cloud at that time.

How do you know if you are in VMC?

I’ve devised a set of easy rules to help you remember the cloud clearance and visibility requirements for VMC.

Haze Layer

The air on our planet is full of particles of moisture. These are often visible as haze, usually in calm air or on humid days. Did you know that on many days, when the air is stable, you can get above the haze and the visibility goes way up?

Click on the picture below to view it full size. You can see the haze extending to the horizon and up to a clear “haze line”. Above this line the visibility is much higher. It will be easier to see other planes at this altitude.

Haze Layer – click to view full size

Calculating the top of the haze layer

The haze layer will generally be present in stable air where the temperature is above the dewpoint. At an altitude above the haze layer, the temperature will be below the dewpoint, so any moisture in the air will condense into droplets of mist of rain. Then gravity will pull these droplets down until they are back in the haze layer where they will turn back into water vapor (haze).

So the simplest way to find the top of the haze layer is to determine where the temperature and dewpoint will match.

The air is colder at altitude, but do you know how much colder? There is something called the adiabatic lapse rate, which is the rate at which the temperature drops as you climb.

It varies depending on the moisture of the air but as a rule of thumb just assume it is 2 degrees Celsius per 1000 feet of altitude. In dry air, it may be as high as 3 degrees Celsius.

So when you listen to the AWOS at sea level and it reports “temperature 12, dewpoint 6” then you can figure out the approximate top of the haze layer. That is a 6-degree difference, so we will need to climb 3000 feet.

This quick rule of thumb will let you make the haze layer calculation very quickly, just find the difference and divide it by 2 to get the altitude of the haze line.

for example:

Temperature: 10C, Dewpoint: 5C = Top of Haze Layer: 2500 feet

Temperature: 15C, Dewpoint 13C = Top of Haze Layer: 1000 feet

The next time you are flying start looking for the haze layer. It isn’t there every time, but it is there often enough (depending on where you fly).