Temperature Conversion

°C = (°F - 32) x 5/9

°F = (9/5 x °C) + 32

Rule of Thumb

Since 0°C = 32°F, simply add or subtract 5°C for each 9°F or vice versa.

Example: 30°C  is 0°C + 30°C, or 0°C + (6 x 5°C), or 32°F + (6 x 9°F), which is 32°F + 54°F, or 86°F.

Standard Temperature For A Given Altitude:

The Standard Lapse Rate for temperature is 2°C (3.6°F) per 1000 feet increase in altitude.

ISA temperature in °C at any altitude = 15°C - (2 x number of thousands of feet)

Example: ISA at 9000 ft = 15°C - (2 x 9) = -3°C

ISA temperature in °F at any altitude = 59°F - (3.6 x number of thousands of feet).

Example: ISA in degrees F at 9000 ft = 59 - (3.6 x 9) = 26.6°F.

Determining Freezing Level

If the air temperature is 6°C at an airport elevation of 1200 ft and a standard temperature lapse rate exists, what will be the approximate freezing level?

4200 MSL

6°C at the surface divided by the average temperature lapse rate of 2°C results in a 3000 ft freezing level above the ground, converted to MSL by adding the 1200 ft airport elevation.

Temperature Change At Night

After sunset, the temperature drops approximately 2°F each hour, reaching the lowest temperature at sunrise.

Daylight Remaining

To obtain a rough estimate of the amount of daylight remaining:

Extend your arm and hold up the back of your hand toward the sky between the sun and the horizon. Each stacked hand-width (4 fingers) between the sun and the horizon represents approximately 1 hour of daylight.

Determining Cumulous Cloud Bases

Water absorbs heat energy if it moves to a higher energy state (from ice to water to vapor) and gives off heat energy if it moves to a lower energy state (vapor to water to ice). This heat energy is known as latent heat. At its dew point, the air will be fully saturated—its relative humidity will be 100%. If air becomes cooler than its dew point, then the excess water vapor will condense as visible water droplets (clouds) or, in sub-freezing temperatures, deposit as ice crystals, snow, or frost. The actual value of the dew point temperature for a particular parcel of air varies, depending on the amount of water vapor it contains. If the air is moist (for instance over a tropical ocean), the dew point temperature may be quite high, say 25°C; if the air is dry, the dew point temperature may be quite low.

The air temperature/dew point spread will decrease at approximately 2.5°C (4.4°F) in rising unstable air. This is considered the moist adiabatic lapse rate. The height of the bases of cumulus clouds above ground level (AGL) can be determined by dividing the temperature / dew point spread by 2.5 (if the temperature is in ºC) or 4.4 (if the temperature is in ºF) and then multiplying by 1000.

Example:

Airport elev: 316 ft. MSL

Temp./Dew point spread: 26°C / 21°C

26 - 21 = 5

5/2.5 = 2

2 x 1000 = 2000 ft AGL

The base of the cumulous clouds is 2316 ft. MSL

Rule of Thumb

When the temperature / dew point spread is given in °C, simply multiply the spread by 400 to get the cloud bases in AGL.

Example:

Airport elev : 316 ft. MSL

Temp./Dew point spread:  26°C / 21°C

26 - 21 = 5

5 x 400 = 2000ft AGL

The base of the cumulous clouds is 2316 ft. MSL

Pressure Changes

Standard Atmosphere: Sea Level Pressure (SLP) is 29.92 Hg at 15 degrees C

Standard Lapse Rate For Pressure: 1" Hg decrease per 1000 ft increase in altitude

If Sea Level Pressure is 29.92 in., then atmospheric pressure at 1000 ft. is 28.92 in.

Pressure Altitude 

Pressure altitude is used in aircraft performance charts and is required to calculate density altitude. To determine pressure altitude set the altimeter to 29.92 in. and read the pressure altitude from the dial. The following formula can also be used:

Pressure Altitude = Field Elevation + [(29.92 - Current Altimeter Setting) x 1000 ft.]

Example:

Field Elevation = 96 ft.

Altimeter Setting = 30.14

Pressure Altitude = 96 + [(29.92 - 30.14) x 1000]

= 96 + [-.22 x 1000]

= 96 + [-220]

= -124 ft.

For every .01 in. Hg altimeter setting, the pressure altitude changes 10 ft.

Density Altitude

Density Altitude is pressure altitude corrected for nonstandard temperature. High density altitude reduces an airplane's performance—climb performance is lower, takeoff distance is longer, and propellers have less efficiency because there is less air for the propeller to grip. However, the same indicated airspeed is used for takeoffs and landings, regardless of altitude or air density. Density Altitude increases or decreases 120 ft. for each 1 degree Celsius (or 70 ft for each 1 degree F) that the outside air temperature (OAT) varies from standard temperature (ISA).

Density Altitude = Pressure Altitude + [120 x (Outside Air Temp - ISA)]

Example:

Pressure Altitude = 6000 ft.

OAT = 13 degrees C

ISA = 3 degrees C 

Density Altitude = 6000 + 120 (13 - 3)

= 6000 + 1200 

= 7200

Crosswind

Rule of Thumb

To Determine Crosswind Component:

1. Determine the wind angle to the runway (the difference between the magnetic wind direction and the magnetic runway heading).

2. Add 20 to this number.

3. Use the total number from Step 2 as a percentage and multiply it by the windspeed to determine the crosswind component.