Wind Chill

This program calculates the wind chill temperature in degrees Fahrenheit using the traditional Siple-Passel formula. It prompts the user for an ambient temperature in Fahrenheit and a wind speed in miles per hour, then applies the formula WC = 0.0817 × (3.71 × √S + 5.81 − 0.25 × S) × (T − 91.4) + 91.4. A guard clause at line 80 handles the meteorological convention that wind chill is undefined below 4 mph, simply returning the ambient temperature in that case. After displaying the result with a long PAUSE of approximately 40 seconds, the screen clears and the program restarts automatically.

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Program Analysis

Program Structure

The program follows a simple linear input–calculate–output loop across three phases:

1. Input (lines 10–60): Prompts for temperature T and wind speed S, echoing each value back after entry.
2. Calculation (lines 70–80): Applies the wind chill formula and handles the low-wind edge case.
3. Output and restart (lines 90–120): Displays the result, pauses, clears the screen, and loops via RUN.

Lines 9998–9999 are utility lines outside normal execution: SAVE "WINDCHIL%L" saves the program (the %L represents an inverse-video ‘L’ in the filename), and the trailing RUN restarts immediately after loading.

Wind Chill Formula

Line 70 implements the Siple-Passel empirical wind chill formula:

WC = 0.0817 × (3.71 × S^0.5 + 5.81 − 0.25 × S) × (T − 91.4) + 91.4

This is the classic pre-2001 formula expressing wind chill in degrees Fahrenheit, where S is wind speed in mph and T is air temperature in Fahrenheit. The constant 91.4°F represents normal skin temperature. The **.5 exponentiation is used to compute the square root of wind speed rather than a dedicated SQR function, which is mathematically equivalent.

Notable Techniques

• Tab-comma formatting: The double commas (,,) in PRINT statements at lines 10, 40, and 90 advance the print position to the third print zone, centering the prompts on the display.
• Input echo: Lines 30 and 60 print back T and S respectively. On this platform, INPUT accepts values without leaving them visibly confirmed on screen in all modes, so this serves as user feedback.
• Long PAUSE for display: PAUSE 40000 at line 100 holds the result on screen for roughly 40 seconds (at 50 Hz) before auto-clearing, avoiding the need for a keypress to continue.
• RUN as loop: Using RUN at line 120 rather than GO TO 10 restarts from scratch, clearing all variables — a clean but slightly heavier-weight approach to looping.

Edge Case Handling

Line 80 checks whether wind speed is below 4 mph:

80 IF S<4 THEN LET WC=T

This correctly replaces the formula result with the ambient temperature, as the Siple-Passel model is not valid at very low wind speeds (the formula can produce values higher than the ambient temperature at near-zero wind, which is physically nonsensical).

Variable Summary

Variable	Purpose
T	Ambient temperature in degrees Fahrenheit
S	Wind speed in miles per hour
WC	Calculated wind chill temperature in degrees Fahrenheit