REM PROGRAM D8_CI
NC = 100
DIM X(NC + 1), Z(NC + 1), R(NC + 2)
DIM XOLD(NC + 1), ZOLD(NC + 1), ROLD(NC + 2)
SCREEN 2: SCREEN 12
LINE (0, 0)-(640, 10), 10, BF
LINE (0, 0)-(10, 480), 10, BF
LINE (630, 0)-(640, 480), 10, BF
LINE (0, 470)-(640, 480), 10, BF
COLOR 13: LOCATE 3, 30: PRINT "MAKING A CIRRUS CLOUD": SLEEP 2
LOCATE 6, 17: PRINT "CIRRUS CLOUDS ARE TRAILS OF FALLING ICE CRYSTALS"
LOCATE 7, 31: PRINT "TWISTED BY THE WINDS"

SLEEP 2: COLOR 10: LOCATE 9, 5: : PRINT "MOST CIRRUS CLOUDS CONSIST OF"
SLEEP 2: LOCATE 10, 8: PRINT "1. A LEADING HEAD IN WHICH ICE CRYSTALS ARE MANUFACTURED"
SLEEP 2: LOCATE 11, 8: PRINT "2. A TRAILING, COMMA-SHAPED TAIL OF FALLING ICE CRYSTALS"
SLEEP 2: COLOR 12: LOCATE 13, 5: PRINT "THE HEAD USUALLY LEADS THE TAIL BECAUSE"
LOCATE 14, 11: PRINT "WIND SPEED DECREASES DOWNWARD FROM CLOUD LEVEL"
SLEEP 2: COLOR 11: LOCATE 16, 5: PRINT "THE TAIL OFTEN RESEMBLES A COMMA THAT LEVELS OFF AT BOTTOM BECAUSE"
LOCATE 17, 11: PRINT "CRYSTALS EVAPORATE AS THEY FALL"
LOCATE 18, 11: PRINT "AND FALL MORE SLOWLY AS THEY EVAPORATE"
SLEEP 2: COLOR 14: LOCATE 20, 5: PRINT "OTHER FEATURES"
SLEEP 2: LOCATE 21, 8: PRINT "1. TAILS SWIRL WHEN WIND DIRECTION CHANGES WITH HEIGHT"
SLEEP 2: LOCATE 22, 8: PRINT "2. TAILS BEND WHEN WIND SPEED CHANGES ABRUPTLY WITH HEIGHT"
SLEEP 2: LOCATE 23, 8: PRINT "3. LINES AND BANDS OF CIRRUS FORM FROM WAVES OR JET CONTRAILS"
SLEEP 2: COLOR 12:
DO WHILE INKEY$ = ""
LOCATE 27, 27: PRINT "HIT ANY KEY TO START PROGRAM": SLEEP 1
TT = TIMER + .25
DO UNTIL TIMER > TT
LOCATE 27, 27: PRINT "                            "
LOOP
LOOP
CLS : COLOR 15

WINDOW (0, 0)-(640, 480)
COLOR 13: LOCATE 2, 28: PRINT "GROWING AND MOVING CIRRUS"
COLOR 14: LOCATE 6, 65: PRINT "FAST WIND SPEED"
LOCATE 15, 65: PRINT "SLOW WIND SPEED"
COLOR 11: LOCATE 7, 65: PRINT "FAST FALL SPEED"
LOCATE 16, 65: PRINT "SLOW FALL SPEED"
LOCATE 18, 31: PRINT "CRYSTALS EVAPORATE": COLOR 15
LINE (60, 100)-(580, 100), 6
LINE (0, 60)-(640, 60), 6
LINE (60, 100)-(0, 60), 6
LINE (580, 100)-(640, 60), 6
PAINT (320, 80), 6
RANDOMIZE TIMER
FOR II = 1 TO 40
ZT = (60 - II) * RND + 103
Z2 = ZT - 103
XT = 100 + RND * 440
HUEMT = 7 + INT(II / 21)
LINE (XT - Z2, 101)-(XT, ZT), HUEMT
LINE (XT + Z2, 101)-(XT, ZT), HUEMT
LINE (XT - Z2, 101)-(XT + Z2, 101), HUEMT
PAINT (XT, ZT - 2), HUEMT
NEXT II
PI = 3.141593
UTERM = -.5
CU = .006
NDT = 4500
DT = .04
Z1 = 400
RC = 6: DRDT = .002
RCLD = 3 * RC
IDT = 200
CX = IDT * CU
FOR I = 1 TO NC
Z(I) = Z1
X(I) = 30 + CX * Z(I) * DT * (I - 1)
R(I) = RC
NEXT I
CIRCLE (30, 410), RCLD, , 0, PI
LINE (30 - RCLD, 410)-(30 + RCLD, 410)
PAINT (30, 420)
FOR K = 1 TO NDT
CIRCLE (30 + K * DT * CU * 400, 410), RCLD, , 0, PI / 2
CIRCLE (30 + K * DT * CU * 400, 410), RCLD, 0, PI / 2, PI
FOR JJ = 0 TO 4
LINE (12 - JJ + K * DT * CU * (395 - 40 * JJ), 398 - 40 * JJ)-(15 - JJ + K * DT * CU * (395 - 40 * JJ), 395 - 40 * JJ), 14
LINE (12 - JJ + K * DT * CU * (395 - 40 * JJ), 392 - 40 * JJ)-(15 - JJ + K * DT * CU * (395 - 40 * JJ), 395 - 40 * JJ), 14
LINE (K * DT * CU * (395 - 40 * JJ), 400 - 40 * JJ)-(K * DT * CU * (395 - 40 * JJ), 390 - 40 * JJ), 0
NEXT JJ
IC = 1 + INT(K / IDT)
FOR I = 1 TO IC
R(I) = RC - DRDT * (K - IDT * (I - 1))
W = UTERM * R(I)
U = Z(I) * CU
Z(I) = Z(I) + W * DT
X(I) = X(I) + U * DT
IF ROLD(I) > 0 THEN
        PAINT (XOLD(I), ZOLD(I)), 0
        CIRCLE (XOLD(I), ZOLD(I)), ROLD(I), 0
END IF
IF R(I) > 0 THEN
        CIRCLE (X(I), Z(I)), R(I), 11
        PAINT (X(I), Z(I)), 11
END IF
REM WHILE INKEY$ = "": WEND
NEXT I
FOR I = 1 TO IC
XOLD(I) = X(I)
ZOLD(I) = Z(I)
ROLD(I) = R(I)
NEXT I

NEXT K