Trasformare il tipo di "el.nino" in numero (es. cold=0, warm=1, neutral=2)

Shown below is the data set with the numbers of Atlantic Basin 
tropical storms and hurricanes for each year from 1950 to 1997. 
The variable storm index, is an index of overall intensity of the 
hurricane season. (It is the average of number of tropical storms, 
number of hurricanes, the number of days of tropical storms, the 
number of days of hurricanes, the total number of intense hurricanes, 
and the number of days they last—when each of these is expressed 
as a percentage of the average value for that variable. 
A storm index score of 100, therefore, represents, essentially, 
an average hurricane year.) Also listed are whether the year was 
a cold, warm, or neutral El Nino year; a constructed numerical variable 
temperature that takes on the values -1, 0, and 1 according to 
whether the El Nino temperature is cold, neutral, or warm; 
and a variable indicating whether West Africa was wet or dry 
that year. It is thought that the warm phase of El Nino 
suppresses hurricanes while a cold phase encourages them. It is also 
thought that wet years in West Africa often bring more hurricanes. Analyze 
the data to describe the effect of El Nino on 
(a) the number of tropical storms, (b) the number of 
hurricanes and (c) the NTC, after accounting for the effects 
of West African wetness and for any time trends, if appropriate

year	el.nino	temperature	west.africa	storms	hurricanes	storm.index
1950	cold	-1	1	13	11	243
1951	warm	1	0	10	8	121
1952	neutral	0	1	7	6	97
1953	warm	1	1	14	6	121
1954	cold	-1	1	11	8	127
1955	cold	-1	1	12	9	198
1956	neutral	0	0	8	4	69
1957	warm	1	1	8	3	86
1958	neutral	0	1	10	7	140
1959	neutral	0	1	11	7	99
1960	neutral	0	1	7	4	101
1961	cold	-1	1	11	8	222
1962	neutral	0	0	5	3	33
1963	neutral	0	0	9	7	118
1964	cold	-1	1	12	6	168
1965	warm	1	1	6	4	86
1966	neutral	0	0	11	7	140
1967	cold	-1	1	8	6	97
1968	neutral	0	0	7	4	41
1969	neutral	0	1	17	12	157
1970	cold	-1	0	10	5	65
1971	cold	-1	0	13	6	95
1972	warm	1	0	4	3	28
1973	cold	-1	0	7	4	52
1974	cold	-1	0	7	4	76
1975	cold	-1	1	8	6	92
1976	warm	1	0	8	6	85
1977	warm	1	0	6	5	46
1978	neutral	0	0	11	5	86
1979	neutral	0	0	8	5	96
1980	neutral	0	0	11	9	135
1981	neutral	0	0	11	7	114
1982	warm	1	0	5	2	37
1983	warm	1	0	4	3	32
1984	neutral	0	0	12	5	77
1985	cold	-1	0	11	7	110
1986	warm	1	0	6	4	38
1987	warm	1	0	7	3	48
1988	cold	-1	1	12	5	121
1989	cold	-1	1	11	7	140
1990	neutral	0	0	14	8	104
1991	warm	1	0	8	4	59
1992	warm	1	0	6	3	62
1993	warm	1	0	8	4	55
1994	warm	1	0	7	3	37
1995	cold	-1	1	19	11	237
1996	cold	-1	0	13	9	198
1997	warm	1	0	7	3	54