1. 15 th EMS / 12 TH ECAM, 7-11 September 2015,Sofia, Bulgaria HAIL CHARACTERISTICS IN NORTHERN GREECE Dimitrios Foris (1) and Vasileios Foris (2) (1) Meteorological Applications Center, ELGA, Thessaloniki, Greece, d.foris@yahoo.comd.foris@yahoo.com (2) Physics Department, Aristotle University of Thessaloniki, Greece 1. INTRODUCTION During the warm period of the year, Greece is exposed to the natural hazard of hail. In order to alleviate its consequences to agriculture, a National Hail Suppression Program (NHSP) was established in 1984 and is running since then. The aim of hail suppression is accomplished by cloud seeding with AgI delivered to potential hailstorms from specially equipped aircrafts. One of the project areas is the plain of Central Macedonia. A hailpad network, consisting of 150 hailpads is installed in this area. Only recently, hailpads started to be digitally analyzed with the aid of specially designed software, IMAGE PRO PLUS. In this study data of the years 2009 – 2013 were taken into account. 2. RESULTS According to the official definition, hail is constituted of stones with diameter > 5 mm. For the 5 years 2009-2013, a total of 27,404 hailstones were recorded. These stones were produced out of 141 hailstorms (28 of which non-seeded), which occurred on 75 hail days and from the analysis of 357 hailpads (36 of which refer to non-seed conditions). From the 27,404 stones only 2474 refer to non-seed cases (9%). On the average, 15 hail days occur each year, 71 hailpads are hit by 28 hailstorms and 5481 hailstones are recorded. The monthly distributions of hail days, hailstorms, pads hit and pads per storm appear in Fig.1.  There is a great variability from year to year (depending on the activity).  May and June show a maximum of hail days and storms.  Hit pads show a pronounced max in June, which though is largely due to one supercell on 10 June 2013.  The vast majority of storms (60%) hit only one hailpad, the outliers indicating intense and extended storms sweeping out the hailpad network. Fig.1: Monthly distributions of hail days, hailstorms, pads hit and pads per storm 3. HAIL SIZE DISTRIBUTION In the classical classification scheme, 95% of the stones fall in the pea-size category (5-12 mm), 4.7% in grape-size (12-20 mm) and only 0.3% in walnut- size (21-33 mm). The 42% of stones have size 6-7 mm, 86% are of size 5-10 mm and only 10% have size 11-15 mm (Fig.2) 4. SEEDED AND NON-SEEDED CASES Seeding seems to reduce small diameters (<7 mm) and increase the bigger ones. One reason is that the sample of non-seeded storms is small (<10%) and this behavior may change if more cases will be included. Another reason may be that non-seeded storms are less intense and so are of second priority for seeding (Fig.3). 5. HAILSTONE CHARACTERISTICS (average of all hailpads) These are presented in Table 1. Hailstones are in general elliptical. Their orientation reveals also the commonest W to E motion of storms. The KE distribution according to TORRO scale reveals that 70% of all hit pads falls in H0 category, 22% in H1, 7.5% in H2 and less than 0.5% in H3 (Fig.4). This reflects the success of hail suppression method. Number of hailstones (per m 2 )961 Average size of hailstones (mm per m 2 )7.634 Maximum size of hailstones (mm per m 2 )12.157 Mass of hailstones (kg per m 2 )0.294 Momentum of hailstones (kg.m/s per m 2 )3.901 Kinetic energy of hailstones (J per m 2 )26.863 Area occupied by hailstones (mm 2 )21.824 Area hit (percentage of area exposed)2.313 Aspect (ratio of semi-axes)1.544 Orientation (of major semi-axis) (deg)90.720 Table 1. Hailstone characteristics. 6. SPATIAL DISTRIBUTION Isopleths of hail frequency (for 27 years) reveals that storms overcoming mountains are orographically enhanced (Fig.5). Fig.5 7. RESULTS  Convective activity is max in May & June  Most hailstones are of pea size  Seeding seems to reduce the number of small-size stones  92% of pads are in H0-H1 TORRO scale  Storms are affected by orography