A windproof umbrella comprises a lower umbrella including a plurality of ribs, a lower canopy having a plurality of substantially trapezoidal sectors, and a center vent hole; and an upper umbrella including a plurality of rigid ribs, and an upper canopy having a plurality of sectors, wherein each rib of the upper umbrella is integral with each rib of the lower umbrella by means of a connection of a securing device for forming a gap between a corner of two adjacent sectors of the upper canopy and a corner of two corresponding adjacent sectors of the lower canopy, and the upper canopy is in covering relation to and capable of elastically separating from the main umbrella for exhausting the air from the space between the lower canopy and the upper canopy.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The present invention relates to umbrellas, and more particularly to an
 umbrella that is resistant to inversion from the wind.
 2. Description of Related Art
 Frequently, a wind will catch an umbrella user unaware and exert a force
 against the inner surface of the canopy which will cause the canopy to
 invert from its normal operable position to an upwardly convex position.
 Accordingly, specially designed umbrellas for rainy, windy weather have
 been proposed.
 As shown in FIGS. 1 and 2, this disclosed windproof umbrella C has an upper
 canopy C2 in covering relation to a lower canopy C1 in which the canopy C1
 has a plurality of (e.g., eight) vent holes H each correspondingly
 arranged in a sector C3. The upper canopy C2 is disposed over the vent
 holes H. The area of the upper canopy C2 is about three fifth of the area
 of the lower canopy C1. In order to secure the bottom edge of the upper
 canopy C2 to the umbrella C, eight elastic strips E are provided, wherein
 each of the elastic strips E with one end sewn to a corner of adjacent
 sectors C4 of the upper canopy C2, and the opposite end fitted tightly
 onto a tip of a rib G. During rainy, windy weather, wind I that is caught
 beneath the lower canopy C1 exit through the vent holes H and applies
 pressure to the underside of the upper canopy C2. This forces the upper
 canopy C2 lift away from the lower canopy C1 by means of the elastic
 strips E which are stretched to provide a path for wind I to escape the
 umbrella C. However, because the elastic strips E often stretched, its
 elasticity will be worn out after used for a predetermined times. As a
 result, the escape space for wind I will become larger.
 Thus, heavy rain may enter the space between the lower canopy sectors C3
 and the upper canopy sectors C4 and fall through bottom edges H1 of the
 vent holes H in the lower canopy C1 onto the user. Further, the
 manufacturing process of the windproof umbrella C is time consuming. For
 example, the upper canopy C2 comprises eight sectors C4. Two edges of each
 canopy sector C4 are secured to the ribs G by means of stitching
 respectively. As stated above, the corner of adjacent canopy sectors C4 is
 sewn to one end of the elastic strip E, and the opposite end of the
 elastic strip E is fitted tightly onto the tip of the rib G. Furthermore,
 the lower canopy C1 and the upper canopy C2 (via the stretched elastic
 strips E) both exert a force onto the tip of the rib G and thus, the
 elastic strips E are very taut. As a result, the elastic strips E are
 prone to be separated from the tips of the ribs G in case of the tip of
 the ribs G damaged. Once the elastic strips E are separated from the tips
 of the ribs G, upper canopy C2 will be lifted by the upward wind I caught
 beneath the canopy C1. Thus, the windproof function of the umbrella C will
 be completely worthless because the bottom edges H1 of the vent holes H
 are not covered anymore.
 An umbrella C which is similar in construction is disclosed in FIGS. 3 and
 4. A circular vent hole H is provided in the center portion of a canopy
 C1. A small circular piece of fabric C2 is positioned over the canopy C1
 in covering relation to the vent hole H. The area of the piece of fabric
 C2 is about two fifth of the area of the canopy C1. Accordingly, an
 elastic strip E, which attached from a corner of adjacent fabric sectors
 C3 to a tip G, will have a relatively longer length as compared with the
 elastic strip E illustrated in FIGS. 1 and 2. Further, because the price
 of elastic strip is higher than that of fabric, the cost of manufacturing
 the umbrella shown in FIGS. 3 and 4 will be higher than that of FIGS. 1
 and 2 in spite of the saving in fabric. Furthermore, the umbrella of FIGS.
 3 and 4 has the same disadvantage as that of FIGS. 1 and 2 in respect of
 the elastic fatigue of the elastic strips.
 Thus, it is desirable to provide a windproof umbrella to overcome the above
 drawbacks of prior art.
 SUMMARY OF THE INVENTION
 It is an object of the present invention to provide a windproof umbrella
 which comprises a lower umbrella including a shank, a lower hub of a
 double-layer stationary hub, a movable ring being capable of sliding along
 the shank, a plurality of ribs each having a fixed end and a free end, a
 plurality of spreaders each with one end pivotally secured to the ring and
 the other end pivotally secured to a predetermined position of one of the
 ribs, a lower canopy having a plurality of substantially trapezoidal cloth
 sectors in which a seam between sectors of the lower canopy is stitched
 from a top to a bottom of each of the ribs along its length to the free
 end portion thereof, and a substantially polygonal vent hole provided in a
 center portion of the lower umbrella; an upper umbrella including an upper
 hub of a double-layer stationary hub, a plurality of ribs each having a
 free end, and an upper canopy having a plurality of substantially
 triangular cloth sectors; and a ferrule on a top of the upper umbrella;
 wherein each of the ribs of the upper umbrella is integral with each of
 the ribs of the lower umbrella by means of a connection of a securing
 device for forming a gap between a corner of two adjacent sectors of the
 upper canopy and a corner of two corresponding adjacent sectors of the
 lower canopy, and the upper canopy is in covering relation to and capable
 of elastically separating from the lower umbrella for exhausting the air
 from the space between the lower canopy and the upper canopy.
 It is another object of the present invention to provide a windproof
 umbrella which does not employ any elastic strip.
 It is a further object of the present invention to provide a durable
 windproof umbrella by providing a securing device for connecting the upper
 umbrella and the lower umbrella.
 It is still a further object of the present invention to provide a
 windproof umbrella which is time saving in assembly and cost effective by
 eliminating the process of stitching the upper canopy to the ribs of the
 upper umbrella.
 The above and other objects, features and advantages of the present
 invention will become apparent from the following detailed description
 taken with the accompanying drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
 Referring to FIGS. 5 to 11, a windproof umbrella of the present invention
 includes a lower umbrella 1, an upper umbrella 2 in covering relation to
 the lower umbrella 1, and a ferrule 3 on a top of the upper umbrella 2.
 The lower umbrella 1 comprises a shank 10, a lower hub 21B of double-layer
 stationary hub 21, a ring 11 being capable of sliding along the shank 10,
 a plurality of ribs 13 and having a fixed end pivotally secured to the
 lower hub 21B and a free end, each free end having a tip 14, a plurality
 of spreaders 12 each with one end pivotally secured to the ring 11 and the
 other end pivotally secured to a predetermined position of one of the ribs
 13, and a lower canopy 15 having a plurality of (e.g., eight) generally
 trapezoidal cloth sectors 150 in which the seam between sectors 150 is
 stitched from the top to the bottom of each rib 13 along its length to the
 tip 14. A substantially polygonal (e.g., octagonal) vent hole 16 is
 provided in the center portion of the lower umbrella 1.
 The upper umbrella 2 comprises an upper hub 21A of the double-layer
 stationary hub 21, a plurality of rigid ribs 23 each having a fixed end
 pivotally secured to the upper hub 21A of the double-layer stationary hub
 21, and an upper canopy 25 having a plurality of (e.g., eight) generally
 triangular cloth sectors 250 where a small cloth bag member 251 is
 provided to the outer perimeter of each sectors 250 and positioned
 thereunder. A free end of each of the ribs 23 is allowed to be received
 within the bag member 251 for being integral with the cloth sector 25 for
 forming a whole upper umbrella 2.
 As best shown in FIGS. 7 and 9, an I-shaped securing device 4 comprises a
 lower tube member 41, an upper tube member 43, and a rod member 42 for
 connecting the tube members 41 and 43.
 The assembly procedure of the windproof umbrella of the present invention
 is best shown in FIGS. 7 and 8. In assembly, first the cloth sectors 150
 are stitched together to form the lower canopy 15 and then each sector 150
 is further stitched at one portion of each rib 13 and the tip 14 for
 forming the lower umbrella 1; second the cloth sectors 250 are stitched
 together to form the upper canopy 25; insert an upper end of each of the
 ribs 13 going through the corresponding lower tube member 41 and then
 pivotally secured to the lower hub 21B, and the upper end of each of the
 ribs 23 also going through the corresponding upper tube member 43 and then
 pivotally secured to the upper hub 21A, a predetermined distance such that
 the free end of the rib 23 will be received within the bag member 251; and
 finally, insert the ferrule 3 onto a top of the double-layer stationary
 hub 21.
 It is apparent that a plurality of gaps 161 exist between the upper canopy
 25 and the lower canopy 15 due to the provision of the securing devices 4.
 As such, during windy weather, wind that is caught beneath the lower
 canopy 15 exists through the vent hole 16 and applies pressure to the
 upper canopy 25. This forces the upper canopy 25 lift away from the lower
 canopy 15 a predetermined distance (i.e., at least the height of the rod
 member 42) to provide a path through the gap 161 for the wind to escape
 the lower umbrella 1.
 As best shown in FIGS. 10 and 11, the vent hole 16 is completely covered by
 the upper canopy 25, i.e., the upper canopy 25 extends to a distance
 beyond a perimeter of the vent hole 16. As such, during rainy weather,
 rain may not enter the gap 161 between each upper canopy sector 250 and
 each lower canopy sector 150 and fall through bottom edge of the vent hole
 16 on the user.
 As seen from the FIG. 14, an another preferred embodiment of a Z-shaped
 securing device 4A of the present invention is shown. The Z-shaped
 securing device 4A comprises a lower tube member 41A, an upper tube member
 43A and a flexible member 42A for elastically connecting the tube members
 41A and 43A.
 The purpose of provision of the flexible member 42A between tube members
 41A and 43A is rendered to have the lower end of the upper umbrella 2 to
 be closely contacted with the lower umbrella 1, it means that the outlet
 of the gap 161 between each two lower end of ribs 23 of the upper umbrella
 2 and the corresponding two ribs 13 of the lower umbrella 1 is closed in
 normal, as shown in FIGS. 12 and 13, however, the outlet of the gap 161
 will be automatically opened during windy weather. In other words, when
 wind is caught beneath, the lower canopy 15 through the vent hole 16 and
 applies pressure to the upper canopy 25, this force will make the outlet
 of the gap 161 to be open for the wind to escape out of the umbrella 1, as
 best seen in FIG. 15.
 Advantages Over The Prior Art
 As described above, the invention is embodied without any elastic strip
 provided, and thus is durable. Further, the invention can preserve the
 umbrella's appearance by providing a relatively small upper umbrella over
 the relatively large lower umbrella. Furthermore, the assembly of the
 invention is easy by eliminating the process of stitching the upper canopy
 to the ribs of the upper umbrella, and thus is cost effective. Above all,
 the invention can prevent the inversion of the umbrella while also
 preventing rain from falling on the user.
 While the invention herein disclosed has been described by means of
 specific embodiments, numerous modifications and variations could be made
 thereto by those skilled in the art without departing from the scope of
 the invention set forth in the claims.